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3.3: Natural Selection and Adaptive Evolution

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• Page ID 94601

• Tara Jo Holmberg
• Northwestern Connecticut Community College

Natural Selection and Adaptive Evolution

• Please read and watch the following Mandatory Resources
• Reading the material for understanding, and taking notes during videos, will take approximately 1.5 hours.
• Optional Activities are embedded.
• If on a mobile device, use the Contents menu at the top of the page OR the links at the bottom of the page.

Learning Objectives

• Explain how natural selection leads to adaptive evolution
• Contrast stabilizing selection, directional selection, and diversifying selection.
• Describe frequency-dependent selection
• Explain the effects of sexual dimorphism on the reproductive potential of an organism

Introduction to Natural Selection

Natural selection drives adaptive evolution by selecting for and increasing the occurrence of beneficial traits in a population. Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles and, thus, increasing their frequency in the population, while selecting against deleterious alleles and, thereby, decreasing their frequency. This process is known as adaptive evolution (Figure \(\PageIndex{1}\)). Natural selection does not act on individual alleles, however, but on entire organisms. An individual may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce. If that same individual also carries an allele that results in a fatal childhood disease, however, the fecundity phenotype will not be passed on to the next generation because the individual will not live to reach reproductive age. Natural selection acts at the level of the individual. It selects for individuals with greater contributions to the gene pool of the next generation, known as an organism’s evolutionary fitness .

Evolutionary fitness is often quantifiable as the contribution of genetic information by an individual to subsequent generations. However, it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population. This concept, called relative fitness , allows researchers to determine which individuals are contributing additional offspring to the next generation and, thus, how the population might evolve.

Genetic Variation and Natural Selection

Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against. Acquired traits, for the most part, are not heritable. For example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not necessarily grow up to be a bodybuilder. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child.

Genetic variation is a measure of the genetic differences (diversity of alleles and genotypes) that exist within a population. The genetic variation of an entire species is often called genetic diversity. Genetic variations are the differences in DNA segments or genes between individuals and each variation of a gene is called an allele. For example, a population with many different alleles at a single chromosome locus has a high amount of genetic variation. Genetic variation is essential for natural selection because natural selection can only increase or decrease the frequency of alleles that already exist in the population. 

Genetic variation is caused by:

• mutation (discussed further in Section 2.2.4 )
• random mating between organisms (discussed further in Section 2.2.4 )
• random fertilization
• genetic recombination during meiosis

Mutations are the only method of creating new alleles; mutations are ultimately the engine that drives evolution. Sexual reproduction (including random mating, random fertilization, and crossing over) are ways to reshuffle and recombine those genes in new ways, giving offspring combinations that differ from their parents and from others. 

This 4-minute video provides a brief introduction to variation in humans and a method for identifying traits that are encoded by genetics and those determined by the environment. Question after watching: In most cases, when a person who has an identical two develops a disease like cancer, their identical twin does not. What does this tell you about the causes of cancer?

Evolution and Adaptation to the Environment

Variation allows some individuals within a population to adapt to the changing environment. Because natural selection acts directly only on phenotypes, more genetic variation within a population usually enables more phenotypic variation. Some new alleles increase an organism’s ability to survive and reproduce, which then ensures the survival of the allele in the population. Other new alleles may be immediately detrimental (such as a malformed oxygen-carrying protein) and organisms carrying these new mutations will die out. Neutral alleles are neither selected for nor against and usually remain in the population. Genetic variation is advantageous because it enables some individuals and, therefore, a population, to survive despite a changing environment (Figure \(\PageIndex{2}\)).

Geographic Variation

Some species display geographic variation as well as variation within a population. Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or when they are under selection pressures from a different environment. One example of geographic variation are clines , gradual changes in a trait across an organism's geographic distribution.

Species of warm-blooded animals, for example, tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. This is considered a latitudinal cline (Figure \(\PageIndex{3}\)). Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of a mountain, known as an altitudinal cline . 

Environmental Variance

Genes are not the only players involved in determining population variation. Phenotypes are also influenced by other factors, such as the environment (Figure \(\PageIndex{4}\)). Some major characteristics, such as gender, are determined by the environment for some species. For example, some turtles and other reptiles have temperature-dependent sex determination (TSD). TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range. 

