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9.3: Evidence for Evolution - Biology

9.3: Evidence for Evolution - Biology


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A Horse Is a Horse, of Course, of Course

This drawing was created in 1848, but it's likely that you recognize the animal it depicts as a horse. Although horses haven't changed that much since this drawing was made, they have a long evolutionary history during which they changed significantly. How do we know? The answer lies in the fossil record.

Evidence from Fossils

Fossils are a window into the past. Scientists who find and study fossils are called paleontologists. How do they use fossils to understand the past? Consider the example of the horse, outlined in figure (PageIndex{2}). Fossils spanning a period of more than 50 million years show how the horse evolved.

The oldest horse fossils show what the earliest horses were like. They were only 0.4 m tall, or about the size of a fox, and they had four long toes. Other evidence shows they lived in wooded marshlands, where they probably ate soft leaves. Over time, the climate became drier, and grasslands slowly replaced the marshes. Later fossils show that horses changed as well.

  • They became taller, which would help them see predators while they fed in tall grasses. Eventually, they reached a height of about 1.6 m.
  • They evolved a single large toe that eventually became a hoof. This would help them run swiftly and escape predators.
  • Their molars (back teeth) became longer and covered with hard cement. This would allow them to grind tough grasses and grass seeds without wearing out their teeth.

Evidence from Living Species

Scientists can learn a great deal about evolution by studying living species. They can compare the anatomy, embryos, and DNA of modern organisms to help understand how they evolved.

Comparative Anatomy

Comparative anatomy is the study of the similarities and differences in the structures of different species. Similar body parts may be homologous structures or analogous structures. Both provide evidence for evolution.

Homologous structures are structures that are similar in related organisms because they were inherited from a common ancestor. These structures may or may not have the same function in the descendants. Figure (PageIndex{3}) shows the upper appendages of several different mammals. They all have the same basic pattern of bones, although they now have different functions. All of these mammals inherited this basic bone pattern from a common ancestor.

Analogous structures are structures that are similar in unrelated organisms. The structures are similar because they evolved to do the same job, not because they were inherited from a common ancestor. For example, the wings of bats and birds, shown in the figure that follows, look similar on the outside and have the same function. However, wings evolved independently in the two groups of animals. This is apparent when you compare the pattern of bones inside the wings.

Comparative Embryology

Comparative embryology is the study of the similarities and differences in the embryos of different species. Similarities in embryos are likely to be evidence of common ancestry. All vertebrate embryos, for example, have gill slits and tails. All of the embryos in Figure (PageIndex{4}), except for fish, lose their gill slits by adulthood, and some of them also lose their tail. In humans, the tail is reduced to the tail bone. Thus, similarities organisms share as embryos may no longer be present by adulthood. This is why it is valuable to compare organisms in the embryonic stage.

Vestigial Structures

Structures like the human tail bone are called vestigial structures. Evolution has reduced their size because the structures are no longer used. The human appendix is another example of a vestigial structure. It is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food, but it serves no purpose in the human body today. Why do you think structures that are no longer used shrink in size? Why might a full-sized, unused structure reduce an organism’s fitness?

Comparing DNA

Darwin could compare only the anatomy and embryos of living things. Today, scientists can compare their DNA. Similar DNA sequences are the strongest evidence for evolution from a common ancestor. Look at the diagram in Figure (PageIndex{5}). The diagram is a cladogram, a branching diagram showing related organisms. Each branch represents the emergence of new traits that separate one group of organisms from the rest. The cladogram in the figure shows how humans and apes are related based on their DNA sequences.

Evidence from Biogeography

Biogeography is the study of how and why organisms live where they do. It provides more evidence for evolution. Let’s consider the camel family as an example.

Biogeography of Camels: An Example

Today, the camel family includes different types of camels (Figure (PageIndex{6})). All of today’s camels are descended from the same camel ancestors. These ancestors lived in North America about a million years ago.

Early North American camels migrated to other places. Some went to East Asia via a land bridge during the last ice age. A few of them made it all the way to Africa. Others went to South America by crossing the Isthmus of Panama. Once camels reached these different places, they evolved independently. They evolved adaptations that suited them for the particular environment where they lived. Through natural selection, descendants of the original camel ancestors evolved the diversity they have today.

Island Biogeography

The biogeography of islands yields some of the best evidence for evolution. Consider the birds called finches that Darwin studied on the Galápagos Islands (Figure (PageIndex{7}))). All of the finches probably descended from one bird that arrived on the islands from South America. Until the first bird arrived, there had never been birds on the islands. The first bird was a seed eater. It evolved into many finch species, each adapted for a different type of food. This is an example of adaptive radiation. This is the process by which a single species evolves into many new species to fill available ecological niches.

Eyewitnesses to Evolution

In the 1970s, biologists Peter and Rosemary Grant went to the Galápagos Islands to re-study Darwin’s finches. They spent more than 30 years on the project, but their efforts paid off. They were able to observe evolution by natural selection actually taking place.

While the Grants were on the Galápagos, a drought occurred, so fewer seeds were available for finches to eat. Birds with smaller beaks could crack open and eat only the smaller seeds. Birds with bigger beaks could crack open and eat seeds of all sizes. As a result, many of the smaller-beaked birds died in the drought, whereas birds with bigger beaks survived and reproduced. As shown in Figure (PageIndex{8}), within 2 years, the average beak size in the finch population increased. In other words, evolution by natural selection had occurred.

Review

  1. How do paleontologists learn about evolution?
  2. Describe what fossils reveal about the evolution of the horse.
  3. What are vestigial structures? Give an example.
  4. Define biogeography.
  5. Describe an example of island biogeography that provides evidence of evolution.
  6. Humans and apes have five fingers they can use to grasp objects. Are these analogous or homologous structures? Explain.
  7. Compare and contrast homologous and analogous structures. What do they reveal about evolution?
  8. Why does comparative embryology show similarities between organisms that do not appear to be similar as adults?
  9. What does a cladogram show?
  10. Explain how DNA is useful in the study of evolution.
  11. A bat wing is more similar in anatomical structure to a cat forelimb than to a bird wing. Answer the following questions about these structures.
    1. Which pairs are homologous structures?
    2. Which pairs are analogous structures?
    3. Based on this, do you think a bat is more closely related to a cat or to a bird? Explain your answer.
    4. If you wanted to test the answer you gave to part c, what is a different type of evidence you could obtain that might help answer the question?
  12. True or False. Fossils are the only type of evidence that supports the theory of evolution.
  13. True or False. Adaptive radiation is a type of evolution that produces new species.

