Sperm and eggs are the vessicles for passing on genetic material to the next generation. It is often easy to think about motile sperm competing with each other to fertilize a finite set of eggs. It is easy to see a sperm with it's swimmy little tail, and think of how many swimmy little things could compete.You can imagine the faster sperm "winning", or the evolution of cooperation among sperm to get to the egg first.
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| By ScienceGenetics (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons |
It is harder to think about competition among eggs. To do this, we need need to take a step back and think not about competition between eggs, but about competition between the genetic content during egg formation. Competition in egg formation comes at the level of producing the egg, where you might have some alleles (variants of a gene) that distort inheritance so that instead of having a 50:50 chance of inheriting alleleA versus alleleB, the probability might be distorted to 90:10. This kind of distortion happens, but it doesn't happen all the time. Why?
To answer that, we need to remember that sperm and egg do not evolve independently.
Brandvain and Coop investigate co-evolution fo sperm and egg in a series of models:
Evolution. 2015 Feb 9. doi: 10.1111/evo.12621.
Sperm should evolve to make female meiosis fair.
Brandvain Y, Coop G.
The title gives away their major result: Sperm evolve to enforce fairness in female meiosis.
Check it out.
