Okay, I know some of you are thinking, like me, " Story of my life!" but this here's actually a biology node, so put on yer spectacles and get out your field notebook:

In evolutionary biology, reproductive isolation is a set of circumstances that prevent members of two different species from interbreeding. Reproductive isolation is one of the main criteria of the Biological Species Concept -- according to that version of what makes a species a species, if two populations readily interbreed, then they should be considered part of the same species.

A number of types of "isolating mechanisms" have been observed that prevent gene flow between populations in nature:

  • Geographical isolation. When two populations aren't in spatial proximity, they can't interbreed. This alone is not enough to argue that the populations are in different species, because if they could interbreed when in the same region, they would still be considered the same species. However, geographic isolation and the reduced gene flow it results in can allow two populations to diverge enough in their genetic makeup that other isolating mechanisms arise.
  • Behavioral isolation. Different behavior patterns in two populations can keep them reproductively isolated from each other. There are several forms of this:
    • Habitat isolation. If two populations exist in the same region, but prefer different habitats, they may be sufficiently isolated from one another to prevent interbreeding.
    • Temporal isolation. If members of two populations are active at different times of the day (e.g., one is active at night, and the other during the day), or if they breed or flower during different seasons, this can prevent interbreeding.
    • Incompatible mating behaviors can prevent two populations from interbreeding. For example, the great variety of cichlid species in Africa's Lake Malawi are probably kept separate due to differences in their mating rituals, combined with some habitat isolation.
  • Mechanical isolation. Differences in sex organs or body shape and size can prevent mating, thus preventing gene flow between two populations. For example, the populations of Great Danes and Chihuahuas are , for all practical purposes, reproductively isolated.
  • Hybrid inviability. Gene flow between populations can be prevented if the result of mating between members is an individual which dies before reaching sexual maturity. This can happen just after fertilization or later, during the juvenile stage.
  • Hybrid sterility. Gene flow can also be prevented if hybrids between two species are viable, and live a normal lifespan, but are sterile and incapable of reproducing. For example, mules, which are the result of mating a horse and a donkey, are sterile; their sex organs are normal, but they do not produce viable gametes.
In "real life", there are often several of these isolating mechanisms at work, keeping species apart.

It's also interesting to note that reproductive isolating mechanisms that work well in nature sometimes fall apart in captivity or when ecological change occurs. Returning to the example of the Lake Malawi cichlids, species that would never interbreed in the wild quite often do so in the aquariums of tropical fish hobbyists.