Explore More

The Galapagos finches remain one of our world's greatest examples of adaptive radiation. Watch as these evolutionary biologists detail their 40-year project to document the evolution of these famous finches:


Evidence for evolution 5.1

There are different types of evidence which support the theory of evolution, from fossils to selective breeding. This topic covers a range of evidence and some of the concepts related to inheritance, natural selection and evolution.

Key concepts

Learn and test your biological vocabulary for 5.1 Evidence for evolution using these flashcards.

Essentials - quick revision through the whole topic

These slides summarise the essential understanding and skills in this topic.
They contain short explanations in text and images - good revision for all students.

Read the slides and look up any words or details you find difficult to understand.

Exam style question about pentadactyl limbs

The question below, requires an understanding of the structure of the pentadactyl limb and the differences and adaptations which can be found in different animals. In this case the examples are birds and humans.

Compare and contrast the pentadactyl limb of mammals and birds. [4]

Describe how the structure of the wing and hand is adapted to each different function [2]

Explain how the existence of these structures in birds and mammals provides evidence supporting the theory of evolution. [3]

Click the + icon to see a model answer.

Model answer

The skill of this answer is to ensure that the points make logical connections between the two species.

Compare and contrast the pentadactyl limb of mammals and birds. [4]

  • The bird wing and the human arm both have a humerus, ulna and a radius bone.
  • Both limbs also have several wrist bones
  • They are an example of homologous structures - evolving from a single origin.
  • There are only three "finger bones" in the bird wing and five in the human hand.
  • The finger bones are longer in the bird wing than in the human hand.

Describe how the structure of the wing and hand is adapted to each different function. [2]

The skill in this answer is to link a structure to it's function

  • The wrist bones of birds have long shapes, allowing the attachment of feathers.
  • The position of the thumb opposite the other fingers on the human hand allow it to hold tools

Explain how the existence of these structures in birds and mammals provides evidence supporting the theory of evolution. [3]

This answer requires knowledge beyond the data provided.

The pentadactyl limb is found in Birds Mammals as well as reptiles and amphibians

They all share the same organisation of bones even though these animals use a wide variety of methods of locomotion. This is known as 'adaptive radiation'.

The simplest explanation of the existence of this bone arrangement in so many different animals is that it evolved once in history in a common ancestor of all the animals. These animals have since evolved by divergent evolution to use the structures in a variety of ways.

It would be very unlikely that two separate evolutionary processes would arrive at the same structure as complex as a pentadactyl limb.

Revision summary list for 5.1 Evidence for evolution

  • A change in the heritable characteristics of a species is evolution.
  • Evidence for evolution comes from
    • the fossil record.
    • Selective breeding of domesticated animals
    • "Adaptive radiation" can explain similar structures with different functions.
    • Isolated populations can produce separate species by divergent evolution.
    • Continuous variation across the geographical range of related populations matches the concept of gradual divergence.
    • Describe how the dark bark leads to the development of melanistic insects in polluted areas.
    • Compare and contrast the pentadactyl limb of mammals, birds, amphibians and reptiles.
    • Describe how each limb structure is adapted to each different method of locomotion.

    Mindmaps

    These diagram summaries cover the main sections of topic 4.3 Carbon cycle.
    Study them and draw your own list or concept map, from memory if you can.

    Test yourself - multiple choice questions

    This is a self marking quiz containing questions covering the topic outlined above.
    Try the questions to check your understanding.

    5.1 Evidence for evolution 1 / 1

    Which of the following statements is not true about Evolution?

    It's a gradual cumulative change.

    It is learned by animals during their lifetime.

    It concerns characteristics which can be inherited.

    It happens at the species level.

    "A change in the heritable characteristics of a species" is evolution.
    Most frequently this is a gradual cumulative change.

    Changes to individual organisms during their lifetime are not evolution, as the change has to be heritable.
    Evolution occurs a the species level because individuals survive or die which changes the composition of the members of the species.

    Which of the points below provide evidence in support of evolution?

    Selective breeding of food crops.

    The wings of birds and bats

    Evidence for evolution in this list comes from

    What might be expected to happen if two populations of a species of snail are separated by a river which has very different environmental conditions on each bank, for example a woodland and a golf course?

    One population will become better adapted to the environment.

    Interbreeding of the two population.

    If the are different selection pressures in the two environments, then snails will be affected by different natural selection. This is divergent selection.

    Lake Malawi has a surface area of almost 30 000 km 2 and is home to 1000 species of Cichlid fish.

    The diagram of Cichlid fish varieties illustrates a continuous variation in their features across the geographical range of the lake.

    It is understood that all but five species are descendants of a single original colonist species?

    What is the name of the process which forms this sort diversity?

    Gradual production of new species.

    Continuous variation across the geographical range of related populations matches the concept of gradual divergence.

    The image below shows adaptations in the wings of modern insects.

    There is evidence that ancestors of all these insects had two pairs of wings.

    What is the biological term for structures like these?

    All the insects share the feature of having 2 pairs of 'wing-like' structures although there are differences in each animal.

    This suggests that they all shared a common ancestor, with 2 pairs of wings.

    What is the best description of selective breeding?

    A dog breeder selects the animals with preferred features for his breeding programme.

    A farmer selects sheep with the highest quality wool at a market.

    A zoologist in a safari park puts animals together for breeding to increase genetic diversity.

    A pet owner keeps carefully selected aquarium fish so there are no predators in the tank.

    Selective breeding provides evidence for evolution because it shows that when a breeder chooses the animals for the breeding programme which have specific features, the offspring have more of these features.

    What natural process causes the increase in frequence of the melanistic form of the Peppered moth if pollution causes the vegetation it lives on to be covered in smoke and become darker?

