How do such Female Choices Arise? -- Sensory Drive
Male-Male Competition -- Alternative Male Strategies
Summary of Population Genetics Theory
The Assumption of Random Mating
The Assumption of Population Size- Genetic Drift
The Big Question -- Accounting for genetic variation
Interaction of evolutionary factors are very important
The Adaptive Landscape
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.
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.
Let us assume that females come in two types:
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.
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.
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:
Many Organisms display two of the three types described above.
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:
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.
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.
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.
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.
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
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
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.
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.
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.
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