Adaptive Coloration and Gene Flow as a Constraint to Local Adaptation in the Streamside Salamander, Ambystoma barbouri

Abstract

Predation is an important selective force that influences animal color patterns. Some larval populations of the streamside salamander, Ambystoma barbouri, inhabit streams with fish predators. Other larval salamanders are found in shallow, ephemeral streams that are predator-free. Quantitative melanophore cell counts and estimates of percent body area pigmented indicated that larval coloration is strongly correlated with stream type. Larvae that coexist with fish tend to be lighter than larvae from streams that are Ashless and ephemeral. Two approaches demonstrated that lightly pigmented salamander larvae better match the common background in relatively permanent streams and are less conspicuous to fish than dark larvae. First, using a model based on the spectral sensitivity of the fish and reflectance properties of salamanders and natural stream backgrounds, we showed that light larvae are three times more cryptic than dark larvae on rocks. Second, lighter larvae had higher survival than darker salamanders on rocks in a predator- choice experiment. It is not clear why larvae in ephemeral streams are darker. Larvae in ephemeral streams should be active to feed and develop rapidly and reach sufficient size to metamorphose before seasonal drying. Several hypotheses may explain why larvae tend to be darker in ephemeral streams, such as increased thermoregulatory ability, better screening of ultraviolet radiation (in these shallower streams), or better background matching to terrestrial predators. Among populations where salamander larvae coexist with fish, there are differences in relative crypsis. Larvae from populations with fish and relatively high gene flow from ephemeral populations (where larvae are dark) tend to be darker (with more melanophores) and more conspicuous to predators than those from more genetically isolated populations, where larvae are lighter and more cryptic. These differences illustrate the role of gene flow as a constraint to adaptive evolution.

Pigmentation plasticity enhances crypsis in larval newts: associated metabolic cost and background choice behaviour

“…in response to environmental factors such as temperature and ultraviolet (UV) radiation, water turbidity and substrate colour, with degree of plasticity varying over larval ontogeny75767778. Together with protection against UV-induced damage297980, prevention from visual predators appears to be the most relevant function of such colour plasticity in Ambystoma larvae and the one demonstrated here in the genus Lissotriton 3284041 (but see ref. 81).…”

“…By achieving colour patterns resembling habitat features, both anurans and salamanders avoid detection from visual predators, thereby improving their fitness28293031. In addition to constitutive adaptations, amphibians can increase concealment by expressing plasticity in skin coloration.…”

How Much of the Variation in Adaptive Divergence Can Be Explained by Gene Flow? An Evaluation Using Lake-Stream Stickleback Pairs

“…For example, traits that influence mate choice might be particularly important in ecological speciation (Schluter 2000;McKinnon et al 2004). Reflecting this uncertainty about the direction of causation, negative associations between gene flow and adaptive divergence have been variously interpreted as evidence that the former constrains the latter (Storfer and Sih 1998;Storfer et al 1999;Hendry et al 2001Hendry et al , 2002Calsbeek and Smith 2003) or the latter constrains the former (Gíslason et al 1999;Lu and Bernatchez 1999;Ogden and Thorpe 2002). Determining the direction of causality may be particularly germane for threespine stickleback because they have been advanced as providing evidence that gene flow constrains adaptive divergence (Bell and Richkind 1981;Bell 1982;Bourgeois et al 1994;Hendry et al 2002) and that adaptive divergence constrains gene flow (McPhail 1994;Schluter 2000;Reusch et al 2001;McKinnon and Rundle 2002;McKinnon et al 2004).…”

“…Several studies have adopted a paired-sample correlative approach for testing associations between gene flow and adaptive divergence. Most of these, however, have not used independent pairs (e.g., Smith et al 1997;Storfer and Sih 1998;Storfer et al 1999;Ogden and Thorpe 2002;Calsbeek and Smith 2003), and those that have used few such pairs: N ϭ 4 independent pairs for char morphs (Gíslason et al 1999), N ϭ 6 for benthic and limnetic whitefish (Lu and Bernatchez 1999), N ϭ 4 for benthic and limnetic stickleback (Hendry et al 2001), and N ϭ 4 for two Neotropical fish species (Langerhans et al 2003). In the present study, we quantify the relationship between adaptive divergence and gene flow across eight independent population pairs of threespine stickleback (Gasterosteus aculeatus).…”

Adaptive Color Polymorphism and Unusually High Local Genetic Diversity in the Side-Blotched Lizard, Uta stansburiana

“…If the phenotypic variation in this population is not due to individual phenotypic plasticity, this species can be used to understand the balance between gene flow and adaptive evolution in reptiles. Depending on the strength of natural selection and rates of migration, gene flow may obstruct local adaptation by homogenizing populations subject to different selection pressures [14]–[15]. Alternatively, strong disruptive selection may overwhelm even substantial gene flow and lead to population differentiation [16].…”