Lecture: Sexual Selection

Goals: to study evolution through sexual selection, which occurs because of non-random mating resulting from competition for mates or selection of mates. To understand why sexual selection typically causes the evolution of elaborate traits in males, but not females. To study three hypotheses for the evolution of female choice of males: honest advertisement, runaway sexual selection, and sensory exploitation.

Related Textbook Material: Freeman and Herron (2001) Chapter 9

Lab Manual Questions over this material are in Lab Manual Chapter XXI

The Lecture:

The fact that sexual reproduction has evolved has consequences for further evolution within species. The degree to which individuals with certain traits reproduce, and therefore pass those traits on from generation to generation, will depend on how well they compete for mates and how well they attract mates, in addition to how well they survive. Early on, we noted that one assumption of Hardy-Weinberg Equilibrium was random mating. If individuals do not mate at random -- if they choose mates, or if some compete better for mates than others -- then evolution can occur.

Sexual selection is evolution that occurs because some individuals are better than others at either competing for mates or at attracting mates -- it is the evolution of traits based on differences in mating success among individuals. Sexual selection can occur in two ways:

1. some traits may increase the ability to compete with individuals of the same sex for access to mates.
2. some traits increase the ability to attract individuals of the opposite sex.

The first thing to note about the kinds of trait that evolve through sexual selection is that they do NOT typically increase survival in the individuals who have them. In fact, they may HARM survival. Having a big brightly colored tail is likely to attract predators and make it harder for a peacock to escape. The reason these traits evolve even though they are bad for survival is that they increase mating, and therefore increase reproduction. A peacock with a small, dull colored tail might survive much better, but if he did not attract females his genes would not be passed to the next generation, and a small, dull tail would not evolve.

The next thing to note is that in most species, it is the males, not the females, that have these structures that have evolved through sexual selection. Sexual selection typically occurs because males compete with other males for access to females, or because females have a preference for males with certain traits. In other words, when sexual selection occurs because of competition among individuals of the same sex, it is typically occurring though male-male competition. When sexual selection occurs because some traits attract individuals of the opposite sex, it is typically males who have traits that attract females, so sexual selection occurs through female choice.

Why do these traits that increase mating success but decrease survival evolve in males but not females? The answer is that in most species males can increase their actual reproduction a great deal by attracting more mates, but females can not. Think of mammals. A female only needs to attract one male to get pregnant, and once she's pregnant she will not increase her reproduction by attracting more males until that pregnancy is finished and she has successfully raised those young. In contrast, during the same period of time, if a male can attract many females he can potentially be the father to many offspring. This same principle applies to other species. Females typically put more time and energy into producing each young and can not increase their reproduction as much by attracting many mates. This starts with gamete production -- eggs, produced by females, are large and energy rich, while sperm are smaller and energetically cheaper to produce.

The result is that having a trait that increases mating but decreases survival is likely to increase the lifetime reproduction of a male, but much less likely to increase the lifetime reproduction of a female. So we predict sexual selection to cause the evolution of traits in males much more often than females, and that is what we observe.

There are a few species of bird in which females are larger and more brightly colored than are males. They thus have the opposite of the predicted pattern. Interestingly, in these species it is the males who incubate the eggs and care for the young. So in these species it is females who can increase their reproduction most by attracting more mates -- the females are not putting much time and energy into raising the young, and the males are. The general rule, then, is that the sex that invests most time and energy in raising the young will not increase the number of young produced as much by attracting a large number of mates, so will not evolve traits to attract or compete for mates through sexual selection. The sex that puts less energy into caring for young can increase reproduction more by attracting many mates, and it is in this sex that traits that attract or compete for mates will evolve through sexual selection.

When Darwin first proposed sexual selection as a means through which evolution occurs, other evolutionary biologists agreed that male-male competition was a likely explanation for traits such as antlers and large size in males. They were less sure about female choice, however. They posed an important question: why should females prefer to mate with brightly colored males? If sexual selection occurs through female choice of males, then we need to explain the evolution of female choice as well as the evolution of the traits of males.

Early evolutionary biologists doubted whether sexual selection through female choice occurred at all, because they did not think female choice should evolve. In the past 20 years, however, female choice has been demonstrated to occur in a large number of species, including various birds, reptiles, fish, and insects. So it seems that sexual selection through female choice does occur, and we need to explain why female choice would evolve. Three main hypotheses for the evolution of female choice have been proposed. These are:

