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5.21: Homologous and Analogous Traits - Biology

5.21: Homologous and Analogous Traits - Biology


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Learning Objectives

  • Compare homologous and analogous traits

Scientists must collect accurate information that allows them to make evolutionary connections among organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny, evolutionary investigations focus on two types of evidence: morphologic (form and function) and genetic. In general, organisms that share similar physical features and genomes tend to be more closely related than those that do not. Such features that overlap both morphologically (in form) and genetically are referred to as homologous structures; they stem from developmental similarities that are based on evolution. For example, the bones in the wings of bats and birds have homologous structures (Figure 1).

Notice it is not simply a single bone, but rather a grouping of several bones arranged in a similar way. The more complex the feature, the more likely any kind of overlap is due to a common evolutionary past. Imagine two people from different countries both inventing a car with all the same parts and in exactly the same arrangement without any previous or shared knowledge. That outcome would be highly improbable. However, if two people both invented a hammer, it would be reasonable to conclude that both could have the original idea without the help of the other. The same relationship between complexity and shared evolutionary history is true for homologous structures in organisms.

Misleading Appearances

Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. Similarly, unrelated organisms may be distantly related, but appear very much alike. This usually happens because both organisms were in common adaptations that evolved within similar environmental conditions. When similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is called an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely different. These are called analogous structures (Figure 2).

Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin; analogous organs have a similar function. For example, the bones in the front flipper of a whale are homologous to the bones in the human arm. These structures are not analogous. The wings of a butterfly and the wings of a bird are analogous but not homologous. Some structures are both analogous and homologous: the wings of a bird and the wings of a bat are both homologous and analogous. Scientists must determine which type of similarity a feature exhibits to decipher the phylogeny of the organisms being studied.

Molecular Comparisons

With the advancement of DNA technology, the area of molecular systematics, which describes the use of information on the molecular level including DNA analysis, has blossomed. New computer programs not only confirm many earlier classified organisms, but also uncover previously made errors. As with physical characteristics, even the DNA sequence can be tricky to read in some cases. For some situations, two very closely related organisms can appear unrelated if a mutation occurred that caused a shift in the genetic code. An insertion or deletion mutation would move each nucleotide base over one place, causing two similar codes to appear unrelated.

Sometimes two segments of DNA code in distantly related organisms randomly share a high percentage of bases in the same locations, causing these organisms to appear closely related when they are not. For both of these situations, computer technologies have been developed to help identify the actual relationships, and, ultimately, the coupled use of both morphologic and molecular information is more effective in determining phylogeny.


The Difference Between Analogy and Homology in Evolution

There are many types of evidence that support the Theory of Evolution. These pieces of evidence range from the minute molecular level of DNA similarities all the way up through similarities within the anatomical structure of organisms. When Charles Darwin first proposed his idea of natural selection, he used mostly evidence based on anatomical features of organisms he studied.

Two different ways these similarities in anatomical structures can be classified is as either analogous structures or homologous structures. While both of these categories have to do with how similar body parts of different organisms are used and structured, only one is actually an indication of a common ancestor somewhere in the past.


5.21: Homologous and Analogous Traits - Biology

Since a phylogenetic tree is a hypothesis about evolutionary relationships, we want to use characters that are reliable indicators of common ancestry to build that tree. We use homologous characters — characters in different organisms that are similar because they were inherited from a common ancestor that also had that character. An example of homologous characters is the four limbs of tetrapods. Birds, bats, mice, and crocodiles all have four limbs. Sharks and bony fish do not. The ancestor of tetrapods evolved four limbs, and its descendents have inherited that feature — so the presence of four limbs is a homology.

Not all characters are homologies. For example, birds and bats both have wings, while mice and crocodiles do not. Does that mean that birds and bats are more closely related to one another than to mice and crocodiles? No. When we examine bird wings and bat wings closely, we see that there are some major differences.

Bat wings consist of flaps of skin stretched between the bones of the fingers and arm. Bird wings consist of feathers extending all along the arm. These structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor with wings. This idea is illustrated by the phylogeny below, which is based on a large number of other characters.

Bird and bat wings are analogous — that is, they have separate evolutionary origins, but are superficially similar because they have both experienced natural selection that shaped them to play a key role in flight. Analogies are the result of convergent evolution.

