WHAT IS a SPECIES
KARDONG 159-165ISH

Species
The most widely held modern view of biparental biological species is individuals that interbreed or have the potential to interbreed with each other and not with other species. This ensures that the amount of exchange of genetic variation is low enough to share enough similarities between the parents, keeping the offspring in this same distinct category.
Linnaeus more than likely was using the morphospecies species definition when listing all of the then known species.

speciation
Modern biologists believe the four steps to speciation are what have caused most biparental species to come into existence.
1. Barrier separates species
2. Reproductive isolation
3. Speciation
4. Barrier is gone, no breeding between populations.
Darwin believed evolution cause the new species but did not require physical isolation of the subsets.
Most biologists think reproductive isolation is a prerequisite for most cases of speciation because it disrupts gene flow. Without the isolation, species will continue to mate and genes will be passed on the same way they have, producing the same variation of offspring.
Some biological barriers that could occur after having a physical barrier and being isolated:
Incompatible mating signals or pheromones
Ecological, behavioral, temporal isolation could occur due to environmental changes from the barrier
Mechanical isolation (structural differences) and gamete incompatibility.
Prezygotic mechanisms are the isolations that keep different species from trying or effectively mating.
Postzygotic mechanisms are used after