    Natural selection causes an increase in the number of dark moths because the birds which eat the moths cannot see the dark forms so easily, as they have better camoflage in polluted environments.


    Biological Evidence

    Biogeography

    The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time. Broad groups that evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana. The presence of members of the plant family Proteaceae in Australia, southern Africa, and South America is best due to their appearance prior to the southern supercontinent Gondwana breaking up.

    The great diversification of marsupials in Australia and the absence of other mammals reflect Australia’s long isolation. Australia has an abundance of endemic species—species found nowhere else—which is typical of islands whose isolation by expanses of water prevents species migration. Over time, these species diverge evolutionarily into new species that look very different from their ancestors that may exist on the mainland. The marsupials of Australia, the finches on the Galápagos, and many species on the Hawaiian Islands are all unique to their one point of origin, yet they display distant relationships to ancestral species on mainlands.

    Molecular Biology

    Like anatomical structures, the structures of the molecules of life reflect descent with modification. Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material and in the near universality of the genetic code and the machinery of DNA replication and expression. Fundamental divisions in life between the three domains are reflected in major structural differences in otherwise conservative structures such as the components of ribosomes and the structures of membranes. In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that would be expected from descent and diversification from a common ancestor.

    DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events that allow the free modification of one copy by mutation, selection, or drift (changes in a population’s gene pool resulting from chance), while the other copy continues to produce a functional protein.


    Evidence for Evolution

    There is much evidence for the process of evolution and that natural selection is the mechanism by which it occurs.

    Palaeontology

    In the oldest rocks there are very few types of organism that have been found. Those that have been discovered have a relatively simple structure.

    In younger rocks there is a greater variety of organisms and they are much more complex. Species do appear and disappear from the fossil record. This disappearance is probably due to climate or other environmental changes.

    The record is not continuous though, which can lead to questions about how much store we should put by evidence from fossils.

    It is not surprising though that there are gaps since many organisms would decompose quickly, well before they would have had a chance to fossilize.

    Other organisms would have been eaten, many would have been soft bodied and of course, many have not even been found.

    It has also been suggested that evolution is not a gradual process, occurring all the time. Instead, it may not occur at all for a long period of time and then, as conditions change, occur very rapidly. If this is the case, so-called "missing links" may never be found, as change occurred so rapidly.

    Many island populations will evolve rapidly, due to high levels of competition and several niches may be filled by different species originating from a single ancestor.

    Comparative anatomy

    Also called adaptive radiation.

    When a group of organisms all possess a structure that appears to have come from a common ancestor and which has the same microscopic structure and body position, as well as other features, they are said to have homologous structures.

    These may have been specialised in each group for a particular function (e.g, the pentadactyl limb. All mammals share this, but in some it is used as a hand, in others as a flipper, in others as a wing and so on.

    They look different in different species and are used for different things because of the particular selection pressures on those species.

    Convergent evolution

    This is the opposite of adaptive radiation it is where structures that on first appearance are similar but are actually found to be unrelated.

    These structures are said to be analogous. In these cases, selection pressures from the environment in which each species lives have caused particular structures to be advantageous.

    Comparative Biochemistry

    Various chemicals have been studied in order to find evidence of evolution, DNA and proteins such as cytochrome c in the electron transport chain of respiration are often used.

    Looking at the order of bases in the lengths of DNA and the order of amino acids in a protein, it is possible to determine how similar they are in different species.

    Species that are closely related have the most similar DNA and proteins, those that are distantly related share far fewer similarities. A comparison of DNA sequences show that it is 99.9% certain that chimpanzees are humans' closest relatives.


    A little clarification:

    The standard contains this clarification statement:

    Emphasis is on a conceptual understanding of the role each line of evidence has relating to common ancestry and biological evolution. Examples of evidence could include similarities in DNA sequences, anatomical structures, and order of appearance of structures in embryological development.

    Let’s look at some lines of evidence and their role in common ancestry and evolution:

    DNA Evidence

    DNA technology is the most advanced and accurate understanding of inheritance available to science. By genotyping a parent and their offspring, we can actually “read” the genetic code and see which genes each parent contributed to the offspring. Further, scientists have documented and watched the process of cellular division under a microscope, confirming Mendel’s Laws of Inheritance.

    Because all organisms on Earth share the same basic format of DNA, as well as the basic mechanisms for replicating DNA and expressing proteins, it can also be assumed that all of life on Earth arose from a common ancestor (or at least a very small handful of primitive, bacteria-like organisms). We can actually measure how closely related two species are by comparing their DNA. Bananas and humans, for example, share 25% of their DNA. While this may seem like a lot, humans and chimpanzees share about 99% of our DNA. This shows that humans, chimpanzees, and bananas all share a common ancestor. But, it also tells us that humans and chimpanzees share a common ancestor much more recently than humans and bananas.

    Anatomical Structures

    Before the days of DNA, scientists had to rely on more primitive methods to model inheritance. Mendel simply counted phenotypic ratios of offspring, because he had no way to actually analyze the genotypes of organisms. Punnett squares are also used as a way to visualize the alleles of different genes, and how they interact.

    One common method of (fairly) accurately determining how closely two species are related is computational phylogenetics. In the simplest form of this method, scientists create a spreadsheet containing a number of individual traits and anatomical features present on three or more different animals. The animals that share the most traits are considered more closely related than animals that share fewer traits. (A more accurate version of this compares actual genes carried by different animals to see which sets of genes various animals share.)

    Embryology

    The study of embryology is the study of embryos as they develop. Though this entire field was created after the theories of common ancestors and biological evolution, embryology has many empirical lines of supporting evidence.

    For example, all vertebrate embryos go through an embryological phase in which they have gill slits. That suggests that all vertebrates are related to fish, though the actual formation of functional gills is replaced by lungs in terrestrial vertebrates. Vertebrate embryos have several other traits that are seen across vertebrate species, while adults may not retain these traits. Looking at the first few rounds of cellular division also shows that animals essentially employ two different methods of forming different tissue layers, allowing animals to broadly be classified into deuterostomes and protostomes.

