How identical *is* species DNA anyway?
Is the genome like a template? If one could put the entire DNA strands of a
species on an overhead projector acetate, could one overlay all the G-C and
A-T base pairs on top of one another would they all line up? Could you say
that any variation within that line-up would be something like 'different
colored eyes, and so same species' or 'different all together and so a
hybrid'? I mean, how can one isolate individual variation within a
species? And how would one know that the aberrant base pairs weren't
individual variation but the result of hybridization?
This _is_ actually my field, for once... And yes, the genome is a
template, if you aligned all of the DNA for all of the individuals in a
species, it would line up pretty darn well. The last estimate I heard
for the human genome was one SNP (single nucleotide polymorphism) for
every 1000 bases or so. Not every SNP (in fact very few) actually does
anything significant to a gene. We don't know enough now to read the
genome quite like a book. The main problem is that there are very few
single locus traits, most are multiple loci working in various degrees
of collaboration. It isn't easy to read a book when the sentences
bounce around between chapters and the grammar isn't entirely worked out.
How do you know the difference between individual variation and
hybridization? In a grossly oversimplified view: Most of the genes
for two closely related species will be very close in sequence. This is
as it should be, if they are derived from a common ancestor. But, if
you look at the right genes, there should be a consistent difference in
the sequence between two species. If you know that difference, then it
is easy to look and see if an organism has two copies of organism A's
gene, two copies of organism B's gene, or one of each. You need to
look at many different genes, usually. Actually, a variation of this is
exactly how a paternity test works (children are hybrids too).
Note that is only the closely related things that we have a problem
with... We can all tell that Paph. rothschildianum and Paph. armeniacum
are distinct species. We can get a pretty good idea that Paph.
Dollgoldii is a hybrid. We don't need DNA evidence for that...
Who is to say what species DNA is? Lord knows there's enough bickering and
infighting amongst taxonomists who classify species according to set
international rules. Who would be the agency that would determine and stamp
approve what 'species DNA' looks like? And how would they know? What if
they missed a few individuals? Like if a Greater Alien Space Race came to
the Earth they'd think by preponderance of numbers that the human species
was Asian, and take that DNA as criteria of human-ness. The tiny Bushman
from the Kalihari Desert would be shit out of luck because they don't look
Asian. What then? Would The Aliens be justified in eating Bushmen because,
according to their definition of human-ness, they weren't human?
I actually have a talk about this worked up for orchids... The
answer is that there is no magic answer when you use DNA, any more than
there is when you use phenotypic markers for taxonomy. But, you can get
good estimates of how far apart things are. The boundaries are always a
mess, and always will be. DNA is just another tool. A powerful tool.
Somebody still needs to decide just how far apart is far enough to
qualify as a separate species.
What makes something a distinct species is genetic isolation and
time. Given enough of each, you don't fancy science to tell them
apart. It is those fuzzy edges that kick you in the head. When Al's
aliens come to pick him up, they will realize that his DNA is 99.9999%+
similar to both the Kalihari bushman and the Eskimo. It is
extraordinarily similar to that of a chimpanzee. It is remarkably
similar (in both sequence and the organization of the genes on the
chromosomes) to the DNA of a mouse. It is somewhat (disturbingly)
similar to the sequence and organization of the genes on the chromosome
of the zebra fish. The farther apart we get, the more difference there
is, but there is still remarkable conservation.
Besides, we all know that the aliens (or monsters) eat the cute,
mysteriously underdressed girls, who get separated from their group of
friends by strange plot twists. I learned that from the movies.
Rob
--
Rob's Rules:
http://www.msu.edu/~halgren
1) There is always room for one more orchid
2) There is always room for two more orchids
2a. See rule 1
3) When one has insufficient credit to purchase
more orchids, obtain more credit