How is it that Dalmatians and Chihuahuas are so different but are members of the same species, yet two sparrows that look nearly identical are different species?
In: Biology
Because looks have not necessarily a lot to do with how closely related two organisms are.
1. Two organisms are commonly said to be in the same species if they can produce fertile offspring. This is at some point no longer the case when two branches of the same species evolved far into different directions and their genes are no longer compatible enough, they then become two species.
2. In evolution, selection decides how fast and in which direction a branch of a species develops. So the two species of sparrows might have developed into different directions for a very long time, such that they have a lot of genetic differences. But only few of them might be related to looks if there was no selection pressure in this regard (= no survival advantage with different looks).
3. Dogs have been selectively bred by humans for a relatively short time (in evolutionary terms) but selected *specifically* for looks and aesthetics. So dogs are still genetically very close to each other (= close enough to be the same species and produce offspring), but the few differences they *do* have are very concentrated on looks.
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