Quick semantic question...
Due to evolution in a macro sense does the word "species" have any real meaning? I was thinking about the recent studies showing that modern humans have Neanderthal DNA but I've always been given the definition of species that two different species are incapable of interbreeding. Or does that mean we are in fact two breeds of the same species?
On the first page, it was quoted that Tiger/Lion hybrids can be fertile, so the definition needs to be modified.
Here's a scenario... A population becomes divided in two and continues to evolve into sexually incompatible populations. Are either of the populations still the original species? Are they both new species? If one is the original species, how do you determine which one? If they are both new species, then does that mean the definition is too limited? Over time, one species evolves into another species, despite always being sexually compatible within the modern population?
Well yes, but thats how evolution works. The lines are going to get really fuzzy eventually. We obviously caught these two groups, the tigers and lions, at the edge of their speciation, so they are diverging in physical characteristics, adapting to their different environments but their common ancestor was not long ago so they are still able to produce offspring that are fertile. These two groups will continue to diverge through geographical isolation and sexual selection until they cannot produce offspring. The tiger is still the same genus as the lion, so they are extremely similar, (Panthera tigris), and the lion is (Panthera leo).
When we do breed these two together, they have to be a male lion and a female tiger because of this genetic divergence. The male lion has a growth promoting gene, but in the female tiger its absent, so they end up being freaking huge. And since again, the genetic differences are accumulating in the populations, the male ligers are completely sterile but the females remain fertile...but how would this "species" survive without any fertile males? It wont. So this is the beginning stages of speciation, producing offspring that aren't fertile.
And your scenario, the population A then splits into 2 populations, but this is only one way that speciation happens. Population A, the founder group, may stay the same and the new population B, begins to change. Population A then can also change into a different species but normally speciation occurs in the group that breaks off, a smaller population, because the genetic changes, or allele changes, are able to be passed on more easily.
The original species is normally found by genetic changes in alleles that can be determined in the lab. The amount of similarity in the DNA will show you the divergence time of 2 different species, but more often than not, the original is long gone, as in our common ancestor for the homo genus and the pan genus (chimp). The chimp isnt any closer to the original than we are, but our ancestor group had a split and that involved a main group 7 million years ago, and our group which was quite small and diversified quickly because of the size and environmental pressures. The chimp population also broke from our original ancestors and diverged right along with us, leaving the original population in the dust. We changed because selection pressures were so strong that our ancestral population was just not fit enough, and started to change, so that's why its no longer around.
Does any of that make sense or did i just rant lol.
Okay, everything you said made sense, but you completely sidestepped the defining of species again.
You spoke as if the hybrid animal would be a new species, but the point I was making is that two separate species formed fertile offspring, which according to your definition, would make them the same species. If you separated all of the ligers, this population wouldn't be fertile, but you have made the error of assuming it is a different species than lions and tigers, which means your definition of species is different than what you have defined on here (though it seems you do have some definition in mind).
The rest was just a rant that also didn't answer any of the questions. I would say it sounds like a professor's response when he/she doesn't know the answer to a question. ;)
Also, you may find that you don't know the answers to these questions. If there were hard answers, then there wouldn't be a controversy about how to define species in the first place.
"A species is a population that can interbreed and have fertile offspring, therefore when populations cannot interbreed with another, whether they produce infertile offspring or cannot produce offspring at all, they become 2 different species."
This is a species, I posted this above. But this is a very straight forward definition. I am fully aware that its not a perfect definition, but we have to start somewhere.
But yes, the definition of species is grey, just like every other category us humans make to understand things. There are several situations where the straight forward definition for species breaks down, but this is absolutely expected. An example of this is the horizontal gene transfer when two different groups exchange genes, like in the theory of the horizontal gene transfer between different prokaryote groups.
I don't see where there is any sidestepping here. Lions and tigers are separate species because they are reproductively isolated--they do not produce reproductively viable offspring (to use a more general term than "fertile"). If you are only producing the occasional fertile female, that's a genetic dead end. There is no reproduction. Admittedly there is the possibility of hybridization if a male lion (or tiger?) mates with a fertile female liger. Situations like this are to be expected, given what we know of speciation. There usually isn't a sudden leap from one species to the next, but a, well, evolution. Lions and tigers have speciated, but less than horses and donkeys, which have speciated less than humans and chimps.
The "rant" was intended to clarify that it doesn't really make sense to talk which of two contemporary related species is the "original" one--they both keep evolving.
The problem isn't with defining species. Reproductive isolation is a good rule of thumb because that is what drives speciation. The problem is with nature--it doesn't organise itself into neat categories ;)
You are assuming the ligers must be isolated and considered as one group, because you are assuming that the offspring are a different species. This is because you are using a different definition of species than merely "able to produce fertile offspring." If you re-introduce the liger into a lion or tiger population and it continues to reproduce, which I assume it can since that is the only way to prove it is fertile in the first place (that I can think of), then it is of the same species as tigers and lions, which were not separate species in the first place.
You might argue that the majority of the time, the offspring produced will not be fertile, in which case frequency of producing fertile offspring is an additional detail in your definition which needs to be specified (no pun intended).
In my example, the evolution of the species was the important part. My point was that the scenario presents a problem, that you have apparently failed to understand, for the definition of species. The example was intended to make this obvious, so try re-reading it. I will now explain the fundamental problem.
At any given point in time in the history of a species' evolution, there is another point in history at which the species would not be able to produce fertile offspring with itself. I have no idea what number it might be (I am very ignorant about this.), and it probably varies a lot, but let's just pretend it's 100,000 years. 100,000 years ago, we can define whatever we were then as not the same species as us. 50,000 years ago, we were still the same species. Great, except that according to the animals of that year, the species was able to produce fertile offspring with itself 100,000 years ago (because there was only a 50,000 year separation), which would mean those two animals were the same species, even though we are only the same species as one of them.
The "fertile offspring" definition has limitations, two of which I have demonstrated.
"Species" is a useful concept, but I wouldn't get too hung up on its objective reality. Evolutionarily, species are lines drawn through continua. Looking for the line when one species became another species is like looking for the line when Latin became French. Reproductive isolation is a good rule of thumb for defining species, but there are others as well. A lot depends on what "work" the researcher wants the species concept to do. Wikipedia actually has a pretty good discussion of different definitions in its entry on species.
Anatomically Modern Humans (AMH) and Neandertals apparently could interbreed, but in other ways they were so behaviourally and physical distinct that I am really not sure it is useful to think of them as the same species. But there have certainly been debates as to whether they are Homo Neandertalensis or Homo Sapiens Neandertalensis.