    Fossil Evidence

    The fossil record, though incomplete and hard to study, offers many insights into evolution and common ancestors. One of the main concepts from the fossil record that supports evolution is the presence of transitional forms – organisms that have traits from both extinct groups and extant groups. Great examples here include feathers on dinosaurs and aquatic adaptations on the (mostly) terrestrial ancestors of whales.


    Evidence for Evolution

    You are probably aware that the concept of evolution still generates controversy today, despite its wide acceptance. In The Origin of the Species, Darwin mentioned humans only once, predicting, ”Light will be thrown on the origin of man and his history.” Nevertheless, some people immediately distorted its far-reaching message about the unity of life into nearsighted shorthand: humans “came from” monkeys (Figure 1).

    Figure 1: In Darwin’s time and today, many people incorrectly believe that evolution means “humans come from monkeys.” This interpretation does not do justice to Darwin’s theory, which holds that all species share common ancestry.

    Evolution relates all of life – not just humans and monkeys. Biological evolution, like all scientific theories, is much more than just an opinion or hypothesis, it is based on evidence.

    In science, a theory is an explanation which ties together or unifies a large group of observations. Scientists accept theories if they have a great deal of supporting evidence. In The Origin of the Species, Darwin took the time to compile massive amounts of fossil and biological evidence to support his ideas of natural selection and descent from common ancestors. He clearly and effectively compared animal breeding (artificial selection), which was familiar to most people, and natural selection. Because Darwin provided so much evidence and used careful logic, most scientists readily accepted natural selection as a mechanism for change in species. Since Darwin’s time, additional fossil and biological data and new fields of biology such as genetics, molecular biology, and biogeography have dramatically confirmed evolution as a unifying theory – so much so that eminent biologist Theodosius Dobzhansky wrote that “Nothing in biology makes sense except in the light of evolution.”

    The Fossil Record: Structural Changes Through Time

    Few would argue that dinosaurs roamed Earth in the past, but no longer exist. The fossil record is a revealing window into species that lived long ago. Paleontologists have carefully analyzed the preserved remains and traces of animals, plants, and even microorganisms to reconstruct the history of life on Earth. Relative (rock layer position) and absolute (radioisotope) dating techniques allow geologists to sequence the fossils chronologically and provide a time scale. Geology also reveals the environmental conditions of past species.

    For many reasons, the fossil record is not complete. Most organisms decomposed or were eaten by scavengers after death. Many species lacked hard parts, which are much more likely to fossilize. Some rocks and the fossils they contained have eroded and disappeared. Moreover, much of evolution happens in the small populations that survive changes in environmental conditions, so the chance that intermediates will fossilize is low. Nevertheless, the current record includes billions of fossils – over 300 million from Los Angeles’ LaBrea Tar Pits alone, and an estimated 800 billion in South Africa’s Beaufort Formation. Analysts have identified 250,000 species among these remains.

    Although the fossil record is far more detailed today than in Darwin’s time, Darwin was able to use it as powerful evidence for natural selection and common descent. Throughout geological history, species that appear in an early rock layer disappear in a more recent layer. Darwin argued that a species’ appearance recorded its origin, and that its disappearance showed extinction. Moreover, he noted remarkable similarities among structures in differing species, supporting common ancestry. Finally, he could often correlate environmental conditions with structures, supporting his idea that natural selection led to adaptations which improved survival within certain habitats.

    As an example, let’s analyze a relatively complete set of fossils which record the evolution of the modern horse. Figure 2 sequences five species which show major evolutionary changes. The oldest fossil shows a fox-sized animal with slender legs and nearly vertical digits: Hyracotherium bit and chewed soft leaves in wooded marshlands. Geology and paleontology suggest that the climate gradually dried, and grasslands slowly replaced the marshes. Mesohippus was taller, with fewer, stronger digits – better able to spot and run from predators, and thus more likely to survive and reproduce in the new grasslands. Merychippus was taller still, and kept only one, enlarged digit – a hoof to run fast on the hard ground. By Pliohippus time, molar teeth had widened and elongated to grind the tough grasses. These fossils show gradual structural changes which correspond to changes in the environment. They appear to show a smooth, linear path directed toward the “goal” of the modern horse, but this is deceiving. These five fossils are merely “snapshots” of a bushy family tree containing as many as 12 genera and several hundred species. Some transitions are smooth progressions others are abrupt. Together, they support natural selection and descent with modification from common ancestors.

    Figure 2: The fossil record for relatives of the modern horse is unusually complete, allowing us to select a few which show major change over time. These changes can be correlated with environmental changes, supporting the ideas of evolution and natural selection. However, the linear arrangement is misleading addition of all known fossils would show a branching, bushy path of descent and common ancestry.

    Comparative Anatomy and Embryology

    The evidence Darwin presented in The Origin of Species included not only fossils but also detailed comparisons of living species at all life stages. Naturalists in Darwin’s time were experts in comparative anatomy – the study of the similarities and differences in organisms’ structures (body parts). At different times during his life, Darwin studied the comparative anatomy of closely related species of marine mammals, barnacles, orchids, insectivorous plants, and earthworms.

    Figure 3: Darwin’s Theory of Evolution explains both the similarities and the differences among living things. All flowering plants share leaves, petals, stamens, and pistil, but orchids have parallel-veined leaves and flowers with lips and fused stamens and pistil, while sundews have leaves with branching veins and flowers with equal petals and separate stamens and pistil. The two species of orchid (A and B) share a recent common ancestor, whereas all three species share a more distant common ancestor.

    Species which share many similarities are closely related by a relatively recent common ancestor. For example, all orchids share parallel-veined leaves, two-sided flowers with a “lip,” and small seeds (Figures 3A and 3B). Species which share fewer similarities, sharing only basic features, are related by relatively distant ancestor. The sundew, one of the insectivorous plants Darwin studied, shares leaves and petals with orchids, but the leaves are wide with branching veins and the flowers are radially symmetrical rather than two-sided (Figure 3C). The many species of orchids, then, share a recent common ancestor, but they also share a more distant ancestor with the sundew.

    Homologous and Analogous Structures

    Similarities can show two different kinds of relationships, both of which support evolution and natural selection.