Types of Natural Selection

There are several ways selection can affect population variation (each is described in detail below):

• stabilizing selection
• directional selection
• diversifying selection
• frequency-dependent selection
• sexual selection

As natural selection influences the traits in a population, individuals can either become more or less genetically similar to one another and the phenotypes displayed can become more similar or more disparate. In the end, natural selection cannot produce perfect organisms from scratch, it can only generate populations that are better adapted to survive and successfully reproduce in their environments through the aforementioned selections.

In this 3-minute video, David Attenborough explains how iguana evolved to swim on the Galpagos islands. Question after watching:  Propose a series of steps that could have led to this evolution in the iguana population in the Galpagos. Specifically, explain how they evolved to live for short periods underwater from their land ancestor.

Stabilizing, Directional, and Diversifying Selection

Stabilizing selection.

If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. 

Stabilizing selection : Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic diversity decreases. In the graph above, the x-axis indicates the variation in a trait. The y-axis indicates the percentage of individuals in a population that have a particular trait. In stabilizing selection, the average becomes more common over time, with less variation in the population.  

For example, in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. Assuming the ground is a fairly consistent shade of brown, those mice whose fur most closely matches that color will most probably survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. As a result of this stabilizing selection, the population’s genetic variance will decrease.

Directional Selection

When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation.

Directional selection : Directional selection occurs when a single phenotype is favored, causing population to shift towards one end of the trait variation.  In the graph above, the x-axis indicates the variation in a trait. The y-axis indicates the percentage of individuals in a population that have a particular trait. In directional selection, the average shifts one direction or another but the total variation in the population remains.

A classic example of this type of selection is the evolution of the peppered moth ( Biston betularia ) in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.

Over time, the frequency of the melanic (dark) form of the moth increased because their darker coloration provided camouflage against the sooty tree; they had a higher survival rate in habitats affected by air pollution (Figure \(\PageIndex{5}\)). Similarly, a hypothetical mouse population may evolve to take on a different coloration if their forest floor habitat changed. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.

Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages. In these cases, the intermediate phenotypes are often less fit than their extreme counterparts.

Diversifying (or disruptive) selection : Diversifying selection occurs when extreme values for a trait are favored over intermediate values. This type of selection often drives speciation. In the graph above the x-axis indicates the variation in a trait. The y-axis indicates the percentage of individuals in a population that have a particular trait. In directional selection, the average is selected against, leading to selection for the rarer variations of a trait.

This is seen in many populations of animals that have multiple male mating strategies, such as lobsters. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

This 2.5-minute video highlights directional, stabilizing, and disruptive selection. Question after watching: In the video, the narrator speaks of disruptive selection (this textbook called it diversifying selection). What is being disrupted?

Frequency-Dependent Selection

Another type of selection, called frequency-dependent selection , favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection).

Negative Frequency-Dependent Selection

Negative frequency-dependent selection  increases the population’s genetic variance by selecting for rare phenotypes. An interesting example of this type of selection is seen in a unique group of lizards in the deserts of the United States. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females; blue males are medium-sized and form strong pair bonds with their mates; and yellow males are the smallest and look a bit like female, allowing them to sneak copulations.

Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. The big, strong orange males can fight off the blue males to mate with the blue’s pair-bonded females; the blue males are successful at guarding their mates against yellow sneaker males; and the yellow males can sneak copulations from the potential mates of the large, polygynous orange males (Figure \(\PageIndex{7}\)). 

In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes. In one generation, orange might be predominant and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored. So, orange (strong) > blue (cooperation) > yellow (sneaky) > orange (strong).

This 3.5-minute video illustrates the side-blotched lizard case study of negative frequency-dependent s election. Question after watching: The video mentions that the blues can be altruistic, which is to say exhibit behaviour that would benefit not themselves but another animal. Propose a hypothesis about the evolutionary advantage of altruism in this case.

Positive Frequency-dependent Selection

Positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes. An example of positive frequency-dependent selection is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless.

The scarlet kingsnake ( Lampropeltis elapsoides ), a harmless species, mimics the coloration of the eastern coral snake (Micrurus fulvius) , a venomous species typically found in the same geographical region (Figure \(\PageIndex{8}\)). Predators learn to avoid both species of snake due to the similar coloration. As a result, the scarlet kingsnake becomes more common and its coloration phenotype becomes more variable due to relaxed selection. This phenotype is, therefore, more “fit” as the population of species that possess it (both dangerous and harmless) becomes more numerous. In geographic areas where the coral snake is less common, the pattern becomes less advantageous to the kingsnake, and much less variable in its expression, presumably because predators in these regions are not “educated” to avoid the pattern.

