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BACKGROUND: Introgression likely plays a significant role in evolution,
but understanding the extent and consequences of this process requires a
clear identification of species boundaries in each focal group. The
delimitation of species, however, is a contentious endeavor. This is true
not only because of the inadequacy of current tools to identify species
lineages, but also because of the inherent ambiguity between natural
populations and species paradigms. The result has been a debate about the
supremacy of various species concepts and criteria. Here, we utilized
multiple separate sources of molecular data, mtDNA, nuclear sequences, and
microsatellites, to delimit species under a polytypic species concept
(PTSC) and estimate the frequency and genomic extent of introgression in a
Neotropical genus of cichlid fishes (Cichla). We compared our inferences
of species boundaries and introgression under this paradigm to those when
species are identified under a diagnostic species concept (DSC). RESULTS:
We find that, based on extensive molecular data and an inclusive species
concept, 8 separate biological entities should be recognized rather than
the 15 described species of Cichla. Under the PTSC, fewer individuals are
expected to exhibit hybrid ancestry than under the DSC (~2% vs. ~12%), but
more of the species exhibit introgression from at least one other species
(75% vs. 60%). Under either species concept, the phylogenetic breadth of
introgression in this group is notable, with both sister species and
species from different major mtDNA clades exhibiting introgression.
CONCLUSIONS: Introgression was observed to be a widespread phenomenon for
delimited species in this group. While several instances of introgressive
hybridization were observed in anthropogenically altered habitats, most
were found in undisturbed natural habitats, suggesting that introgression
is a natural but ephemeral part of the evolution of many tropical species.
Nevertheless, even transient introgression may facilitate an increase in
genetic diversity or transfer of adaptive mutations that have important
consequences in the evolution of tropical biodiversity.
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