Interestingly, though bird and bat wings are analogous as wings, as forelimbs they are homologous. Birds and bats did not inherit wings from a common ancestor with wings, but they did inherit forelimbs from a common ancestor with forelimbs.


5.21: Homologous and Analogous Traits - Biology

Different species of living organisms often have similar physical features. These features are sometimes used for classification. Classifications that reflect evolutionary relationships are generally the most useful, because if you know about one organism in the group, you can make relatively accurate predictions about other group members, even if you haven't seen them before.

For example, all frogs are related to each other. Based on what you know about our local frogs, you can predict that most African frogs probably hop, like wet habitats, eat insects, and make chirping noises.

On the other hand, what if you had a group of organisms called 'greens', which included any animal that was mostly green? You'd have a hard time predicting the habits of all green animals based just on what you know about grasshoppers. Green coloration has evolved many times in many different groups of animals. It doesn't indicate that all green animals are closely related.


Identifying Analogous Structures

Scientists usually identify analogous structures by looking at the known relatives of the two species being studied.

If a line of common inheritance can be found – such as humans and monkeys both having fingers, when we have a fossil record showing that humans and monkeys shared a common ancestor, who also had fingers – the structures are not considered analogous.

But if no common ancestor which shares these features is found – such as in the case of bats and insects, whose shared ancestor did not fly at all – the structures would be considered analogous.


Shared Characteristics

Organisms evolve from common ancestors and then diversify. Scientists use the phrase “descent with modification” because even though related organisms have many of the same characteristics and genetic codes, changes occur. This pattern repeats over and over as one goes through the phylogenetic tree of life:

  1. A change in the genetic makeup of an organism leads to a new trait which becomes prevalent in the group.
  2. Many organisms descend from this point and have this trait.
  3. New variations continue to arise: some are adaptive and persist, leading to new traits.
  4. With new traits, a new branch point is determined (go back to step 1 and repeat).

If a characteristic is found in the ancestor of a group, it is considered a shared ancestral character because all of the organisms in the taxon or clade have that trait. The vertebral column in Figure 1 is a shared characteristic. Now consider the amniotic egg characteristic in the same figure. Only some of the organisms in Figure 1 have this trait, and to those that do, it is called a shared derived character because this trait derived at some point but does not include all of the ancestors in the tree.

The tricky aspect to shared ancestral and shared derived characters is the fact that these terms are relative. The same trait can be considered one or the other depending on the particular diagram being used. Returning to Figure 1, an amniotic egg is a shared derived trait for amniotes as a clade, because the immediate ancestor of amniotes, as well as other groups descended from the ancestor of amniotes, do not have it. However, it is a shared ancestral trait for any particular group of amniotes, such as the lizard, rabbit, or human (shown in Figure 1) because they all stem from an ancestor with that trait. These terms help scientists distinguish between clades in the building of phylogenetic trees.

Choosing the Right Relationships

Imagine being the person responsible for organizing all department store items properly—an overwhelming task. Organizing the evolutionary relationships of all life on Earth proves much more difficult: scientists must span enormous blocks of time and work with information from long-extinct organisms. Trying to decipher the proper connections, especially given the presence of homologies and analogies, makes the task of building an accurate tree of life extraordinarily difficult. Add to that advancing DNA technology, which now provides large quantities of genetic sequences for researchers to use and analzye. Taxonomy is a subjective discipline: many organisms have more than one connection to each other, so each taxonomist will decide the order of connections.

To aid in the tremendous task of describing phylogenies accurately, scientists often use the concept of maximum parsimony , which means that events occurred in the simplest, most obvious way. For example, if a group of people entered a forest preserve to hike, based on the principle of maximum parsimony, one could predict that most would hike on established trails rather than forge new ones.

For scientists deciphering evolutionary pathways, the same idea is used: the pathway of evolution probably includes the fewest major events that coincide with the evidence at hand. Starting with all of the homologous traits in a group of organisms, scientists look for the most obvious and simple order of evolutionary events that led to the occurrence of those traits.

These tools and concepts are only a few of the strategies scientists use to tackle the task of revealing the evolutionary history of life on Earth. Recently, newer technologies have uncovered surprising discoveries with unexpected relationships, such as the fact that people seem to be more closely related to fungi than fungi are to plants. Sound unbelievable? As the information about DNA sequences grows, scientists will become closer to mapping the evolutionary history of all life on Earth.


