Adaptive Radiations

The seemingly accelerated rates of evolution that occur in adaptive radiations have long been recognized by biologists and paleontologists. G. G. Simpson, who did much to reconcile paleontological data with Neo-Darwinian notions of adaptive change and speciation, referred to the phenomenon as 'quantum evolution', to distinguish it from the 'background' processes of adaptation and spe-ciation that characterize most of a lineage's history. However, Simpson emphasized that quantum evolution did not entail any novel evolutionary mechanism. Rather, he saw the process as a consequence of the fact that external circumstances (such as a vacated ecological niche) imposed much stronger diversifying selection than that encountered by most taxa during their normative history.

That adaptive radiations can be seen as adaptation and speciation under special external circumstances has support from classical population genetics theory. In an important early paper, R. A. Fisher demonstrated that mutations with large fitness consequences have a higher probability of fixation early in the history of a species, with adaptive change having 'diminishing returns' as time progressed. This is the basic pattern with many evolutionary radiations - large-scale differences are established first, followed by lower-level changes later in the history of a lineage.

The basic argument is that if the space of possible changes in phenotype is represented in a multidimensional Cartesian coordinate system. In this multidimensional space, an ancestral genotype is some distance from an optimum. At first, mutations with large phenotypic effects have a high probability of moving the population closer to the optimum and increasing mean fitness. However, as the population approaches the optimum, mutations with large phenotype effects are more likely to move the population further from the optimum and reduce fitness, so that only mutations with small effects have a high probability of fixation and increasing mean fitness. While the details of Fisher's analysis have been disputed, this general result has been vindicated in a number of recent papers. A related argument, based on the declining probability of moving to higher adaptive 'peaks' on a fitness landscape with multiple local optima, was advanced by S. Kauffman and other 'complexity' theorists.

The same principles that apply to tracking optima with intraspecific variation are applicable to differences between species. In the case of an ancestral species encountering a large number of open niches, each new niche colonized represents a large distance to be reached in genotype and phenotype space. Since there are many such niches, initially mutations with large-scale pheno-typic effects are likely to be favored because they are likely to take the genotype to a better evolutionary place. Afterward, when at the 'coarse-grained' level the descendant lineages have occupied the new niches, mutations with large effects are more likely to move them into regions of inferior fitness. At that point, mutations with small phenotypic effects are likely to be fixed as the populations optimize their fitness within the new niche (i.e., the type of 'fine-grained' adaptation that constitutes normal adaptive evolution).

This dynamics has been observed in a number of computer simulations of adaptive evolution, and is consistent with the observed pattern in the phylogenies of many higher taxa where the deeper nodes define major changes in phenotype associated with colonizing new regions of ecological space, while the differences among lower-level taxa (e.g., interspecific differences within a genus) usually involve relatively minor differences in morphology and ecology.

In other words, adaptive radiations reflect adaptive evolution and speciation on fitness landscapes that are intrinsically coarser grained than those typically encountered by most evolving lineages. However, while such a perspective accounts for the types of adaptive radiations typically discussed in the literature (e.g., differences in trophic modes and coloration in African lake cichlids, beak morphology in finches on the Galapagos, etc.), it still leaves unanswered the other aspect of evolutionary radiations seen in the early fossil record - the origin of truly novel morphologies.

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