bhayes@glacier.ARPA (Barry Hayes) (05/23/86)
When all is said and done "genus" and "family" distinctions are the creation of human minds. Some draw their taxonomy from morpholory, others from genetics. But the division of organisms into species is fundamental, representing the genetic barrier of producing fertile offspring. But one thing that has always troubled me is the arising of a new species from an old. Given just about any situation I cannot see how a distinct species which cannot interbreed with any previous species can come out of a population. What could do it other than two or more simultaneous identically placed mutations? It doesn't seem as if a species could get that lucky as often as it has happened. So what gives? How does it happen? Has it ever been observed to happen?
michaelm@3comvax.UUCP (05/23/86)
In article <7730@glacier.ARPA> bhayes@glacier.ARPA (Barry Hayes) writes: >But one thing that has always troubled me is the arising of a new >species from an old. Given just about any situation I cannot see >how a distinct species which cannot interbreed with any previous >species can come out of a population. What could do it other than >two or more simultaneous identically placed mutations? It doesn't >seem as if a species could get that lucky as often as it has happened. > >So what gives? How does it happen? Has it ever been observed to >happen? I believe (correct me if I'm wrong, oh ye more knowledgeable types) that *isolation* is nearly always the crucial element. Either a small population is isolated, so genetic drift can produce a distinction from the very beginning, or similar populations are isolated from each other. Either way, groups are isolated in differing environmental circumstances so natural selection can do its work making the two groups more and more distinct. (Groups can even be "isolated" at opposite ends of a species own range. It often occurs that individuals of a "species" are interfertile at every point of the species' range, but individuals taken from opposite extremes of the range are not!) Given isolation, an increasing load of differences eventually results in two groups no longer being interfertile. This can happen for a variety of reasons: genetically, the fertilized egg at some point in its development may just not be viable -- like trying to use Chevy parts to repair a Ford, mechanisms taken from two divergent organisms may not function together. Or, it might simply be a matter of one group being active during the day, the other at night, so they are never simultaneously active to mate. Or, they might live in different habitats, swamp and forest, say, so their paths never cross. Or, the mating "dance" -- the recognition system for finding mates which most species have in one form or another -- may have diverged to the point that members of one group are no longer attractive to the other. For example, how many humans are attracted to chimpanzees of the opposite sex? And, King Kong aside, I daresay that chimpanzees feel similarly about humans. Humans, chimpanzees, and gorillas are good examples of how separate species can evolve. These species apparently diverged from each other quite recently as these things go -- some 4 million years ago. Each shares approximately 99 percent of the others' genetic message. This relationship is close enough that in other groups, such as the big cats, offspring between species can occur. For example, lions and tigers are more distantly related than chimpanzees and humans, and yet they can, and sometimes do, have live offspring (although I believe these are usually sterile "mules"). The horse and the donkey, of course, are more familiar examples. I've read one report which says that chimpanzees and humans no longer have the same number of chromosomes. If true, this would be a powerful factor preventing human-chimpanzee cross-fertility. However, even without a possible genetic barrier between the two species, the number of individuals in either species who *desire to try* such an experiment must be low. Such "attractiveness" barriers between different species really do work! -- Michael McNeil 3Com Corporation "All disclaimers including this one apply" (408) 970-1835 {hplabs|fortune|idi|ihnp4|tolerant|allegra|glacier|olhqma} !oliveb!3comvax!michaelm Not to go on all-fours; that is the Law. Are we not Men? Not to suck up Drink; that is the Law. Are we not Men? Not to eat Flesh or Fish; that is the Law. Are we not men? Not to claw Bark or Trees; that is the Law. Are we not men? Not to chase other Men; that is the Law. Are we not Men? H. G. Wells, 1896, *The Island of Doctor Moreau*
palmer@cit-vax.Caltech.Edu (David Palmer) (05/25/86)
Organization : California Institute of Technology In article <7730@glacier.ARPA> bhayes@glacier.ARPA (Barry Hayes) writes: >... But the division of organisms into species >is fundamental, representing the genetic barrier of producing >fertile offspring. > >But one thing that has always troubled me is the arising of a new >species from an old. Given just about any situation I cannot see >how a distinct species which cannot interbreed with any previous >species can come out of a population. What could do it other than >two or more simultaneous identically placed mutations? It doesn't >seem as if a species could get that lucky as often as it has happened. Speciation is not as fundamental as you might think. Interbreeding is not transitive, if A can breed with B and B can breed with C, then A cannot necessarily breed with C (where "breeding" means the creation of fertile offspring). For example, I very much doubt that a chihuahua (sp) and a great dane are interfertile, but we class them in the same species because a chihuahua can breed with a dacschund (sp) and a dacschund can breed with a spaniel, etc. until you get to the great dane. There are more extreme examples, for instance clines, where you can have a set of animals spread geographically along a line. Two animals that are near each other on the line can interbreed, but the breeding gets harder and harder as the distance between them along the line gets longer and longer, and so the animals at opposite ends of the line are intersterile. There is a good example involving a type of hawk along the Arctic circle. This cline actually has both ends at the same point (the line having passes all the way around the pole) and the two types of hawk cannot interbreed. David Palmer "God is as real as I am," the old man said. My faith was restored, for I knew that Santa would never lie.