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Re: definition of a species
From: Scott <HORTON@BCRSSU.AGR.CA>
> What is the definition of a species? Is it an organism that cannot produce
> fertile offspring with another species?
This is not exactly an easy question to answer, as the issue is
However, I would say the definition you mention is too
tight. I would suggest something more like "a species is
a group of interbreeding populations that does not *effectively*
interbreed with other such groups".
The idea here is that any *intrinsic* factor which prevents
the formation of hybrids (including innate behavioral factors),
or which makes the hybrids less viable reproductively, can
become a segregation factor.
Thus any of the following would mark seperate species:
- the different morphs do not normally mate in the wild
- the hybrids tend to fail at reproduction in the wild
- the different forms breed at different times
and so on.
> How can you tell if that is the case by
> looking at fossils?
This is difficult, and can only really be done in exceptional cases.
However, applying known principles of population genetics makes
it possible to at least estimate seperation.
For instance, unless a trait is governed by a few distinct
alleles, or is gender-related, the distribution of morphs *within*
a breeding tend to show a unimodal distribution. Therefore, a
bimodal character distribution is indicative of some biological
factor, and is probably either a sex difference or a species
Also, certain taphonomic situations typically indicate a mono-
specific bone bed. On these cases one has fairly solid evidence
that the range of morphs found constitutes one species.
[Examples include the Maiasaura herd bed, the Chasmosaurus
mariscalensis bed in Texas, and the "First Family" series
of Australopithecus fossils].
> What about eg. lion/tiger crosses (ligers and tigons)? Are
> they sterile and therefore not new species?
- they do not seem to do well in the wild
- lions and tigers do not generally interbreed in the wild
- it is likely that they have a hard time finding mates
in the wild, since neither parent species will really
Also, they do not form a population of their own, rather they
tend to mate with either lions or tigers when they actually
manage to mate. Thus ligers and tigons are simply interspecific
*If* a sufficient group of them should become established in
some habitat where they could survive, *and* they had a strong
tendency to mate with one another as opposed to with the two
parent species, *and* the two parent species tended to avoid
mating with them, then they would have become a new species
that originated by hybridization.
Such things apparently do happen, even in animals. A recent
article in one of the science mags I read suggested that some
of the Galapagos Finch species (aka "Darwin's Finches") actually
originated this way.
> And what about dogs (Canis domesticus?).
> Aren't they one species?
Just barely. They constitute what is called a "ring species",
that is the end forms (the extrema) cannot interbreed, but
these end forms are conneced by a series of intermediates that
allow gene flow between them over time. This intermediate
state between one and multiple species has been declared to
be officially one species.
>Yet if we found the fossil remains of a Great Dane and
> a Chihuahua, would we have classified them as one species?
Single fossils, probably NOT.
However, please note that such extreme variation between
forms in a population chain do not normally evolve in nature.
So it would really be a good first estimate that they *were*
seperate species. And in fact it is only the existance of
toy poodles, and boxers, and collies and such like that keep
them in a single species. So, in the absence of such linking
forms they *would* be seperate species.
Now, a good large sample of dog fossils would show them to
be the end-points of a continuous chain of intermediate forms,
with no clear morphological dividing lines, and would reveal
the real situation.
> Would this much
> variation be encountered naturally (esp. in dinosaurs), or is this strictly
> result of mankind manipulating a species' breeding. I'm confused (obviously)!
That level of variation is pretty much restricted to cases of
human intervention. Under normal conditions, such variance could
not develop without isolation, and would thus ne part and parcel
of a speciation event.
The peace of God be with you.