•Gametic isolation occurs when gametes of two species do not form a zygote because of incompatibilities preventing fusion or other mechanisms.
–In species with internal fertilization, the environment of the female reproductive tract may not be conducive to the survival of sperm from other species.
–For species with external fertilization, gamete recognition may rely on the presence of specific molecules on the egg’s coat, which adhere only to specific molecules on sperm cells of the same species.
–A similar molecular recognition mechanism enables a flower to discriminate between pollen of the same species and pollen of a different species.
Monophyly is the goal because it shows common ancestries
Parsimony: searching for simplest phylogenetic tree