Natural selection is a process influencing phenotypes within and among populations. This process is the result of differential success in the fitness of some individuals relative to their conspecifics. In general, established populations are well adapted to their specific environment due to the action of natural selection; however, rapid environmental changes can cause shifts in selective pressure and after generations, phenotypic means may shift in response. Shifts in phenotypic means have frequently been observed following colonization of new environments, and are most likely attributed to the new selective pressures the colonizing population experiences.
In Arkansas, diamond-backed watersnakes (Nerodia rhombifer) can be found on most ponds and lakes where they prey primarily on fish. As gape-limited predators, N. rhombifer are constrained in the size of prey they can consume. Many populations have moved from these natural areas to inhabit fish farms where they tend to encounter a more narrow range of prey sizes than natural populations due to the tendency of fish farms to specialize in the species they raise. Using snakes from five different populations (two large-prey sites, two small-prey sites, and one natural site), I examined patterns of variation in cranial morphology (i.e. head length, lower jaw length, and quadrate length).
Since some farms raise large-bodied fish and others raise small-bodied fish, I would expect cranial elements to respond to the new selective pressure imposed by the narrow prey range with a shift towards larger relative cranial elements at sites where large prey are more prevalent. However, this study suggests that any differences among populations in relative size of cranial elements are likely not a consequence of natural selection. While there is no evidence of selection on head size, there is strong evidence suggesting body size is an important target of selection since females at large-prey sites reached greater maximal body size than females from small-prey sites.
When testing hypotheses concerning natural selection, it is important to establish a genetic basis for any traits suspected to be under natural selection because while selection acts on the phenotype it can only stimulate an evolutionary response on traits with a genetic basis. For this reason, I estimated the heritabilities (the proportion of the phenotype explained by the additive genetic variance) for each cranial element at each fish farm and the natural area using neonates from 164 families. I found significant heritabilities for all elements at each site indicating adequate genetic variance in cranial morphology to respond to selective pressures. The pattern of heritabilities among populations for quadrate length, while not significant, suggests the action of past selection for longer quadrates at large-prey sites and variable-prey sites.
|Advisor:||Gifford, Matthew E.|
|Commitee:||Adams, Gianetta, Adams, Reid|
|School:||University of Central Arkansas|
|School Location:||United States -- Arkansas|
|Source:||MAI 56/02M(E), Masters Abstracts International|
|Keywords:||Evolution, Herpetology, Morphology, Natural selection, Snake|
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