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Sexual Selection and Summary of Population Genetics

Barry Sinervo©1998

Sexual Selection

Runaway Sexual Selection

How do such Female Choices Arise? -- Sensory Drive

Male-Male Competition -- Alternative Male Strategies

The Rock-Paper-Scissors Game

See Barry's Web Site

Summary of Population Genetics Theory

The Assumption of Random Mating

The Assumption of Population Size- Genetic Drift

The Assumption of Gene Flow

Natural Selection

The Big Question -- Accounting for genetic variation

Interaction of evolutionary factors are very important
The Adaptive Landscape

Sexual Selection

Sexual selection is distinguished from natural selection by Charles Darwin.

Sexual selection is designated as variance in the number of mates.

Because females are the limiting sex, and females invest more in offspring than males, males tend to be competing for females.

Thus, males tend to develop ornaments for attracting females or engaging other males in contests. These are referred to as sexual dimorphisms. Note: this is not necessarily always the case in the animal kingdom. For example, in pipefish, a form of sea horse, males are limiting because they brood the offspring in a pouch. In this case females compete for mates, and in some pipefish species, females are more brightly colored than males.

Sexual selection leads to non-random mating -- a violation of the 5th important assumption of the Hardy-Weinberg Law.

There are generally two modes of sexual selection:

  1. Female Choice: Intersexual selection, in which females choose males based upon elaborate ornamentation or male behaviors.
  2. Male Competition: Intrasexual selection, in which males compete for territory or access to females, or places on mating grounds where displays take place. Male-male competition can lead to intense battles for access to females and elaborate armaments (e.g., horns of many ungulates).


Today we will explore Female Choice as a model for sexual selection. This theory was original proposed by Sir Ronald Fisher in which he believed that a correlation would be set up between genes for female choice and the genes for male traits, which would lead to a Runaway Process.

We will also explore models of frequency dependent sexual selection that explain Male competition for Mates -- Evolutionary Game Theory.

Runaway Sexual Selection

Let us assume that females come in two types:

  1. Choosy females that prefer males with elaborate ornaments
  2. Non-choosy females that have no preference, but will pick males with bright ornaments 1/2 of the time and plain males 1/2 of the time:


Notice that the ornamented male has a fitness of 3/2 and the plain male has a fitness of 1/2.

What happens each generation is that both the gene for female choice and the male trait become correlated. Note that 1/2 the progeny have both genes for choice and the exaggerated trait.

If we continue this process, one more generation, all of the daughters of the choosy females have both the choice gene and the male trait gene. This means that females will be producing sons and daughters with both choice and exagerated trait genes together. Because the exagerated males have an advantage, Female Choice and the male trait spread, and they spread together linked by assortative mating, almost like a wild fire, or as Fisher termed a Runaway Process.

How do such Female Choices Arise? -- Sensory Drive

A recent theory (Endler, Ryan, etc.) of the 1980's is that there exists a sensory bias in the nervous/sensory receptors of females that pre-disposes them to pick some male traits, not because they perceive them as sexy per se, but because they are "attracted to them", probably for reasons other than mate choice.

Certain stimuli (e.g., colors, shapes, movement) may be useful in other contexts (e.g., feeding and foraging) and the nervous system of females (and males) are honed by natural selection to be efficient at picking out food items from a world that is overly rich in extraneous stimuli.

In a sense, these parts of the nervous system/sensory system may be co-opted by sexual selection and males that show a trait that triggers a hightened response in females may have an advantage.

Do sensory biases exist?

Basalo looked a genus of Sword-tailed fish, Xiphoporus, which have elongate swords. A phylogeny of Xiphophorus indicates that most recent members of the "clade" have swords. One member of the genus, the most "ancestral" type lacks a sword.

Basalo asked whether females from this ancestral species preferred males of their own species which lack a sword, or males of their own species with swords tied on. The overwhelming choice was for males that had a Sword!!! She interpreted these results to imply that there existed an "ancestral" bias, for swordedness in these fish, that in turn led to a Runaway Process.

