Question:
Explain what is meant by the following statement:

Mating systems play two roles in eukaryotic evolution:
1) they provide pre-zygotic isolation systems that foster the process of speciation, and
2) they are often subject to sexual selection within a species.

Answer:
1) Mating systems provide pre-zygotic isolation systems through a variety of behavioral, temporal, mechanical, ecological, and biochemical mechanisms that ensure the sperm of one species cannot join the egg of another species and result in a hybrid which would be an evolutionary dead end. For example, the proteins on the coat of a pib's sperm cellss are incompatible with the receptors of a frog's egg cells, so that even if mating occurred, no offspring would result.

This process can foster speciation if some percentage of a species displays a new pre-zygotic barrier. Then, this individual will only be compatible with certain others, which will eventually constitute a breeding group. If this group is in pre-zygotic isolation long enough, the mating systems will evolve away from each other so far that mating no longer produces viable offspring, and thus speciation has occured.

2) Mating systems subject to sexual selection cause evolution within the same species by increasing the frequency of one trait relative to another. Sexual selection arises as both sexes choose mates that demonstrate possession of genes that are good for survival (i.e., fitness indicators). As this mating is non-random, traits that encourage survival will become more common in the gene pool of the species as a whole. As time passes, this leads to overall evolution of the species as traits better for survival are passed on to more offspring, which in turn get more mates, ad infinitum.