1. Honest Advertisement. This hypothesis states that females increase their fitness by selecting certain males, because the characteristics of those males indicates that they have higher quality in some way that will increase the survival of the female and/or her offspring. So the traits females selection -- bright color, for example -- advertise some other quality of the male. There are two main kinds of quality that could be advertised:
• Direct benefits. These directly increase female or offspring survival. A brightly colored male, might, for example, be better and providing food or be less likely to have a disease that would be transmitted to the female or the offspring. In house finches (a kind of bird that is very common around here) for example Geoff Hill (a professor at Auburn University) found that males differ in how red their heads are, that females prefer males with very red heads, and that males with the reddest heads had the highest rate of bringing food to females while the females were sitting on nests. Thus females benefited directly from selecting males with bright red heads. The relationship between having a red head and bringing food to the nest may seem odd, but it turns out that red coloration in birds depends on getting red pigments in the diet of the birds (they can't biochemically synthesize the reds themselves, they have to get them from food.) So a bird that is good at finding food may turn out to be redder, and that could be why males with very red heads bring food to the nest at the highest rate.
• "Good genes." It may be that the female receives no direct benefits from mating with a high quality male, but the male may have high genetic quality so that if a female mates with a high quality male her offspring will inherit the alleles for high quality. A female who mates with a male with high genetic quality should therefore have more surviving offspring. A potential problem with this hypothesis is that for most traits, alleles for high fitness become quickly fixed, so we do not expect males to differ much in genetic quality for most traits. There are some traits, however, for which variation in fitness should be maintained over time; a major kind of trait for which this is true is disease resistance. As we have seen, diseases quickly evolve to overcome host defenses. As a result, the host population keeps evolving, and variation in traits that confer disease resistance is not lost. So it has been predicted that the traits in males that females prefer may indicate that they have good genes for disease resistance. In several species of bird, males with parasites or other diseases develop duller colored feathers, and females prefer males that are more brightly colored, and have fewer parasites.
2. Sensory Exploitation (also called sensory bias.) This hypothesis states that the male traits that evolve through sexual selection evolve because they attract the attention of females because of biases in the female sensory system. The idea is that some colors, sounds, sizes, or shapes are more noticeable to females because of the way their sensory systems work, and as a result if males have these traits females will notice them and therefore be more likely to mate with them than with males with traits that do NOT attract the attention of the female sensory system. According to this hypothesis, then, the female preference is present, because of these biases in the sensory system, before the male trait evolves. This hypothesis has been tested in some groups phylogenetically. Using a phylogeny, as we have seen, it is possible to determine the most likely ancestor within which certain traits have evolved. The sensory exploitation hypothesis predicts that female preference for traits evolves in an ancestor earlier than the ancestor in which the male evolved the preferred trait. In some fish called swordtails, for example, males have a sword-like extension on the tail fin. Females in this species prefer to mate with males with swords than males without swords. Females in outgroups to the swordtails, in which males do NOT have swords, also prefer males if an artificial sword is added to their tail fin. This suggests that the female preference was primitive, and already present before the sword evolved, and supports the hypothesis of sensory bias.
3. Runaway Sexual Selection (also called arbitrary sexual selection.) This hypothesis states that the male traits that evolve do not have to indicate male quality, or stimulate a pre-existing bias in the sensory system -- the traits that evolve can be arbitrary, and evolve because non-random mating of females with a preference for a certain male trait mate with males with that trait, causing the alleles for preference and the alleles for the trait to become correlated -- they tend to occur together in the same individuals. The way this works can be illustrated by the following explanation of how males could evolve long tails, and females could evolve preference for males with long tails. This occurs through the following steps:
1. We start with a population with genetic variation for tail length and genetic variation for preference for tail length. This variation has arisen (as variation does) through chance mutation. We assume that long tails are expressed only in males, but that both males and females can carry alleles for long tails. This can occur if expression depends on the presence or absence of certain hormones, for example -- only males, with male sex hormones, and without female sex hormones, would express the trait of having long tails. We also assume that only females express the trait of preference for certain males, but that both males and females can carry preference alleles. Both of these traits are assumed to be polygenic (quantitative) traits (if you need to review the lecture explaining quantitative traits, Click here.
2. The females who have the alleles for preference for males with longer tails will mate with the longer tailed males (because they prefer them. ) As a result, in the next generation, the daughters of these pairs will inherit preference for longer tails from their mothers. They will also carry, but not express, alleles for longer tails, inherited from their fathers. The sons of these pairs will inherit long tails from their fathers. They will carry, but not express, alleles for preference for long tails, inherited from their mothers. So in this generation there will be individuals like the following:

 



3. The females with preference for males with longer tails will again mate with males with longer tails. In their offspring, females will inherit preference for longer tails from both parents. Since variation is additive, this will result in females preferring even longer tails. Similarly, males will inherit long tails from both parents, and will have even longer tails. Females will also inherit, but not express, alleles for longer tails from both parents, so they will carry even more long-tail alleles than their parents did. Males will also inherit, but not express, alleles for preference for longer tails from both parents, so they will carry even more alleles for preference for longer tails than their parents did. So in this generation there will be individuals like the following:

 



4. The process described in the previous step will continue each generation, so each generation there will be preference for still longer tails in females, and males will have still longer tails. The whole process, over several generations, is diagrammed here (and in your lab manual):

 



5. It may sound like this should lead to infinitely long tails. Eventually, something stops the process. One thing that could happen is that genetic variation could be lost -- all alleles would be fixed, and tails would be as long as they could be. Another possibility is that the survival disadvantage to having very long tails could be so strong that selection against long tails because of poor survival (natural selection) could balance the effect of the mating advantage of males with long tails (sexual selection.) Either way, eventually tails would be expected to stabilize at some long length.
The process outlined above for tail length shows how runaway sexual selection can work; the same process could apply to other characteristics.
 

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