I have been asked many times to explain how so called “macro-evolution” works. There are of course many excellent books and articles online and off that cover this, but I thought it might be a good idea to have a blog post that explains it quickly and simply that I can refer to when asked. The following is adapted from a book manuscript (which might get published some day).
To see how macroevolution (the origin of new species) works, we can use a hypothetical animal, maybe one in the cat family. Let’s call it a lipard. And let’s say that there is a population of these large cat-like carnivores living on a large plain with plenty of prey animals. The lipards have gotten better and better at hunting thanks to several improvements (microevolution) in vision, muscle strength, digestion of meat, and other traits. And all of these positive changes eventually got shared by the whole population of lipards, due to breeding and natural selection (like all examples of microevolution).
But now the population of lipards becomes divided so that there are two groups of lipards that cannot interbreed. Perhaps one group crossed a river, a desert, or a mountain range and couldn’t get back, or they just wandered so far away that it wasn’t convenient to find mates in the other group.
Now both groups of lipards continue to accumulate new genetic variations through mutations, but because they are no longer interbreeding with each other, the new variantions in one group do not spread to the other group. With time, each group begins to differ in their variations. Now, for each group, natural selection could choose different genetic variants to be successful than in the other group.
Its also possible that some differences in the two groups could arise by chance, and not have any important effects on survival (like slight differences in skin color). But the key point is that none of the changes will spread to the other group, because the two populations cannot interbreed.
With the passage of time, different traits will appear in one of the groups that are not found in the other. In one group, the skin could become darker, and in the other, males could develop a large mane of hair around their heads. Both groups continue to change independently of each other, and after a long enough time, neither group resembles the original lipards. One group has become lions, and the other has become leopards.
They still have a lot in common, but they are now two separate species. Please note that no lion turned into a leopard or vice versa. Both lions and leopards share a common ancestral species, the lipard, which now no longer exists. It didn’t go extinct – it evolved. Lipards themselves had evolved from an ancestor that they had in common with tigers and snow leopards, and even further back with cheetahs and domestic cats. And they all became separate species the same way: population isolation, separate genetic changes in the separate populations, and continued evolution by natural selection. This is what Darwin observed among species of finches in separate islands of the Galapagos chain.
We can keep looking backwards in biological history. All the cat-like animals are descended from a no-longer-living ancestor shared with bears, wolves, hyenas, badgers, and other carnivores. If we keep going we will find a common ancestor for all mammals, and then all vertebrates, and so on. For an excellent book that describes all of this in beautiful detail, there is nothing to match The Ancestor’s Tale by Richard Dawkins.
Is there any evidence for this scenario of how the diversity of life arose? Yes, tons. There is so much evidence, both in fossil records and from genetics, that there is no doubt at all that the theory of evolution for the origin of species is correct (though perhaps not complete – see “New Ideas in Evolutionary Biology”).