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45.2: Life Histories and Natural Selection - Biology

45.2: Life Histories and Natural Selection - Biology


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Skills to Develop

  • Describe how life history patterns are influenced by natural selection
  • Explain different life history patterns and how different reproductive strategies affect species’ survival

A species’ life history describes the series of events over its lifetime, such as how resources are allocated for growth, maintenance, and reproduction. Life history traits affect the life table of an organism. A species’ life history is genetically determined and shaped by the environment and natural selection.

Life History Patterns and Energy Budgets

Energy is required by all living organisms for their growth, maintenance, and reproduction; at the same time, energy is often a major limiting factor in determining an organism’s survival. Plants, for example, acquire energy from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich seeds to produce the next generation. Animals have the additional burden of using some of their energy reserves to acquire food. Furthermore, some animals must expend energy caring for their offspring. Thus, all species have an energy budget: they must balance energy intake with their use of energy for metabolism, reproduction, parental care, and energy storage (such as bears building up body fat for winter hibernation).

Parental Care and Fecundity

Fecundity is the potential reproductive capacity of an individual within a population. In other words, fecundity describes how many offspring could ideally be produced if an individual has as many offspring as possible, repeating the reproductive cycle as soon as possible after the birth of the offspring. In animals, fecundity is inversely related to the amount of parental care given to an individual offspring. Species, such as many marine invertebrates, that produce many offspring usually provide little if any care for the offspring (they would not have the energy or the ability to do so anyway). Most of their energy budget is used to produce many tiny offspring. Animals with this strategy are often self-sufficient at a very early age. This is because of the energy tradeoff these organisms have made to maximize their evolutionary fitness. Because their energy is used for producing offspring instead of parental care, it makes sense that these offspring have some ability to be able to move within their environment and find food and perhaps shelter. Even with these abilities, their small size makes them extremely vulnerable to predation, so the production of many offspring allows enough of them to survive to maintain the species.

Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities, sometimes at the expense of their own health. This is the case with many mammals, such as humans, kangaroos, and pandas. The offspring of these species are relatively helpless at birth and need to develop before they achieve self-sufficiency.

Plants with low fecundity produce few energy-rich seeds (such as coconuts and chestnuts) with each having a good chance to germinate into a new organism; plants with high fecundity usually have many small, energy-poor seeds (like orchids) that have a relatively poor chance of surviving. Although it may seem that coconuts and chestnuts have a better chance of surviving, the energy tradeoff of the orchid is also very effective. It is a matter of where the energy is used, for large numbers of seeds or for fewer seeds with more energy.

Early versus Late Reproduction

The timing of reproduction in a life history also affects species survival. Organisms that reproduce at an early age have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that start reproducing later in life often have greater fecundity or are better able to provide parental care, but they risk that they will not survive to reproductive age. Examples of this can be seen in fishes. Small fish like guppies use their energy to reproduce rapidly, but never attain the size that would give them defense against some predators. Larger fish, like the bluegill or shark, use their energy to attain a large size, but do so with the risk that they will die before they can reproduce or at least reproduce to their maximum. These different energy strategies and tradeoffs are key to understanding the evolution of each species as it maximizes its fitness and fills its niche. In terms of energy budgeting, some species “blow it all” and use up most of their energy reserves to reproduce early before they die. Other species delay having reproduction to become stronger, more experienced individuals and to make sure that they are strong enough to provide parental care if necessary.

Single versus Multiple Reproductive Events

Some life history traits, such as fecundity, timing of reproduction, and parental care, can be grouped together into general strategies that are used by multiple species. Semelparity occurs when a species reproduces only once during its lifetime and then dies. Such species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive. Examples of semelparity are bamboo, which flowers once and then dies, and the Chinook salmon (Figure (PageIndex{1})a), which uses most of its energy reserves to migrate from the ocean to its freshwater nesting area, where it reproduces and then dies. Scientists have posited alternate explanations for the evolutionary advantage of the Chinook’s post-reproduction death: a programmed suicide caused by a massive release of corticosteroid hormones, presumably so the parents can become food for the offspring, or simple exhaustion caused by the energy demands of reproduction; these are still being debated.

Iteroparity describes species that reproduce repeatedly during their lives. Some animals are able to mate only once per year, but survive multiple mating seasons. The pronghorn antelope is an example of an animal that goes into a seasonal estrus cycle (“heat”): a hormonally induced physiological condition preparing the body for successful mating (Figure (PageIndex{1})b). Females of these species mate only during the estrus phase of the cycle. A different pattern is observed in primates, including humans and chimpanzees, which may attempt reproduction at any time during their reproductive years, even though their menstrual cycles make pregnancy likely only a few days per month during ovulation (Figure (PageIndex{1})c).

a

b

c

Figure (PageIndex{1}): The (a) Chinook salmon mates once and dies. The (b) pronghorn antelope mates during specific times of the year during its reproductive life. Primates, such as humans and (c) chimpanzees, may mate on any day, independent of ovulation. (credit a: modification of work by Roger Tabor, USFWS; credit b: modification of work by Mark Gocke, USDA; credit c: modification of work by “Shiny Things”/Flickr)

Link to Learning

Play this interactive PBS evolution-based mating game to learn more about reproductive strategies.

