Species of living things are usually defined either as populations that can breed and produce fertile offspring, or populations that do. One understanding of how independent species are established over time from a single genus is that they are allopatric, or that they have 'different fathers'. This may be due to geographic separation, say by a nearly impassible mountain range, or by their divergence into different ecological niches that have to do with environmental factors like specialized sources of food. Over time, members of a genus may diverge enough geographically or in their life habits so that they no longer have the opportunity to breed, which then leads them to diverge genetically so that they become incompatible for breeding if the opportunity were to arise.
There is a very interesting alternative to these more obvious ways that allopatric species are established. Certain organisms, like the tropical sea urchin Echinometra, have indiscriminate mechanisms of mating. These urchins use 'broadcast' fertilization, in which males put sperm into seawater, and a highly mixed and non-selective process brings these sperm to females. If just any male, fit or unfit, can mate with a female, her genes may become unfit. Females in such a predicament may evolve resistance to sperm, and males will evolve adaptations to this resistance. This is called "sexual conflict\, and it may result in reproductive isolation in the form of mating 'cliques'.
In one species of Echinometra, several subspecies are emerging that all have the ability to mate and produce fertile offspring, but their fertilization proteins have started to bunch up into such cliques. The affinities of sperm and egg docking proteins have undergone changes due to genetic drift, and each subgroup is becoming less likely to be cross-fertilized by the others.
Even though they mate in the same waters, they are losing their family ties.
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