Analogy

Analogy refers to the similarity in function of two different organisms due to convergent evolution and not common ancestry.

Analogy in Animals

Analogous organs are the opposite of homologous organs, which have similar functions but different origins. An example of an analogous trait would be the wings of insects, bats and birds that evolved independently in each lineage separately after diverging from an ancestor without wings. The wings of insects originate from the inner or outer surface of the insect&rsquos body. Feathers of birds originate from their forelimbs, and the wings of bats originate from both the fore limb and the membranous skin of the abdomen.

Another example of analogous animals is sugar gliders and flying squirrels. These two animals can glide in air using their gliding wings. Both species are different from each other in many ways. Flying squirrel are placental mammals, where as sugar gliders are marsupial mammals like kangaroos. To adapt a common function, the flying squirrel and sugar glider evolved similar gliding wings.

Analogy in plants

The leaves of opuntia and peepal are analogous organs in plants. In opuntia the stem is modified into a broad succulent leaf like structure that performs photosynthesis like leaves. Peepal leaf is a normal leaf that performs photosynthesis. So both the opuntia and peepal leaves perform common function through photosynthesis, so they are analogous.

Many of the cacti and African euphorbias are similar in appearance, being succulent, spiny, water-storing, and adapted to desert conditions generally. But these two plants belong to different families though they share traits according to the similar environmental conditions they are placed in.

Potato and sweet potato also show similar characteristics, but have different origins. Potato is a modified stem meant for storage of food. Sweet potato is a modified root also meant for the storage of food, so they are analogous.


Examples of Homologous

As stated above, “homologous” can be used to describe two things:

Climbers, Flyers, and Swimmers

What do squirrels, birds, and whales have in common? The obvious answer is that they breathe, have beating hearts, and use their upper appendages to move. Let’s explore this latter idea a bit more, using the squirrel’s ability to climb, the bird’s ability to fly, and the whale’s ability to swim as examples:

Examine the images below, focusing on the squirrel’s arm, the bird’s wing, and the whale’s fin. Note some similarities and differences.

One of the things these illustrations show is that each example consists of three main parts: the humerus, or the “upper arm,” the radius and ulna, which form the “forearm,” and the metacarpals, which form the “fingers.”

On the other hand, we can also identify differences. The whale’s humerus, for instance, tends to be shorter, wider, and flatter. In fact, some whales have a patella, or “shoulderblade,” instead of a humerus. Likewise, the bird has no fingers its metacarpals taper into something that resembles a dagger.

To quote the popular Darwinist phrase, you are 98% chimp. While technically true, this information can mislead beginning biologists who have yet to explore it further.

The genetic code of most animals contains four nucleotide bases, also called nucleobases, and marked as A, T, C, and G. In different combinations, they account for features such as the color and location of body hair, nose size, blood type, and even ear lobe attachment. More recent research even suggests that nucleobases also determine whether you will develop a psychological or personality disorder.

Nonetheless, most genetic expression, or the manner in which nucleobase combinations manifest themselves, is relatively benign. We don’t give much thought, for instance, to the nucleobase combinations that make our bones hard, our heart muscular, or our liver able to regenerate. In fact, geneticists estimate that only 0.1% (that’s one-tenth of a percent) of our genes actually account for the features we see. The other 99.9% rests dormant as “junk DNA,” or makes up features that we take for granted.

The four nearly-universal nucleobases in the genetic code, when paired with the fact that some human DNA remains dormant, permits us to understand the phrase “You are 98% chimp” more deeply. In short, however, humans and chimps have a homologous genetic code. The differences lie in how that code is expressed.

Your Mother’s Eyes, but Your Father’s Hands

If you know your birth parents, you have probably noticed that you have inherited some features from your mother, and some features from your father. You may have also had relatives and friends tell you resemble one or the other.

For better or for worse, you probably resemble both of your parents. This is because, during conception, you inherited a set of 23 chromosomes from your mother’s egg, and a set of 23 chromosomes from your father’s sperm. The same genetic information is stored on each set, in similar locations.

Because you can “match” one of your mother’s chromosomes with one of your father’s, they are homologous.