Evolution Connection

Energy Budgets, Reproductive Costs, and Sexual Selection in Drosophila

Research into how animals allocate their energy resources for growth, maintenance, and reproduction has used a variety of experimental animal models. Some of this work has been done using the common fruit fly, Drosophila melanogaster. Studies have shown that not only does reproduction have a cost as far as how long male fruit flies live, but also fruit flies that have already mated several times have limited sperm remaining for reproduction. Fruit flies maximize their last chances at reproduction by selecting optimal mates.

In a 1981 study, male fruit flies were placed in enclosures with either virgin or inseminated females. The males that mated with virgin females had shorter life spans than those in contact with the same number of inseminated females with which they were unable to mate. This effect occurred regardless of how large (indicative of their age) the males were. Thus, males that did not mate lived longer, allowing them more opportunities to find mates in the future.

More recent studies, performed in 2006, show how males select the female with which they will mate and how this is affected by previous matings (Figure (PageIndex{2})).1 Males were allowed to select between smaller and larger females. Findings showed that larger females had greater fecundity, producing twice as many offspring per mating as the smaller females did. Males that had previously mated, and thus had lower supplies of sperm, were termed “resource-depleted,” while males that had not mated were termed “non-resource-depleted.” The study showed that although non-resource-depleted males preferentially mated with larger females, this selection of partners was more pronounced in the resource-depleted males. Thus, males with depleted sperm supplies, which were limited in the number of times that they could mate before they replenished their sperm supply, selected larger, more fecund females, thus maximizing their chances for offspring. This study was one of the first to show that the physiological state of the male affected its mating behavior in a way that clearly maximizes its use of limited reproductive resources.

These studies demonstrate two ways in which the energy budget is a factor in reproduction. First, energy expended on mating may reduce an animal’s lifespan, but by this time they have already reproduced, so in the context of natural selection this early death is not of much evolutionary importance. Second, when resources such as sperm (and the energy needed to replenish it) are low, an organism’s behavior can change to give them the best chance of passing their genes on to the next generation. These changes in behavior, so important to evolution, are studied in a discipline known as behavioral biology, or ethology, at the interface between population biology and psychology.

Summary

All species have evolved a pattern of living, called a life history strategy, in which they partition energy for growth, maintenance, and reproduction. These patterns evolve through natural selection; they allow species to adapt to their environment to obtain the resources they need to successfully reproduce. There is an inverse relationship between fecundity and parental care. A species may reproduce early in life to ensure surviving to a reproductive age or reproduce later in life to become larger and healthier and better able to give parental care. A species may reproduce once (semelparity) or many times (iteroparity) in its life.

Footnotes

  1. 1 Adapted from Phillip G. Byrne and William R. Rice, “Evidence for adaptive male mate choice in the fruit fly Drosophila melanogaster,” Proc Biol Sci. 273, no. 1589 (2006): 917-922, doi: 10.1098/rspb.2005.3372.

Glossary

energy budget
allocation of energy resources for body maintenance, reproduction, and parental care
fecundity
potential reproductive capacity of an individual
iteroparity
life history strategy characterized by multiple reproductive events during the lifetime of a species
life history
inherited pattern of resource allocation under the influence of natural selection and other evolutionary forces
semelparity
life history strategy characterized by a single reproductive event followed by death

241 Life Histories and Natural Selection

By the end of this section, you will be able to do the following:

  • Describe how life history patterns are influenced by natural selection
  • Explain different life history patterns and how different reproductive strategies affect species’ survival

A species’ life history describes the series of events over its lifetime, such as how resources are allocated for growth, maintenance, and reproduction. Life history traits affect the life table of an organism. A species’ life history is genetically determined and shaped by the environment and natural selection.

Life History Patterns and Energy Budgets

Energy is required by all living organisms for their growth, maintenance, and reproduction at the same time, energy is often a major limiting factor in determining an organism’s survival. Plants, for example, acquire energy from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich seeds to produce the next generation. Animals have the additional burden of using some of their energy reserves to acquire food. Furthermore, some animals must expend energy caring for their offspring. Thus, all species have an energy budget : they must balance energy intake with their use of energy for metabolism, reproduction, parental care, and energy storage (such as bears building up body fat for winter hibernation).

Parental Care and Fecundity

Fecundity is the potential reproductive capacity of an individual within a population. In other words, fecundity describes how many offspring could ideally be produced if an individual has as many offspring as possible, repeating the reproductive cycle as soon as possible after the birth of the offspring. In animals, fecundity is inversely related to the amount of parental care given to an individual offspring. Species, such as many marine invertebrates, that produce many offspring usually provide little if any care for the offspring (they would not have the energy or the ability to do so anyway). Most of their energy budget is used to produce many tiny offspring. Animals with this strategy are often self-sufficient at a very early age. This is because of the energy tradeoff these organisms have made to maximize their evolutionary fitness. Because their energy is used for producing offspring instead of parental care, it makes sense that these offspring have some ability to be able to move within their environment and find food and perhaps shelter. Even with these abilities, their small size makes them extremely vulnerable to predation, so the production of many offspring allows enough of them to survive to maintain the species.

Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities, sometimes at the expense of their own health. This is the case with many mammals, such as humans, kangaroos, and pandas. The offspring of these species are relatively helpless at birth and need to develop before they achieve self-sufficiency.

