Incipient speciation is the evolutionary process in which new species form but are still capable of interbreeding; it can be the first part of the larger process of speciation. Incipient speciation can arise from natural selection and/or genetic drift, and is further reinforced by these factors, as well as others, such as sexual selection. Examples of incipient speciation in nature are the focus of much biological research.
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Initiation
Two populations of a species can begin speciation by adapting to ecological niches that differ spatially, temporally, or in the amount or type of resources, predators, parasites, etc. This is known as ecological speciation. An example is the fish Gobiomorphus cotidianus of New Zealand. Within the same rivers live migratory and non-migratory populations, and these populations, although of the same species, are genetically distinct and spawn in different seasons.
Geographically isolated populations can also begin speciation simply through random genetic drift. Isolated populations can diverge more quickly if they have differing resources, however, as they adapt through natural selection to their different environments (ecological speciation). This has been described in the mosquitofish of the blue holes of the Bahamas. Isolated populations in holes with different predators diverge much more quickly than isolated populations with the same predators.
Reinforcement
Once speciation is initiated, it can be further reinforced by a variety of factors other than simple adaptation to a specific ecological niche. In the example of G. cotidianus mentioned above, different spawning seasons severely limit interbreeding between populations and so genetically isolate the populations from one another, increasing speciation through genetic drift. Any such trait that encourages non-random mating that reinforces speciation is known as a “magic trait”.
Different populations can also reinforce their isolation through differing mate preferences, or sexual selection. For example, fruit fly females will only mate with males who exhibit a proper courtship ritual, and the details of this ritual (which are inherited genetically) differ between some populations of the same species, increasing their isolation from interbreeding. The population of Drosophila kikkawai in New Guinea is considered as undergoing incipient speciation based on this criterion of mating ritual. Theoretical computer models have also shown that a completely arbitrary change in mate preference has the capacity to drive speciation, without any outside pressure from natural selection.
Orca whales exhibit a similar form of incipient speciation, although mating preferences are based on culturally, instead of genetically, inherited traits. Separate populations of orcas hunt different prey, use different hunting techniques, and sing distinct songs. These behaviors are taught to young orcas by their mother and family group and are not genetically heritable, making these behaviors the culture of distinct orca populations. Orcas use these cultural cues to pick mates generally only within their cultural population, reinforcing or perhaps causing ecological speciation.
There is much debate over whether speciation is a byproduct of local adaptation and driven primarily by natural selection, or whether sexual selection is instead the primary mechanism. The precise genetics of incipient speciation are still being studied to determine if certain traits or genes tend to diverge early in speciation, especially traits for mating preferences. If such traits quickly diverged, it would cause sexual reinforcement to act early and powerfully on speciation.