Systematics and the Origin of Species On Ernst Mayr's 100th Anniversary (2005) / Chapter Skim
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5 Developmental Plasticity and the Origin of Species Differences--MARY JANE WEST-EBERHARD
5 Developmental Plasticity and the Origin of Species Differences--MARY JANE WEST-EBERHARD
Pages 69-90
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From page 69... ...
followed by the genetic accommodation of change. Because selection acts on phenotypes, not directly on genotypes or genes, novel traits can originate by environmental induction as well as mutation, then undergo selection and genetic accommo dation fueled by standing genetic variation or by subsequent mu tation and genetic recombination.
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From page 70... ...
. The origin of species differences, not reproductive isolation, were the main focus of Darwin's book On the Origin of Species by Means of Natural Selection (Darwin, 1858)
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From page 71... ...
, and selectable variation means phenotypic variation, whether it has a genetic component or not. It is adaptive evolution, or a genetic response to selection, that requires genetic variation among the selected entities, not selection (differential reproductive success)
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From page 72... ...
Genetic accommodation, or genetic change in the regulation or form of a novel trait (West-Eberhard, 2003) , is the process by which new developmental variants become established within populations and species because of genetic evolution by selection on phenotypic variation when it has a genetic component.
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From page 73... ...
If the phenotypic variation has a genetic component, selection leads to "genetic accommodation," that is, adaptive evolution that involves gene-frequency change. Genetic accommodation of regulation adjusts the frequency, timing, and circumstances of the novel response (e.g., by adjusting the threshold for its expression)
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From page 74... ...
In either case, the genetic accommodation of novelty need not await mutation as long as there is a standing pool of genetic variation. As I discuss below, such variation is likely to be sufficient to support a response to selection on virtually any novel trait.
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From page 75... ...
drew attention to the contribution of developmental sources of the origin of the distinctive maize phenotype. However, it is impossible to tell, from current information, whether the loci now identified as involved in the change are the result of genetic accommodation based on alleles at low frequency in natural populations of teosinte or are products of later mutations, whose phenotypic effects may have been modified (amplified or reduced)
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From page 76... ...
. It is not known whether the bipedal goat's abnormal front legs were due to a genetic or an environmentally induced defect, but in either case the inducer acted as a novel switch mechanism that in effect controlled the expression of a whole suite of correlated and adaptive changes in behavior, muscle, and bone.
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From page 77... ...
. DEVELOPMENTAL RECOMBINATION AND PARALLEL SPECIES PAIRS Recurrent phenotypes, similar or identical phenotypic traits with discontinuous phylogenetic distributions, are quite common in a wide diversity of taxa (West-Eberhard, 2003)
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From page 78... ...
. This discussion is not to deny the importance of natural selection in contrasting habitats for the evolution of species differences and their possible effects on the origin of reproductive isolation (Schluter, 2000; also see below)
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From page 79... ...
Their origins are, in fact, inseparable events: you cannot have a change in the phenotype, a novel phenotypic state, without an altered developmental pathway. In terms of gene expression, developmental recombination means that a set of ancestral genes are now coexpressed, or their products used, in a new combination or a new context, and the ancestral regulatory mechanisms are now triggered by a new inducer or an old one in some new sequence or environmental context.
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From page 80... ...
. This description applies best to genetic assimilation, a special case of genetic accommodation that begins with environmental induction and proceeds toward fixation of the novel trait (Waddington, 1953)
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From page 81... ...
Thus, although standing genetic variation usually must be sufficient to produce a response to selection (Orr and Betancourt, 2001) , genetic accommodation may in some cases, like that of directional asymmetry, await mutation (Palmer, 2004)
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From page 82... ...
An environmental factor can affect numerous individuals at once, whereas a mutation initially can affect only one. The larger the population affected, the greater the likelihood that an environmentally induced novelty occurs in a favorable genetic, phenotypic, or selectively advantageous environment in at least some subpopulation of the individuals affected, and the larger the probability of genetic variation that can result in an evolutionary response to selection.
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From page 83... ...
THE ORIGIN OF SPECIES DIFFERENCES: BEFORE OR AFTER REPRODUCTIVE ISOLATION? Trait origin by developmental recombination predicts several properties of species and their genetics as follows: (i)
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From page 84... ...
shows how traits such as body size, which have diverged under natural selection, can contribute to the origin of reproductive isolation. Variation in body size is often associated with the evolution of condion-sensitive, faculatatively expressed alternative phenotypes within species, including in fishes and other
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From page 85... ...
Not only are species differences often parallel to intraspecific alternative phenotypes, as already mentioned, but the rampant speciation and associated phenotypic diversification that characterize some of the most spectacular adaptive radiations known can be linked to particular kinds of developmental plasticity. The beak and trophic diversification of Darwin's finches in the Galapagos Islands has involved learned associations between beak size and shape and feeding preferences; the niche diversification of African lake cichlids is associated with dietary flexibility in mouth morphology and behavior; and the larval habitat diversification of Hawaiian Drosophila may have involved biochemical versatility within species (reviewed in WestEberhard, 2003)
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From page 86... ...
loci that are actually subject to selection in the evolution of species differences, beginning with differences that arise because of developmental recombination without reproductive isolation. Comparative genomics has the potential to illuminate the contribution of developmental-genetic processes to speciation.
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From page 87... ...
(1858) On the Origin of Species by Means of Natural Selection (Murray, London)
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From page 88... ...
(2003) The role of phenotypic plasticity in driv ing genetic evolution.
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From page 89... ...
(1995) Parallel speciation by natural selection.
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