Indeed. When a species generates a new branch, observe that the two species begin at the same point and slowly grow apart. I doubt there is some point in time where interbreeding between these two branches becomes impossible. I expect that it gradually becomes less successful over time.
Can animals breed across species?
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Yes, because taking a swab of cotton to the inside of his mouth would cause irreparable harm to his quality of life.
I'm not advocating experimenting in him.
Precisely, as with so much else in the natural sciences, it is likely merely a matter of probabilities. Once the probability has eroded to a certain point, we can confidently label the two branches 'different species', but that isn't to say all possibility is lost.
Perhaps a better definition would replace 'cannot breed to produce fertile offspring' with 'usually cannot' or 'have not been known to'.
Here's a bit of an analogy I like to use when explaining evolution to people.
What we categorize as individual species are snapshots of the now and of the past, like moments in time.
The fossil record is like looking at still pictures, each one representing a different species in a particular evolutionary branch.
Speciation along that branch is like watching the face morphing in the MJ "Black or White" video, with each individual person representing a different species.
Evolution never stops. The fossil record is so incomplete it's staggering. We have access to such a tiny fraction of all of the species that have ever existed, and virtually no record of the microscopic outside of the organisms that have directly carried on.
What we categorize as individual species are snapshots of the now and of the past, like moments in time.
The fossil record is like looking at still pictures, each one representing a different species in a particular evolutionary branch.
Speciation along that branch is like watching the face morphing in the MJ "Black or White" video, with each individual person representing a different species.
Evolution never stops. The fossil record is so incomplete it's staggering. We have access to such a tiny fraction of all of the species that have ever existed, and virtually no record of the microscopic outside of the organisms that have directly carried on.
Indeed.
I hope that's true that evolution never stops because us humans sure do have a long way to go.
Humans are going to be a very interesting evolutionary path, because we have developed a society in which those who, in the wild, would probably succumb to selective pressures and not end up breeding, can live healthy and productive lives. It will be interesting for future humans to see what evolution does with this new concept - a species that actively works against it.
Considering that our definition of a species (in animals anyway) is the biospecies (as opposed to the morphospecies in plants) is the ability to (consistently) produce fertile offspring, then no. If animals from two different species would be able to produce fertile offspring, they'd actually be part of the same species.
Two closely related species can sometimes produce hybrids (as mentioned above) but they are not fertile -except by 'accident'.
When a species is evolving by diverging or isolation, at one point they will end up practically unable to cross-breed. At that point, they will have become separate species.
In plants however, the species concept is much more fluid and fertile hybrids are very common (leading to taxonomical constructs like the Rubus fruticosus complex in which about 2,000 species are classified since they are almost impossible to distinguish) - so a species is defined as a population or group of populations that differs morphologically from other populations. Which isn't saying much, of course. =)
Two closely related species can sometimes produce hybrids (as mentioned above) but they are not fertile -except by 'accident'.
When a species is evolving by diverging or isolation, at one point they will end up practically unable to cross-breed. At that point, they will have become separate species.
In plants however, the species concept is much more fluid and fertile hybrids are very common (leading to taxonomical constructs like the Rubus fruticosus complex in which about 2,000 species are classified since they are almost impossible to distinguish) - so a species is defined as a population or group of populations that differs morphologically from other populations. Which isn't saying much, of course. =)
When a species is evolving by diverging or isolation, at one point they will end up practically unable to cross-breed. At that point, they will have become separate species.
Except what Jadzia and I are positing is this is unlikely to be an overnight thing - one generation can breed to produce fertile offspring, the next simply can't - except perhaps in rare cases resulting from a mutation that directly affects breeding. Odds are, we're usually talking a decline of probabilities over a number of generations, and a distinct 'grey area' when it comes to defining speciation. After all, the definition of species is more than just the breeding issue, it is a definition which is controversial even in its own field as that deceptively simple approach ignores a great many subtleties which turn out to be quite important (this thread has thrown up a few already).
When a species is evolving by diverging or isolation, at one point they will end up practically unable to cross-breed. At that point, they will have become separate species.
Except what Jadzia and I are positing is this is unlikely to be an overnight thing - one generation can breed to produce fertile offspring, the next simply can't - except perhaps in rare cases resulting from a mutation that directly affects breeding. Odds are, we're usually talking a decline of probabilities over a number of generations, and a distinct 'grey area' when it comes to defining speciation. After all, the definition of species is more than just the breeding issue, it is a definition which is controversial even in its own field as that deceptively simple approach ignores a great many subtleties which turn out to be quite important (this thread has thrown up a few already).
Oh of course, I didn't mean to imply that these things happen fast.
Some evolution theorems do allow for very rapid speciation (punctuated equilibria or the outdated hopeful monsters) - but even those processes would take say, 20 000 years.
I can't believe no one has posted this yet.
