lyOk, I've been reading this book by Richard Dawkins called The Ancestors Tale,
and its one of the most thought provoking books I've ever read. Great
book, even for non-biologists, I'd recommend everyone read a few
chapters. He traces humanity back through generations and talks about
species that join up as he takes us back through our lineage. Anyways,
One really cool idea I just came across: Gulls in the UK. There are two
separate gull species that nest there, one with black wings, the other
mostly white. They dont interbreed, and scientists have given them two
different species names. One species proliferates only westward, the other
only eastward. However, if you start sampling the gulls in either direction,
the further you get away from the islands, the more grey they become...
until around halfway across the globe, the two separate species both
look very very similar, and in fact interbreed. Its like a spectrum,
where each individual is able to interbreed in say, a westward
direction... and it wraps around so far that by the time you reach the
end of the spectrum, the individuals are separate species. Freaking
crazy. Two different species morph into the same one as you travel around the globe. Each bird can mate sucessfully with other gulls in its breeding ring... but the genetic differences increase over the distance as you wrap around the globe, so much in fact that the one white species turns dark, and upon meeting its white colored cousins, are so different, they can no longer breed successfully, and thus are DIFFERENT SPECIES (by definition). CRAZY.
Still confused? Maybe another example will illustrate this idea better: Say I go and pick up a
pigeon. I take it in my time machine and go back 1000 years, where I
find one of its great great grandparents with my pigeon-locater and
snag him. Now, the two pigeons would be close enough genetically to
breed and produce viable offspring. I go back another 1000 years and
pick up another great grandparent of the original pigeon. Though this
one cannot produce a viable offspring with that original pigeon, it can
breed with the ancestor from 1000 years earlier. If I keep going back,
which will take a while, probably more time than I have in my lifespan,
I could set up a chain of ancestors. Each ancestor could breed and make
viable offspring with its 1000 year old ancestor and its 1000 year old
descendant, but no other. Even 5 forms away, the lineage would start to
look very very different. Eventually, I'd be picking up reptilian
ancestors, which are clearly not the same species, even family of life
form. But they're still 'linked' together through their ability to
interbreed with those around them. There is no clear distinction that would make these 'different' species unless you start comparing them 2000 years apart. Different species melt as you get closer and closer in.
This can be applied to a broader range however. We really dont have
separate species... ever. Its just that the 'links' between species
have died out. For example, if some of the ancestral apes that walked Africa still existed, its reasonable to believe that members of our
species could breed with them, and those could breed with others...
until there was a chain of interbreeding that directly linked chimps to
us. Humans and chimps could never interbreed, but through all the
intermediate ancestors that have died out (via competitive exclusion, or just from evolutionary die offs), we would cease to be separate species....
This is back to the pigeon idea as well. In the fossil record, we never
really have distinct species. All we see are snapshots of an ever
changing morphology. In fact, its good that we have gaps in our fossil
record... if we didnt, and had the bones of every single species that
ever lived, we would never see distinct species, just transitional
forms. A cool analogy would be to think of evolution like a tea kettle.
Theres never a point in the water where it goes from 'cold' to 'hot',
yet it still boils. People need to stop thinking of evolution or
speciation on the whole as specific and distinct.
Check this out for a good illustrated example. This is the classic species idea, right? Though all these transitional forms between the 'lines' of
speciation have died out, you could take the previously mentioned
seagull example and apply it here. If the forms never died out via
competitive exclusion (lets say a half chimp half human didnt have any
selective advantage or disadvantage), then the chart would get filled
in... sorta greyed in. Instead of direct lines you would just have a
sea or dots, each an individual ancestor. Pick one out between chimps
and humans and it would look like a hybrid. It would be able to breed
with the dots immediately around it in both the x and y directions, but
no other. These 'seas' exist in some populations (like the gulls as
mentioned, where the same interbreeding specie can get so spread out
that its ends no longer can breed), while in others, like in humans and
chimps, the intermediary forms do not exist because way back when, they
died out, probably for an assortment of reasons.
Anyways, cool rant. Richard Dawkins is a great author, really thought provoking stuff. Check it out!