In this short 1-minute narrated by David Attenborough, you will see several examples of biomimicry, where animals evolved shapes and colors that make them look like something else. Question after watching: What appears to be the purpose of biomimicry in most organisms? At which step in a process of natural selection would the mimicry provide an advantage?

Sexual Selection

Sexual selection , the selection pressure on males and females in mating, can result in traits designed to maximize sexual success. In species with sexual selection, typically one sex is less-limited due to their ability to produce an overabundance of gametes and the other is more-limited due to their higher parental investment (e.g. produce eggs, gestate a fetus, raise young, etc.). The more-limited sex typically reproduces fewer times and has higher parental investment than the less-limited sex; they are typically more discerning about who they mate with in order to optimize those reproductive events. 

Sexual selection in animals takes two major forms: intersexual selection in which, the less-limited sex (typically males) are chosen by the more-limited sex (typically females) based on certain characteristics and intrasexual selection in which members of the less-limited sex compete aggressively among themselves for access to the more-limited sex (Figure \(\PageIndex{9}\)).

This 3-minute video highlights extreme behaviors that arise from intense sexual selection pressure. Question after watching: What can you surmise about which of the sexes are 'choosier' in most mammals, based on morphological and behavioural dimorphism?

Sexual Dimorphism in Animals

Males and females of certain animal species are often quite different from one another in ways beyond the reproductive organs. Male vertebrates are often larger, for example, and display many elaborate colors, adornments, and behaviors such as seen in the figure below, while females of the same species tend to be smaller and less attention-seeking. These differences are called sexual dimorphisms and arise from the variation in reproductive success (Figure \(\PageIndex{10}\)).

Animal females do not have trouble finding mates for reproduction, while reproduction is not guaranteed for males. The bigger, stronger, or more decorated males usually obtain the vast majority of the total reproductive events, while other males receive none. This can occur because the males are better at fighting off other males (intrasexual selection), or because females will choose to mate with the bigger, brasher, or more decorated males (intersexual selection). In either case, this variation in reproductive success generates a strong selection pressure among males to compete for mates, resulting in the evolution of bigger body sizes or elaborate ornaments in order to increase their chances of mating. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to be selective in their mates.

Sexual dimorphism varies widely among species; some species are even sex-role reversed. In such cases, females tend to have a greater variation in their reproductive success than males and are, correspondingly, selected for the bigger body size and elaborate traits usually characteristic of males.

The Handicap Principle

Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual’s survival because they maximize its reproductive success. For example, while the male peacock’s tail is beautiful, and the male with the largest, most colorful tail will more probably win a female's attention, it is not practical or advantageous (Figure \(\PageIndex{11}\)). In addition to making the males more visible to predators, it slows down any needed escapes. There is some evidence that this risk is why females like the big, bright tails. Because large tails carry risk, only the best males can survive that risk, and therefore the bigger the tail the more fit the male must be. This idea is known as the handicap principle .

The Good Genes Hypothesis

The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be so selective because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring.

This 8.5-minute video is an overview of natural and sexual selection. Questions after watching: How is sexual selection an example or type of natural selection? Explain the following statement: “evolution is just a numbers game”

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4 Chapter 4: Sexual Selection

Mason Tedeschi; Lisa Limeri; and Anastasia Chouvalova

Learning Objectives

By the end of this section, students will be able to:

4.1 Compare and contrast artificial, natural, and sexual selection.

4.2 Describe the relationship between sexual selection and sexual dimorphism

4.3 Contrast intrasexual selection and intersexual selection.

4.4 Describe the hypothesized mechanisms of intersexual selection (sexy sons, good genes, handicap, and direct benefits hypotheses).

Sexual Selection

When discussing natural selection and adaptations, we often focus on traits that help an organism survive – surviving cold winters, avoiding predators, obtaining food, etc. But what about traits that do not directly aid in survival? Or better yet, what about traits that actually hinder survival? Consider the elaborate train feathers of a peacock (Fig 4.1). These feathers, while visually stunning, make it nearly impossible for an individual to fly. Not only that, but these feathers also make it easier for a predator to spot a peacock from a distance and they can also be used by a predator to apprehend a peacock so it cannot escape. Rather than helping a peacock survive, their tail feathers actually make it more difficult to escape predation. So does such a trait evolve?