Nonetheless, the alleles, or modes of expression, of these genes may differ. This is why you may have inherited your mother’s brown eyes, a dominant allele, but not your father’s cleft chin, a recessive allele. Typically, however, the fact that you have colored eyes or a chin in the first place indicates that your parents’ chromosomes both carried the information necessary for creating them.


These Examples of Analogous Structures Will Surely Surprise You

The structural features that serve a common function in various species, but have different ancestral origins are called analogous structures, and this phenomenon is called analogy. Read this BiologyWise post to know more about such structures.

The structural features that serve a common function in various species, but have different ancestral origins are called analogous structures, and this phenomenon is called analogy. Read this BiologyWise post to know more about such structures.

Insane Similarity!

Though differing in their evolutionary pathways, the eyes of humans and octopuses are almost the same regarding structure and appearance. Both organisms have stereoscopic vision, even if they belong to totally unrelated classes, i.e., Vertebrata and Cephalopoda, respectively.

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Convergent evolution has played a very important role in the development of complex life forms since the time of appearance of the first organisms. The features in organisms that perform the same functions, but have different evolutionary history are called analogous/convergent structures. This type of development of structures is also called homoplasy. For example, wings have the same functions for insects and birds, but there is no evolutionary similarity between them.

Wings of birds and bats look similar in structure though they have evolved independently, but are related to each other because they adapted to a similar environment. This process is known as convergent evolution. Analogous traits are adopted due to this type of evolution. The tusks of elephants and the gnawing front teeth of the beaver are incisor teeth types. They have been inherited from common ancestors, but are modified with due course of evolution according to the respective uses, and now look very different from each other. Analogous and homologous structures are similar to each other, but show a major difference in their ancestral history. The latter are developed in different types of organisms, and have been derived from a common ancestor. The former do not have a common ancestor, yet are observed in organisms that have evolved separately.

Wings

Analogous structures are easily identifiable when wings of different organisms are studied. For example, the wings of a bird and insect perform the same function, i.e., flying or movement through the air. But, insects have evolved separately, whereas birds are the modern versions of the ancient dinosaurs both the classes are not related in any manner, though they exhibit similar features. Similarly, another example includes the study of wings of bats and butterflies. Both look very different from each other, but perform the same function of flying. It must be noted that in this case, bat is a flying mammal species, whereas butterflies are included under the category of insects. The comparison between both these organisms is represented in the accompanying image.

Limbs

Limbs of tetrapods and arthropods are analogous to each other, and they evolved after the Cambrian explosion, approximately 530 million years ago. Tetrapods evolved from fish more than 370 million years ago, while the ancestors of arthropods were terrestrial invertebrates. Thus, these structures were independently evolved. The legs of vertebrates and insects serve the same purpose, but both have different structures and evolutionary histories. Both the classes have been derived from two different origins. In the accompanying image, a representation can be seen, which compares the limbs of four different types of mammals: humans, horses, dolphins, and bats. The colored index points out to the difference in size and shape of the various body parts. In spite of this variation, the limbs are used for the same function, i.e., locomotion, whether it is in water, on land, or in the air.

One of the most easily observed analogous similarity is with respect to the fins of various animals. For instance, the fins present in birds, like penguins, and mammals, like dolphins, serve the same purpose. But as both belong to a different class of vertebrates, their evolutionary line must be totally different. In spite of this, both organisms developed body parts that appear similar, but have more or less the same function. The same can be observed in case of fins belonging to sharks and dolphins. The former are categorized as a fish, while the latter are a variety of mammals that thrive in water. Thus, the fins have the same function in both organisms, i.e., navigation in the water, but the organisms themselves are totally unrelated to each other regarding their evolutionary path.

Storage of Food

A classic example of homoplasy regarding an inherent aspect of the organism is the similarity between sweet potato and potato. Both the species of vegetables have evolved along different lines and show the same function. i.e., food storage in their tuberous mass. But, potato is in the form of a stem that is buried underground, whereas sweet potato is a root. Similarly, potato and cactus are also majorly categorized as stems. But potato can be grown in agricultural fields, whereas cacti are mostly desert plants. Both of these plant species have evolved differently, but in their stem form, food is stored inside the tissues.

