A Reduction of "Species" Resolves the Species Problem ----- Jody Hey, January 1997
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THE CAUSES OF MULTIPLE SPECIES
There are two kinds of events that can cause a single genetic species to become two genetic
species. First is physical distance or the emergence of a physical barrier between organisms so that
they do not draw from the same pool of resources. This geographic barrier to drift may be reversed if
organisms are mobile or if geography changes. Second, is the appearance of a mutation that changes
the sequence of a DNA so that an organism and its descendants undergo genetic drift separately from
other organisms not carrying the mutation. This kind of speciation can be reversed only if all of the
descendants of the organism fail to reproduce so that all copies of the new sequence ceased to exist.
Back mutation could not reverse the speciation event unless all descendant copies of the mutant DNA
underwent back mutation.
The effects of geographic barriers and environmental heterogeneity on genetic drift do not
change as a function of sex. If geography constrains the replacement of some individuals by the
descendants of others, then it also constrains the process of recombination between some pairs of
individuals. This constraint on genetic drift will occur for all portions of the genome regardless of
sex.
The kinds of mutations that can create genetic species are different for sexual and asexual
organisms. In the absence of sex, a new advantageous mutation causes an organism and its
descendants to undergo a different pattern of genetic drift from those organisms not carrying the
advantageous mutation. Within a genetic species of sexual organisms, favorable mutations do not
contribute directly to the multiplication of species. Regions of the genome under tight linkage to the
site of the mutation will experience accelerated genetic drift and have a shortened gene tree history
(Kaplan et al., 1989) while the gene trees of unlinked portions of the genome will not be affected. In
short, sex prevents favorable mutations from causing one genetic species to split into two - organisms
that lack the beneficial mutation are not excluded from the birth and death process that occurs among
organisms that carry the mutation. However, there is a class of mutation that, in obligately sexual
organisms, can contribute to the formation of new genetic species. These are mutations that cause
recombination either not to occur between some individuals or cause the results of recombination to
fail to reproduce. Included within this class of events are genomic changes that shift the mode of
reproduction or the ploidy level of the genome. For example, interspecific hybrids of sexual
organisms may be polyploid or parthenogenetic. In either case, the progeny of the hybrids can no
longer exchange genes with the original species and do not share in a common process of genetic drift
with either original species.
The description of the causes of multiple species is intended as a simplification. By posing the
discussion in terms of gene tree histories and genetic drift, mutations and barriers to the movement of
DNAs appear to be the only possible causes of species diversity. Similarly the irreversible aspect of
mutation, as a cause of diversity, contrasts with the reversible effect of barriers, and this distinction
follows directly from the genetic drift perspective. However, these mechanisms are not novel ideas,
but rather a simple version of the causes of speciation that have been discussed in other contexts. In
particular, geographic isolation and the evolution of barriers to sex form the central model of
speciation under the biological species concept (Mayr, 1942; Dobzhansky, 1951). Also, a similar
depiction of the impact of different kinds of mutations, and their shifting effect as a function of sex, is
contained within the cohesion species concept (Templeton, 1989).
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© 1997 Jody Hey
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