Anisogamy

This post follows in line with several people at ScienceBlogs (Also Larry Moran and Greg Laden) in an attempt to generate a series of posts that cover what they (we) think are essential.. See nice posts on Fitness, evolution, clade, 8th grade math, and teaching resources. See the current/complete list here.

Most basic definition:

an- without, not

iso- same

gamy- referring to gamete.

Anisogamy describes the situation where there are 2+ differently sized gametes. It is the logical opposite of isogamy.

Why is anisogamy important?

Anisogamy is the fundamental cause for the majority of differences between males and females, both in terms of macro-scale physiology, post mating sex-roles, reproductive strategies, and sets the stage for sexual conflict. For instance, anisogamy is the fundamental reason why:

1. Males are more likely to cuckold their female mate than the opposite.

2. The ESS for males, ceteris paribus, is polygyny. (That the optimal # of sexual partners is greater for males)

3. Females (usually) are the ones providing parental care

4. We males are less choosy than females when it comes to reproduction.

In fact, anisogamy serves as the foundation for the biological definitions of male (the ones with the small gametes) and female (the ones with the big gametes).

How did anisogamy evolve?

Although it is Geoff Parker's 1972 model has gained near-universal acceptance, there are several other models that attempt to explain the evolution of anisogamy. All of them, however, suppose 2 distinct and diametrically opposed (as resources are limited) selective pressures.

1. Selection for increased numbers of gametes (the more the merrier, huh)

2. Selection for increased size (as this influences gamete survival)

Notice that you can have a lot of gametes, or big ones, but not both...

Parkers idea- (a good one) was to note that given the above conditions and isogamy, disruptive selection drive gamete size to the extremes- some very big, some very small... Later work expanded on this intuition, showing that anisogamy is indeed an ESS.

Outcomes- For brevity- I'll just describe one case.

So wait a second- why are males more likely to try for polygyny than females??? Think about your average male, being the lucky possessor of X million sperm, ready to be put to good use in a moments (literally) notice. Which strategy will likely afford him higher reproductive success? Inseminate 1 female, and stay with her, or inseminate 1 female, stay for a minute or 2 to see the no other male copulates with her, then proceed to the next, and next, and next... It's pretty obvious who which strategy wins here. Now on to the female.. She has only 1 big egg per estrus cycle. Her options are to mate with one male and have her egg fertalized, then stop- or to mate with many, but with the caveat that it is the 1^st males sperm that did the duty.. In the case where she continues to copulate, she accrues all the costs of reproduction (risk of predation, disease, energetics, etc), but no benefits (Let's ignore the acquisition of direct benefits).

Given the above factors- it seems quite logical that the optimal number of sexual partners is much greater for males than for females...

Other Reading

Parker, 1972. The origin and evolution of gamete dimorphism.

Hoekstra, 1987. The Evolution of Sexes- in The Evolution of Sex and its Consequences

Maynard-Smith, 1982. Evolution and the Theory of Games

Bell, 1978. The Evolution of Anisogamy

Charlesworth, 1978. Population genetics of Anisogamy