Cpncept which emphasizes reproductive isolation as major cause
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Genetic Analysis of Reproductive Isolation: Principles
Reproductive isolation is unique in evolution because it is not a trait possessed by members of a single species, but a composite character that is the joint property of a pair of species. A single species can be reproductively isolated only with respect to another. Moreover, by its very nature, reproductive isolation is a trait that almost always involves epistatic interaction between alleles – but alleles occurring in different species. Hybrid inviability, for example, results from genes that produce normal viability in members of their own species but are lethal when interacting with alien genes in hybrids. Similarly, sexual isolation is caused when females evolved to prefer traits of conspecific males encounter different traits in other species. This composite and epistatic nature of reproductive isolation guarantees that speciation will not only show emergent genetic and phenotypic properties not seen in studies of a single species (e.g., Haldanes rule; see below), but also that mathematical theories of speciation will be different – and perhaps more complicated – than models of evolution in single lineages. While genetic analysis of reproductive isolation has occurred since the mid-1930s, mathematical theories of speciation are only now beginning to appear.
There are several reasons for studying the genetic basis of speciation. First, just as with a trait that evolves within a lineage, one wants to know whether a reproductive isolating mechanism has a ‘simple’ genetic basis (i.e., involves only a few genes of large effect) or is based on the accumulation of many genes. The number of genes involved may, in turn, allow inferences about the evolutionary process producing reproductive isolation. For example, if the difference in plumage color between males of two sexually dimorphic bird species is due to many genes of small effect, one may posit that these differences arose by sexual selection during which the male trait evolved step-by-step in concert with the female preference for that trait.
Similarly, the pattern of genetic differences causing reproductive isolation may give clues to the underlying evolutionary processes. It has been found, for example, that there are often many more genes causing hybrid male than female sterility between closely related species of Drosophila. This has led to the idea that hybrid sterility may result from sexual selection acting in isolated populations. Such selection, based on female choice, may cause more evolutionary change in males than in females, leading to the preferential sterility of male hybrids as an accidental outcome. Finally, genetic analysis can help localize small sections of chromosomes containing genes causing reproductive isolation, a necessary prelude to cloning and sequencing these genes. Such molecular work is essential for understanding the developmental basis of reproductive isolation, including the question of how a gene that works normally within a species causes deleterious effects in hybrids. At this writing we understand the developmental basis of only one case of reproductive isolation: the formation of lethal melanomas in hybrids between the swordfish and platyfish. This hybrid lethality is based on an oncogene in one species that is normally suppressed by another gene in the same species; the absence of suppression in hybrids causes the appearance of tumors.
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Genetic Analysis of Reproductive Isolation: Principles
Reproductive isolation is unique in evolution because it is not a trait possessed by members of a single species, but a composite character that is the joint property of a pair of species. A single species can be reproductively isolated only with respect to another. Moreover, by its very nature, reproductive isolation is a trait that almost always involves epistatic interaction between alleles – but alleles occurring in different species. Hybrid inviability, for example, results from genes that produce normal viability in members of their own species but are lethal when interacting with alien genes in hybrids. Similarly, sexual isolation is caused when females evolved to prefer traits of conspecific males encounter different traits in other species. This composite and epistatic nature of reproductive isolation guarantees that speciation will not only show emergent genetic and phenotypic properties not seen in studies of a single species (e.g., Haldanes rule; see below), but also that mathematical theories of speciation will be different – and perhaps more complicated – than models of evolution in single lineages. While genetic analysis of reproductive isolation has occurred since the mid-1930s, mathematical theories of speciation are only now beginning to appear.
There are several reasons for studying the genetic basis of speciation. First, just as with a trait that evolves within a lineage, one wants to know whether a reproductive isolating mechanism has a ‘simple’ genetic basis (i.e., involves only a few genes of large effect) or is based on the accumulation of many genes. The number of genes involved may, in turn, allow inferences about the evolutionary process producing reproductive isolation. For example, if the difference in plumage color between males of two sexually dimorphic bird species is due to many genes of small effect, one may posit that these differences arose by sexual selection during which the male trait evolved step-by-step in concert with the female preference for that trait.
Similarly, the pattern of genetic differences causing reproductive isolation may give clues to the underlying evolutionary processes. It has been found, for example, that there are often many more genes causing hybrid male than female sterility between closely related species of Drosophila. This has led to the idea that hybrid sterility may result from sexual selection acting in isolated populations. Such selection, based on female choice, may cause more evolutionary change in males than in females, leading to the preferential sterility of male hybrids as an accidental outcome. Finally, genetic analysis can help localize small sections of chromosomes containing genes causing reproductive isolation, a necessary prelude to cloning and sequencing these genes. Such molecular work is essential for understanding the developmental basis of reproductive isolation, including the question of how a gene that works normally within a species causes deleterious effects in hybrids. At this writing we understand the developmental basis of only one case of reproductive isolation: the formation of lethal melanomas in hybrids between the swordfish and platyfish. This hybrid lethality is based on an oncogene in one species that is normally suppressed by another gene in the same species; the absence of suppression in hybrids causes the appearance of tumors.
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