    (1) Similarities shared by closely related species (species who share many characteristics) are homologous, because the species have descended from a common ancestor which had that trait. Homologous structures may or may not serve the same function. Figure 4 shows the forelimbs of mammals, considered homologous because all mammals show the same basic pattern: a single proximal bone joins a pair of more distal bones, which connect to bones of the wrist, “hand,” and digits. With this basic pattern, bats build wings for their lives in the air, whales form fins for their lives in the sea, and horses, as we have seen, construct long, hoofed legs for speed on land. Therefore, homologous structures support common ancestry.

    Figure 4: Homologous structures are similarities throughout a group of closely related species. The similar bone patterns in bat’s wings, dolphin’s flippers, and horse’s legs support their descent from a common mammalian ancestor.

    (2) Similarities shared by distantly related species may have evolved separately because they live in similar habitats. These structures are analogous because they serve similar functions, but evolved independently. Figure 5 compares the wings of bats, bird, and pterosaurs. Bats evolved wings as mammals, pterosaurs as dinosaurs, and birds from a separate line of reptiles. Their wings are analogous structures, each of which evolved independently, but all of which suit a lifestyle in the air. Note that although the wings are analogous, their bones are homologous: all three share a common but more distant vertebrate ancestor, in which the basic forelimb pattern evolved. Because analogous structures are independent adaptations to a common environment, they support natural selection.

    Figure 5: The wings of pterosaurs, bats, and birds illustrate both homologous and analogous structures. Similarities in the patterns of bones are due to descent from a common vertebrate (reptilian) ancestor, so they are homologous. However, the wings of each evolved independently, in response to similar environments, so they are analogous, and provide evidence for natural selection.

    Embryology is a branch of comparative anatomy which studies the development of vertebrate animals before birth or hatching. Like adults, embryos show similarities which can support common ancestry. For example, all vertebrate embryos have gill slits and tails, shown in Figure 6. The “gill slits” are not gills, however. They connect the throat to the outside early in development, but in many species, later close only in fish and larval amphibians do they contribute to the development of gills. In mammals, the tissue between the first gill slits forms part of the lower jaw and the bones of the inner ear. The embryonic tail does not develop into a tail in all species in humans, it is reduced during development to the coccyx, or tailbone. Similar structures during development support common ancestry.

    Figure 6: Comparative embryology reveals homologies which form during development but may later disappear. All vertebrate embryos develop tails, though adult humans retain only the coccyx. All vertebrate embryos show gill slits, though these develop into gill openings only in fish and larval amphibians. In humans, gills slits form the lower jaw and Eustachian tube. Many scientists consider developmental homologies evidence for ancestry, although some embryologists believe that these particular drawings exaggerate the similarities.

    Vestigial Structures

    Structures which are reduced and perhaps even nonfunctional, such as the human tail and the human appendix, are considered vestigial structures. The tail, of course, functions for balance in many mammals, and the human appendix may have served digestive functions in herbivorous ancestors. Whales, which evolved from land mammals, do not have legs or hair as adults both begin to develop in embryos, but then recede. Vestigial leg bones remain, buried deep in their bodies, shown in Figure 7A.

    Figure 7: Vestigial structures show evolutionary reduction or loss of unneeded structures which were useful to ancestors. A: Whales retain remnants of their mammalian ancestors’ leg bones (c). B: Cavefish lack the eyes and pigments important to their relatives who live in lighted habitats. C: True flies have reduced insects’ second pairs of wings to balancing knobs. D: We still show the reflex which raises hairs for insulation in cold air in our furry relatives, but all we have to show for our follicle’s efforts are goosebumps.

    True flies have reduced the second pair of wings found in most insects to halteres for balance shown in Figure 7B. Cavefish lose both eyes and pigment, because both would require energy to build and are useless in the lightless habitat they have adopted shown in Figure 7C. You are probably very familiar with a fine example of a vestigial behavior: goosebumps raise the sparse hairs on your arms even though they are no longer sufficiently dense to insulate you from the cold by trapping warm air next to your skin in most mammals, this reflex is still quite functional shown in Figure 7D. Most vestigial structures are homologous to similar, functioning structures in closely related species, and as such, support both common ancestry and (incomplete!) natural selection.

    Molecular Biology

    Did you know that your genes may be 50% the same as those of a banana?

    Unknown in Darwin’s time, the “comparative anatomy” of the molecules which make up life has added an even more convincing set of homologies to the evidence for evolution. All living organisms have genes made of DNA. The order of nucleotides – As, Ts, Cs, and Gs - in each gene codes for a protein, which does the work or builds the structures of life. Proteins govern the traits chosen (or not) in natural selection. For all organisms, a single Genetic Code translates the sequence of nucleotides in a gene into a corresponding chain of 20 amino acids. By itself, the universality of DNA genes and their code for proteins is strong evidence for common ancestry. Yet there is more.

    If we compare the sequence of nucleotides in the DNA of one organism to the sequence in another, we see remarkable similarities. For example, human DNA sequences are 98-99% the same as those of chimpanzees, and 50% the same as a banana’s! These similarities reflect similar metabolism. All organisms have genes for DNA replication, protein synthesis, and processes such as cellular respiration. Although metabolic processes do not leave fossils, similar DNA sequences among existing organisms provide excellent evidence for common ancestry.

    The differences in DNA sequences are even more intriguing. Many are single base substitutions resulting from mutations accumulated through time. Assuming mutations occur randomly, the number of differences in bases between any two species measures the time elapsed since two organisms shared a common ancestor. This type of ”molecular clock” has confirmed traditional classification based on anatomy. Most scientists consider it sufficiently powerful to clarify or correct our understanding of evolutionary history. For example, human DNA differs 1.2% from chimpanzees, 1.6% from gorillas, and 6.6% from baboons we can infer from this data that humans and chimpanzees share a relatively recent common ancestor, and that the common ancestor we share with gorillas lived much longer ago. Figure 8 shows a cladogram depicting hypothetical evolutionary relationships constructed with this data. Similarities and differences in the sequences of amino acids in proteins support common ancestry in the same way, because they are determined by DNA.

    Figure 8: Cladograms use comparison data to construct diagrams showing evolutionary relationships. This cladogram uses comparisons of DNA nucleotide sequences to reveal patterns of descent from common ancestors. Molecular biology has supported and extended our understanding of evolutionary relationships based on traditional anatomy.