This question can be answered by viewing this trait not as aiding in survival, but as a trait that aids in reproduction. Exaggerated traits, like the tail feathers of a peacock, are used to attract females for mating, ensuring that a male passes his genes on to the next generation.

Elaborate peacock trains (Fig 4.1) represent an example of sexual selection. Sexual selection is a “special case” of natural selection in which individuals compete for mates in order to pass on their genes to future generations. The peacock’s train is used to attract females for mating, ensuring that a male passes his genes on to the next generation. In essence, sexual selection acts on an individual’s ability to successfully reproduce- even if that ability comes at a cost to survival.

Darwin identified a special case of natural selection that he called sexual selection. Sexual selection acts on traits that affect an individual’s ability to attract mates and thus produce offspring. Sexual selection often leads to the evolution of dramatic traits that often appear maladaptive in terms of survival but persist because they give their owners greater reproductive success. Sexual selection can be so strong that it selects traits that are actually detrimental to the individual’s survival.

Sexual selection occurs through two mechanisms:  intrasexual selection of mates, also known as mate competition and [latex]null[/latex] and intersexual selection , also known as mate selection/choice.

Mechanisms of Sexual Selection

Sexual selection, the process through which individuals compete for mates, primarily takes two forms: intersexual selection and intrasexual selection. Intersexual selection, often referred to as mate choice, involves individuals of one sex choosing among members of the opposite sex based on the attractiveness of certain traits that those individuals possess. Intrasexual selection, also called mate competition, involves one sex competing with members of the same sex for access to mates.

Sexual selection results from competition over mates. Which sex competes for mates and which sex is choosy? In general, the sex that invests more resources in offspring is the one that will be more choosy, because they have more to lose by making a bad choice about a mate. This investment difference begins with gametes. Females produce eggs, which are much larger in size (and thus more costly to produce) and males produce sperm, which are small and energetically cheap (Fig 4.2). This difference in gamete investment is known as anisogamy .

This early investment difference resulting from anisogamy causes the general trend that in most species, females are the choosy sex and males are the sex competing for female to mate with. Another reason why females are typically the choosy sex has to do with the level of investment in offspring care, known as parental care. For example, following sexual reproduction and fertilization , most mammals develop within the body of their mothers. The developing offspring of most mammals then get their food and oxygen from the blood of their mothers through a spongy organ called the placenta. Even marsupial offspring, though not fully developed when born, are usually carried by their mothers in a pouch until they are able to walk on their own.

However, it is important to note that not all species follow this trend. In some animals, males provide a great deal of parental care to their offspring. For example, in emperor penguins each female produces a single egg. She then transfers the egg to her male mate and leaves to spend the winter in the open ocean in search of food and other resources. During the Antarctic winter, which lasts about four months, male emperor penguins huddle in groups, guarding their eggs and keeping them warm. Another example is seahorses, where males incubate  eggs and care for young in a pouch. When they mate, a female deposits eggs into the male’s pouch and leaves, providing no further parental care. Thus, male seahorses invest far more resources into offspring than females do, and it’s the males who are the choosy sex and females compete for male mates.

Question #1

Which sex is typically the choosy sex? The sex that…

A. invests more resources into offspring. B. produces a larger number of gametes. C. is larger in size. D. has more elaborate coloration and ornamentation.

Intrasexual Selection: Competition

Intrasexual (within sex) competition takes the form of conflicts between members of the same sex competing for mates. These intrasexual competitions are often ritualized, but may also pose significant threats to the competitors’ survival. Intrasexual selection involves mating displays and aggressive mating rituals such as rams butting heads—the winner of these battles is the one that is able to mate. Many of these rituals use up considerable energy but result in the selection of the healthiest, strongest, and/or most dominant individuals for mating. Sometimes the competition is for territory, with prospective mates more likely to mate with individuals with higher quality territories.

Intersexual Selection: Mate Choice

Intersexual (between sexes) selection occurs when members of the choosy sex select a mate based on a trait or suite of traits, such as feather colors, the performance of a mating dance, or the building of an elaborate structure. There are several, non-exclusive models of how and why mating preferences evolve. Broadly, there are two types of fitness benefits that drive mate choice: direct benefits and indirect benefits.

