Selection, in which the absence of selection on some traits or alleles between Researchers hypothesize thatĭifferential gene flow across species boundaries is a pattern resulting from Hybrid zone to understand the evolutionary forces at work (Payseur et al., 2004). University of Arizona has analyzed the spread of traits and alleles across this House mouse hybrid zone is found in central Europe, and the Nachman lab at the Studied extensively for variance in gene exchange across the genome. In addition to fruit flies, house mice have also been Inversions in the persistence of hybridizing species. Although there is still much to learn, this system suggests a role for Suggesting that the inversions themselves cannot readily cross the speciesīoundary. Genetically, allelic exchange (also known as introgression) between the speciesĪppears highest away from inverted regions and lowest within inverted regions (Noor et al., 2007), Species hybridize at a low level in nature. Three chromosomal inversions (reversals in a segment of the DNA sequence).ĭespite their limited nucleotide divergence and genomic rearrangements, these These co-occurring species are less than 1 million years diverged and differ by Well-studied example involves reduced gene flow associated with inversionĮvents on whole chromosomes in the North American Drosophila species D. Large genomic regions, such as whole chromosomes or chromosomal regions. This observation has been reportedĪcross many different organisms, and it is associated with both single genes and Species boundaries more easily than others. Not necessarily homogeneous, because alleles of some genes disperse across When different genomes come into contact, gene exchange is These artificial hybrids resemble natural hybrids in adaptive allelic combinations, demonstrating that hybridization may be a unique way for species to invade novel habitats with the generation of novel allelic combinations at many genes in a single generation. Loren Rieseberg's research team has tested this hypothesis directly by generating artificial hybrids with the same genetic origins as the ancient hybrid species found in nature (Rieseberg et al., 2003). Each of these three hybrid species inhabits extreme environments relative to the parental species, suggesting that hybridization has generated novel adaptive traits (Figure 1). For instance, reticulation drove diversification within the sunflower genus Helianthus, in which researchers have uncovered three species of hybrid origin. This special case in which genomes from different species fuse to generate a new hybrid species lineage, a process called reticulation, has been most intensively studied in plants. Just as sexual reproduction can bring different sets of alleles together in a common genetic background to facilitate adaptation, hybridization between species can allow alleles from one genetic background to integrate into another if favored by selection. Gene flow and recombination can act to generate novel haplotypes to facilitate Researchers understand those forces that limit hybridization, as well as how Scientists' systematic understanding of the taxa involved, but also helping Study of hybridization has yielded valuable insights, not only refining Molecular tools have enhanced hybridization research in a wealth of biological Initially came from fossil or extant species morphology, but more recently, Since then, scientists have been able to uncover manyĮxamples of hybridization across several branches of life. "rare" or "exceptional," and they instead discussedįavored the strengthening of discrimination to maintain species (Dobzhansky, 1937 Mayr, In contrast, zoologists suchĪs Theodosius Dobzhansky and Ernst Mayr considered animal hybrids to be Ledyard Stebbins focused on hybridization as an important generator of geneticĭiversity (Stebbins, 1950). After Darwin, botanists and zoologists tookĭifferent paths to understanding hybridization. Species in nature is not as uncommon as this early result would imply (Mallet, 2005). Surprisingly, however, hybridization between Species by interbreeding them and demonstrating that mating between groups canīe difficult (Darwin, 1859). In the mid-nineteenth century, Darwin explored the nature of Species exchange genes with one another in nature. Species-those instances in which the rules of differentiation are broken, and Understanding these natural processes requires looking at the interface between Natural processes that generate and preserve biodiversity on this planet. With human-induced destruction of habitats andĬlimate-change issues pervading the news, researchers have renewed interest in
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