Plants with low fecundity produce few energy-rich seeds (such as coconuts and chestnuts) with each having a good chance to germinate into a new organism plants with high fecundity usually have many small, energy-poor seeds (like orchids) that have a relatively poor chance of surviving. Although it may seem that coconuts and chestnuts have a better chance of surviving, the energy tradeoff of the orchid is also very effective. It is a matter of where the energy is used, for large numbers of seeds or for fewer seeds with more energy.

Early versus Late Reproduction

The timing of reproduction in a life history also affects species survival. Organisms that reproduce at an early age have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that start reproducing later in life often have greater fecundity or are better able to provide parental care, but they risk that they will not survive to reproductive age. Examples of this can be seen in fishes. Small fish, like guppies, use their energy to reproduce rapidly, but never attain the size that would give them defense against some predators. Larger fish, like the bluegill or shark, use their energy to attain a large size, but do so with the risk that they will die before they can reproduce or at least reproduce to their maximum. These different energy strategies and tradeoffs are key to understanding the evolution of each species as it maximizes its fitness and fills its niche. In terms of energy budgeting, some species “blow it all” and use up most of their energy reserves to reproduce early before they die. Other species delay having reproduction to become stronger, more experienced individuals and to make sure that they are strong enough to provide parental care if necessary.

Single versus Multiple Reproductive Events

Some life history traits, such as fecundity, timing of reproduction, and parental care, can be grouped together into general strategies that are used by multiple species. Semelparity occurs when a species reproduces only once during its lifetime and then dies. Such species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive. Examples of semelparity are bamboo, which flowers once and then dies, and the Chinook salmon ((Figure)a), which uses most of its energy reserves to migrate from the ocean to its freshwater nesting area, where it reproduces and then dies. Scientists have posited alternate explanations for the evolutionary advantage of the Chinook’s post-reproduction death: a programmed suicide caused by a massive release of corticosteroid hormones, presumably so the parents can become food for the offspring, or simple exhaustion caused by the energy demands of reproduction these are still being debated.

Iteroparity describes species that reproduce repeatedly during their lives. Some animals are able to mate only once per year, but survive multiple mating seasons. The pronghorn antelope is an example of an animal that goes into a seasonal estrus cycle (“heat”): a hormonally induced physiological condition preparing the body for successful mating ((Figure)b). Females of these species mate only during the estrus phase of the cycle. A different pattern is observed in primates, including humans and chimpanzees, which may attempt reproduction at any time during their reproductive years, even though their menstrual cycles make pregnancy likely only a few days per month during ovulation ((Figure)c).


Play this interactive PBS evolution-based mating game to learn more about reproductive strategies.

Energy Budgets, Reproductive Costs, and Sexual Selection in Drosophila
Research into how animals allocate their energy resources for growth, maintenance, and reproduction has used a variety of experimental animal models. Some of this work has been done using the common fruit fly, Drosophila melanogaster. Studies have shown that not only does reproduction have a cost as far as how long male fruit flies live, but also fruit flies that have already mated several times have limited sperm remaining for reproduction. Fruit flies maximize their last chances at reproduction by selecting optimal mates.

In a 1981 study, male fruit flies were placed in enclosures with either virgin or inseminated females. The males that mated with virgin females had shorter life spans than those in contact with the same number of inseminated females with which they were unable to mate. This effect occurred regardless of how large (indicative of their age) the males were. Thus, males that did not mate lived longer, allowing them more opportunities to find mates in the future.

More recent studies, performed in 2006, show how males select the female with which they will mate and how this is affected by previous matings ((Figure)). 1 Males were allowed to select between smaller and larger females. Findings showed that larger females had greater fecundity, producing twice as many offspring per mating as the smaller females did. Males that had previously mated, and thus had lower supplies of sperm, were termed “resource-depleted,” while males that had not mated were termed “non-resource-depleted.” The study showed that although non-resource-depleted males preferentially mated with larger females, this selection of partners was more pronounced in the resource-depleted males. Thus, males with depleted sperm supplies, which were limited in the number of times that they could mate before they replenished their sperm supply, selected larger, more fecund females, thus maximizing their chances for offspring. This study was one of the first to show that the physiological state of the male affected its mating behavior in a way that clearly maximizes its use of limited reproductive resources.


These studies demonstrate two ways in which the energy budget is a factor in reproduction. First, energy expended on mating may reduce an animal’s lifespan, but by this time they have already reproduced, so in the context of natural selection this early death is not of much evolutionary importance. Second, when resources such as sperm (and the energy needed to replenish it) are low, an organism’s behavior can change to give them the best chance of passing their genes on to the next generation. These changes in behavior, so important to evolution, are studied in a discipline known as behavioral biology, or ethology, at the interface between population biology and psychology.

Section Summary

All species have evolved a pattern of living, called a life history strategy, in which they partition energy for growth, maintenance, and reproduction. These patterns evolve through natural selection they allow species to adapt to their environment to obtain the resources they need to successfully reproduce. There is an inverse relationship between fecundity and parental care. A species may reproduce early in life to ensure surviving to a reproductive age or reproduce later in life to become larger and healthier and better able to give parental care. A species may reproduce once (semelparity) or many times (iteroparity) in its life.

Review Questions

Which of the following is associated with long-term parental care?

Which of the following is associated with multiple reproductive episodes during a species’ lifetime?

Which of the following is associated with the reproductive potential of a species?

Critical Thinking Questions

Why is long-term parental care not associated with having many offspring during a reproductive episode?