Direct Benefits

Direct benefits increase the fitness of choosy individuals through material resources. Members of the competing sex will sometimes provide members of the choosy sex with a food gift before mating. These resources, called nuptial gifts , provide nourishment to prospective mates that they may not otherwise get. For example, male great grey shrikes- a predatory bird- will present prey items (e.g., rodents, other birds, lizards) to females immediately before mating. A female great grey shrike will choose a mate according to the size of the prey item presented to her. Nuptial gifts are observed in many insects and spiders where males present nuptial food gifts to females in the hopes that she will choose to mate with him.

In extreme cases, the competing sex will even sacrifice parts or all of themselves to members of the choosy sex. For example, in some species of ground crickets, females receive a nuptial gift by chewing on a specialized spur structure on the male hind tibia (i.e. leg) while mating. Most predatory species of preying mantids practice a type of extreme nuptial feeding known as sexual cannibalism, in which a female will eat her mate prior to, during, or after copulation. Most often, a female mantid will begin feeding by biting off the head of a male, as they would with regular prey. Because copulatory movement in males is controlled by nerves in the abdomen, not the head, removal of a male’s head does not affect mating, sperm transfer, or proper fertilization. The reason for sexual cannibalism has been heavily debated. Experiments show that females on low quality diets are more likely to cannibalize her mates, compared with females given high quality diets. Thus, nuptial gifts are typically considered a direct benefit, because they enhance a female’s survival and reproduction. Some suggest that males that submit to females and are cannibalized gain a selective advantage by producing higher quality offspring. In any event, this type of sexual behavior is quite rare because the costs are often assumed to out-weigh the benefits, particularly for males.

Question #2

What are nuptial gifts?

A. A form of indirect benefits to the choosy sex. B. Resources typically provided by the competitive sex to the choosy sex. C. Resources typically provided by the choosy sex to the competitive sex. D. A form of intrasexual competition.

Indirect Benefits

Indirect benefits do not directly benefit the choosing individual, but rather, indirectly benefit them by increasing the fitness of their offspring. There are multiple hypothesized mechanisms of indirect benefits. Three of the most common and well-supported are the Sexy Sons, Good Genes and Handicap Hypotheses.

The Sexy Sons Hypothesis: The sexy sons hypothesis postulates that members of the choosy sex who select mates with attractive traits will benefit by producing offspring who also possess the attractive traits and thus will be reproductively successful. As such, the attractive (sexy) sons will be more likely to attract females, and thus the choosy female’s genes will continue to spread.

The Good Genes Hypothesis. The good genes hypothesis posits that males develop impressive ornaments to show off their efficient metabolism or effectiveness at obtaining food, or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring.

The Handicap Hypothesis. Exaggerated traits, such as the Peacock’s train, that exist to attract mates can reduce the owner’s survival. Why would females prefer to mate with males that have traits that reduce survival? The Handicap hypothesis poses that a male with a large, elaborate train must be especially strong and fast to escape predators while having a handicap, and thus makes an ideal mate.

In both the handicap principle and the good genes hypothesis, the trait is an honest signal of the males’ quality, thus giving females a way to find the fittest mates— males that will pass the best genes to their offspring.

Question #3

Individuals with a large, elaborate traits are ideal mates because they must be especially strong and fast to escape predators despite having such an unwieldy trait. This describes which hypothesized mechanism of sexual selection?

A. Sexy Sons hypothesis B. Good genes hypothesis C. Handicap hypothesis D. Direct benefits hypothesis

Post-Copulatory Sexual Selection

Sexual selection does not come to a halt after animals have mated. If a female mates with multiple males, such that sperm from several individuals remains in her body for an extended period of time, sexual selection can continue long after a male and female have mated. Just like with sexual selection before mating, post-copulatory sexual selection occurs in two forms: cryptic female choice – an extension of mate choice; and sperm competition – an extension of mate competition.

Cryptic female choice

Using physical or chemical mechanisms, females can bias paternity and affect male reproductive success by choosing whether certain sperm are successful in fertilizing their eggs. The term “cryptic” is used to describe an internal, and thereby hidden, process that females employ to choose the sperm from males that they prefer. The research suggests that cryptic female choice is likely a consequence of sexual conflict regarding the frequency and mode of mating. While males increase their fitness by successfully mating with as many females as possible, and thereby fertilizing as many eggs as possible from different females, females can incur fitness costs associated with mating with many males. Cryptic female choice reduces these costs by allowing females to mate multiply (as males wish to do), but then only select sperm from the favorable males afterwards. Here, females benefit by influencing paternity in favor of the males they prefer- possibly because they provide some direct or indirect benefit to her.

