Question

Using Miller’s hypothesis for adaptive evolution of thrashers (Toxostoma) from a mockingbird (Mimus) ancestor as representing the standard evolutionary genetic model of the modern synthesis, explain how each of the following hypothesized processes of evolutionary diversification differ: (1) West-Eberhard’s “alternative adaptations,” (2) West-Eberhard’s phenotypic and genetic accommodation, and (3) McKay and Zink’s “Sisyphean evolution.” Include in part 2 a discussion of how genetic accommodation resembles and differs from Waddington’s hypothesis of genetic assimilation.

Answer 1

(1) West-Eberhard’s “alternative adaptations:

Alternative adaptations—different adaptive phenotypes maintained in the same life stage and the same population but not necessarily simultaneously expressed in the same individual—represent contrasting character sets produced by the same genome, in effect allowing a single species to occupy more than one sympatric niche. Such alternatives are particularly likely to give rise to novel adaptations because of selection for extreme dissimilarity between them and because established traits buffer populations against extinction while independently expressed alternatives evolve in new directions. Particular alternatives can be suddenly fixed in populations with little or no genetic change, leading to a period of rapid evolution (especially, of morphology) exaggerating the characteristics of the newly fixed form. This burst of change would facilitate rapid speciation and could produce “punctuated” patterns of evolution. Evidence from a wide variety of organisms shows that alternative phenotypes are exceedingly common in nature and that they are probably important in speciation and macroevolution. Although many of these ideas and observations have been noted piecemeal by previous authors, bringing them together demonstrates the probable importance of alternative adaptations in the origin of major evolutionary novelties and calls for a revision of current and traditional ideas about the role of behavior and ontogeny in the genesis of organic diversity.

(2) West-Eberhard’s phenotypic and genetic accommodation

Phenotypic accommodation is adaptive adjustment, without genetic change, of variable aspects of the phenotype following a novel input during development. Phenotypic accommodation can facilitate the evolution of novel morphology by alleviating the negative effects of change, and by giving a head start to adaptive evolution in a new direction. Whether induced by a mutation or a novel environmental factor, innovative morphological form comes from ancestral developmental responses, not from the novel inducing factor itself. Phenotypic accommodation is the result of adaptive developmental responses, so the novel morphologies that result are not "random" variants, but to some degree reflect past functionality. Phenotypic accommodation is the first step in a process ofDarwinian adaptive evolution, or evolution by natural selection, where fitness differences among genetically variable developmental variants cause phenotype-frequency change due to genefrequency change.

(3) McKay and Zink’s “Sisyphean evolution.

The trajectory of speciation involves geographic isolation of ancestral populations followed by divergence by natural selection, genetic drift or sexual selection. Once started, the process may experience fits and starts, as sometimes diverging populations intermittently reconnect. In theory populations might cycle between stages of differentiation and never attain species status, a process we refer to as Sisyphean evolution. We argue that the six putative ground finch species (genus Geospiza) of the Galápagos Islands represent a dramatic example of Sisyphean evolution that has been confused with the standard model of speciation. The dynamic environment of the Galápagos, closely spaced islands, and frequent dispersal and introgression have prevented the completion of the speciation process. We suggest that morphological clusters represent locally adapted ecomorphs, which might mimic, and have been confused with, species, but these ecomorphs do not form separate gene pools and are ephemeral in space and time. Thus the pattern of morphological, behavioural and genetic variation supports recognition of a single species of Geospiza, which we suggest should be recognized as Darwin's ground finch (Geospiza magnirostris). We argue that instead of providing an icon of insular speciation and adaptive radiation, which is featured in nearly every textbook on evolutionary biology, Darwin's ground finch represents a potentially more interesting phenomenon, one of transient morphs trapped in an unpredictable cycle of Sisyphean evolution. Instead of revealing details of the origin of species, the mechanisms underlying the transient occurrence of ecomorphs provide one of the best illustrations of the antagonistic effects of natural selection and introgression.