David B. Wake, 2006
Problems with Species: Patterns and Processes of Species
Formation in Salamanders.
Ann. Missouri Bot.Gard., Vol.93, pp. 8-23.
How many species one recognizes within a given taxon remains a difficult
question, especially when morphology is relatively stable or when clinal
variation is present, thus complicating diagnosis. I accept the general
lineage concept of species, and my goal is to recognize historically distinct
evolutionary lineages that are likely to remain distinct. Here I analyze
this task with respect to patterns of species formation in two genera of
plethodontid salamanders in California. Ensatina is a ring species
complex surrounding the Central Valley of California. At
present it is a single species with seven subspecies that are linked by
apparent clinal variation in intergrade zones, but there are some narrow
hybrid zones where morphologically and ecologically differentiated forms
interact. In contrast, Batrachoseps, which has much the same distribution,
has about 20 species in California, most occurring in sympatry with Ensatina.
Divergence of the two taxa is based on two fundamentally different phenomena,
and yet there are some common themes. Adaptive divergence in coloration
is the dominant theme in Ensatina, whereas differentiation is largely perceived
at the molecular level in Batrachoseps. Yet both have evolved in
the same region and have been affected by many of the same climatic and
earth historical phenomena. Within the Ensatina complex, different
adaptations related to predatory avoidance have evolved. Coloration has
diverged in different directions in coastal and inland populations, even
though genetic interactions continue to take place. Where coastal populations
meet other coastal populations, ecologically and morphologically similar
populations merge genetically even if well differentiated in molecular traits.
In contrast where the ring is crossed and where ecologically and morphologically
differentiated populations meet, they hybridize narrowly or are sympatric
and behave as if they are species. Within the ring-like distribution, clinal
patterns of variation occur. The current polytypic taxonomy is retained,
even though it is problematic, because alternatives are even less appropriate.
In contrast, where genetically differentiated populations of Batrachoseps
meet they typically do not merge. Instead, they replace one another spatially,
in part because they are so similar ecologically. Apparently the periods
of isolation were sufficiently long that even in the absence of adaptive
divergence there has been divergence of isolating mechanisms. Analysis of
patterns of genetic differentiation in allozymes and mtDNA in relation to
geological history in California is used to generate biogeographic scenarios
to help explain the contrast between Batrachoseps and Ensatina.
distribution of Ensatina.
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