Parental care is not feasible for organisms having many offspring because they do not have the energy available to take care of offspring. Most of their energy budget is used in the formation of seeds or offspring, so there is little left for parental care. Also, the sheer number of offspring would make individual parental care impossible.

Describe the difference in evolutionary pressures experienced by an animal that begins reproducing early and an animal that reproduces late in its lifecycle.

A species that reproduces early in its life cycle is under evolutionary pressure to reach sexual maturity as soon as possible. Animals that mature earliest will be able to reproduce the most times, and therefore more of the next generation will carry their early maturation genes.

A species that reproduces late in its life cycle will only generate offspring from parents that were able to survive in their habitats. This increases the likelihood that the next generation will be well-adapted to its environment.

Footnotes

    Adapted from Phillip G. Byrne and William R. Rice, “Evidence for adaptive male mate choice in the fruit fly Drosophila melanogaster,” Proc Biol Sci. 273, no. 1589 (2006): 917-922, doi: 10.1098/rspb.2005.3372.

Glossary


Natural Selection

Natural selection is the process through which species adapt to their environments. It is the engine that drives evolution.

On the Origin of Species

English naturalist Charles Darwin wrote the definitive book outlining his idea of natural selection, On the Origin of Species. The book chronicled his studies in South America and Pacific islands. Published in 1859, the book became a best seller.

Photograph by Ian Forsyth via Getty Images

English naturalist Charles Darwin developed the idea of natural selection after a five-year voyage to study plants, animals, and fossils in South America and on islands in the Pacific. In 1859, he brought the idea of natural selection to the attention of the world in his best-selling book, On the Origin of Species.

Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits&mdashtraits that give them some advantage&mdashare more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population. Through this process of natural selection, favorable traits are transmitted through generations.

Natural selection can lead to speciation, where one species gives rise to a new and distinctly different species. It is one of the processes that drives evolution and helps to explain the diversity of life on Earth.

Darwin chose the name natural selection to contrast with &ldquoartificial selection,&rdquo or selective breeding that is controlled by humans. He pointed to the pastime of pigeon breeding, a popular hobby in his day, as an example of artificial selection. By choosing which pigeons mated with others, hobbyists created distinct pigeon breeds, with fancy feathers or acrobatic flight, that were different from wild pigeons.

Darwin and other scientists of his day argued that a process much like artificial selection happened in nature, without any human intervention. He argued that natural selection explained how a wide variety of life forms developed over time from a single common ancestor.

Darwin did not know that genes existed, but he could see that many traits are heritable&mdashpassed from parents to offspring.

Mutations are changes in the structure of the molecules that make up genes, called DNA. The mutation of genes is an important source of genetic variation within a population. Mutations can be random (for example, when replicating cells make an error while copying DNA), or happen as a result of exposure to something in the environment, like harmful chemicals or radiation.

Mutations can be harmful, neutral, or sometimes helpful, resulting in a new, advantageous trait. When mutations occur in germ cells (eggs and sperm), they can be passed on to offspring.

If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection. Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct. Dinosaurs are one example. An invasive species, a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species.

Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record.

English naturalist Charles Darwin wrote the definitive book outlining his idea of natural selection, On the Origin of Species. The book chronicled his studies in South America and Pacific islands. Published in 1859, the book became a best seller.


Principles of Natural Selection

There is an incredible variety of selective forces in the natural world, ranging from interspecies competition, to predator-prey dynamics, to sexual selection between the different genders. The defining characteristic of natural selection is that it is a force that allows some organisms to reproduce more than others. Natural selection does not always lead to the “right” answer, as some people tend to think.

Natural selection is an imperfect process. It cannot create new DNA spontaneously, or change the DNA it is given in meaningful ways. It can only slow or stop the reproduction of some DNA while allowing other DNA to persist. Every population has the opportunity to adapt, migrate to different conditions, or go extinct in the face of natural selection.

The process of natural selection screens the DNA it is given, with the minor mutations and recombination that occurs during replication, and simply does not let some DNA pass. Sometimes, the screen is random, as in a lighting strike killing a single tree. Other times, the screen is biased towards certain types of organisms, causing a selection to happen. This can be seen in the pine beetle invasion in North America. The pine beetles are being selected for because they are exploiting a rich food source. The pine trees, on the other hand, are being selected against for not having adequate defenses against the beetles.


20 Examples of Natural selection

The process of natural selection refers to one of the mechanisms of evolution of the species of living beings, proposed by Charles Darwin and Alfred Russel Wallace, from which they explained the design of nature.

Natural selection occurs thanks to the progressive adaptation of the species to its environment. When individuals with certain characteristics have a higher survival rate than other members of a population, they pass these inheritable genetic characteristics to their offspring.

Evolution

Natural selection is the central basis of all evolutionary change, being also the process through which the best-adapted organisms displace the less adapted by the slow and progressive accumulation of genetic changes.

The contribution of an individual to the next generation is recognized as biological efficacy, and it is a quantitative character that encompasses many others, related to the survival of the fittest and the differential reproduction of different genotypes.

The fundamental thesis of natural selection is that the traits are hereditary, but nevertheless, there is variability in the trait between different specimens. In this way, there is a biological adaptation to the environment, and only certain characteristics of the new apparitions extend to the entire population.

The generations are in permanent evolution, and it is precisely the set of variations that occur throughout the generations that constitute the evolutionary process.