Behavioral Characteristics

Apart from the body part aspect, research in this field also tells us about homoplasy regarding the behavior of organisms. This has been observed especially when two birds of differing origins when kept together, may develop skills to voice out the same kind of a bird song. However, this is just possible on an experimental basis. Among birds, both New and Old World vultures look very similar, both have featherless necks and heads, and feed on carrion. But, apart from these similarities, they belong to different families. The former ones belong to the Cathartidae family, whereas the latter belongs to the Accipitridae family. New World vultures use both, sense of smell and vision to hunt for prey, whereas Old World vultures use only senses of vision for preying purposes and can spot a 3-foot prey from approximately 4 miles away. New World vultures have a stronger sense of smell than the Old World vultures. One of the best examples in this category is that of the duck-billed platypus, which is an egg-laying mammal. This behavioral evolution is dominantly seen in birds in case of vertebrates. Though the platypus looks like a bird and lays eggs, it is not related to the avian family by any other characteristic.

Other Examples

✦ Analogy is quite different from homology where the structures are similar because they have a common embryonic origin. There are many reasons because of which the animals in nature resemble each other. Two insects of the same species might look similar due to the same color of spots inherited from the ancestors.

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✦ Human beings, lizards, and whales have similar skeleton structures, although they have a different habitat and lifestyle.

✦ Humans use their limbs to throw a ball, whales use fins to swim, and lizards use their limbs to climb the walls. Each one of them are similar in structure, but vary with respect to detailed morphology.

Evolutionary biologists call these structures as analogous.

The ongoing process of evolution can be traced with the help of comparative anatomy. Several other evidences can be studied with the help of bio-geography, fossil records, and molecular records. Analogy is an aspect of evolutionary biology, which says that the structures are similar not because of embryonic origin, but due to the similarities in functions. Analogies evolve when the challenges and problems faced by two species are similar. Evolution then shapes both as being similar to each other, and hence, their structures are evolved.

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12.2 Determining Evolutionary Relationships

Scientists collect information that allows them to make evolutionary connections between organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny, evolutionary investigations focus on two types of evidence: morphologic (form and function) and genetic.

Two Measures of Similarity

Organisms that share similar physical features and genetic sequences tend to be more closely related than those that do not. Features that overlap both morphologically and genetically are referred to as homologous structures the similarities stem from common evolutionary paths. For example, as shown in Figure 12.6, the bones in the wings of bats and birds, the arms of humans, and the foreleg of a horse are homologous structures. Notice the structure is not simply a single bone, but rather a grouping of several bones arranged in a similar way in each organism even though the elements of the structure may have changed shape and size.

Misleading Appearances

Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. For example, chimpanzees and humans, the skulls of which are shown in Figure 12.7 are very similar genetically, sharing 99 percent 1 of their genes. However, chimpanzees and humans show considerable anatomical differences, including the degree to which the jaw protrudes in the adult and the relative lengths of our arms and legs.

However, unrelated organisms may be distantly related yet appear very much alike, usually because common adaptations to similar environmental conditions evolved in both. An example is the streamlined body shapes, the shapes of fins and appendages, and the shape of the tails in fishes and whales, which are mammals. These structures bear superficial similarity because they are adaptations to moving and maneuvering in the same environment—water. When a characteristic that is similar occurs by adaptive convergence (convergent evolution), and not because of a close evolutionary relationship, it is called an analogous structure . In another example, insects use wings to fly like bats and birds. We call them both wings because they perform the same function and have a superficially similar form, but the embryonic origin of the two wings is completely different. The difference in the development, or embryogenesis, of the wings in each case is a signal that insects and bats or birds do not share a common ancestor that had a wing. The wing structures, shown in Figure 12.8 evolved independently in the two lineages.

Similar traits can be either homologous or analogous. Homologous traits share an evolutionary path that led to the development of that trait, and analogous traits do not. Scientists must determine which type of similarity a feature exhibits to decipher the phylogeny of the organisms being studied.

Concepts in Action

This website has several examples to show how appearances can be misleading in understanding the phylogenetic relationships of organisms.

