Biology Reference
In-Depth Information
Response to Thyroid Hormone
Low
High
Large
Late
Early
Small
Permanent
Ephemeral
Habitat stability
Figure 8.5 Model showing relationships among salamander life history variation, phys-
iology, and habitat stability. Paedomorphosis is associated with low thyroid hormone
responsiveness, permanent aquatic habitats, delayed metamorphic timing, and large
body size. Metamorphosis is associated with contrary values for these variables.
low TH response alleles in biphasic populations when habitats allow for ex-
tensions of the larval period, as these alleles will delay metamorphic timing
and increase adult body size. Under directional selection for mutations that
increase the length of the larval period, central mechanisms of TH-release
become progressively desensitized to intrinsic and extrinsic signals that
provide reliable cues for initiating metamorphosis within the context of a
biphasic life history. We hypothesize that central mechanisms are
desensitized because TH-response loci become fixed for alleles that are less
responsive to positive TH feedback. As genetic background changes, the
likelihood of paedomorph expression increases. In support of this model,
studies of facultative paedomorphic populations (
A. talpoideum
) have dem-
onstrated the efficacy of selecting for higher paedomorph expression in rel-
atively few generations (
Semlitsch & Wilbur, 1989
). If directional selection
occurs over many generations and within the context of large and stable
aquatic habitats, as has happened independently in the case of Tiger salaman-
ders species in Mexico (
Shaffer, 1984; Shaffer & McKnight, 1996; Shaffer &
Voss, 1996
), further changes in genetic background are predicted to yield
individuals that are no longer capable of initiating metamorphosis in nature.
Many studies have shown that environmental factors, and especially low
temperature, are often associated with paedomorphic populations (
Eagleson &
McKeown, 1980; Snyder, 1956; Sprules, 1974
). It is possible that some examples