Examples of natural selection

  1. The evolution of medicine is based precisely on the fact that from the use of antibiotics for viruses or bacteria it is possible to kill some of them, but those that survive become more resistant.
  2. The white fur of the arctic animals, which allows them to hide in the snow.
  3. The camouflage of grasshoppers, which makes them look like leaves.
  4. The movements of the male blue-legged gannet, to attract its mate.
  5. The giraffes, of which the longest neck survived.
  6. The color change of a chameleon when it has prey, or to protect itself.
  7. The process of cloning, constantly developing but already proven in the facts, could potentially interfere with natural selection.
  8. The brown beetles have a greater chance of survival, and have more descendants, the population becoming frequent.
  9. The case of the totality of the species that disappeared, and still continue to do so.
  10. The cheetahs, which have survived the fastest.
  11. The evolution of the human being in different species, called hominids.
  12. The deformation of the jaw of the snake to swallow larger prey.
  13. The change of coloration of some moths, motivated by the industrial revolution in England. (Here the change in the environment was generated by man)
  14. The bobbing dance of bees.
  15. The resistance to insecticides of some insects, which show the question of selection as a source of survival.
  16. The shape of the beak of the finches was modified over time because after the droughts they hardened allowing to eat harder seeds.
  17. The ability of human beings to learn to speak.
  18. The orchids that are capable of deceiving wasps so that they ‘mate’ with them.
  19. The non-poisonous king snakes, which mimic poisonous coral snakes.
  20. The courtship rituals of the birds.

Linear and continuous process?

The question of evolution implies an additional consideration because if the characteristics pass along the evolutionary process as explained, a linear sequence of species could be traced, getting to connect each of the genetic variabilities that were appearing.

Under this premise is that the evolutionary chain under which the idea of ?? a missing link was interpreted, a variability that is missing to fully describe an evolution. However, this is not what happens: evolution is endowed with ramifications, with mixtures between species and modifications according to different adaptations to the environment, which is a correction that leaves out this idea of a missing link.

The generalization of Darwinism

The question of natural selection was replicated through the analogy for other areas, and by extension, the idea of Darwinism explained precisely these areas, where the strongest and capable is the one that survives while those that are not so adapted do not. When it comes to social processes, it is evident that Darwinism is a very cruel and aggressive situation.

For the natural selection process to occur, it is necessary that the differential biological effectiveness exists, that the phenotypic type is variable, and that this variation occurs through inheritance.


Natural Selection and Adaptive Evolution

Natural Selection and the Evolution of Populations

Though each has been tested and shown to be accurate, none of the observations and inferences that underlies natural selection is sufficient individually to provide a mechanism for evolutionary change Footnote 6 . Overproduction alone will have no evolutionary consequences if all individuals are identical. Differences among organisms are not relevant unless they can be inherited. Genetic variation by itself will not result in natural selection unless it exerts some impact on organism survival and reproduction. However, any time all of Darwin's postulates hold simultaneously—as they do in most populations—natural selection will occur. The net result in this case is that certain traits (or, more precisely, genetic variants that specify those traits) will, on average, be passed on from one generation to the next at a higher rate than existing alternatives in the population. Put another way, when one considers who the parents of the current generation were, it will be seen that a disproportionate number of them possessed traits beneficial for survival and reproduction in the particular environment in which they lived.

The important points are that this uneven reproductive success among individuals represents a process that occurs in each generation and that its effects are cumulative over the span of many generations. Over time, beneficial traits will become increasingly prevalent in descendant populations by virtue of the fact that parents with those traits consistently leave more offspring than individuals lacking those traits. If this process happens to occur in a consistent direction—say, the largest individuals in each generation tend to leave more offspring than smaller individuals—then there can be a gradual, generation-by-generation change in the proportion of traits in the population. This change in proportion and not the modification of organisms themselves is what leads to changes in the average value of a particular trait in the population. Organisms do not evolve populations evolve.

Adaptation

The term “adaptation” derives from ad + aptus, literally meaning “toward + fit”. As the name implies, this is the process by which populations of organisms evolve in such a way as to become better suited to their environments as advantageous traits become predominant. On a broader scale, it is also how physical, physiological, and behavioral features that contribute to survival and reproduction (“adaptations”) arise over evolutionary time. This latter topic is particularly difficult for many to grasp, though of course a crucial first step is to understand the operation of natural selection on smaller scales of time and consequence. (For a detailed discussion of the evolution of complex organs such as eyes, see Gregory 2008b.)

On first pass, it may be difficult to see how natural selection can ever lead to the evolution of new characteristics if its primary effect is merely to eliminate unfit traits. Indeed, natural selection by itself is incapable of producing new traits, and in fact (as many readers will have surmised), most forms of natural selection deplete genetic variation within populations. How, then, can an eliminative process like natural selection ever lead to creative outcomes?

To answer this question, one must recall that evolution by natural selection is a two-step process. The first step involves the generation of new variation by mutation and recombination, whereas the second step determines which randomly generated variants will persist into the next generation. Most new mutations are neutral with respect to survival and reproduction and therefore are irrelevant in terms of natural selection (but not, it must be pointed out, to evolution more broadly). The majority of mutations that have an impact on survival and reproductive output will do so negatively and, as such, will be less likely than existing alternatives to be passed on to subsequent generations. However, a small percentage of new mutations will turn out to have beneficial effects in a particular environment and will contribute to an elevated rate of reproduction by organisms possessing them. Even a very slight advantage is sufficient to cause new beneficial mutations to increase in proportion over the span of many generations.

