11 05 2012 Recognition of species boundaries in natural populations can be surprisingly difficult It hinges on the intertwined attributes of common descent reproductive continuity and possession of diagnostic characteristics Two processes that generate biological diversity Anagenesis the accumulation of heritable changes altering the characteristics of species selection drift gene flow etc o Forms the basis for microevolution change in allelic frequencies over time Cladogenesis speciation the process by which new species form o Forms the basis for macroevolution formation of lineages Definitions concepts of what constitutes a biological species Morphological Typological species concept o Pre darwinian concept in which species were viewed as distinct unchanging entities characterized by invariable distinguishing features o Problems with this concept Geographic variation Ex viceroy butterfly when they live in the range of the common monarch butterfly they mimic them If they live in the range of the florida queen butterfly they mimic them Sexual dimorphism Biological species concept T Dobzhansky and E Mayr o A species is a reproductive community of populations reproductively isolated from others that occupies a specific niche in nature o Represents a minimal criterion for a common evolutionary trajectory reproductive and genetic continuity o Problems with this concept Populations are partially inter fertile Populations are potentially inter fertile but are geographically isolated Populations are strictly asexual Phylogenetic species concept Niles Eldridge and Joel Cracraft o Emphasizes genealogy o A species is a distinct group of organisms within which there is an exclusive pattern of ancestry and descent o Cladogenesis represents the formation of two distinct evolutionary lineages from a common ancestor Speciation the formation of distinct species by genetic divergence leading to reproductive isolation Begins when gene flow is interrupted genetic isolation Happens most often when populations are geographically separated from one another allopatric speciation Under some condition this can occur without geographic separation sympatric speciation How new species are typically formed in nature Allopatric speciation assumes that the establishment of geographic isolation between populations halts gene flow between them and allows them to diverge genetically Over time the accumulation of genetic differences may be sufficient to result in reproductive incompatibility o Via colonization of islands or new habitats Ex Hawaiian honey creepers 1 Dispersal and colonization 2 Genetic drift and selection 3 Two populations are isolated o Speciation with geographic isolation Ex antelope squirrels Elimination of gene flow leads to genetic divergence among the separated populations through selection genetic drift and or mutation o Allopatric speciation is most common mode of speciation in animals o Vicariance event Closure of the Isthmus of Panama divided a formerly unitary marine fauna inducing allopatric speciation on a massive scale Snapping shrimp populations were separated by the closing of the isthmus Separated populations diverged genetically now different species on each side of isthmus Sympatric speciation emphasized that possibility of genetic divergence and speciation proceeding in geographically overlapping populations Includes rapid speciation due to changes in chromosomal complements and also patterns of non random mating that restrict gene flow Formation of prezygotic reproductive barriers Changes in mating behaviors jumping spider courtship sexual selection Sexual selection in cichlid fishes has caused explosion of sympatric speciation in the African great lakes Lake victoria harbors a unique species rich flock of more than 500 endemic cichlid fishes o They are severely threatened by the introduction of exotic predators like the Nile perch and by environmental changes such as increased turbidity caused by human induced runoff Assortative mating depends on visual markers Remove the markers i e turn off the light and mating is random If water visibility if impeded fish can interbreed without loss of fertility so are genetically compatible o In this way human activities that increase turbidity destroy both the mechanism of diversification and that which maintains diversity Temporal Isolation periodical cicadas o Species have 13 or 17 year life cycles spend most of their lives as underground larvae emerge en masse after 13 or 17 years to molt into adults mate very noisily and die 13 year and 17 year broods can diverge genetically even in sympatry because of their temporally offset life cycles Habitat Isolation o Apple maggot flies mate on fruit of native hawthorne where their larvae bore into the fruit When apples were introduced into North America some flies switched to mate on apple fruit The populations diverged in sympatry due to reproductive isolation Gametic Isolation o Sea urchin Gamete Recognition proteins show high rates of evolution and may cause speciation if the sperm and egg receptors diverge so that gamete fusion is inhibited Mechanical isolation o Like many insects fleas have very complex genitalia especially in males All the parts must properly fit or stimulate the female reproductive tract in order for mating to be successful Many insect species can only be recognized morphologically based on differences in their genitalia Postzygotic Barriers Hybrid inviability o Hybrid embryos or juveniles die Ex hybrid frogs in genus Rana are frail and usually don t complete development Hybrid infertility o The liger is a hybrid between a male lion and a female tiger Ligers are typically sterile Sympatric speciation via polyploidy increase in the number of complete sets of chromosomes 2 basic modes o Autopolyploidy o Allopolyplodiy 11 05 2012 11 05 2012