    Heritability and variation in traits are essential parts of Darwin’s theory of evolution by natural selection. Since he published The Origin of the Species, rediscovery of Mendel’s identification of genes and how they are inherited has confirmed Darwin’s ideas. Molecular biology has clarified the nature of genes and the sources of variation. Comparative analysis of DNA and proteins continues to give us an exquisitely detailed view of patterns of variation, common ancestry, and how evolution works.

    Biogeography

    Australia, Africa, and South America occupy the same latitude, at least in part, and therefore have roughly the same climate. If plants and animals were distributed only according to their adaptations to habitat, we would expect the same species to occupy similar regions of these continents. However, the short-tailed monkeys, elephants, and lions in Africa differ significantly from the long-tailed monkeys, llamas, and jaguars of South America, and even more from the koalas, kangaroos, and Tasmanian devils of Australia. Biogeography studies the distribution of plants and animals and the processes that influence their distribution – including evolution and natural selection. Only geologic change and evolution can explain the distributions of many species, so biogeography is another kind of evidence for the theory of evolution.

    Figure 9: Alfred Russel Wallace identified six major biogeographic regions: Nearctic, Neotropical, Palaearctic, Ethiopian, Oriental, and Australian Regions. Wallace explained the distributions of many animals and plants as a result of changes in geography and evolution.

    Alfred Russel Wallace, who developed his own ideas of evolution and natural selection at the same time as Darwin, explained the distributions of many species in terms of changes in geography (such as formation of land bridges) and environment (for example, glaciations) and corresponding evolution of species. Figure 9 shows the six biogeographical regions he identified: Nearctic, Neotropical, Palaearctic, Ethiopian, Oriental, and Australian.

    Figure 10: Biogeography explains the distribution of camel-like animals as a result of geographical changes and independent evolution. Today, the descendants of early camel ancestors are the dromedary in Africa, the Bactrian camel in Asia (center), and the guanaco (right) and llamas of South America.

    Let’s consider just the camel family as an example, shown in Figure 10 of how biogeography explains the distribution of species. Fossils suggest that camel ancestors originated in North America. Distant fossils show structural similarities which suggest that their descendants migrated across the Bering land bridge to Asia and across the Isthmus of Panama into South America. These two isolated populations evolved in different directions due to differences in chance variations and habitat. Today’s descendants are llamas and guanacos in South America, and camels in Asia. Asian camels continued to migrate west into Africa, giving rise to two species – the dromedary in Africa, and the Bactrian in eastern Asia.

    Figure 11: The locations of fossils such as Glossopteris on widely separated continents form contiguous patterns if the continents are joined. These patterns led to the theory of plate tectonics. Gondwanaland, a supercontinent of long ago, played an important part in evolution, natural selection and the history of life.

    The distribution of some older fossils shows an opposite pattern for example, fossils of a single species of fern, Glossopteris, have been found in South America, Africa, India, Antarctica, and Australia (Figure 11). Putting together many such distributions and a great deal of geologic data, Alfred Wegener showed that the continents were long ago united as Gondwanaland, and have since drifted apart. His theory of continental drift and its modern form, plate tectonics, help to further explain patterns of evolutionary descent in space and time.

    Island Biogeography

    Island biogeography studies archipelagos (oceanic island chains) as isolated sites for evolution. Both Darwin and Wallace used examples from isolated oceanic islands, such as the Galapagos and Hawaii, in their arguments for evolution and natural selection. Until humans arrived, terrestrial mammals and amphibians were completely absent on these islands. Darwin and Wallace showed that the animals and plants which were present had blown or drifted from one of the continents, or had descended – with modifications which suited the new habitats – from one of the original colonists. Terrestrial mammals and amphibians, having no powers of dispersal across oceans (until humans came along), were understandably absent.

    Darwin's Finches

    Only long after returning from his voyage did Darwin, with help from ornithologist John Gould, realize that the Galapagos birds he had collected but dismissed as uninteresting blackbirds, grosbeaks, finches, and a wren, were actually all closely related descendants of a single ancestral finch which had relatives on the South American mainland. Careful analysis showed that each of the 12 new species was confined and adapted to a specific habitat on a specific island. The finches, now known as “Darwin’s finches” (Figure 12A), clearly support both descent with modification and natural selection. Hawaiian honeycreepers (Figure 12B) are a more colorful but also more endangered example of the same evolutionary process of adaptive radiation. Bills ranging from thick and heavy (finch-like) for seed-eaters to long and curved for probing flowers illustrate the variations by which descendants of a single, original finch-like colonizer adapted to multiple ecological niches on the islands. Unfortunately, human destruction of habitat and introductions of rodents, the mongoose, and the mosquito which carries avian malaria have caused the extinction of 15 honeycreeper species, and still threaten the species which remain.

    Figure 11: Darwin’s finches (above) on the Galapagos and honeycreepers (right) on Hawaii show the adaptive radiation of single finch ancestors which first colonized the islands. Each species show descent with modification, and the variety of bill shapes show adaptation to a specific niche. Many similar examples from island biogeography support evolution and natural selection. Honeycreepers are the finch-like palila (top right), the flower-probing I’iwi (center), and another nectar feeder, the amakihi (bottom).

    Scientific Evidence

    Altogether, the fossil record, homologies, analogies, vestigial structures, molecular uniformity and diversity, and biogeography provide powerful scientific evidence for the descent of today’s species from common ancestors. Some details of natural selection have been and are still being modified. However, the remarkable biological discoveries of the 150 years since Darwin published The Origin of the Species have dramatically strengthened support for his theory. Moreover, Darwin’s theory continues to enlighten new discoveries. Perhaps we could paraphrase Dobzhansky: Everything in biology makes sense in the light of evolution. The only piece still missing from the evidence puzzle is direct observation of the process itself. Darwin thought that humans could never witness evolution in action because of the vast time periods required. For once, however, he was mistaken.


    Fossils are the preserved remains of previously living organisms or their traces, dating from the distant past. The fossil record provides snapshots of the past that, when assembled, illustrate a huge picture of evolutionary change over the past four billion years. This picture may be smudged in places and may have bits missing, but fossil evidence clearly shows that life is old and has changed over time.

