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?
Please forgive me for depending on Wikipedia, but I don't have much time to devote to this. A quick google on ligers brought me to this finding on the Wiki article:
The fertility of hybrid big cat females is well documented across a number of different hybrids. This is in accordance with Haldane's rule: in hybrids of animals whose sex is determined by sex chromosomes, if one sex is absent, rare or sterile, it is the heterogametic sex (the one with two different sex chromosomes e.g. X and Y).
According to Wild Cats of the World (1975) by C. A. W. Guggisberg, ligers and tiglons were long thought to be sterile: In 1943, a fifteen-year-old hybrid between a lion and an 'Island' tiger was successfully mated with a lion at the Munich Hellabrunn Zoo. The female cub, though of delicate health, was raised to adulthood."
So, if this is the case, then likely the DNA was passed through matrilineally (is that a word?)...
/hardly an expert on biology. I just like looking stuff up.
Unless someone can point me in a better direction, I also found a post on Wikipedia explaining the "species problem" that basically explains that since we are all transitional forms any division between species is going to be somewhat arbitrary. Also since Neanderthals seem to have headed up to Europe sooner than H.Sapiens, it looks like we might be a good candidate for the ring species defintion.
Defining a species can be difficult. Wolves, coyotes, and dogs are considered to be separate species, yet they can interbreed successfully AND produce fertile offspring, as anyone who owns wolf-dog hybrids well knows. There are also many coyote-dog hybrids, and I have heard of a few wolf-coyote crosses, as well.
Horses, donkeys, and zebras are also separate species, and can interbreed quite successfully, yet they cannot generally produce fertile offspring. The same is true of most big cat hybrids.
When it comes down to it, we classify these animals as separate because they look different, have different ranges, eat different types of food, and so on. They might be better called sub-species, I guess, since they can still mate with each other. I think that these various taxa--the canines, equines, and felines--are living examples of one type of animal dividing into many different types due to the variety of biological and environmental pressures that they must deal with. As a population separates, the individuals gradually lose the ability to produce offspring amongst one another, first going through a phase where they produce offspring that are sterile, and ending with completely distinct populations that cannot interbreed at all.
All this talk about species and interbreeding caused this thought to come to me:
What if scientists impregnated a female chimp with human sperm?
What a topic! I think that would cause a huge sensation in the entire world.
First problem: If scientists defined species as physical characteristics, then there would never be two organisms that appear the same but are different species (or two organisms that appear different but are the same species). A different definition of species would have to be used for this to happen.
Second problem: The rest of your post assumes a definition of species that you never defined, unless you want to clarify what it means to "belong within the same gene pool."
Third problem: You say, with evolution, a species "eventually" becomes two different species, but this ambiguity is the current problem with the definition. Where do you draw the line? And by defining where the line is, you define what a species is.
Reproduction is the key to the understanding of how new species form, and new species arrive through barriers such as geographic, morphological, behavioral, as well as other types of isolation. Homo neanderthalensis and Homo sapien as we see them today seem to have had most of these isolations as our last common ancestor lived 700,000ya, and our divergence from them is set at about 300,000ya. At the time period around our last common ancestor, most likely the Homo heidelbergensis line split, leading to the northern Homo neanderthalensis, who then became reproductively isolated from the ancestral population of Homo heidelbergensis in Africa.
As far as we know for the next 320,000 years the main populations of these 2 lines did not continue to interbreed as neanderthalensis began to come into its distinctive features and adaptations for colder climates in Europe and the sapien population was developing in Africa in the warmer drier climate. It did not take long for this heidelbergensis population in Europe to develop into neanderthalensis and it is no doubt because of the extreme environment that they lived in. The population changed so quickly anatomically that before we had DNA, their physical differences alone told us the 2 lines were different. If you put great selective pressure upon a small population, (which no doubt this group was), mutations that allow for a better chance of reproducing and surviving are going to be selected and passed on, and quickly. Punctuated equilibrium at its finest.
Now, genetic analysis reveals that the 2 lines did interbreed and they were able to produce fertile offspring that eventually prospered. This tells us that they cannot be 2 completely separate species, but should be labeled as two sibling subspecies of the original African populations. It has been proposed before that instead of them being Homo sapien and Homo neanderthalensis it should be Homo sapien and Homo sapien neanderthalensis. I think we should stick with that.
The gene flow between the neanderthalensis population and modern humans shows that the interbreeding must have occurred before the spread of sapien into Europe, Asia, and Papua. In summary, 400,000ya a population migrated north through the Middle East into Europe and parts of Asia and became subjected to harsh natural selection. Between 80,000 -50,000ya another population from the original population in Africa followed that same path into the Middle East and the 2 populations met. I believe that the neanderthalensis line did not go “extinct” but instead some groups died out at 30,000ya but other groups interbred with the sapiens and their children survive today.