Variation Arising from Adaptation among Frog Species
Darwin’s theory of natural selection argues that the survival of a particular organism is mediated upon by its favourable traits. From this line of argument, only the “fittest” organism is the one who can continue to live and reproduce in a particular environment. However, the current crop of researchers assert that “morphologies are equally adaptive and that differences among them are a result of the particular phylogenetic histories of the involved taxa (Emerson 1984),” which necessarily means that there are no “favorable traits” and that all traits and characteristics are the result of needs of the organisms for survival in the particular environment where they are found. Schopf (1975), for instance, argues that “only complicated animals evolve” based on the observation that “groups based on a few characters are longer lived and are commonly polyphyletic in comparison with groups based on many characters.”
Based on this, the dominant notion that organisms of the same taxa possess the same morphological, physiological, and behavioral characteristics have been debunked. Lately, there have also been concerted efforts to disprove the assumption that “all members of a species exhibit the same behavior” or species-typical behavior with various studies showing that differences exist even in organisms of the same species (Foster and Endler 1999). It is from this perspective that this paper attempts to explore whether the variations between species and within species in the anuran order of the vertebrate taxa do exist and how these variations are manifested.
Adaptive Differences in Frog Species
Similar to the rest of the vertebrate taxa, frogs undergo genetic and environmentally-induced differentiation in order to survive. The fact that frogs, which belong to the order of Anura, are probably one of the most diverse groups of vertebrates with 5,000 species (Wikipedia 2006), is a testament to the immense differentiation within the same taxa brought about by adaptive and surivival pressure on frogs. Likewise, although generally classified as amphibians, emergent research provides new and interesting insights on the evolving characteristics and habitats of frogs (Schmid 1965; Relyea 2001, 2004).
Reproduction and Metamorphosis
One of the most interesting findings in the study of the evolution of frogs are the recent evidence of direct development or absence of the usual metamorphosis among some anuran species. The most extreme example of direct development is the Eleutherodactulus coqui or the Puerto Rican tree frog, a terrestrial species whose eggs hatch directly into tiny frogs, similar to the development of young birds and reptiles, but in stark contrast to the metamorphosis of majority of frog species which involve a unique larval stage—as tadpoles—before growing into matured organisms (Elinson 2001). It is suggested that the loss of the tadpole morphology in the E. coqui is derived, an adaptive response to the fact that the Puerto Rican tree frog never goes in the water and lays its eggs on land (Elison 2001).
On the other hand, environmental factors such as temperature and altitude are known to influence the differences in the reproductive cycle and the number of eggs produced among different and same frog species. For instance, equatorial frogs Rana erythraea and Bufo melanossicius are observed to reproduce throughout the year compared to species found in habitats which experience winter or extremely cold temperatures (Feder, M. and Burggren _____). The findings of another study also reveal that altitude affected the reproductive process of the Rana Sylvatica, in which “the age at first reproduction of mountain females was very sensitive to environmental differences and mountain females exhibited a greater plasticity in the age at which they matured relative to lowland females (Berven 1982).” Another study on the development of the Rana sylvatica suggest adaptive differences within the same species regarding to breeding patterns in those found in the North versus those in Southern Alaska that was influenced by temperature effects on embryonic development (Herreid and Kinney 1967).
There are also several variations on egg protection across frog species to minimize the damage of predators. In the Argentinian and Chilean Darwin’s Frog or Rhenoderma darwinii, the eggs are gathered in the males’ vocal sacs while the males of the European midwife toad Alytes obstetricians keep their egg strands wound around their waists. Other frogs such as the tree frog Gastrotheca sp. carry their eggs in the females’ dorsal brood-pouch while the females of the Australian gastric-brooding frog suppresses stomach-acid secretion and peristalsis and keeps the fertilized eggs in its stomach (Bartlett and Bartlett 1996).
With respect to physical characteristics, among the most notable adaptive differences found among frogs are variations in 1) size, 2) skin texture and permeability, 3) leg and feet musculature and characteristics, and 4) laryngeal morphologies related to mating and mate selection.
Perhaps the most directly observable adaptive variation among the anuran species is the differences in frog body size. Nevo (1973), based on his comparative study of Acris crepitans and Acris grylius, contends that the difference in body sizes among cricket frogs is an adaptive mechanism to humidity, the larger frogs being more adapted to arid climates since they are prone to loss less water than smaller sized ones.
