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
Humans are really good at guided selection breeding programs. We have taken wild plants and turned them into potatoes, zucchini, and cauliflower. We’ve taken wild animals and turned them into dairy cows, turkeys, and an entire rainbow of different pet dogs.
But we haven’t started yet on sparrow breeding. If birds are even real, I mean. Natural selection has created those sparrows, and that takes effing forEVER compared to human breeding. Those untouched animals take thousands of generations of natural selection to change a species as much as human guided breeding can do in ten generations. Those species of sparrow separated a million years ago, while the dogs are practically cousins.
Because how things look is a bad indicator of how related things are. Modern genetic knowledge has changed, and continues to change, the family trees we’ve drawn based on purely physical traits.
In the case of dogs, we’ve selectively bred them extremely rapidly, but the genes that control body size or leg length don’t make sperms and eggs incompatible. 20,000 years isn’t enough time for regular genetic drift to give different breeds different # of chromosomes or something. And, breeds aren’t exactly completely separate populations. Regularly getting gene pool infusions from other breeds keeps massive genetic differences from building up.
Physiology is easy to modify, as it’s basically selective breeding of traits inherited by parents over quick generations.
Like, breeding small dogs to have smaller and smaller offsprings. Or breeding white dogs with black spots, etc.
Main issues are: Genetic disorders, and inapt genetic adaptation (brain size, etc.).
Sparrows populations would have diverted slowly over thousands of generations. Isolation preventing the genetic pool to maintain coherent genetic code that allows reproduction, the population divide in two species.
Yet, you don’t change a winning model, at least in nature. Sparrows are adapted to rather specific environment, food, etc, and there’s only so much way a sparrow population can “evolve” physiologically. After all, a bird is a bird.
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.
We have decided that the definition of species includes those two dogs, but not those two birds. That’s pretty much it. There are many definitions of species, but what they all have in common is how *useful* they are. No serious definition of species will define a chihuahua and a fir tree to be the same species for a variety of differences. Those differences are the same ones used to determine a chihuahua and a dalmatian to be the same species, but two different sparrows as different ones.
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