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SPECIATION How do new species arise? Note that mutation, recombination, and selection change the average characteristics of a population This provides for “evolution” as adaptation But it does not by itself explain “speciation,” the production of new species and increase in the number of species What are species? Taxonomic definition: the members of different species have consistent differences in a significant number of traits Taxonomic definition same species clustered traits different species divergence of traits Genetic definition: the members of different species are reproductively isolated Genetic definition same species interbreeding different species reproductive isolation Genetic speciation implies taxonomic speciation For interbreeding populations, any new allele (trait), generally small, will spread throughout the population; clusters of new alleles will tend to disperse randomly; differences will be gradual For non-interbreeding-- “reproductively isolated” -populations, alleles will not be shared; differences in new alleles and clusters of new alleles will accumulate; populations will diverge; the more they diverge, the more isolated they become How are populations reproductively isolated? Prezygotic mechanisms prevent fertilization Different habitat Sexual maturation at different seasons Different mating behavior Different sexual organs Gamete incompatibility (no sperm-egg fusion) Postzygotic mechanisms limit hybrid survival or reproduction Hybrids inviable Hybrids sterile (no gonads) Hybrids sterile (abnormal meiosis, incomplete gametes) F2 breakdown Every natural species distinguished from every other species by one or more of these factors But how do populations BECOME reproductively isolated? Geographical isolation (allopatric speciation) Mutation, recombination, selection in differing environments stabilize different traits, some producing reproductive Isolation Once isolated, populations diverge further, giving different genera, etc. Examples of speciation through reproductive isolation Fruit flies in the Hawaiian islands Examples of speciation through reproductive isolation Darwin’s finches Examples of speciation through reproductive isolation Evolution of horses (from fossil evidence) Sympatric speciation Polyploidy: change in chromosome number gives Immediate reproductive isolation Diploid Chromosome Duplication, Mitosis Without Cytokinesis Tetraploid Examples of polyploidy as speciation: Tobacco Nicotiana otophora 2n = 12 x Nicotiana sylvestris 2n = 12 Nicotiana tabacum 2n = 24 (occurred naturally, but verified experimentally) Rado-cabbage radish (Raphanus sativus) 2n = 18 x cabbage (Brassica oleracea) 2n = 18 “radocabbage” 2n = 36 (radish head, cabbage root) (experimental, but shows possibility of intergeneric speciation) The tempo of evolution: is evolution gradual? Fossil evidence shows : Species morphology is stable over long periods Species appear and disappear abruptly (in evolutionary time) “Punctuated equilibrium” may describe life history better than “evolution” time Gradual change Morphological trait Punctuated equilibrium Morphological trait Flower compatibility may affect the tempo of evolution: Self compatibility -> genetic constancy -> high speciation, high extinction -> low diversification Self incompatibility -> genetic variability -> low speciation, lower extinction -> high diversification Self-compatible Speciation rate Self-incompatible Diversification rate Science 22 Oct 2010, p. 493 Summary Evolution of a population does not itself imply the formation of species Speciation occurs through reproductive isolation Reproductive isolation can occur through geographical isolation Tetraploidy is one method of sympatric isolation The appearance of new species, observed through study of fossils, occurs in a “punctuated equilibrium” pattern Other questions: Are there any cases of gradual phenotypic change (instead of punctuated equilibrium)? Are there alternatives to selection that can explain the pattern of changes in fossil samples? (Raup: the changes are not significantly different from random; also consider genetic drift) What is the best way to sample fossils to estimate the pattern of increase in species over geologic time? (Right: number of marine genera estimated from fossils in three different ways)