|
A Reduction of "Species" Resolves the Species Problem ----- Jody Hey, January
1997
<
PREVIOUS||NEXT>
In a gene tree view of the history of a sample of DNAs, sex is synonymous
with recombination, and can be defined as any process that causes different
portions along the sequence of a set of DNAs to have different gene tree
histories. In the absence of sex, the gene tree history of a sample of DNAs
is the same for all parts of the sequence. With sex, it is possible that
the speed of genetic drift for one portion of a DNA is different from another
portion. If there is a high recombination rate, then a large sample of DNAs
will have a history of many different gene trees, perhaps as many as there
are base pairs in the sequence.
For sexual organisms, a genetic species is the same as a Mendelian population,
as defined by Dobzhansky:
A Mendelian population is a reproductive community of sexual and
cross-fertilizing individuals which share in a common gene pool. . . .The
smallest Mendelian populations are panmictic units (Wright, 1943), which
are groups of individuals any two of which have equal probability of mating
and producing offspring. (Dobzhansky, 1950)
Thus by definition, organisms within a Mendelian population share in a
probabilistic process of reproduction, and all pairs of organisms are equally
subject to reproductive failure and equally likely to reproduce. Within a
Mendelian population, each generation occurs with some distribution of
reproductive success among the component organisms. The shape of this
distribution may vary across generations, but at any point in time the particular
pattern of reproduction is a major determinant of the gene tree for all portions
of the genome. A sample of DNAs for a short region of the genome will have
a particular history, while a different genomic region will have a different
history; yet all of these histories must run through the same historical
procession of organisms, with a different group of reproductives each generation.
Thus a Mendelian population carries genomes with numerous gene trees that
were all shaped by a common birth and death process.
From a genetic perspective, natural selection can be defined as variation
in reproductive success caused by genotypic variation (Lewontin, 1970), and
it is often cast as a directed force of evolutionary change in contrast to
the random force of genetic drift. However at the level of DNA where there
is linkage, natural selection on functional DNA sequence variation contributes
to the genetic drift that occurs among linked sequences. In a genetic species
of asexual organisms, a mutation that changes a DNA sequence and causes natural
selection, also causes a new pattern of genetic drift among organisms that
carry that mutation. In effect, a new genetic species is created by the mutation;
although one of the species will probably be replaced by the other. For the
DNAs of organisms with recombination, the acceleration of genetic drift by
natural selection depends on the degree of linkage, the number of sites of
functional variation, and the strength of natural selection on the functional
variation (Hill and Robertson, 1966; Felsenstein, 1974).
Natural selection on functional genotypic variation may play a major role
in the formation of new genetic species. However, shared genetic drift, and
not natural selection, is the appropriate description of the essence of genetic
species. Genetic species will share in the process of natural selection on
functional DNA sequence variation, and thus will share adaptations. However
this process proceeds both concomitantly with, and as a contributor to, genetic
drift. Furthermore, from a genealogical perspective
(Fig. 2), genetic drift proceeds even
in the absence of DNA sequence variation and in the absence of natural selection
caused by DNA sequence variation.
<
PREVIOUS||NEXT>
© 1997 Jody Hey
|
