sarima@gryphon.CTS.COM (Stan Friesen) (05/15/88)
In article <819@ucsd.EDU> akkana@brain.UCSD.EDU (Akkana) writes: >In article <564@pedsga.UUCP> lae@pedsga.UUCP (Leslie Ann Ellis) writes: >>I seem to recall from my Physical Anthropology course that speciation >>is defined as occuring when the parts of a population that become >>physically isolated from each other either: >>1) Can no longer breed successfully, >>or >>2) Produce sterile offspring. > >I've heard that, but it doesn't seem consistent with current taxonomy. >What about dogs (Canis familiaris) and wolves (C. lupus) and >coyotes (C. latrans or something like that)? ... >Are dogs and wolves now considered to be the same species? No, they are not. The actual definition of species as it is now used is quite subtle. It is based on failure to interbreed when the opportunity arises *under *natural* conditions*. The dog-coyote and dog-wolf hybrids are due to the disturbance caused by human habitation and the ecological chaos it produces. There are quite a number of other examples where reporductive isolation breaks down between two otherwise well seperated species in areas of significant man-made disturbance. It seems that cleared and altered landscapes so change the interactions of organisms that normal isolating mechanisms fail to work. (Actually a case could be made for including dogs in Canis lupus, since they are apparently derived from a dwarf variety Asian Wolf) -- Sarima Cardolandion sarima@gryphon.CTS.COM aka Stanley Friesen rutgers!marque!gryphon!sarima Sherman Oaks, CA
winalski@psw.enet.dec.com (Paul S. Winalski) (06/15/91)
In article <7760@mace.cc.purdue.edu>, larsenp@mace.cc.purdue.edu (Paul Larsen) writes: |> |>In article <1991Jun14.195209.12987@cs.cmu.edu> vac@cs.cmu.edu (Vincent Cate) writes: |>> |>>No. If A and B can not cross, then they are not of the same species. |>>However, if they can cross it does not mean that they are the same |>>species. Look at dogs and wolves, or donkeys and horses. So just |>>having two different species (names and all!!) does not mean |>>that they can not cross. We name different species because the |>>gene pools have been separated for some time (i.e. they have not |>>been crossing (much) and have significant differences). |>> |>Your understanding of speciation is faulty. A species is defined as a |>group of organisms which are able to VIABLY reproduce amongst themselves. |>Therefore, we have different species because the gene pools have diverged |>to the point where interspecific crosses do NOT produce viable offspring. If 2 |>separate types of organisms can produce viable offspring then I think that |>they are classified as subspecies It isn't that simple. Mallard and pintail ducks, if they mate with each other, produce perfectly viable and fertile hybrids. This is a common occurrance in captivity, requiring no intervention by man (except setting up the captivity in the first place, of course). However, it is a very rare occurrance in the wild. What separates the two duck species (and there is no argument among taxonomists; all agree they are distinct species) in this case is behavior: the two species of ducks have different mating seasons that do not overlap. This sort of situation is termed "sibling species". So there is more to it than viability and fertility. Two breeding populations are considered to be of separate species when they would retain distinct gene pools even if they shared the same geographic range. Even this definition gets bent a little when you consider situations like that of the clouded sulphur and alfalfa butterflies (Colias philodice and Colias eurytheme) where their ranges overlap in eastern North America. --PSW
ronald@uhunix1.uhcc.Hawaii.Edu (Ronald A. Amundson) (06/16/91)
In article <23529@shlump.lkg.dec.com> winalski@psw.enet.dec.com (Paul S. Winalski) writes: > >It isn't that simple. Mallard and pintail ducks, if they mate with each >other, produce perfectly viable and fertile hybrids. This is a common >occurrance in captivity, requiring no intervention by man (except setting >up the captivity in the first place, of course). However, it is a very >rare occurrance in the wild. What separates the two duck species (and there >is no argument among taxonomists; all agree they are distinct species) in this >case is behavior: the two species of ducks have different mating seasons >that do not overlap. This sort of situation is termed "sibling species". > > >--PSW Genuine phenomenon, wrong term. Sibling species are pairs which are morphologically identical (at least they look to us to be identical), but reproductively isolated. Mallards and pintails are easily distinguishable on morphological grounds, so they're not sibling species. (See Mayr's "Populations, Species, and Evolution" p. 22 ff.) Difference of breeding seasons is one of many isolating mechanisms which separate hybridable populations (including species). Mating behavior is another ... and odd mating behaviors become more acceptable when there is no "normally" behaving member of the opposite sex around -- as in captivity. This is Mother Nature's version of the principle that everyone gets better-looking when the bar is about to close. Also, from a naturalist's (e.g. Mayr's) viewpoint, one cannot conclude that a hybrid is "perfectly viable" from observing it in captivity. (Mayr p. 73) It may well be at a selective disadvantage in its natural environment, even if there are sufficient hybrids around for it to cross with. There are several cases of stable "hybrid zones" where hybridization persists only at the boundaries of two species, and doesn't penetrate either of the species's ranges. The general point is right, though. Species are messy. Ron Amundson