One of the most striking evolutionary patterns biologists have observed is called adaptive radiation. To radiate means to spread outward. In this case, this isn't meant in the sense of speading out physically; it refers to one or a few species which diversify ("spread out") and generate multiple daughter species. Something like this
The most common situations which result in adaptive radiation occur following mass extinctions, or when species move into new, unoccupied regions. The feature shared by these two situations is the availability of a variety of ecological niches. How this can lead to the creation of a variety of new species is initmately tied to the ecological phenomena described in the Niches essay: fundamental and realized niches, resource partitioning and competitive exclusion.
In a stable ecosystem, a species' realized niche is typically much smaller than its fundamental niche. This is because in a complex ecosystem, a lot of resident species generally have overlapping niches. To avoid competitive exclusion, the species in the ecosystem partition resources--each species actually reduces its niche size to avoid arguing with the "neighbors," so to speak. The ecosystem has many niches, and those niches are all pretty much occupied by the various different species.
But a mass extinction wipes out a large percentage of the species in ecosystems, leaving a lot of unoccupied niches--a lot of ecological space, so to speak. Any surviving species would have a lot more ecological opportunities than it would have had before the extinction event. And if a species migrates into a new, unoccupied region, leaving behind the other species from its former home, it finds itself with essentially no potential competitors. Again, the species would have a lot of ecological opportunities not previously available to it.
In both of these situations, the species would have the opportunity to greatly increase their realized niches. In essence, this is a relaxation of the normal selective pressure which inhibits the development of a lot of potential diversity. These species become more and more diverse as they take advantage of their new opportunities.
Eventually, success breeds its own problems. As the populations increase, they become their own competitors. Different factions of the population begin to specialize in portions of their broadened niche, and to develop variations which favor their chosen specialties. In other words, resource partitioning begins to occur within the species, leading to different varieties.
Having different varieties within one species is not the same thing as having several species. What is necessary for the varieties to become new species is some degree of reproductive isolation. A geneticist would say that something needs to prevent "gene flow" between the different varieties. All sorts of phenomena can create reproductive isolation--physical barriers, behavioral differences, space preferences. Some separation, some time for the development of genetic differences, and voila! New species.
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