Biology Reference
In-Depth Information
Whereas metamorphic and facultative taxa are capable of dispersal among
aquatic habitats, obligatorily paedomorphic taxa are confined to isolated bod-
ies of water. This suggests that biphasic or facultative ancestors colonized
aquatic habitats and ecological conditions (e.g., permanent aquatic habitat
and/or arid terrestrial conditions) selected for paedomorphic life histories.
The expression of paedomorphic life histories is known to affect population
structure and may affect the probability of speciation (
Shaffer, 1984; Shaffer &
Breeden, 1989
). The fitness benefits of paedomorphosis are an earlier time to
first reproduction, more than one breeding event per year, larger clutch size,
and a higher probability of mating success (
Krenz & Server, 1995; Ryan &
Semlitsch, 1998; Scott, 1993
). It is important to note that paedomorphosis
is not a pathology or default life history strategy—a failure to undergo meta-
morphosis. Paedomorphic species are highly adapted to their aquatic habitats.
For example,
Ambystoma dumerilii
has secondarily evolved webbed-feet and
extra gill filaments that develop at the time metamorphosis occurs in related
metamorphic species (
Fig. 8.3
). Thus, evolution of paedomorphosis may re-
lease tissues from the constraint of TH remodeling, allowing new functions to
evolve that better adapt adults to aquatic habitats. The combination of selec-
tion and genetic drift acting on isolated paedomorphic lineages may explain
rapid divergence of morphology and gene expression within some species
groups (
Page et al., 2010; Shaffer & Voss, 1996
).
Paedomorphic life history strategies are widespread among salamanders,
occurring in nine out of 10 families with a total of 57 species. Studies show
that paedomorphic salamanders have evolved multiple times in different lin-
eages (
Denoel et al., 2005; Shaffer & Voss, 1996; Weins & Hoverman, 2008
).
Four salamander families (Amphiumidae, Sirenidae, Proteidae, and
Cryptobranchidae) are comprised entirely of paedomorphic species, having
no extant biphasic species. Some salamanders, such as species in the families
Cryptobranchidae and Amphiumidae, are said to undergo an incomplete
metamorphosis—meaning they undergo some anatomical changes, but not
others, and remain aquatic throughout their lives. For example, adult Hell-
benders (
Cryptobranchus alleganiensis
) lose their larval external gills, but do
not develop eyelids and retain a single gill-slit that appears as a circular opening
on the neck (
Larson, Weisrock, & Kozak, 2003
). The most species-rich sal-
amander family, Plethodontidae, includes many biphasic and direct develop-
ing species, with the Tribe Hemidactyliini (Subfamily Plethodontinae)
containing numerous paedomorphic species that are cave dwelling and
exhibit reduced pigmentation and vision (
Larson et al., 2003
). Perhaps the
best-studied salamander families with respect to life history variation are
