Speciation is simply the process whereby a single lineage splits into two lineages. In other words, new species arise from existing species.
But what constitutes a species and how does speciation occur? The following sections explain.
Scientists have a pretty good handle on what constitutes a species for sexually reproducing animals: the biological-species concept. According to this concept, a species is a group of organisms that can interbreed and produce viable and fertile offspring.
Individuals can mate and reproduce with members of their own species but not with members of other species. The defining characteristic separating one species from another is that they are reproductively isolated from each other. When a speciation event occurs — when evolution results in members of one species developing into another species — that group of individuals can no longer interbreed with members of the original species.
The biological-species concept best applies to sexually reproducing animals. It doesn't adequately define what bacterial species are. In fact, defining the term species in other cases is an active area of evolutionary biology. For more on bacterial species, head to the later section "A Species Concept for Bacteria."
When new species arise from existing species, you have speciation. Here's how it works: Two different populations of the same species evolve in different ways. They become progressively more different until they are so different that they are no longer able to interbreed. That's all there is to it.
You've heard about speciation, and it might be one of the major reasons you bought this book. Can it be that such a thing really happens? It's clear that some people don't even like the idea of it. How do we know that the whole idea of speciation wasn't just something that Darwin concocted with after one too many beers? Because of ring species, which are explained in the next section. At this point, suffice it to say that by studying ring species, scientists know that a gradual accumulation of small differences is sufficient to cause two populations of the same species to become reproductively isolated.
Of course, ring species aren't the only interesting thing in this chapter. I hope you read the rest of the chapter, too, because speciation is a fascinating topic and one that evolutionary biologists think about a lot. But even if you read no more than the next section, you'll see how scientists — and now you — know that speciation can indeed happen.
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