While it has been commonly held that all frog species are of amphibious nature, there are studies which reveal that there are salient differences with regards to water tolerance across the spectrum of frog species. A study conducted by Schmid (1965) found out that terrestrial frogs tolerated the loss of significant levels of body water but were unable to survive prolonged hydration. Hydration tolerance in frogs was likewise found to be directly related to differences in skin permeability to water and plasma concentration, wherein terrestrial skin was poorly equipped to regulate the influx of water (Schmid 1965). This implied that as frogs became more terrestrial in nature, they “have specialized in such a way that aquatic habitats are no longer suitable for them.” This adaptive character is evidenced by the differences in winter habitats among frogs. For instance, Rana pipiens and Rana clamitans which wintered in water exhibited high tolerance to hydration during Schmid’s experiment while Rana sylvatica which is a terrestrial species favoured dry conditions.
Another adaptive strategy found in frogs is the development of varied leg and feet characteristics according to their habitat. Flying frogs of the Hylidae, who are arboreal species, have evolved enlarged, extensively webbed hands and feet that enable them to glide within an arboreal canopy and also an adapted limb position that improved the aerodynamic performance of the frogs to ensure that they landed where intended and minimized the impact of the fall (McCay 2003; Emerson and Koehl 1990). These frogs were also observed to possess the ability to grip vertical surfaces with their feet through capillarity action (Emerson and Diehl 1980). Meanwhile, while aquatic frog species shared the characteristic of the webbed hands and feet, a difference appears in the degree of webbing between semi-aquatic and fully aquatic species, with the more terrestrial ones having less webbed hands and feet, if at all (Ford and Canatella 1993).
Within terrestrial burrowing frogs, hindfeet digging led to the development of postero-laterally hind-limbs and musculature, as evidenced by the Glyphoglossus molossus or microhylidae (Emerson 2005). Adaptation to headfirst burrowing among the Hemisus marmoratus (Ranidae), on the other hand, has brought forth an “extensive reorganization of the pectoral-cranial morphology compared to that of a non-borrowing confamilial species (Emerson 2005).”
Meanwhile, adaptive morphological changes to the larynx was seen to be an outcome of mating pressures. In a study conducted by McClelland et al. (1996), it was contended that the males of the frog species were forced to adapt specific acoustic parameters in vocalization to increase the chances of mate selection which induced changes on the larynx. Further study also supported the contention that there were mating call differences within the same species of frogs based on the study of cricket frogs Acris crepitans and that a subsequent adaptation of the larynx to create these varied calls was also possible (McClelland 1998).
The evolution of defense mechanisms in frogs as a result of predation has been the subject of many researchers. In an experiement conducted by Relyea (2001), it was observed that “some wood frog populations (Rana sylvatica) are adapted to the local conditions of their natal pond and that localized selection by predation and competition may be the underlying mechanism.” This meant that even among the same species, different defense mechanisms may be evolved based on the nature of predator and competition present in their environment. Another study by the same researcher supported these findings which showed that “tail depth exhibited high heritability with predators but low heritability without predators (Relyea 2005),” suggesting that genetic differentiation and heredity of induced or derived defense mechanisms was highly influenced by adaptive needs of the species in a particular environment.
Likewise, the wood frog Rana Sylvatica was found to be able to tolerate extremely low and freezing temperature by the ability to “build up glucose levels as cryoprotectant (Garland and Adolph 1991) while the members of the Rana muscosa were found to be able to depress their metabolic rate by lying dormant under the water that was crucial in energy conservation and maximized oxygen use in its environment (Bradford 1983). This findings tend to mirror the findings previously mentioned on the differences in the degree of permeability of the skin among frog species which allowed some species to hibernate underwater and killed some others.
The data presented clearly support the argument that interspecific and intraspecific differences brought about by adaptation to particular environments do exist among organisms as demonstrated by the adaptive differences among species and within the same species of frogs. Common factors associated with a high level of variation primarily involved environmental and predator-induced pressures which then significantly changed the morphological, behavioral, and physiological characteristics of a particular species which could be inherited based from the degree of need for adaptive mechanism, leading to the birth of new phenotypes among the same species.
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