Why is it that male gametes are smaller and produced in larger quantities than female gametes that are large and lesser in number. Why aren't the gametes equally sized, or their sizes the other way around. Is there an evolutionary advantage to this, or is it just how things evolved and would it not have mattered if the sizes and quantities were reversed?

  • $\begingroup$ Regarding the question "Why aren't the gametes sizes the other way around?", it's simply impossible by definition: in an oogamic system the female gamete is the bigger gamete. So, if the male gamete were bigger it would be automatically named female gamete. Regarding your first question ("Why aren't the gametes equally sized"), this excerpt from Wikipedia explains it succinctly: "The prevalence of oogamy in higher animals (sic) leads to the conclusion that this specialization of the gametes..." $\endgroup$ – user24284 Jan 15 '18 at 11:56
  • $\begingroup$ "...results in their performing their respective tasks better and more efficiently than those tasks could be performed by generalist isogametes, particularly the ability to concentrate high-energy substances in a smaller number of ova." $\endgroup$ – user24284 Jan 15 '18 at 11:56

Gerardo is correct that female designates the bigger gamete, so there's nothing important about the terms "male" and "female" here. But I'd like to also respond to your initial question of how the difference in size would've evolved because I don't think it's trivial.

I can provide a rough summary of one explanation that was summarized in Dawkin's The Selfish Gene which was summarized from G. A. Parker's 1972 paper "The origin and evolution of gametic dimorphism and the male-female phenomenon."

It goes something like this:

The asymmetry could have evolved from originally equal sized gametes through the evolution of two divergent sexual strategies - a large investment strategy which opened the way for a small-investment exploitative strategy. The idea is that when gametes were all roughly the same size, some would have happened to be bigger than others. There may have been some advantage to this because the larger gamete could provide a larger initial food supply. However, this also could have created a condition for exploitation by individuals producing smaller than average gametes, especially if they could ensure their smaller gametes fused with the larger than average ones. This could be achieved by making the small ones more mobile and able to actively seek out the larger ones. Making smaller gametes would have allowed that individual to make more of them and potentially have more descendants, as they're not investing as much energy into any one gamete. The interplay of these two strategies would've continued in a runaway fashion, with the larger gametes evolving larger and larger to compensate for the ever-smaller contributions by the exploiters. Eventually the larger gamete could become immobile since it's being sought out by the smaller gametes anyway. You can see where this is going, with the larger gamete becoming what we know of as eggs and the smaller as sperms.

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