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.

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 exagerated 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.

How about 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.

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. See the slide show for more details on the ESS analysis of the rock-paper-scissor game (B. Sinervo and C. M. Livley (1996) Nature 340:240-243).

The rock-paper-scissors game can be readily modelled 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.