09-ch13-evolution2 - Populations Populations are the units...

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Unformatted text preview: Populations Populations are the units of evolution X individuals are selected populations evolve evolution – (1) all changes that have evolution transformed life on earth from its beginnings to its current diversity, resulting from (2) microevolution – any change in the genetic makeup of a population across generations population – localized group of individuals that are capable of interbreeding and producing fertile offspring gene pool – all genes (i.e., collection of allele variations) in a population at any one time recall that genes are carried on chromosomes; sexually reproducing species are diploid (carry two alleles for each character) (carry two alleles for each character) modern synthesis = Darwinian evolution Mendelian Darwinian evolution + Mendelian genetics Variation is extensive in most populations polymorphism – existence of two or more distinct phenotypes in the population Hawaiian happy face spiders http://evolution.berkeley.edu/ northern cricket frog (check your local ponds) Mutation Mutation & sexual recombination generate variation Mutation is the raw material for natural selection mutation – change in DNA nucleotide sequence; corresponding change in mRNA codon may alter protein synthesis • mutation = source of all heritable variation • many mutations do not affect phenotype • most that do are deleterious that do are deleterious • once in a while... under certain conditions... mutations can be beneficial conditions mutations can be beneficial • individual with beneficial mutation can produce more offspring than competitors produce more offspring than competitors (natural (natural selection in action) consider a population of Ghostus pacmanus which yields a new generation... yields new generation what happened here? several generations later... generations later hypotheses? In In addition to natural selection, gene flow & genetic drift can cause evolution gene flow – genetic input from other populations can change allele frequencies Our ghost population is joined by a few migrant orange ghosts from the other side of the labyrinth... can we predict the consequences? genetic drift – random fluctuation in gene pool (especially important in small populations) Examples Examples of genetic drift include bottleneck & founder effects bottleneck effect – sudden event dramatically reduces population size (at random with respect to individual phenotypes)... consider consequences in future gene pool if this population reaches its previous size over generations, will the genetic composition be similar to that prior to the bottleneck effect? similar to that prior to the bottleneck effect? predict how it will be different... bottleneck bottleneck effect – sudden event dramatically reduces population size; future variation northern elephant seal (California coast) hunting reduced pop size to ~20 by 1890s genetic variation much reduced (i.e., compared to southern elephant seal) threats include disease, pollution, El Niño black-footed Ferret (Wyoming high desert) http://www.pbs.org/wnet/nature/episodes/the-loneliest-animals/video-full-episode/4935 “The Loneliest Animals” min 42:12-50:00 42:12- Examples Examples of genetic drift include bottleneck & founder effects founder effect – same result as bottleneck, different mechanism: isolated group starts new population with no further gene flow from source population these two individuals migrate to new habitat these two individuals migrate to a new habitat (where (where a geographical barrier prevents further interaction with the rest of the group). genetic variation for color has been eliminated (red allele is fixed in future generations) founder founder effect – same result as bottleneck, different mechanism: isolated group starts new population with no further gene flow new population with no further gene flow from from source population Pennsylvania Amish increased frequency of Ellis-van Creveld syndrome frequency of Ellis Creveld syndrome (polydactylism / dwarfism / heart defects) inbreeding inbreeding (from bottleneck; bottleneck; founder; artificial selection) gene flow – genetic input from other gene populations can change allele frequencies how does this affect variation? inbreeding is a consequence of the bottleneck & founder effects; artificial selection http://news.bbc.co.uk/2/hi/uk_news/7569064.stm vs. natural selection in feral dogs dingo, desert (Canaan) dog, New Guinea singing dog, pariah dog, Carolina dog 50 years of artificial selection in Russian foxes How fur industry efforts to improve How fur industry efforts to improve the the efficiency of fox farms revealed unexpected details about the domestication process... http://www.youtube.com/watch?v=lR-GHmuumAw&NR=1 http://cbsu.tc.cornell.edu/ccgr/behaviour/Index.htm Natural Natural selection can alter variation in a population in three ways directional selection – natural selection that th favors individuals at one end of the phenotypic range disruptive selection – favors individuals on both extremes of phenotypic range stabilizing selection – favors intermediate variants by acting against both extremes Natural Natural selection is the primary mechanism of adaptive evolution http://www.hhmi.org/biointeractive/evolution/animations.html Pocket mice exhibit polymorphism for coat color (dark vs. light). Which is more fit? sexual selection natural selection for mating success; may produce may produce sexual dimorphism distinct male vs. male vs female adult phenotypes (secondary sex (secondary sex characteristics) http://darwin-online.org.uk http://www.pbs.org/wgbh/evolution/library/01/6/l_016_09.html intrasexual selection – direct competition (often by males) for potential mates coho salmon salmon marine phase spawning “hooknose” spawning female “intra” = within Finding Finding Beauty in Dung http://www.npr.org/blogs/pictureshow/2009/05/dung_beetles.html intersexual selection – mate choice; individuals of one sex (usually females) are choosy in selecting their mates selecting their mates “inter” = between Sexual Sexual selection may exert pressure opposite other selective pressures Sometimes being sexy has its drawbacks. Popular male frogs with loud calls are easy pickings for predators Sorry dude, nature is with good ears. full of such “trade-offs.” “trade- Unfortunately, sometimes the trade-offs tradeare not so natural. not so nat http://www.npr.org/templates/story/story.php?storyId=113717563 balancing selection – maintains stable frequencies of multiple phenotypic forms for example, via... frequency dependent selection – success of one phenotypic morph declines when it becomes too common in the population Thus, balancing balancing selection helps to preserve variation! Alternative Alternative mating strategies are a fascinating example of frequency-dependent selection! frequency- freshwater isopods have three different male morphs Check out the cricket cartoon! (follow the link below) http://evolution.berkeley.edu/evolibrary/article/0_0_0/sneakermales_01 Alternative Alternative mating strategies are a fascinating example of frequency-dependent selection! frequency- coho salmon salmon marine phase spawning “hooknose” spawning female vs. sneaker “Jack” ...
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This note was uploaded on 12/12/2011 for the course BIO 2 taught by Professor Poenie during the Fall '08 term at University of Texas at Austin.

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