There is a Princeton article that discusses a study on speciation. Some researchers observed a single finch that became isolated from its own species. The island it ended up on, though, had several other species of finch. So, it crossed the species divide to mate with one of the other populations.
That alone questions the very meaning of species. It was neither genetics nor behavior that kept these breeding populations separate. It was simply geographic distance. Eliminate that geographic factor and hybridization quickly follows. The researchers argue that this hybridization represents a new species. But their observations are over a short period of time. There is no reason to assume that further hybridization won’t occur, causing this population to slowly assimilate back into the original local population, the genetic variance lessening over time (as with populations of homo sapiens that hybridized with other homonids such as neanderthals).
All this proves is that our present definition of ‘species’ isn’t always particularly scientific, in being useful for careful understanding. Of course, it’s not hard to create a separate breeding population. But if separate breeding populations don’t have much genetic difference and can easily interbreed, then how is calling them separate species meaningful in any possible sense of that word? Well, it isn’t meaningful.
This study showed that sub-populations can become isolated for periods of times. What it doesn’t show is that this isolation will be long-lasting, as it isn’t entirely known what caused the separation of the breeding populations in the first place. For example, we don’t know to what extent the altered bird songs are related to genetics versus epigenetics, microbiome, environmental shifts, learned behavior, etc. The original lost and isolated finch carried with it much more than the genetics of its species. It would be unscientific to conclude much from such limited info and observations.
The original cause(s) might change again. In that case, the temporary sub-population would lose the traits, in this case birdsong, that have separated it. That probably happens all the time, temporary changes within populations and occasional hybridized populations appearing only to disappear again. But it’s probably rare that these changes become permanent so as to develop into genuinely separate species, in the meaningful sense of being genetically and behaviorally distinct to a large enough degree.
Also, the researches didn’t eliminate the possible explanation of what in humans would be called culture. Consider mountain lions. Different mountain lion populations will only hunt certain prey species. This isn’t genetically determined behavior. Rather, specific hunting techniques are taught from mother to cub. But this could create separate breeding populations for, in some cases, they might hunt in different areas where the various prey are concentrated. Even so, this hasn’t separated the mountain lion populations into different species. They remain genetically the same.
Sure, give it enough time combined with environmental changes, and then speciation might follow. But speciation can’t be determined by behavior alone, even when combined with minor genetic differences. Otherwise, that would mean every human culture on the planet is a separate species. The Irish wold be a separate species from the English. The Germans would be a separate species from the French. The Chinese would be a separate species from the Japanese. Et cetera. This is ludicrous, even though some right-wingers might love this idea and in fact this was an early pre-scientific definition of races as species or sub-species. But as we know, humans have some of the lowest levels of genetic diversity as seen among similar species.
Our notion of species is too simplistic. We have this simplistic view because, as our lives are short and science is young, we only have a snapshot of nature. Supposed species are probably a lot more fluid than the present paradigm allows for. The perceived or imposed boundaries of ‘species’ could constantly be changing with various sub-populations constantly emerging and merging, with environmental niches constantly shifting and coalescing. The idea of static species generally seems unhelpful, except maybe in rare cases where a species becomes isolated over long periods of time (e.g., the ice age snails surviving in a few ice caves in Iowa and Illinois) or else in species that are so perfectly adapted that evolutionary conditions have had little apparent impact (e.g., crocodiles).
We easily forget that modern science hasn’t been studying nature for very long. As I often repeat, our ignorance is vast beyond comprehension, much greater than our present knowledge.
As an amusing related case, some species will have sex with entirely different species. Hybridization isn’t even possible in such situations. It’s not clear why this happens. An example of this is a particular population of monkeys sexually mounting deer and, as they sometimes get grooming and food out of the deal, a fair number of the deer tolerate the behavior. There is no reason to assume these deer-mounting monkeys have evolved into a new species, as compared to nearby populations of monkeys who don’t sexually molest hoofed animals. Wild animals don’t seem to care all that much what modern humans think of them. Abstract categories of species don’t stop them from acting however they so desire. And it hasn’t always stopped humans either, whether between the supposed races within the human species or across the supposed divide of species.
From the lascivious monkey article (linked directly above):
“Finally, the researchers say, this might be a kind of cultural practice. Japanese macaques display different behaviors in different locations — some wash their food, or take hot-spring baths, or play with snowballs.
“Adolescent females grinding on the backs of deer might similarly be a cultural phenomenon. But it has only been observed at Minoo within the past few years.
“The monkey-deer sexual interactions reported in our paper may reflect the early stage development of a new behavioural tradition at Minoo,” Gunst-Leca told The Guardian.
“Alternatively, the paper notes, it could be a “short-lived fad.” Time will tell.”