Biologists sometimes describe beneficial mutations as “spreading” or “sweeping” through a population, but this shorthand is misleading. Rather, beneficial mutations simply increase in proportion from one generation to the next because, by definition, they happen to contribute to the survival and reproductive success of the organisms carrying them. Eventually, a beneficial mutation may be the only alternative left as all others have ultimately failed to be passed on. At this point, that beneficial genetic variant is said to have become “fixed” in the population.

Again, mutation does not occur in order to improve fitness—it merely represents errors in genetic replication. This means that most mutations do not improve fitness: There are many more ways of making things worse than of making them better. It also means that mutations will continue to occur even after previous beneficial mutations have become fixed. As such, there can be something of a ratcheting effect in which beneficial mutations arise and become fixed by selection, only to be supplemented later by more beneficial mutations which, in turn, become fixed. All the while, neutral and deleterious mutations also occur in the population, the latter being passed on at a lower rate than alternatives and often being lost before reaching any appreciable frequency.

Of course, this is an oversimplification—in species with sexual reproduction, multiple beneficial mutations may be brought together by recombination such that the fixation of beneficial genes need not occur sequentially. Likewise, recombination can juxtapose deleterious mutations, thereby hastening their loss from the population. Nonetheless, it is useful to imagine the process of adaptation as one in which beneficial mutations arise continually (though perhaps very infrequently and with only minor positive impacts) and then accumulate in the population over many generations.

The process of adaptation in a population is depicted in very basic form in Fig. 2. Several important points can be drawn from even such an oversimplified rendition:

Mutations are the source of new variation. Natural selection itself does not create new traits it only changes the proportion of variation that is already present in the population. The repeated two-step interaction of these processes is what leads to the evolution of novel adaptive features.

Mutation is random with respect to fitness. Natural selection is, by definition, non-random with respect to fitness. This means that, overall, it is a serious misconception to consider adaptation as happening “by chance”.

Mutations occur with all three possible outcomes: neutral, deleterious, and beneficial. Beneficial mutations may be rare and deliver only a minor advantage, but these can nonetheless increase in proportion in the population over many generations by natural selection. The occurrence of any particular beneficial mutation may be very improbable, but natural selection is very effective at causing these individually unlikely improvements to accumulate. Natural selection is an improbability concentrator.

No organisms change as the population adapts. Rather, this involves changes in the proportion of beneficial traits across multiple generations.

The direction in which adaptive change occurs is dependent on the environment. A change in environment can make previously beneficial traits neutral or detrimental and vice versa.

Adaptation does not result in optimal characteristics. It is constrained by historical, genetic, and developmental limitations and by trade-offs among features (see Gregory 2008b).

It does not matter what an “ideal” adaptive feature might be—the only relevant factor is that variants that happen to result in greater survival and reproduction relative to alternative variants are passed on more frequently. As Darwin wrote in a letter to Joseph Hooker (11 Sept. 1857), “I have just been writing an audacious little discussion, to show that organic beings are not perfect, only perfect enough to struggle with their competitors.”

The process of adaptation by natural selection is not forward-looking, and it cannot produce features on the grounds that they might become beneficial sometime in the future. In fact, adaptations are always to the conditions experienced by generations in the past.

A highly simplified depiction of natural selection (Correct) and a generalized illustration of various common misconceptions about the mechanism (Incorrect). Properly understood, natural selection occurs as follows: (A) A population of organisms exhibits variation in a particular trait that is relevant to survival in a given environment. In this diagram, darker coloration happens to be beneficial, but in another environment, the opposite could be true. As a result of their traits, not all individuals in Generation 1 survive equally well, meaning that only a non-random subsample ultimately will succeed in reproducing and passing on their traits (B). Note that no individual organisms in Generation 1 change, rather the proportion of individuals with different traits changes in the population. The individuals who survive from Generation 1 reproduce to produce Generation 2. (C) Because the trait in question is heritable, this second generation will (mostly) resemble the parent generation. However, mutations have also occurred, which are undirected (i.e., they occur at random in terms of the consequences of changing traits), leading to both lighter and darker offspring in Generation 2 as compared to their parents in Generation 1. In this environment, lighter mutants are less successful and darker mutants are more successful than the parental average. Once again, there is non-random survival among individuals in the population, with darker traits becoming disproportionately common due to the death of lighter individuals (D). This subset of Generation 2 proceeds to reproduce. Again, the traits of the survivors are passed on, but there is also undirected mutation leading to both deleterious and beneficial differences among the offspring (E). (F) This process of undirected mutation and natural selection (non-random differences in survival and reproductive success) occurs over many generations, each time leading to a concentration of the most beneficial traits in the next generation. By Generation N, the population is composed almost entirely of very dark individuals. The population can now be said to have become adapted to the environment in which darker traits are the most successful. This contrasts with the intuitive notion of adaptation held by most students and non-biologists. In the most common version, populations are seen as uniform, with variation being at most an anomalous deviation from the norm (X). It is assumed that all members within a single generation change in response to pressures imposed by the environment (Y). When these individuals reproduce, they are thought to pass on their acquired traits. Moreover, any changes that do occur due to mutation are imagined to be exclusively in the direction of improvement (Z). Studies have revealed that it can be very difficult for non-experts to abandon this intuitive interpretation in favor of a scientifically valid understanding of the mechanism. Diagrams based in part on Bishop and Anderson (1990)