Male-Male Competition -- Alternative Male Strategies

Males do not just have to be big and aggressive to gain access to females. There exist in the world of males Alternative Male Strategies of the following types:

  1. Showy, Aggressive males that defend large groups of females -- Polygynous
  2. Mate-guarding males that vigorously defend one or only a few females -- Monogomous
  3. Sneaky males that try to obtained copulations on the fly, many of these males resemble females in morphology (e.g., size and color) and behavior so that they can get closer to the females to fertilize them when the dominant male is around.


Many Organisms display two of the three types described above.

The Rock-Paper-Scissors Game

See Barry's Web Site

An unusual game is being played out in the Coast Range of California. Three alternative male strategies are locked in an ecological "perpetual motion machine" from which there appears little escape. As in the rock-paper-scissors game where rock beats scissors, paper beats rock, and scissors beats paper, three morphs of lizards cycle from the ultra-dominant polygynous orange-throated males, which best the more monogamous mate gaurding blues; the oranges are in turn bested by the sneaker strategy of yellow-throated males, and the sneaker strategy of yellows is in turn bested by the mate guarding strategy of blue-throated males. Each strategy in this game has a strength and a weakness, and there is the evolutionary rub that keeps the wheels spinning.

The rock-paper-scissors game can be readily modeled using a branch of evolutionary theory referred to as game theory or Evolutionary Stable Strategies (ESS) Analysis. In ESS models you consider how well a male type does when it is rare compared to when it is common. In this model, the fitness of a male type when rare is different compared to when it is common. This is described by the Pay-Off matrix which describes the fitness of each male type when rare, competing against each other type when common.
Common Morph of Male
Fitness of Yellow Males Blue Males Orange Males
a Rare Morph Yellow Males 1 1 2
Against a Blue Males 2 1 0.2
Common morph Orange Males 0.4 4 1


Note that:

  1. when a morph competes against its own type (e.g, Y vs Y, B vs B or O vs O) the morphs have the same fitness (W=1).
  2. a rare Yellow beats a common Orange (W=2), but a common Yellow looses to blue.
  3. also, that a rare Blue beats a common Yellow (W=2), but looses to a rare Orange.
  4. finally, a rare Orange beats a common Blue, but looses to a rare Yellow.

Thus, the proportions of morphs in the population will tend to endlessly cycle from Yellow to Blue, from Blue to Orange and from Orange to Yellow.

The Big Lesson

Frequency dependent selection tends to preserve a lot of genetic variation. Each morph has some kind of advantage when rare and it will increase in frequency.

This example with lizards probably takes place in other mating systems as well, even those with only two male types. These simpler systems will not necessarily oscillate, but will settle to an equilibrium frequency where the fitness of the two morphs (on average) are equal. Frequency dependent changes in fitness maintains the equilibrium point.

Summary of Population Genetics Theory

H-W assumes the following:

1) Organism is diploid

2) Reproduction is sexual

3) Generations are non-overlapping (if not Ne is reduced)

4) Mating is random

5) Population size is large

6) Migration is negligible (no gene flow)

7) Mutation is ignored

8) Natural Selection does not affect the locus

1) - 3) are trivial

4) - 8) are evolutionary important.

The Assumption of Random Mating

1) Random mating -- choice of mates is independent of genotype or phenotype

2) Positive assortative mating -- mates are phenotypically more similar than would be expected by chance

3) Disassortative mating (Negative assortative mating) -- mates phenotypically more dissimilar than would be expected by chance

4) Inbreeding -- Assortative mating between relatives (assortative mating by similar genotype).

Non-Random Mating can affect the allele frequencies of H-W in a powerful way.

See the lecture notes on sexual selection above, and my diagrams on inbreeding from lecture.

The Assumption of Population Size- Genetic Drift

With small populations, variation (e.g., copies of alleles) can be lost by sampling effects. Inbreeding becomes a factor in very small populations.

