Asymmetric reproductive isolation among polymorphic salamanders
HOWARD
H.WHITEMAN1,2* and RAYMOND D. SEMLITSCH2,3
Author Affiliations
1Department of Biological Sciences, Murray State University,
Murray, KY 42071 USA
2Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802
USA
3Division of Biological Sciences, University of Missouri, Columbia,
MO 65211 USA
*Author for correspondence (E-mail: howard.whiteman@murraystate.edu )
Abstract
The study of reproductive isolation (RI) as a prerequisite to sympatric
speciation has been limited by a focus on species that have already experienced
isolation. However, a complete understanding of speciation depends on
observing taxa before they complete the speciation process. We estimated
RI in field populations of the polyphenic mole salamander, Ambystoma
talpoideum, by capturing paedomorphic (aquatic) and metamorphic (terrestrial)
adults during the breeding season from two natural populations. We found
evidence for asymmetric RI between morphs, such that paedomorphic males
and metamorphic females had functionally zero RI, whereas metamorphic
males and paedomorphic females had substantial RI. Evidence suggests that
ecological factors such as the abundance of each morph, timing of rainfall,
and water depth of the breeding habitat play a large role in the production
of these asymmetries. Spatial aspects of RI had a greater relative impact
on overall isolation than temporal differences, in part because metamorphic
adults were often captured in shallower water than paedomorphic adults.
However, morph separation varied across populations and year, suggesting
that environmental heterogeneity likely plays a large role in the potential
for RI, particularly between metamorphic males and paedomorphic females.
In addition, body-size variation and behavioural differences could also
influence the RI estimates presented here. Although facultative paedomorphosis
appears to have played a large role in macroevolutionary change via allopatric
speciation in some taxa, our results suggest that there is little potential
for sympatric speciation in the future within these populations. However,
asymmetric RI creates the opportunity for fitness differences between
morphs and sexes that would directly affect the maintenance of this polymorphism.
Our results suggest that further studies on this and other polyphenisms
may provide valuable insight into the evolution of RI and the role of
environmental heterogeneity in the production and maintenance of biological
diversity.
Keywords
body size – breeding phenology – environmental heterogeneity
– facultative
paedomorphosis – fitness – polyphenism – reproductive
behaviour – sympatric speciation.
SREL Reprint #2897
Whiteman,
H. H. and R. D. Semlitsch 2005. Asymmetric reproductive isolation among
polymorphic salamanders. Biological Journal of the Linnean Society 86:265-281.
