Evolutionary Ideas: Species and Speciation
In evolution, speciation is simply the process whereby a single lineage (species) 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.
The biological-species concept
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.
When one species becomes two
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.
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 after a night of carousing?
Because of ring species.
Proof: Ring species
Ring species are species with these specific features:
- Their habitat surrounds an area of hostile environment that they can't cross. Think about a bird species living in the lower elevations around the Himalayan mountain plateau, or a little salamander species living around the edges of California Central Valley. They can move around the edges, but they can't cross over the middle because they couldn't survive.
- Neighboring subpopulations around the circle, or ring, are slightly genetically different from each other. These genetic differences can be measured.
- Most neighboring populations can interbreed with each other. The populations near one another are a little different genetically, but they are still the same species and can therefore mate and produce viable offspring.
- At one place around the ring, the neighboring populations can't interbreed with each other. Each population can breed with its neighbors (because neighboring populations are just a little bit different), yet all those differences add up as you go from one end of the ring to the other. The result is that, by the time you've gone all the way around the ring, the two populations on the "ends" are too different to interbreed. If it wasn't for all the populations in the middle, the two end populations would be different species.
Through the existence of ring species, scientists can say with certainty that small differences can accumulate in nature to the point that two populations of the same species can become reproductively isolated. They can actually go out and see it.