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Re: A Clutter of Duckbills

> Just what keeps two species in the same genus?  And just what keeps them
> separate?

    The way I was taught it was, that two populations of creatures are
regarded as belonging to the same species if genes from one population
make it into the other naturally.  For the kinds of critters we talk
about, that means that one from population A can mate with one from
population B, and have offspring, and the offspring can mate into at
least one of the original populations, thereby completing the
introduction of the new genes.  For example, collies and golden
retrievers can mate and have puppies, and the pups can be mated with
either collies or goldens and have more puppies.  Thus collies and
goldens are the same species.  Alternatively, lions and tigers can mate,
and can have cubs, but the cubs cannot produce offspring upon mating
with anything.  Thus lions and tigers are different species.  And
finally, lions and golden retrievers cannot produce offspring, even
if they should happen to mate.  Thus lions and dogs are different
species.  And note in passing, that reproductive success is not a
yes/no thing; it is not hard to find in nature instance of two
populations which are "almost" separate species, and other instances
which are "just barely" separate species, with reasonable usage of
both words.
    This definition is not as useful as it might be when the
populations in question are sufficiently separated in space or time
that no mating is possible.  In relatively recent geological history,
the North American icecaps split many single species into east coast
and west coast populations for a long time.  Now it is only as these
populations slowly spread across the great plains, in some cases
helped by human alteration of habitat, that we find out whether they
are able to mate and produce fertile offspring, or not.
    When populations are separated in time, the problem is worse;
thus, are we the same species as the hominid population sometimes
called _Homo_erectus_, that existed at a particular time in the past
(say, one million years ago).  Without a time machine, we may never
know.  A relevant related question, if we know the phylogeny of the
population in question, is whether the population ever does split
into two distinct species or not.  When "not", the word "lineage" is
sometimes used to describe the population as it evolves over time.
Thus, suppose you track the descendents of all the _Homo_erectus_
that lived a million years ago, and keep asking the question, "are
they all still one species right now?"  For as long as the answer
to that question is "yes", you have a single "lineage", even though
you have no time machine and so cannot investigate fertility.  (And
incidentally, present evidence suggests that _Homo_sapiens_ and
million-year-old _Homo_erectus_ are the same lineage.)

    Although there is a crudely objective way to decide whether or not
populations of animals are in the same species, decisions about
genera, families, orders, classes and so on up the taxonomic tree are
basically the result of cat fights among biologists <*cough*>
<*cough*>, I mean, the result of thoughtful discussion leading to
rational subjective judgements.  Thus for example, among cats, genus
_Felis_ encompasses species as diverse as house cats and mountain lions,
whereas among hominids, different genera are used even for such closely
related critters as _Australopithecus_africanus_ and _Homo_habilis_.
If you don't like the way things are, all you have to do is intimidate
your professional colleagues sufficiently, and things get changed.

                                             --  Jay Freeman