Sperm competition

Sperm competition, an extension of intrasexual competition, is the process by which sperm from two or more males compete for fertilization of a female’s eggs. Sperm competition is often compared to having tickets in a raffle: a male has a better chance of having their ticket drawn (i.e. fathering offspring) if he has more tickets in the raffle (i.e. he releases more sperm per ejaculate into a female’s reproductive tract). Alternatively, males may not release more sperm, but instead they evolve faster, more motile sperm that allow an individual’s sperm to reach a female’s eggs first. Among the best evidence we have for sperm competition is the evolution of longer sperm tails in animals that have multiple partners (Fig 4.3).

Sexual Dimorphism

Just as with natural selection, sexual selection can lead to changes in the genetic composition of a population that can be seen through physical changes to the way an organism looks. Both mate choice and mate competition can lead to the evolution of elaborate traits, termed secondary sexual traits, (secondary because they are not the primary traits involved in sexual reproduction or sperm transfer). Secondary sexual traits aid in sexual reproduction by improving an individual’s ability to obtain mates. Typically, one sex possesses an elaborate secondary sexual trait or traits, but the other sex does not, a condition called sexual dimorphism (Fig 4.4). Both mate choice and mate competition can involve the evolution of secondary traits that are sexually dimorphic.

Traits that are subject to selection via mate choice are referred to as ornaments  or sexual signals . Ornaments can involve different signal modalities, including visual signals like the bright colors of many birds and butterflies; olfactory (i.e. chemical) signals like the scent patches that many mammal species use to attract mates; auditory signals used by chorusing frogs and some insects like crickets; or even tactile signals like the vibratory signals used by some spiders when they tap their legs on the surface of a leaf to attract mates. Sexual signals can also involve multiple signal modalities. For example, male jumping spiders will often use both visual and vibratory signals when trying to attract females for mating.

Question #4

Which of the following correctly describes sexual dimorphism?

A. An evolutionary force that improves reproductive success in both males and females. B. Traits related to sexual reproduction that are present in both males and females. C. Traits that improve an individual’s ability to survive. D. Differences between males and females within a species resulting from sexual selection.

Question #5

What is the primary difference between natural and sexual selection?

A. Natural selection promotes traits enhancing the likelihood of reproduction whereas sexual selection promotes traits enhancing fitness. B. Natural selection promotes traits enhancing survival whereas sexual selection promotes traits enhancing the likelihood of reproduction. C. While natural selection occurs in individuals, sexual selection occurs on a population level. D. There is no difference between them.

Adapted from Various Authors, Introductory Biology: Evolutionary and Ecological Perspectives. University of Minnesota. Retrieved from https://pressbooks.umn.edu/introbio/

reproduction of individuals with favorable genetic traits that survive environmental change because of those traits, leading to evolutionary change

competition between members of the same sex for a mate

selection of a desirable mate of the opposite sex, contrast to intrasexual selection.

a type of sexual reproduction where male and female organisms produce gametes of unequal size

the union of two haploid cells from two individual organisms of two different sexes.

the act of a bird sitting on their eggs to keep the egg warm and to eventually incite hatching

in sexual reproduction, these are valuable nutritional resources provided by one of the partners to the other partner

in mate selection, this is the act of the female mating with multiple males but latently selecting which mate will fertilize her eggs, without the male knowing

in sexual reproduction, this occurs when sperm from multiple males rival for the fertilization of one female egg

phenotypic difference between a population's males and females

characteristics of organisms that decorate the organism, rather than provide a useful functionality

synonymous with ornaments, that is, characteristics of organisms that decorate the organism, rather than provide a useful functionality

Introductory Biology 2 Copyright © 2023 by Mason Tedeschi; Lisa Limeri; and Anastasia Chouvalova is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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Monika Siekelova

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Department of Psychology, Oakland University Department of Psychology, Rochester, Michigan, USA

