rising@utzoo.uucp (Jim Rising) (02/01/88)
Concerning examples of speciation, it is useful to think of speciation
as either (1) taking place "instantaneously" (in a single generation)
or (2) gradually.
Hybridization could create new species in the former way, and there are
many examples, usually involving polyploidy. A 2N ovum is fertilized
by a 1N sperm of the same species (autopolyploidy) or from another
species (allopolyploidy). 3N offspring is reproductively isolated from
either parent--and if it can reproduce parthenogenetically, a new species.
Of course, can be a 1N ovum and 2N sperm, 2N x 2N etc., and lots of other
combinations. Polyploidy has apparently played a major role in the pro-
liferation of plant species, but occurs in animals as well (insects;
annelids; planarians; amphibians; reptiles (3 different families); but
no records in birds or mammals). Hybridization could lead to new
species even if polyploidy were not involved, but is less clearly
documented (at least in animals?).
As for gradual speciation--the gradual accumulation of differences in
populations over many generations, culminating in the development of
reproductive isolation--I do not think that there are any examples
where biologists I followed the process completely through. The
evidence for this is inferential, based primarily on the geographic
distribution of species. That's not to say that it is unimportant.
I suspect that it is very important, but not something that can easily
be shown in the lab. Any comments?
--Jim Rising
--
Name: Jim Rising
Mail: Dept. Zoology, Univ. Toronto
Toronto, Ontario, Canada M5S 1A1
UUCP: {allegra,ihnp4,linus,decvax}!utzoo!rising