Molecular Comparisons

With the advancement of DNA technology, the area of molecular systematics , which describes the use of information on the molecular level including DNA sequencing, has blossomed. New analysis of molecular characters not only confirms many earlier classifications, but also uncovers previously made errors. Molecular characters can include differences in the amino-acid sequence of a protein, differences in the individual nucleotide sequence of a gene, or differences in the arrangements of genes. Phylogenies based on molecular characters assume that the more similar the sequences are in two organisms, the more closely related they are. Different genes change evolutionarily at different rates and this affects the level at which they are useful at identifying relationships. Rapidly evolving sequences are useful for determining the relationships among closely related species. More slowly evolving sequences are useful for determining the relationships between distantly related species. To determine the relationships between very different species such as Eukarya and Archaea, the genes used must be very ancient, slowly evolving genes that are present in both groups, such as the genes for ribosomal RNA. Comparing phylogenetic trees using different sequences and finding them similar helps to build confidence in the inferred relationships.

Sometimes two segments of DNA in distantly related organisms randomly share a high percentage of bases in the same locations, causing these organisms to appear closely related when they are not. For example, the fruit fly shares 60 percent of its DNA with humans. 2 In this situation, computer-based statistical algorithms have been developed to help identify the actual relationships, and ultimately, the coupled use of both morphologic and molecular information is more effective in determining phylogeny.

Evolution Connection

Why Does Phylogeny Matter?

In addition to enhancing our understanding of the evolutionary history of species, our own included, phylogenetic analysis has numerous practical applications. Two of those applications include understanding the evolution and transmission of disease and making decisions about conservation efforts. A 2010 study 3 of MRSA (methicillin-resistant Staphylococcus aureus), an antibiotic resistant pathogenic bacterium, traced the origin and spread of the strain throughout the past 40 years. The study uncovered the timing and patterns in which the resistant strain moved from its point of origin in Europe to centers of infection and evolution in South America, Asia, North America, and Australasia. The study suggested that introductions of the bacteria to new populations occurred very few times, perhaps only once, and then spread from that limited number of individuals. This is in contrast to the possibility that many individuals had carried the bacteria from one place to another. This result suggests that public health officials should concentrate on quickly identifying the contacts of individuals infected with a new strain of bacteria to control its spread.

A second area of usefulness for phylogenetic analysis is in conservation. Biologists have argued that it is important to protect species throughout a phylogenetic tree rather than just those from one branch of the tree. Doing this will preserve more of the variation produced by evolution. For example, conservation efforts should focus on a single species without sister species rather than another species that has a cluster of close sister species that recently evolved. If the single evolutionarily distinct species goes extinct a disproportionate amount of variation from the tree will be lost compared to one species in the cluster of closely related species. A study published in 2007 4 made recommendations for conservation of mammal species worldwide based on how evolutionarily distinct and at risk of extinction they are. The study found that their recommendations differed from priorities based on simply the level of extinction threat to the species. The study recommended protecting some threatened and valued large mammals such as the orangutans, the giant and lesser pandas, and the African and Asian elephants. But they also found that some much lesser known species should be protected based on how evolutionary distinct they are. These include a number of rodents, bats, shrews and hedgehogs. In addition there are some critically endangered species that did not rate as very important in evolutionary distinctiveness including species of deer mice and gerbils. While many criteria affect conservation decisions, preserving phylogenetic diversity provides an objective way to protect the full range of diversity generated by evolution.

Building Phylogenetic Trees

How do scientists construct phylogenetic trees? Presently, the most accepted method for constructing phylogenetic trees is a method called cladistics . This method sorts organisms into clades , groups of organisms that are most closely related to each other and the ancestor from which they descended. For example, in Figure 12.9, all of the organisms in the shaded region evolved from a single ancestor that had amniotic eggs. Consequently, all of these organisms also have amniotic eggs and make a single clade, also called a monophyletic group . Clades must include the ancestral species and all of the descendants from a branch point.

Visual Connection

Which animals in this figure belong to a clade that includes animals with hair? Which evolved first: hair or the amniotic egg?

Clades can vary in size depending on which branch point is being referenced. The important factor is that all of the organisms in the clade or monophyletic group stem from a single point on the tree. This can be remembered because monophyletic breaks down into “mono,” meaning one, and “phyletic,” meaning evolutionary relationship.