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Siekelova, M. (2018). Healthy Mate Hypothesis. In: Vonk, J., Shackelford, T. (eds) Encyclopedia of Animal Cognition and Behavior. Springer, Cham. https://doi.org/10.1007/978-3-319-47829-6_299-1

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• Proc Biol Sci
• v.266(1414); 1999 Jan 7

Good-genes effects in sexual selection

The magnitude of the effect of good genes as a viability benefit accruing to choosy females remains a controversial theoretical and empirical issue. We collected all available data from the literature to estimate the magnitude of good-genes viability effects, while adjusting for sample size. The average correlation coefficient between male traits and offspring survival in 22 studies was 0.122, which differed highly significantly from zero. This implies that male characters chosen by females reveal on average 1.5% of the variance in viability. The studies demonstrated considerable heterogeneity in effect size; some of this heterogeneity could be accounted for by differences among taxa (birds demonstrating stronger effects), and by differences in the degree of mating skew in the species (high skew reflecting stronger effects). Although these results suggest that viability-based sexual selection is widespread across taxa, they indicate that the effect is relatively minor. Finally, there was also an effect of publication year in that the more recent studies reported reduced effects. This may reflect publication biases during paradigm shifts of this debated issue, but it should also be recalled that the studies have only partly estimated the full fitness consequences of mate choice for offspring.

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How our Genes Lie: Honest and Dishonest Genes in Sexual Selection

Samuel Gascoigne Lake Forest College Lake Forest, Illinois 60045

Natural selection has been understood for over a hundred years, but the mechanisms by which it works have not been identified. One of the forms it takes is sexual selection. Sexual selection is an evolutionary pressure conferred by the opposite sex of the same species. The good genes hypothesis, posed in the 1930s, attempted to reconcile mate choice and the selection for certain traits. The selfish gene hypothesis, first declared in 1976, attempted to explain mate choice as well as our behaviors. With our modern understanding of genetics and DNA that holds the information, these two hypotheses can be applied to identify the honest and dishonest genes that are passed down generation after generation.

Introduction

While the molecular basis is unknown, the role of genes in heredity has been common knowledge since the 1930s. The good genes hypothesis proposed that individuals choose mates on certain phenotypes that pose a genetic advantage for the next generation. To apply this to humans, the attractiveness we prescribe to an individual reflects that individual’s genetic superiority. This is an incomplete model given that different people find different individuals attractive. A possible supplement to the model is the selfish gene hypothesis. The selfish gene hypothesis proposes that our mate choice is a result of our interest to pass our genetic code on to the next generation. A human application of this would be that we choose our mates based on that individual’s similarity to our own genome, thereby probabilistically increasing the longevity of our genes. Both hypotheses have merit but fail to independently explain the presence of honest and dishonest genes; but, together, honest and dishonest genes are made inevitable.

When discussing honest and dishonest genes, it is important to clarify that sexual selection works via the selection of phenotypes, not genotypes. Phenotypes are observable characteristics of an organism and these traits are influenced by the organism’s genes. Since genotypes cannot be seen, phenotypes are used for selection as they are an indirect manifestation of the organism’s genes and experiences. An example of this is if a male peacock has a mutation in a gene important in feather development. A result of this mutation is an upregulation of a hormone responsible for feather growth, thereby increasing the relative size of the peacock’s plumage. Since plumage size is a sexually selected trait in peacocks, the mutated peacock would be selected to a greater degree by hens than a wild-type peacock. The disparity in the selection of males with varying secondary-sexual traits, affected by variation in genotype, is the basis of sexual selection contributing to the evolution of the organism. Yet, while advantageous mutations account for an evolutionary change over the course of multiple generations, genes do not independently explain why a trait is sexually selected. For that, genes must manifest into phenotypes that suggest an evolutionary fitness of the organism. Unfortunately, the path from gene to trait is not without its own set of variation.

Environment plays a key role in phenotype and the development of a sexually selected trait. Genotype does not determine phenotype. Genes code for proteins. Phenotypes can be anything from horn allometry, as in Onthophagus beetles, all the way to call syllables, as in bush crickets. What links genes to corresponding proteins are, most often, a suite of developmental and cellular mechanisms. It is this developmental and cellular link between genes and phenotypes that explain the plasticity of phenotypes. Phenotypic plasticity is the phenomenon that multiple phenotypes can arise from a single genotype; one example is the case of monozygotic twins. Imagine a pair of monozygotic twins, Jim and Jeff. Jim frequents a gym regularly and ensures he maintains a balanced diet. Jeff, on the other hand, frequents a buffet regularly and ensures his freezer is filled with his favorite midnight snacks. It would not be a surprise to find out that Jim has a lower body mass index (BMI) than Jeff despite having the exact same genotype. There was nothing that predisposed Jeff to a higher BMI than Jim. What ensured his increased insulation was the environment he experienced. In summary, genes lead to phenotype, but the phenotype is also moderated by the environment.