General Overviews

Among the general treatments of life history evolution, Stearns 1992 and Roff 1992 provide an excellent starting point for exploring the basic theory and evidence. Roff 2002 revisits many of the topics covered in the author’s previous book (Roff 1992), but with a more evolutionary-genetic framework. Charnov 1993 mainly considers dimensionless ratios of life history variables that appear to be invariant across taxa (for a criticism of this perspective, see Nee, et al. 2005). More recent reviews, including the edited volume Flatt and Heyland 2011 and the book chapter Sibly 2012, have focused on physiological mechanisms underlying animal life history traits, and their associated trade-offs. Marshall, et al. 2012 reviews life history variation of marine invertebrates and Crawley 1997 provides an overview of plant life history ecology.

Charnov, E. L. 1993. Life history invariants: Some explorations of symmetry in evolutionary ecology. Oxford: Oxford Univ. Press.

Reviews much of the author’s previous work, and compares key life history characters across species to establish scaling laws and invariant, dimensionless ratios of trait pairs. This “symmetry” approach has been applied to explain variation in such features as sex ratios, aging, and determinate vs. indeterminate growth.

Crawley, M. J. 1997. Life history and environment. In Plant Ecology. 2d ed. Edited by M. J. Crawley, 73–131. Oxford and Malden, MA: Blackwell Science.

Overview of life history variation in plants. Available online (2009) by subscription.

Flatt, T., and A. Heyland. 2011. Mechanisms of life history evolution: The genetics and physiology of life history traits and trade-offs. Oxford: Oxford Univ. Press.

An edited volume that presents summaries for a large number of topics in life history evolution, with particular attention to genetic and physiological mechanisms underlying traits and trade-offs.

Marshall, D. J., P. K. Krug, E. K. Kupriyanova, M. Byrne, and R. E. Emlet. 2012. The biogeography of marine invertebrate life histories. Annual Review of Ecology, Evolution and Systematics 43:97–114.

Compiles life history and geographic data for more than one thousand species of marine invertebrates, and many traits covary with temperature and local productivity. An excellent example of the utility of the comparative approach, applied here to group whose traits seem less constrained by phylogeny than is true for terrestrial species. Available online for purchase or by subscription.

Nee, S., N. Colegrave, S. A. West, and A. Grafen. 2005. The illusion of invariant quantities in life histories. Science 309.5738: 1236–1239.

Argues that some dimensionless ratios of key life history appear to be “invariant” only because of a lack of independence between the variables compared (e.g., offspring-weaning mass vs. maternal mass). The authors show that, for bounded variables, even random simulations of trait values can give the impression of invariance. Available online for purchase or by subscription.

Roff, D. A. 1992. The Evolution of life histories: Theory and analysis. New York: Chapman & Hall.

A thorough compendium of the theory and evidence used to understand life history evolution. This tour-de-force covers both genetic and optimization approaches for virtually all important traits, and undoubtedly influenced much subsequent work in this field.

Roff, D. A. 2002. Life history evolution. Sunderland, MA: Sinauer.

Updates Roff 1992 and devotes more attention to quantitative-genetic modeling. This book is also organized in an interesting way instead a sequential coverage of the usual life history traits, Roff considers the evolution of relevant traits in three types of environments: constant, stochastic, and predictable.

Sibly, R. M. 2012. Life history. In Metabolic ecology: A scaling approach. Edited by R. M. Sibly, J. H. Brown, and A. Kodric-Brown, 57–66. Chichester, UK and Hoboken, NJ: Wiley-Blackwell.

Metabolic rate, which varies with body size and temperature, influences rates of resource allocation to growth, maintenance, and reproduction. Sibly shows that many life history traits covary with mass, so that the metabolic theory of ecology can help identify broad patterns and constraints.

Stearns, S. C. 1992. The evolution of life histories. New York: Oxford Univ. Press.

An accessible and relatively concise introduction to the theory that is the foundation of life history studies. Provides a useful introductory discussion of the meanings of adaptation and fitness, and their application to the study of life histories.

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Darwin, Charles and Wallace, A. R. 1858a. “On the Tendency of Species to Form Varieties and On the Perpetuation of Varieties and Species by Natural Means of Selection.” Journal of the Proceedings of the Linnaean Society 3: 45–62.

Darwin, Charles. 1959. On the Origin of Species: A Variorum Text. Edited by M. Peckham. Philadelphia: University of Pennsylvania Press.

Darwin, Charles. 1975. Natural Selection. Edited by R. Stauffer. Cambridge: Cambridge University Press.

Owen, Richard. 1849 [2008]. On the Nature of Limbs: A Discourse. Edited with Prefatory Essays, by Ronald Amundson and Brian Hall. Chicago, University of Chicago Press.

Owen, Richard. 1850. “On Dinornis (Part IV): Containing the Restoration of the Feet of That Genus and of Palapteryx, with a Description of the Sternum in Palapteryx and Aptornis, Transactions of the Zoological Society 4: 1–20, and plates i–iv.

Owen, Richard. 1858. “Address of the President.” Report of the 28th meeting of the British Association for the Advancement of Science, held at Leeds: xlix–cxi.

