Mutzy, on Sep 3 2009, 11:31 PM, said:
Lets simplify it, when does the bacteria become an animal with scales, fins, and gills?
At the risk of sounding teleological (which isn't the case; evolution isn't directed to a certain endpoint, it's more a reaction to the various selection pressures), the population of what were once bacteria become fish when their eventual descendants have evolved the various things that fit our criteria for defining what fish are: i.e. and not necessarily in this order; they have gained an aerobic metabolism (if they didn't have one already), they have gained the endosymbionts (mitochondria and possibly some of the other organelles) that make them eukaryotes, they have collected in groups for mutual survival and become metazoans, they have differentiated so that cells perform particular jobs throughout their bodies, they've started reproducing sexually (possibly so that they can more rapidly reshuffle their genetic material each generation to fight off parasites or maybe to increase genetic diversity allowing their descendants to take advantage of new environments more quickly), they,ve gained a one way gut and nervous system, they've gained an internal skeleton (of bone and/or cartilage) and circulatory system, developed limbs, fins, scales, gills etc... Et voila, fish.
It's believed the progression (which is not quite the best thing to call it, but what can you do?) from simple anaerobic bacterium to swimming fishy metazoan took the best part of 3 billion years. So it's not as if it's a speedy process. And given the presumably short generational times of the organisms involved, there's a lot of mutation and selection (that is, sex [more accurately, reproduction] and death) that's gone on.
While all this is going on lineages are splitting off that eventually end up as protists, plants, fungi, worms, arthropods etc. and loads of others that simply died out. Statistically speaking, after all, 99.9% (and probably a few more 9s after that) of all the species that have ever lived are extinct. Evolution has been compared to the Red Queen from
Alice In Wonderland; that is "You have to run as fast as you can to stay where you are." A good(ish) analogy being that if you came second in a race and the next time the race is run everyone in it is twice as fast as they were, you're still going to come second. But if you haven't improved and your opponents have you'll end up last.
This post has been edited by stone monkey: 04 September 2009 - 01:15 AM
If an opinion contrary to your own makes you angry, that is a sign that you are subconsciously aware of having no good reason for thinking as you do. If some one maintains that two and two are five, or that Iceland is on the equator, you feel pity rather than anger, unless you know so little of arithmetic or geography that his opinion shakes your own contrary conviction. … So whenever you find yourself getting angry about a difference of opinion, be on your guard; you will probably find, on examination, that your belief is going beyond what the evidence warrants. Bertrand Russell