Biology Reference
In-Depth Information
any population, especially if molecular techniques are used. This would tend to
greatly increase the number of species (
O'Hara 1994
).
Other species concepts include the cladistic species, cohesion species, com-
posite species, ecological species, genetic species, genotypic cluster concept,
Hennigian species, phenetic species, taxonomic species, and more (
Hey 2001
).
Hey (2001)
suggests that the problem is that “evolutionary biologists try
…
to find a way to have the taxa be the same as the evolutionary groups.” Thus,
depending on which species definition and assumptions are used, species are not
necessary equivalent.
O'Hara (1994)
noted many species are easy to delimit and,
“in those cases where they are not, the difficulty that arises illustrates well
the special historical character of the evolutionary process
…
. Because
evolutionary history is something we are still in the midst of, it will not always
be possible for us to determine which varieties--which distinctive populations
in nature--are temporary and which are permanent, and so our counts of
species across space and through time will always have some measure of
ambiguity in them that we cannot escape.”
As a result, controversy will continue with regard to which genes or other
traits are most suitable for analysis, which model of evolution is more appropri-
ate, and which analysis method and software is best. Whether a phylogeny is
based on molecular or phenetic data, the decision ultimately is affected by the
scientist's biases.
12.10.2 How Many Genes are Involved in Speciation?
Nosil and Schluter (2011)
defined a “speciation gene” as any gene contributing
to the evolution of reproductive isolation, which means that the gene affects
a component of reproductive isolation, that divergence at the locus occurred
before the speciation event occurred, and that the amount of reproductive iso-
lation caused by its divergence can be quantified. Other definitions include any
gene that reduces hybrid fitness, and genes that result in hybrid inviability, ste-
rility, or behavioral aberration.
The genetic basis of speciation is assumed to be due to changes in more than
one gene (
Harrison 1991, Coyne 1992, Hollocher 1998
), with estimates ranging
from 18 to 191 genes in the case of different species of
Drosophila
(
Nosil and
Schluter 2011
). Changes in more than two genes have been considered the mini-
mum required for reproductive isolation. Changes in segments of the genome,
such as inversions or translocations, can result in reproductive isolation as well.
A variety of characters can contribute to speciation, including hybrid sterility,
hybrid inviability, interspecific mate discrimination, and interspecific divergence
in secondary sexual characters (
Hollocher 1998
).
