Abstract: Hybridization between species is commonplace in plants, but is often seen as unnatural and unusual in animals. Here, I survey studies of natural interspecific hybridization in plants and a variety of animals. At least 25% of plant species and 10% of animal species, mostly the youngest species, are involved in hybridization and potential introgression with other species. Species in nature are often incompletely isolated for millions of years after their formation. Therefore, much evolution of eventual reproductive isolation can occur while nascent species are in gene-flow contact, in sympatry or parapatry, long after divergence begins. Although the relative importance of geographic isolation and gene flow in the origin of species is still unknown, many key processes involved in speciation, such as ‘reinforcement’ of post-mating isolation by the evolution of assortative mating, will have ample opportunity to occur in the presence of continuing gene flow. Today, DNA sequence data and other molecular methods are beginning to show that limited invasions of the genome are widespread, with potentially important consequences in evolutionary biology, speciation, biodiversity, and conservation. Hybridization between species is commonplace in plants, but is often seen as unnatural and unusual in animals. Here, I survey studies of natural interspecific hybridization in plants and a variety of animals. At least 25% of plant species and 10% of animal species, mostly the youngest species, are involved in hybridization and potential introgression with other species. Species in nature are often incompletely isolated for millions of years after their formation. Therefore, much evolution of eventual reproductive isolation can occur while nascent species are in gene-flow contact, in sympatry or parapatry, long after divergence begins. Although the relative importance of geographic isolation and gene flow in the origin of species is still unknown, many key processes involved in speciation, such as ‘reinforcement’ of post-mating isolation by the evolution of assortative mating, will have ample opportunity to occur in the presence of continuing gene flow. Today, DNA sequence data and other molecular methods are beginning to show that limited invasions of the genome are widespread, with potentially important consequences in evolutionary biology, speciation, biodiversity, and conservation. two groups of populations or species are allopatric if their ranges are not connected; there is a geographic barrier between them that prevents gene flow. Allopatric speciation is therefore speciation in which the whole process takes place under conditions of geographic isolation. the tendency for ‘like to mate with like’. Species or morphs within a species are said to mate assortatively if mating is biased towards other individuals with a similar phenotype or genotype. the species concept in which species are considered to be reproductively isolated populations [2,6]. Conversely, in the ‘recognition concept’ [13], individuals are members of the same species if they share common fertilization or recognition mechanisms. These two concepts are almost mirror images: in the biological species concept, species are defined by incompatibilities with individuals outside the species; in the recognition concept, species are defined by compatibility with other individuals inside the species. a theory to explain Haldane's Rule. Hybrids between species can suffer deleterious incompatibilities because alleles from two or more genes do not interact well together in hybrids. If these genes are on sex chromosomes and act recessively, the heterogametic sex (i.e. chromosomally XY, as in the male in mammals and Drosophila, and the female in birds and Lepidoptera) is liable to suffer most because it has only a single copy of the deleterious sex-linked allele, which is therefore expressed. In the homogametic sex (i.e. chromosomally XX, as in the female in mammals and Drosophila, and the male in birds or Lepidoptera), the compatible dominant allele from the other species masks the deleterious effect. Dominance theory not only explains why it is the heterogametic sex that usually suffers most in a wide variety of animals, but also why species with large sex chromosomes tend suffer more greatly Haldane's Rule than species with small chromosomes [31]. the tendency for the heterogametic sex of hybrids to suffer greater inviability or sterility than the homogametic sex. The rule was discovered originally by JBS Haldane in the 1920s, who showed that hybrid incompatibilities were usually greater in the heterogametic sex, regardless of whether the heterogametic sex was male or female. For explanation, see dominance theory. transfer of genes between species via some route other than sexual contact and hybridization (e.g. transduction by a virus, transformation by free DNA, or transfer by means of a plasmid). crossing of genetically distinguishable groups or taxa, leading to the production of viable hybrids. In this article, the term is used mainly to refer to hybridization in the wild, as opposed to experimental crosses in the laboratory. invasion of foreign genetic material into a genome. The method of introduction is usually by sexual contact or hybridization; genetic transfer by other means is usually termed horizontal gene transfer. two groups of populations or species are parapatric if they abut at their range boundaries. Gene flow is possible between parapatric populations or species, but is commonly limited to populations at the edge of each species range, and where the ranges of the two forms overlap. If hybridization occurs freely in the overlap, this results in a narrow band of hybridization, or hybrid zone. the species concept in which species are populations considered to differ by one or more fixed differences, or, in other formulations, in which species are considered to be the smallest phylogenetic group that can be defined by shared derived characters (i.e. monophyletic groups). if hybrids between a pair of sympatric species have low viability or sterility, these deleterious effects might ‘reinforce’ reproductive isolation by promoting the evolution of assortative mating, essentially to prevent gametic wastage. populations are sympatric if individuals of each occur within ‘cruising distance’ of the other, so that gene flow is not only possible, but probable if there are no reproductive barriers between the forms. Sympatric speciation is speciation in which the whole process takes place while the diverging populations are in gene flow contact.
Publication Year: 2005
Publication Date: 2005-03-11
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 2048
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