This process:

Leads to a loss of genetic variation within populations

Genetic divergence among populations as the different populations loose different alleles

The Assumption of Gene Flow

Consider the following example:

Imagine an oyster that is found in two estuaries. The following genotype frequencies are found in each estuary:

Estuary 1 -- D: 1280, H: 320, R: 80 and estuary 2 -- D: 80, H: 320, R: 1280. The populations in each estuary are in H-W. Let us assume that gametes are shed in the water column but fertilization takes place in the parental estuary. The larvae, however, disperse and settle randomly in both estuaries. That is larvae take a few weeks to settle and in that time, they are flushed out of the estuaries, mixed at sea, and then come back in to settle after a few weeks. The genotype frequencies of the larvae in both estuary (pooled sample) are given by:

D: 1360, 640, and 1360 (frequencies are an arithmetic average of genotypes from both estuaries.)

If we did not realize that there was this level of population subdivision, we would expect

D: 750, H: 1500, R: 750.

This is an example of the Wahlund effect. It looks like Inbreeding as far as the deviations from our expectation of H-W. And on a grander scale it is. Within each population breeding is non-random, however, the pooled sample among estuaries that contributes to the next generation is not

non-random but reflects that fact that members of estuary 1 tends to interbreed with other members of estuary 1 and members of estuary 2 tend to interbreed with other members of estuary 2. What maintains the different allele frequencies in each estuary (selection???).


Mutation generates variation. Despite its relatively weak affect, mutation can maintain substantial variation even in the face of natural selection. Recall the formula for mutation selection balance

Natural Selection

Selection is the "engine" of evolutionary change, and selection leads to adaptation. However, natural selection eliminates variation. There are a few special cases in which this is not true. For example, overdominance in fitness maintains variation in an equilibrium.

Some useful generalizations concerning mutation and migration. Migration and mutation are analogous. For example, in the case of a single population with an infinite alleles, neutral in the effect on phenotype, migration of unique copies from an outside source for all intents and purposes resembles migration (for low levels of gene flow). We obtain similar population genetic equilibrium solutions.

The Big Question -- Accounting for genetic variation

If selection is acting on a locus, advantageous alleles will become fixed.

The five explanations for the genetic variation we observe:

1) The effects are close to neutral (e.g., s << 1, very small) and genetic drift accounts for variation.

2) The locus is not at an equilibrium, (e.g., a transient polymorphism) and selection is driving one allele to fixation or it is closely linked to other polymorphic genes. Perhaps the selective environment is fluctuating.

3) Fixation is counteracted by mutation. (recall mutation selection balance)

4) Fixation is counteracted by gene flow. (mutation selection balance derived in lecture is analogous to migration selection balance).

5) The "balanced view" a stable polymophism arises from overdominance in fitness.

Interaction of evolutionary factors are very important

We have seen how three of the factors are deterministic: mutation, gene flow, and selection. Given similar parameters, different populations will converge on the same equilibrium solution. There may be probabilistic aspects to mutation, migration, and selection, but the outsome is predictable.

Genetic drift is truly stochastic and works in conjuction with the other forces to determine a distribution of evolutionary outcomes.

For example a weakly selected allele is primarily affected by selection if the population is very large. In a small population the allele behaves as if it were neutral.

mutations and genetic drift

1) A mutation that is slightly advantageous is more likely to be fixed in a large population than a smaller one.

2) The frequency of an allele may wander around an adaptive peak that is set by overdominance in fitness and can in fact go to fixation if the population is small enough.

3) The frequency of a weakly deleterious allele may actually increase in a small population.

These three examples of selection and mutation and genetic drift run counter to the action of mutation and selection considered alone. And thus, genetic drift + selection (or mutation, migration) can accomplish what selection alone cannot accomplish.

The Adaptive Landscape

Fitness peaks on a landscape of gene frequencies as a metaphor for evolutionary processes. See drawing in lecture.

Wright's Shifting Balance Theory of Evolution

Wrights Theory of shifting balance integrates the following 3 forces:

selection -- leads to local adaptation in each of several population

genetic drift -- leads to radically different gene frequencies and perhaps innovation by completely different mechanisms

migration -- inhibits process because it leads to mixing of gene pools

However if migration is cut off then new species might arise.

... Next Lecture we will consider the adaptive landscape and speciation

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