    One of the problems with the fossil record is that it contains gaps. most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. Nonetheless, the fossils that have been collected document the existence of now-extinct past species that are related to present-day species.

    One of the few animals for which we have a fairly complete evolutionary record is the horse, where all the main stages of the evolution of the horse have been preserved in fossil form (see right).


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    The Institute for Creation Research

    This impact pamphlet was written by a scientist, and a science educator, and reviewed by an attorney, to provide a brief summary of the scientific evidence supporting creation. The text materials and references listed at the end together give a more thorough discussion of this scientific evidence.

    Introduction

    Public schools in many localities are teaching two scientific models - the creation model and the evolution model of the origin of the universe, of life, and of man. There is apparent scientific evidence for creation, which is summarized in this pamphlet, just as there is apparent scientific evidence for evolution. The purpose of this pamphlet is to summarize the evidence that shows that:

    "The creation model is at least as scientific as the evolution model, and is at least as nonreligious as the evolution model."

    This scientific evidence for both models can be taught in public schools without any mention of religious doctrine, whether the Bible or the Humanist Manifesto. There are text materials and teacher handbooks that have been prepared for a fair presentation of both models, creation and evolution. There are also seminars and audiovisuals for training teachers to offer both models of origins.

    "This scientific evidence both for creation and for evolution can and must be taught without any religious doctrine, whether the Bible or the Humanist Manifesto."
    "Creation-science proponents want public schools to teach all the scientific data, censoring none, but do not want any religious doctrine to be brought into science classrooms."

    Definitions of the Creation Model and the Evolution Model

    The scientific model of creation, in summary, includes the scientific evidence for a sudden creation of complex and diversified kinds of life, with systematic gaps persisting between different kinds and with genetic variation occurring within each kind since that time. The scientific model of evolution, in summary, includes the scientific evidence for a gradual emergence of present life kinds over aeons of time, with emergence of complex and diversified kinds of life from simpler kinds and ultimately from nonliving matter. The creation model questions vertical evolution, which is the emergence of complex from simple and change between kinds, but it does not challenge what is often called horizontal evolution or microevolution, which creationists call genetic variation or species or subspecies formation within created kinds. The following chart lists seven aspects of the scientific model of creation and of the scientific model of evolution:

    The creation model includes the scientific evidence and the related inferences suggesting that: The evolution model includes the scientific evidence and the related inferences suggesting that:
    I. The universe and the Solar system were suddenly created. I. The universe and the solar system emerged by naturalistic processes.
    II. Life was suddenly created. II. Life emerged from nonlife by naturalistic processes.
    III. All present living kinds of animals and plants have remained fixed since creation, other than extinctions, and genetic variation in originally created kinds has only occurred within narrow limits. III. All present kinds emerged from simpler earlier kinds, so that single-celled organisms evolved into invertebrates, then vertebrates, then amphibians, then reptiles, then mammals, then primates, including man.
    IV. Mutation and natural selection are insufficient to have brought about any emergence of present living kinds from a simple primordial organism. IV. Mutation and natural selection have brought about the emergence of present complex kinds from a simple primordial organism.
    V. Man and apes have a separate ancestry. V. Man and apes emerged from a common ancestor.
    VI. The earth's geologic features appear to have been fashioned largely by rapid, catastrophic processes that affected the earth on a global and regional scale (catastrophism). VI. The earth's geologic features were fashioned largely by slow, gradual processes, with infrequent catastrophic events restricted to a local scale (uniformitarianism).
    VII. The inception of the earth and of living kinds may have been relatively recent. VII. The inception of the earth and then of life must have occurred several billion years ago.

    I. The Universe and the Solar System Were Suddenly Created.

    The First Law of Thermodynamics states that the total quantity of matter and energy in the universe is constant. The Second Law of Thermodynamics states that matter and energy always tend to change from complex and ordered states to disordered states. Therefore the universe could not have created itself, but could not have existed forever, or it would have run down long ago. Thus the universe, including matter and energy, apparently must have been created. The "big-bang" theory of the origin of the universe contradicts much physical evidence and seemingly can only be accepted by faith. 1 This was also the case with the past cosmogonies theories of evolutionists that have been discarded, such as Hoyle&rsquos steady-state theory. The universe has "obvious manifestations of an ordered, structured plan or design." Similarly, the electron is materially inconceivable and yet it is so perfectly known through its effects," yet a "strange rationale makes some physicists accept the inconceivable electrons as real while refusing to accept the reality of a Designer." "The inconceivability of some ultimate issue (which will always lie outside scientific resolution) should not be allowed to rule out any theory that explains the interrelationship of observed data and is useful for prediction," in the words of Dr. Wernher von Braun, the renowned late physicist in the NASA space program.

    II. Life Was Suddenly Created.

    Life appears abruptly and in complex forms in the fossil record, 2 and gaps appear systematically in the fossil record between various living kinds. 3 These facts indicate that basic kinds of plants and animals were created. The Second Law of Thermodynamics states that things tend to go from order to disorder (entropy tends to increase) unless added energy is directed by a conversion mechanism (such as photosynthesis), whether a system is open or closed. Thus simple molecules and complex protein, DNA, and RNA molecules seemingly could not have evolved spontaneously and naturalistically into a living cell 4 such cells apparently were created. The laboratory experiments related to theories on the origin of life have not even remotely approached the synthesis of life from nonlife, and the extremely limited results have depended on laboratory conditions that are artificially imposed and extremely improbable. 5 The extreme improbability of these conditions and the relatively insignificant results apparently show that life did not emerge by the process that evolutionists postulate.

    "One example of the scientific evidence for creation is the sudden appearance of complex fossilized life in the fossil record, and the systematic gaps between fossilized kinds in that record. The most rational inference from this evidence seemingly is that life was created and did not evolve."

    III. All Present Living Kinds of Animals and Plants Have Remained Fixed Since Creation, Other than Extinctions, and Genetic Variation in Originally Created Kinds Has Only Occurred within Narrow Limits.

