A few further issues relating to DNA work:

Just as there are some species easier to tell apart morphologically and
some species that are more variable than others morphologically, there
are species with more DNA differences or with more DNA variability.
Thus, based on data I'm working on right now, I would say that about
1-2 percent difference in the COI gene portion that I sequenced is
suggestive of two closely related species in the genus Pleurobema, but
I would not say that one species couldn't vary that much nor that two
species couldn't have a lower level of difference.

An important thing to do is to try to sample well.  There may be a fair
amount of difference in DNA between two widely separated individuals,
but do the populations in between show a continuous gradient or is
there an abrupt barrier?

Different life history characteristics will promote greater or lesser
genetic variation within a species.  E.g., something with extensive
dispersal capabilities (mostly larvae in mollusks) will tend to be
homogenized genetically, whereas organisms that have very little
dispersal can easily develop local genetic differentiation.

Hybridization can throw DNA studies off track.

There may be multiple versions of a gene (or similar DNA sequences) in
a single animal.  Sequencing one version in one individual and another
version in another individual will confuse you.

Mistakes are probably easier in DNA work than in morphological studies.
 Sometimes I just have a small piece of tissue that someone sent me.
If those get mixed up, I won't know anything is wrong until I get
strange results.  It doesn't help that some people working on DNA know
very little about actual organisms.

When two species first diverge from a common ancestor, their DNA will
be essentially identical.  It takes a while (depending on population
size, lifespan, breeding patterns, etc.) for differences to accumulate
and to spread through the population.  Thus, relatively newly separated
species may be very difficult to distinguish by DNA, even though some
morphological features may seem obvious.

Defining species is also a problem.  How different is enough?  What
typs of differences?  Any one test probably has some known exceptions.

----------------------------------------
Dr. David Campbell
425 Scientific Collections
University of Alabama, Box 870345
Tuscaloosa AL 35487
"James gave the huffle of a snail in
danger But no one heard him at all"  A.
A. Milne

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