Owen, Richard. 1859a. “Summary of the Succession in Time and Geographical Distribution of Recent and Extinct Fossil Mammalia.” Proceedings of the Royal Institution of Great Britain 3: 109–116.

Owen, Richard. 1859b. On classification and geological distribution of the mammalia, with appendices on extinction and apes with references to the transmutation of species, being the Lecture delivered before the University of Cambridge. To which is added an appendix “On the Gorilla” and “On the extinction and transmutation of species.” London, J.W. Parker.

Owen, Richard. 1860a. Paleontology. Edinburgh: Adam & Charles Black (2nd ed., 1861).

Owen, Richard. 1860b. [Anonymous]. “Darwin On the Origin of Species.” Edinburgh Review 111: Article VIII, pp. 487–532.

Owen, Richard. 1866a. “Letter” to the London Review: May 5, p. 516.

Owen, Richard. 1866b. On the Anatomy of Vertebrates, volumes 1–2. London: Longman, Green & Co.

Owen, Richard. 1868a. On the Anatomy of Vertebrates, 3 vols. London: Longman, Green & Co.

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Amundson, Ron. 2008. See Owen, Richard, On the Nature of Limbs, 1849.

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Desmond, Adrian J. 1982. Archetypes and Ancestors: Paleontology in Victorian London 1850–1875. London: Blond and Briggs.

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Insight 7: We Have Not Evolved to Seek the Truth

Humans evolved in small groups under threat of starvation, predation, and exploitation by outsiders—and generally lived brief lives, favoring short-term strategies for consuming resources that could support successful reproduction (59). We have not evolved to think clearly about long-term threats like pandemics—which are statistically abstract and global. And yet, for at least a century, we’ve understood that the threat of a deadly pandemic is real and ever present (60). How should we have responded to this knowledge?

We should have prepared for the next pandemic in advance. But, to do this, we would have had to feel the need to prepare—and been willing to incur actual costs in the face of what could have seemed, in the absence of dead and dying people, like nothing more than morbid speculation.

Unfortunately, most of us are terrible at weighing risks presented as abstract probabilities (61). We also heavily discount the well-being of our future selves (62), along with that of distant strangers (63) and future generations (64), and in ways that are both psychologically strange and, in a modern environment, ethically indefensible. We’re highly susceptible to conspiracy thinking (65), and display an impressive capacity to deceive ourselves, before doing the hard work of deceiving others (66). These predispositions likely endowed our ancestors with advantages (67, 68), but they also suggest that our species is not wired for seeking a precise understanding of the world as it actually is.

Thus, our conversation about most things tends to be a tissue of false certainties and unhedged bets. We look for evidence to support our current beliefs, while ignoring the rest (69). When we encounter friends or family in thrall to some fresh piece of misinformation, we often lack the courage to correct them. Meanwhile, behind a screen of anonymity, we eagerly confront the views of complete strangers online. Paradoxically, the former circumstance presents an opportunity to actually change opinion, while the latter is more likely to further entrench people in their misinformed views (70). Although these predispositions did not cause SARS-CoV-2 to first enter the human population, they are, at least in part, responsible for the pandemic that ensued.

Scientific Agenda.

Evaluate methods to combat shortcomings in reasoning due to mismatches between the demands of the ancestral past and the present, conspiracy thinking, and the spread of misinformation, both in face-to-face communication and on social networks, particularly as they relate to the pandemic and health-relevant information.


Natural mortality: Its ecology, how it shapes fish life histories, and why it may be increased by fishing ☆

A stronger focus on natural mortality may be required to better understand contemporary changes in fish life histories and behaviour and their responses to anthropogenic drivers. Firstly, natural mortality is the selection under which fish evolved in the first place, so a theoretical understanding of effects of natural mortality alone is needed. Secondly, due to trade-offs, most organismal functions can only be achieved at some cost in terms of survival. Several trade-offs might need to be analysed simultaneously with effects on natural mortality being a common currency. Thirdly, there is scattered evidence that natural mortality has been increasing, some would say dramatically, in some fished stocks, which begs explanations. Fourthly, natural mortality most often implies transfer of mass and energy from one species to another, and therefore has foodweb and ecosystem consequences. We therefore analyse a model for evolution of fish life histories and behaviour, where state-dependent energy-allocation and growth strategies are found by optimization. Natural mortality is split into five different components, each specified as the outcome of individual traits and ecological trade-offs: a fixed baseline mortality size-dependent predation risk-dependent growth strategy a fixed mortality when sexually mature and mortality increasing with reproductive investment. The analysis is repeated with and without fishing. Each component of natural mortality has consequences for optimal life history strategies. Beyond earlier models, we show i) how the two types of reproductive mortality sometimes have similar and sometimes contrasting effects on life history evolution, ii) how ecosystem properties such as food availability and predation levels have stronger effects on optimal strategies than changing other mortality components, and iii) how expected changes in risk-dependent growth strategies are highly variable depending on the type of mortality changed.

Highlights

► Natural mortality emerges from individual strategies for behaviour and life history. ► Ecological relationships, including anthropogenic change, shape these strategies. ► Fishing will likely select for increased natural mortality as an adaptive response. ► We show sensitivity to several trade-offs between mortality and life history traits.


Watch the video: Introduction to Evolution and Natural Selection (May 2022).


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