Fractional reduction in heterozygosity Within the entire population A numerical demonstration A numerical demonstration A numerical demonstration Gene flow vs. drift Gene flow and drift opposing forces What happens with changes in migration rates (m or g) and population sizes? Can they achieve an equilibrium? Let?s explore with HYPERLINK "..\\populus 5.3\\Populus Files\\Populus.jar" POPULUS Mendelian genetics, population structure Relationship of gene flow, population size, and FST Relationship of gene flow, population size, and FST Gene flow is a powerful force Low levels of gene flow eliminate the effects of drift If subpopulations exchange only one individual each generation (Nm = 1), FST drops to 0.2 Can obtain estimates of Nm from FST Estimates FST = 1 Nm = 0 (no migration) FST = 0.5 Nm = 0.25 (1 migrant every 4th generation) FST = 0.33 Nm = 0.50 (1 migrant every other generation) FST = 0.2 Nm = 1 (1 migrant each generation) FST = 0.11 Nm = 2 (2 migrants each generation) Data from some real studies How do these processes interact to cause evolution? Evolutionary processes mutation selection drift nonrandom mating gene flow WRIGHT'S SHIFTING BALANCE MODEL OF EVOLUTION Wright?s model Recall adaptive landscapes Increase # of loci, increase complexity How might evolution occur in such a species? Local exploration (within a population) Mutation introduces variants within subpopulations and changes in topography if deleterious, will be eliminated if favorable, will increase in frequency may not occur often... genetic drift allows peak shifts in small population remember drift can overcome selection, allowing subpopulations to cross valleys! How might evolution occur in such a species? When a favorable genotype occurs (through mutation or drift) will climb the peak rapidly (depends on intensity of selection) This has been called mass selection How might evolution occur in such a species? What happens as fitness increases within a population? N increases number of migrants increases How might evolution occur in such a species? If the number of migrants is large and they are more fit, will rapidly cause all other populations to have the same genotype Shifting balance Each population explores the landscape independently Favored genotypes could quickly sweep through all populations Shifting balance Assumes specific model of gene flow Low enough that: Populations evolve independently Drift allows crossing valleys to reach highest peak High enough that: Migrants can move among regions to allow interdemic selection to work Shifting balance As you might guess this is problematic Never been observed Interdemic selection has been demonstrated in flour beetles Serious debate over the importance of this model in the ?real? world Nevertheless, illustrates the significance and potential interplay of the forces we?ve discussed Now that we?ve put it all together, we understand how populations can evolve Where do species come from? Speciation Why are there so many species? How are species generated and what allows them to persist? Some creationists accept microevolution (what we just finished discussing), but do not accept larger scale changes Hard to understand this perspective given evidence of how evolution occurs Hierarchy of taxa Diversity follows a distinctive hierarchy: What is a species? Just a few of the many definitions: Typological ?what a competent taxonomists says it is? Phylogenetic ?irreducible cluster of organisms that are diagnosably distinct from other such clusters and within which there is parental pattern of ancestry and descent? What is a species? Evolutionary ?population or groups of populations that shares a common evolutionary fate through time? Biological ?groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups? Which is good for you? Highly debated as to which is best Choice generally based on area of research interest For convenience, we apply a the biological species concept Process-based (like this class!) doesn?t mean it?s the best, just the one that fits our approach Reproductive isolation Reproductive discontinuity is the key to all species concepts Species must evolve independently What allows them to remain distinct forms when they occur together? Reproductive isolation Several factors responsible for isolation at two levels: Premating prevent mating or fertilization Postmating prevent the development of fertile adults Premating reproductive isolation Temporal flowering time in plants breeding time in frogs Premating reproductive isolation Ecological habitat differences Premating reproductive isolation Behavioral different visual cues songs lacewings HYPERLINK "http://www.eeb.uconn.edu/faculty/henry/Cryptic_songs.html" Premating reproductive isolation Mechanical lock and key mechanism Postmating reproductive isolation Gametic mortality plant incompatibility systems Zygotic mortality fertilized eggs fail to develop and hatch Postmating reproductive isolation Hybrid breakdown Postmating reproductive isolation Hybrid inviability hybrids die before reproducing Postmating reproductive isolation Hybrid sterility individuals viable but unable to produce functional gametes How might barriers to reproduction arise? Variation generated by: Mutation Recombination (e.g., independent assortment) Within population forces: Selection Directional selection (eliminates variation) Underdominance (mostly eliminates variation - in concert with drift) Overdominance (maintains variation) Drift (eliminates variation) Nonrandom mating (eliminates variation) How might barriers to reproduction arise? Among population forces: Selective differences between environments (promotes variation) Drift (promotes variation) Gene flow (homogenizes populations) How do all of these processes interact to produce reproductively isolated lineages? Geography is important! Terms used to describe alternate geographic placement Allopatry populations/species occupy distinct geographic regions (nonoverlapping ranges) Geography is important! Sympatry populations/species occupy same geographic region with ability to interact Geography is important! Parapatry populations/species occupy adjacent geographic regions Why is geography important? A remarkable feature of this state is the Grand Canyon Impressive barrier 5 million years old Different subspecies (?) of squirrels on the north and south side Evidence indicates they have been isolated at least 10,000 years (as many as 5 million) How might this happen? Allopatric speciation Geographic isolation can lead to divergence Factors causing isolation have been common in earth?s history volcanoes mountains rivers glaciers How do we know they will become different species? What happens if barrier is removed? Two possible outcomes If hybrids between A? and B have same fitness as their parents, former isolates fuse back into single species If hybrids are less fit then their parents A? and B (postmating isolation), premating differences will evolve to prevent wastage of gametes How does this work? Allopatric speciation Individuals that can discriminate among A? and B will be favored because: If they mate assortatively they leave more offspring This process is called REINFORCEMENT Of course, they could evolve premating differences by chance while isolated, then they might never hybridize!
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