Biology Reference
In-Depth Information
tions showed no differences in the shape of the skull, however, suggesting
that the differences were largely, if not entirely, due to better nutrition in
the alfalfa field habitat.
The alfalfa field and native woodland populations also differed strik-
ingly in life history features (Patton and Brylski 1987). Alfalfa field popu-
lations were more than 16-fold denser than those in native woodland.The
sex ratio was more heavily biased toward females in the alfalfa field habi-
tat than in native woodland. Alfalfa field females became sexually mature
and bred in their birth season, whereas native woodland females did not
breed until their second year. Litter size was larger and the breeding sea-
son longer in the alfalfa field habitat because of irrigation and greatly
increased food availability. The result of early breeding by alfalfa habitat
females was that their growth in size slowed when they reached sexual
maturity and allocated metabolic resources to reproduction. Males in the
alfalfa field population did not breed in their birth season. They contin-
ued to grow, leading to a larger sexual dimorphism in body size than was
the case in the native woodland animals.These habitat-related ecological
differences could result in patterns of mate competition, so the original
body-size differences could become stabilized by sexual selection (Patton
and Smith 1990). Such a difference would be especially likely if gene flow
between the two populations would be interrupted. Whether or not
genetic differentiation of these two types of pocket gopher populations
has occurred is still uncertain.
Other Evolutionary Considerations
Many native herbivores have thus begun to feed on alien plants that have
appeared in their environment.The use of alien plants is frequent for her-
bivores that have a wide native host range, but many specialist herbivores
have also shifted to alien species that are closely related to their native
hosts. As we saw earlier, oviposition by arthropods on alien plants that
possess secondary chemicals similar to those of their native hosts also leads
to their use by native species.The results of these shifts are quite variable.
Some shifts lead to evolutionary specialization for the new hosts. In other
cases, however, larval development may not always be possible on the new
host (see, e.g., Chew 1977), and the alien species may thus become a
threat to populations of the native herbivore by attracting individuals to
plants on which reproduction fails (see chapter 16).
The successful utilization of alien plant species, however, may lead to
the formation of new genetic races or even species (Futuyma and Peter-
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