Shared Characteristics

Cladistics rests on three assumptions. The first is that living things are related by descent from a common ancestor, which is a general assumption of evolution. The second is that speciation occurs by splits of one species into two, never more than two at a time, and essentially at one point in time. This is somewhat controversial, but is acceptable to most biologists as a simplification. The third assumption is that traits change enough over time to be considered to be in a different state .It is also assumed that one can identify the actual direction of change for a state. In other words, we assume that an amniotic egg is a later character state than non-amniotic eggs. This is called the polarity of the character change. We know this by reference to a group outside the clade: for example, insects have non-amniotic eggs therefore, this is the older or ancestral character state. Cladistics compares ingroups and outgroups. An ingroup (lizard, rabbit and human in our example) is the group of taxa being analyzed. An outgroup (lancelet, lamprey and fish in our example) is a species or group of species that diverged before the lineage containing the group(s) of interest. By comparing ingroup members to each other and to the outgroup members, we can determine which characteristics are evolutionary modifications determining the branch points of the ingroup’s phylogeny.

If a characteristic is found in all of the members of a group, it is a shared ancestral character because there has been no change in the trait during the descent of each of the members of the clade. Although these traits appear interesting because they unify the clade, in cladistics they are considered not helpful when we are trying to determine the relationships of the members of the clade because every member is the same. In contrast, consider the amniotic egg characteristic of Figure 12.9. Only some of the organisms have this trait, and to those that do, it is called a shared derived character because this trait changed at some point during descent. This character does tell us about the relationships among the members of the clade it tells us that lizards, rabbits, and humans group more closely together than any of these organisms do with fish, lampreys, and lancelets.

A sometimes confusing aspect of “ancestral” and “derived” characters is that these terms are relative. The same trait could be either ancestral or derived depending on the diagram being used and the organisms being compared. Scientists find these terms useful when distinguishing between clades during the building of phylogenetic trees, but it is important to remember that their meaning depends on context.

Choosing the Right Relationships

Constructing a phylogenetic tree, or cladogram, from the character data is a monumental task that is usually left up to a computer. The computer draws a tree such that all of the clades share the same list of derived characters. But there are other decisions to be made, for example, what if a species presence in a clade is supported by all of the shared derived characters for that clade except one? One conclusion is that the trait evolved in the ancestor, but then changed back in that one species. Also a character state that appears in two clades must be assumed to have evolved independently in those clades. These inconsistencies are common in trees drawn from character data and complicate the decision-making process about which tree most closely represents the real relationships among the taxa.

To aid in the tremendous task of choosing the best tree, scientists often use a concept called maximum parsimony , which means that events occurred in the simplest, most obvious way. This means that the “best” tree is the one with the fewest number of character reversals, the fewest number of independent character changes, and the fewest number of character changes throughout the tree. Computer programs search through all of the possible trees to find the small number of trees with the simplest evolutionary pathways. Starting with all of the homologous traits in a group of organisms, scientists can determine the order of evolutionary events of which those traits occurred that is the most obvious and simple.

Concepts in Action

Practice Parsimony: Go to this website to learn how maximum parsimony is used to create phylogenetic trees (be sure to continue to the second page).

These tools and concepts are only a few of the strategies scientists use to tackle the task of revealing the evolutionary history of life on Earth. Recently, newer technologies have uncovered surprising discoveries with unexpected relationships, such as the fact that people seem to be more closely related to fungi than fungi are to plants. Sound unbelievable? As the information about DNA sequences grows, scientists will become closer to mapping the evolutionary history of all life on Earth.


Difference between Homologous and Analogous structures

Definition

Homologous structures are structures that evolve in living organisms that have a common ancestor. Analogous structures are those that evolve independently in different living organisms but have a similar or the same function.

Degree of relatedness among organisms

Organisms which have homologous structures are always closely related and share a common ancestral form. Organisms which have analogous structures are not closely related and do not arise from the same ancestor.

Developmental pattern

The developmental pattern in organisms which have homologous features tends to be very similar, and this is often evident when examining the embryos of these organisms. The developmental pattern in organisms which have analogous features tends to be very different.

Functions

Homologous structures may serve the same or different functions. Analogous structures always have the same or very similar functions.

Animal examples

The limbs of vertebrates are examples of homologous structures, and in fact the same bones are present, yet modified from one animal to another. The wings of insects and birds are examples of analogous structures with completely different evolutionary paths and origins.

Plant examples

Examples of homologous structures are the modified leaves of the pitcher plant, Venus fly trap, and cactus. Examples of analogous structures include the leaves of African euphorbia and cacti.


Watch the video: Homologie en analogie 5vwo (May 2022).


Comments:

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  4. Magor

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  5. Tumuro

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