Honest and Dishonest Genes

What determines the honesty of a gene is how accurately it depicts, via a phenotype, the fitness of the organism. From a sexual selection standpoint, the evolution of honest genes would be favorable. In addition, over the course of multiple generations, the scruples of sexually selective pressures would refine the accuracy of the honest genes as it would lead to a sensitive and more prosperous method of selection. This is a case of resolution. Imagine a doe is searching for a buck for mating. Two bucks, Skip and Skippy, appear with similar size and muscle proportion. The only way they differ are their coats and horns. Skip has a relatively dull coat and small horns relative to body size while Skippy has a full shiny coat with a large ornament rack relative to body size. Skippy is favorably selected by the doe for mating. In this situation, genes that synthesize androgenic hormones and genes involved in insulin/insulin-like growth factor signaling (IIS) are honest genes; androgens are positively correlated with hair development and IIS is positively correlated with rack size. This situation is favorable for the doe and Skippy as they both have an increased probability of passing their genes on to the next generation. Skip, on the other hand, draws the short straw in the field of honest genes. He, therefore, favors a dishonest set of cards.

Imagine the doe and two bucks scenario once again with Skippy still being the more sexually favored. Now include a mutant Skip. This Skip has a mutation in genes involved in IIS that increase IIS and, further downstream, upregulate androgenic hormones. Mutant Skip has a glossy coat and large rack relative to body size which catches the doe’s eyes to a greater extent than Skippy’s features. In turn, mutant Skip is selected instead of Skippy. While the genes involved were originally honest, the mutation in Skip’s genome made the environment insubstantial in affecting the final phenotype and thus lead to dishonest phenotypes. In this scenario, the doe and Skip win. However, the doe wins at a probability of smaller magnitude as the offspring may be less fit than the offspring of an honest mate. The disparity in winning magnitude offers logic toward a selective pressure in does to increase their resolution for sexual selection; the better the does are at discerning honest genes, the more likely their genes will survive to the following generations. However, the presence of dishonest genes in species either supports the idea that dishonest genes are inevitable with random mutation or, more poignantly, the disparity in winning magnitude due to potential filial unfitness is not enough to select against dishonest genes.

The Two Theories

The presence of honest and dishonest genes highlights a sexually divergent initiative in sexual selection. The sexual selector prefers honest genes, while the sexual selectee prefers either honest or dishonest genes – whatever offers an advantage in increasing gene longevity. In turn, a theory of sexual selection must reconcile both initiatives.

Together, the good genes hypothesis and the selfish gene hypothesis explain the honest-dishonest genes phenomenon. The good genes hypothesis explains honest genes. In the good genes hypothesis, genes that accurately illustrate the fitness of the organism are preferably selected above inaccurate genes. This theory explains the disparate winning advantage in dishonest selection and offers a selective pressure against dishonest genes. Evidence for this theory can be found in IIS-dependent traits. Almost all animals use IIS for cellular and physiological development. One of the reasons IIS is so conserved is that IIS is upregulated in high nutrition. Therefore, an organism in high nutrition has full or increased development due in part to high IIS. In turn, it makes sense that sexual selection would work on traits that are insulin sensitive, allowing greater selection accuracy of well fed mates. However, the presence of dishonest genes indicates a second manner of sexual selection at work.

The selfish gene hypothesis explains the presence and longevity of dishonest genes despite the selective pressure against them offered by the good genes hypothesis. In the selfish gene hypothesis, animal behavior, including mate choice, is explained to increase the longevity of an individual organism’s genes. An example of this would be the mutant described above, Skip. The mutant Skip illustrates the presence of a dishonest gene via a mutation. According to the selfish gene hypothesis, what offers a dishonest gene its longevity, in addition to the phenotypic advantage, is the fact that organisms with the same gene tend to mate with one another, thus increasing the probable lifetime of the dishonest gene.

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