Speciation is the process by which one species can become two or more separate species. It is my goal here to explain it.

Each species changes over time, somewhat as a whole. A new form of a gene can come along in an individual because of genetic mutation, and the gene becomes mixed into the “gene pool” of the population through breeding with other members of the population. After many generations, this gene may be found on any member of the population. If the new gene was harmful in effect, it might not go on into future generations, because the first individual to have it may not have been able to reproduce or survive because of it. However, if the new gene was highly favorable, then it can be expected that after many generations, it will have spread throughout the population.

In the last case, the population as a whole has taken up the change offered by the initial mutation in one individual. The population can be said to have shifted toward this new trait. Species-level shift can be seen in human evolution; the fossil record shows that over time, the pelvic cavity of our ancestors has increased in size. This is because larger brains are favorable, and the pelvic cavity must be widened to accommodate the birth of babies with larger skulls.

So a population can, somewhat as a whole, change over time. But how does one species or population branch out into two separate ones? For this to happen, there must be something that blocks the flow of genes from one group to another. This is most commonly in the form of a geographic barrier or separation, such as when half a population crosses a land bridge during a time of low sea level, only to be separated from the rest of the population when the sea level returns to normal. This creates two new groups, unable to come together and interbreed.

At this point, any mutation that produced a new form of a gene will stay only in the group it originated in. If a new favorable trait originates in Group A, it cannot spread to Group B, because the two groups are physically separated and cannot interbreed; gene flow has been blocked. The two groups will continue to evolve independently of one another.

What happens if the barrier is removed? Say the land bridge reappears during another time of low sea level. It is possible that the two groups may come together and “blend” genes into a new gene pool. The differences acquired in each group during separation will slowly be averaged out into the new united population.

However, if the two groups have evolved independently of one another long enough, their DNA might not be compatible. The differences may be so great that the combined offspring might be unfit. In the case of horses and donkeys, DNA is similar enough that successful cross-breeding can occur: these are called mules. However, mules are infertile: two mules cannot come together and reproduce. All mules must be made by crossing a horse and a donkey. Therefore, horses and donkeys are considered different species. A species is typically defined as a group in which a member of the group can only produce viable, fertile offspring with other members of the same group. While horses and donkeys produce healthy-but-infertile offspring, other combinations of species may not even produce viable zygotes (the combined egg and sperm). You might imagine something like spontaneous abortion happening in this case.

Moreover, it may be that two groups are so different that they are completely uninterested in cross-breeding in the first place. In fact, this effect can be so powerful that even if two different animals could produce healthy offspring, they won’t do it unless forced. In this case, zoo-keepers and breeders use techniques like artificial insemination to get the job done. An example of this would be the cross between lions and tigers.

Sexual selection can be a very powerful force in the evolution of a species, as well as in the process of speciation. Imagine another example of a geographic barrier, this time maybe something like a group of birds “budding off” from the population and populating a remote island during a time of high winds. If our Group A and Group B have evolved independently so that females of Group A will only copulate with males who sport bright-red feathers, and females of Group B are, say, attracted to males with mad dancing skills (check out this video), then the males of Group A will have evolved to sport bright-red feathers and no dancing skill, while the dancing males of Group B may still be a dull brownish color. The reunited population will not interbreed, because females of each group will still only be interested in their favored traits. This is a hypothetical example of sexual selection being an even stronger force than DNA-incompatibility: it may very well be that the two groups could interbreed with healthy offspring, but they just won’t. The two groups continue to evolve separately, even though they are fundamentally similar and live in the same habitat. Two species have been made.

In conclusion, geographic barriers, DNA compatibility, and sexual selection (as well as other factors) combine to make speciation quite common. It really doesn’t take much. Consider in our two examples that the two groups are physically separated, and therefore in different habitats. These two different habitats may very well place different selection and adaptation pressures on the groups, which will in effect speed up the process of the two groups becoming different. And if you imagine it as being one species becoming two, becoming four, becoming eight, becoming 16, 32, 64, with occasional forks being deleted (mass and localized extinction), and given the span of 3-4 billion years since the earliest evidence of life, then the number of species observed today should not be at all surprising.