    Systematic gaps occur between kinds in the fossil record. 6 None of the intermediate fossils that would be expected on the basis of the evolution model have been found between single celled organisms and invertebrates, between invertebrates and vertebrates, between fish and amphibians, between amphibians and reptiles, between reptiles and birds or mammals, or between "lower" mammals and primates. 7 While evolutionists might assume that these intermediate forms existed at one time, none of the hundreds of millions of fossils found so far provide the missing links. The few suggested links such as Archoeopteryx and the horse series have been rendered questionable by more detailed data. Fossils and living organisms are readily subjected to the same criteria of classification. Thus present kinds of animals and plants apparently were created, as shown by the systematic fossil gaps and by the similarity of fossil forms to living forms. A kind may be defined as a generally interfertile group of organisms that possesses variant genes for a common set of traits but that does not interbreed with other groups of organisms under normal circumstances. Any evolutionary change between kinds (necessary for the emergence of complex from simple organisms) would require addition of entirely new traits to the common set and enormous expansion of the gene pool over time, and could not occur from mere ecologically adaptive variations of a given trait set (which the creation model recognizes).

    IV. Mutation and Natural Selection Are Insufficient To Have Brought About Any Emergence of Present Living Kinds from a Simple Primordial Organism.

    The mathematical probability that random mutation and natural selection ultimately produced complex living kinds from a simpler kind is infinitesimally small even after many billions of years. 8 Thus mutation and natural selection apparently could not have brought about evolution of present living kinds from a simple first organism. Mutations are always harmful or at least nearly always harmful in an organism's natural environment. 9 Thus the mutation process apparently could not have provided the postulated millions of beneficial mutations required for progressive evolution in the supposed five billion years from the origin of the earth until now, and in fact would have produced an overwhelming genetic load over hundreds of millions of years that would have caused degeneration and extinction. Natural selection is a tautologous concept (circular reasoning), because it simply requires the fittest organisms to leave the most offspring and at the same time it identifies the fittest organisms as those that leave the most offspring. Thus natural selection seemingly does not provide a testable explanation of how mutations would produce more fit organisms. 10

    V. Man and Apes Have a Separate Ancestry.

    Although highly imaginative "transitional forms" between man and ape-like creatures have been constructed by evolutionists based on very fragmentary evidence, the fossil record actually documents the separate origin of primates in general, 11 monkeys, 12 apes, 13 and men. In fact, Lord Zuckerman (not a creationist) states that there are no "fossil traces" of a transformation from an ape-like creature to man. 14 The fossils of Neanderthal Man were once considered to represent a primitive sub-human (Homo neanderthalensis), but these "primitive" features are now known to have resulted from nutritional deficiencies and pathological conditions he is now classified as fully human. 15 Ramapithecus was once considered to be partially man-like, but is now known to be fully ape-like. 16 Australopithecus, in the view of some leading evolutionists, was not intermediate between ape and man and did not walk upright. 17 The strong bias of many evolutionists in seeking a link between apes and man is shown by the near-universal acceptance of two "missing links" that were later proved to be a fraud in the case of Piltdown Man (Eoanthropus) and a pig's tooth in the case of Nebraska Man (Hesperopithecus). 18

    VI. The Earth's Geologic Features Were Fashioned Largely by Rapid, Catastrophic Processes that Affected the Earth on a Global and Regional Scale (Catastrophism).

    Catastrophic events have characterized the earth's history. Huge floods, massive asteroid collisions, large volcanic eruptions, devastating landslides, and intense earthquakes have left their marks on the earth. Catastrophic events appear to explain the formation of mountain ranges, deposition of thick sequences of sedimentary rocks with fossils, initiation of the glacial age, and extinction of dinosaurs and other animals. Catastrophism (catastrophic changes), rather than uniformitarianism (gradual changes), appears to be the best interpretation of a major portion of the earth's geology. Geologic data reflect catastrophic flooding. Evidences of rapid catastrophic water deposition include fossilized tree trunks that penetrate numerous sedimentary layers (such as at Joggins, Nova Scotia), widespread pebble and boulder layers (such as the Shinarump Conglomerate of the southwestern United States), fossilized logs in a single layer covering extensive areas (such as Petrified Forest National Park), and whole closed clams that were buried alive in mass graveyards in extensive sedimentary layers (such as at Glen Rose, Texas). Uniform processes such as normal river sedimentation, small volcanoes, slow erosion, and small earthquakes appear insufficient to explain large portions of the geologic record. Even the conventional uniformitarian geologists are beginning to yield to evidences of rapid and catastrophic processes. 19

    VII. The Inception of the Earth and of Living Kinds May Have Been Relatively Recent.

    Radiometric dating methods (such as the uranium-lead and potassium-argon methods) depend on three assumptions: (a) that no decay product (lead or argon) was present initially or that the initial quantities can be accurately estimated, (b) that the decay system was closed through the years (so that radioactive material or product did not move in or out of the rock), and (c) that the decay rate was constant over time. 20 Each of these assumptions may be questionable: (a) some nonradiogenic lead or argon was perhaps present initially 21 (b) the radioactive isotope (uranium or potassium isotopes) can perhaps migrate out of, and the decay product (lead or argon) can migrate into, many rocks over the years 22 and (c) the decay rate can perhaps change by neutrino bombardment and other causes. 23 Numerous radiometric estimates have been hundreds of millions of years in excess of the true age. Thus ages estimated by the radiometric dating methods may very well be grossly in error. Alternate dating methods suggest much younger ages for the earth and life. Estimating by the rate of addition of helium to the atmosphere from radioactive decay, the age of the earth appears to be about 10,000 years, even allowing for moderate helium escape. Based on the present rate of the earth's cooling, the time required for the earth to have reached its present thermal structure seems to be only several tens of millions of years, even assuming that the earth was initially molten. 24 Extrapolating the observed rate of apparently exponential decay of the earth's magnetic field, the age of the earth or life seemingly could not exceed 20,000 years. 25 Thus the inception of the earth and the inception of life may have been relatively recent when all the evidence is considered. 26


    Evidence For Evolution Worksheet Answer Key Biology , Embryology Is The Branch Of Biology That Deals With The Formation, Early Growth, And Development Of Living Organisms.

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