lecture 13 - speciation
lecture 13 - speciation EEOB 3310 - 200
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This 3 page Class Notes was uploaded by an elite notetaker on Wednesday September 30, 2015. The Class Notes belongs to EEOB 3310 - 200 at Ohio State University taught by Joan Herbers,Zakee Sabree in Summer 2015. Since its upload, it has received 32 views. For similar materials see Evolution in Ecology at Ohio State University.
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Date Created: 09/30/15
93015 Lecture 13 Speciation part 1 The theory of evolution suggested that species were not fixed eternal entities name of Darwin s book was The Origin of Species Speciation is the key concept in evolutionary biology but what are species and how are they formed Not necessarily easy to identify the boundaries between species Criteria for identifying species that can be applied to all living things are difficult Many mechanisms promote the isolation of what was once one species and is now two Carl Linnaeus Systema Naturae 1735 first to propose hierarchical system kingdom class order family genus species Morphologically features were used to diagnose species Essentially species are essential fixed entities according to Linnaeus However he advocated the used of many tools that we still find useful today Morphospecies concept species are defined based on morphological similarities and differences rigorous analysis of the phenotype used to identify the limits of species This approach is widely applicable and easy to understand Mayr in 1942 modern synthesis presented a definition of species that united taxonomy animals and population genetic thinking of Fisher Wright Dobzhansky Mayr s book Systematics and the Origin of Species from the viewpoint ofa Zoologist may be the second most in uential book in the science According to Mayr biological species share a gene pool and are separated by other species via reproductive isolation The process by which reproductive isolation forms is important It can form in a variety of ways but geographic isolation is common physical separation When this occurs both local adaptation and genetic drift act to promote genetic divergence and eventual isolation Biological species concept species are groups of interbreeding or potentially interbreeding natural populations that are reproductively isolated from other such groups Mayr 1942 Dispersal leads to founder effect Speciation One island is inhabited another is not and migration leads to Speciation However vicariance is when a large island range subdivides at some point and the two new populations will evolve independently This is allopatric Speciation Studies of allopatry 1 Shrimp where Panama separates the Pacific Ocean with the Caribbean We should find related species on either side with a common ancestor from before formation of panama Found species of shrimp and gathered data from seven pairs of shrimp species Calculated amount of genetic difference Substantial genetic divergence between shrimp not identical population sizes etc lead to differences in amount of divergence Did breeding experiments to see how many species were still reproductively fertile Ability to breed directly correlated with the amount of genetic difference species more genetically similar to each other were better able to reproduce that those with larger genetic difference 2 Thickness of ice sheets 21000 years ago Pacific Northwest rainforests wet forests dominated by cedar and hemlock Mild and wet winters cool and dry summers Formation of Cascade Mountains during Pliocene 52 mya Glaciation occurs How did this affect allopatry In many species salamanders frogs cascades populations and inlands populations were very different from one another gene pools separated during glaciation Some did not speciate most likely due to ability to keep in contact Reconciling paleodistribution models Estimate the environmental requirements and therefore the potential geographic distribution of a species Fundamental niche the set of resources and environmental conditions necessary for an organism to maintain a viable population Realized niche the portion of the fundamental niche that the species actually occupies Habitat isolation is an example of a premating barrier to reproduction Other examples might include behavioral song differences timing Postmating prezygotic barriers Mating or transfer of gametes occur but no zygotes form 1 Mechanical copulation but no transfer of male gametes due to physical incompatibilities 2 Copulatory behavioral isolation failure of fertilization due to incompatible behaviors during copulation 3 Gametic isolation gametes are not transferred due to intrinsic incompatibilities such as sperm precedence or pollen tube precedence Postzygotic barriers hybrid zygotes are formed but have reduced fitness Darwin s dilemma How does postzygotic isolation evolve How can natural selection favor allow the evolution of unfit offspring Intrinsic caused by developmental defects of the hybrids independent of the environment liger Extrinsic environmentally dependent cause by ecological factors BatesonDobzhanskyMuller model BDM 1 Two allopatric populations AampB evolve separately The alleles are optimized for each population due to the environment Forms as a result of local adaptation and hybrid zones 2 Each experiences substitutions over a period of time genetic differentiation of AampB Would a gene from A work if translocated to B Hard to say but it would be unlikely to work better if you re lucky they might not be much worse 4 Some of the genes from A should be expected to perform worse in population B because there will have been some local adaptation The proportion of the A genome that would perform worse is a function of the degree of difference in the local adaptation of AampB 5 It is these genes that lead to extrinsic inviability because the mixture of genes will be a handicap Reinforcement one species undergoes allopatric differentiation If the barrier disappears the two populations Many experiments have been done on Drosophila Allopatric vs sympatric speciation Sympatric taxa have much more prezygotic isolation than do allopatric taxa If Fitness of Hybrids Fitness of Parental Form hybrid zone will be relatively wide and long lived Parental populations will eventually blend together as the 9 differentiation between the parental populations decreases Reabsorption of once separate groups If Fitness of Hybrids lt Fitness of Parental Form hybrid zone will be relatively narrow and short lived Reinforcement of reproductive isolation will lead to speciation Tension zone heterozygote inferiority Natural selection against hybrid genotypes in this situation should lead to reinforcement of barriers If Fitness of hybrids gt tness of parental form size of hybrid zone will depend on whether fitness advantage occurs in ecotone or in a new habitat Eventually either a stable hybrid zone or a new species will form Heterozygote advantage Environment dependent advantage Common on plants Reinforcement in sympatry after allopatric speciation females preferentially mate with their own species to produce more or fitter offspring than those females that mate at random Males benefit from mating preferentially with females of the same species rather than wasting resources on hybrid or heterospecific females Selection will favor alleles that confer mating discrimination Biological species concept is very important process oriented speciation happens over time conservation laws use in way in which species are defined As far as US government is concerned it is how we define species in biological and evolutionary sciences it is useful Speciation begins when populations become isolated physicalreproductive Once you are isolated you may someday come back into contact secondary contact In this case what happens with hybrids is important either reinforcement of barriers selection will reduce the fitness of hybrids and lead to reproductive isolation or hybridization fertile inbreeding gene ow reduces any genetic differentiation that has occurred while the populations were isolated Does this mean hybridization retards speciationinprogress
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