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The Evolution of Populations and the Origin of Species Chapters 23-24 • Evolution happens to populations/ species over time. • Lamarckian evolution - evolution result of change in individual in response to environment (i.e. giraffe stretching its neck to eat) incorrect hypothesis. http://necsi.org/projects/evolution/lamarck/lamarck/giraffes.jpg • Population composed of many different genotypes, phenotypes because of alleles carried in population. • Sum total of all alleles in population - gene pool; variation in gene pool variations in individual phenotypes. http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/graphics/C21_GenePool_2.GIF • Measure of genetic variation in population - allele frequency of gene. • # of copies of allele divided by total # of copies of gene in population. http://anthro.palomar.edu/vary/images/map_of_A_blood_allele.gif http://www.uni-kiel.de/medinfo/mitarbeiter/krawczak/folien/vorlesung2/img002.jpg • If population does not change Hardy-Weinberg equilibrium – no evolution. • Frequencies change - evolution occurring. http://www.southtexascollege.edu/crj/human%20evolution.jpg • Conditions for population to stay at Hardy-Weinberg equilibrium: • 1Random mating occurs. • 2Population large enough to avoid random statistical fluctuations in frequencies. • 3No mutation. http://w3.dwm.ks.edu.tw/bio/activelearner/18/images/ch18c2.jpg • 4No migration into/out of population. • 5No natural selection. • Under conditions - free flow of genes between members of same species. • Alleles shuffled up from 1 generation to next. http://www.artgame.com/images3/migration.jpg • In wildflower population of 500, 80% (0.8) of flower color alleles are R and 20% (0.2) are r. • Each gamete - 1 allele for flower color; gamete drawn from gene pool at random has 0.8 chance of bearing R allele, 0.2 chance of bearing r allele. http://k43.pbase.com/u29/gaocus/upload/17965482.wildflower.jpg • Rule of multiplication - frequencies of 3 possible genotypes in next generation. • RR genotype - probability of picking 2 R alleles is 0.64 (0.8 x 0.8 = 0.64 or 64%). • rr genotype - probability of picking 2 r alleles is 0.04 (0.2 x 0.2 = 0.04 or 4%). • Heterozygous individuals - either Rr or rR - R allele from sperm or egg. • Probability of ending up with both alleles is 0.32 (0.8 x 0.2 = 0.16 for Rr, 0.2 x 0.8 = 0.16 for rR, and 0.16 + 0.16 = 0.32 or 32% for Rr + rR). • p = gene frequency of dominant allele, q = frequency of recessive allele; p + q = 1. • Equation for Hardy-Weinberg principle is: • p2 + 2pq + q2 = 1 • 2pq - # heterozygotes in population. http://bill.srnr.arizona.edu/classes/182/GeneFreqs/HardyWeinberg-lg.jpeg • Hardy-Weinberg equilibrium, frequency of dominant homozygous curly hair (CC) is 64%. Percentage with curly hair? • p = frequency dominant allele (C) q frequency recessive allele (c). • CC frequency 64% so p2 = .64; p = .8. • (p + q = 1) q = 1 - .8 = 2. • Individual with curly hair - either CC or Cc. • Percentage of population with curly hair is p2 + 2pq = .64 + 2(.8*.2) = .96 or 96% of population. Instabilities in populations • Conditions can change HardyWeinberg equilibrium. • Mutations cannot happen in equilibrium; occurs in real world. • Errors in DNA replication accumulate over time as well as mutagenic factors in environment. • Mutations can lead to new alleles not previously in gene pool. http://www.sciencemuseum.org.uk/exhibitions/genes/images/1-3-5-1-2-2-2-2-2-0-0.jpg • Mutations either neutral or harmful on survival of individual. • New phenotypes in population raw material that natural selection acts on to drive evolution; mutations only source of new alleles. • Migration affects equilibrium. • Different populations have different allelic frequencies in gene pools. http://www.geo.arizona.edu/Antevs/nats104/00lect26humigrout.jpg • 1 population breeds with another population, frequencies of alleles change (gene flow). • Small population more likely to have random event than large population. http://web.pdx.edu/~mfish/image019.jpg • Genetic drift - changes in allele frequencies in small population caused by random events. • Even in large population if small # of individuals pass on traits can decrease diversity. • Individuals that do not pass traits on may have harmful alleles - alters gene pool of next generation (2 ways) • 1Bottleneck - large population reduced to small # by disease, natural disaster, overhunting/fishing. • Individuals left eventually reproduce, generations not representative of original gene pool. • Inbreeding usually follows bottleneck; individuals with same recessive genes have more chance of passing harmful gene on. • Population more susceptible to disease/infections that may not have occurred with more diversity in population. http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/graphics/C21_Bottleneck_2.GIF • 2Founder effect - small # of individuals of species migrate into new habitat. • If only a few individuals colonize new area, new population reflect only their gene pool not larger gene pool where they came from. http://www.answersingenesis.org/creation/images/v18/i3/p13_step3.JPG • Nonrandom mating - equilibrium cannot occur. • Individuals must choose mate randomly without respect to phenotype. • If phenotype influences selection, genotypes and phenotypes of population will be changed. http://bioweb.wku.edu/courses/Biol430/wsquirrelHR.jpg • Self-fertilization in plants has this effect. • Reduces # of heterozygotes in population; increases # of homozygotes. • Many species exhibit sexual selection (form of nonrandom mating) http://www015.upp.so-net.ne.jp/shuri/shuri_study/what_is_si.GIF • Natural selection - differential production of offspring based on inherited traits. • Individuals with more favorable phenotypes may survive, reproduce; alters population frequencies. • Fitness - key description of natural selection. http://www.telomere.org/images/Sloth.jpg • Fitness - organism’s ability to contribute alleles, traits to future generations. • Factors involved - ability to survive to reproductive age, mate and produce offspring, raise offspring to maturity. • Other factors - ability to escape predation, gather food, attract mates, or care provided to offspring. http://www.agpix.com/catalog/AGPix_CoTo11/large/AGPix_CoTo11_0092_Lg.jpg • Individual with long life but few offspring - poor fitness if other individuals have more offspring. • Animals that take care of offspring - greater fitness than those that do not. • Balanced by having more offspring that receive little care/fewer offspring that receive more care. http://www.facstaff.bucknell.edu/ddearbor/BFAL_feeds.jpg • Three types of selective pressures that affect natural selection over time. • Any given population, trait distribution bell-shaped. http://www.csulb.edu/~kmacd/346NotesI_files/normalCurve.gif • 1Stabilizing selection - not change average, tends to sharpen curve. • Newborns can have problems if too large or too small at birth – stabilizing selection pushed average to 8 pounds; perfect size for newborn infant. http://img.sparknotes.com/figures/A/a3aa6bb95c7d70781cc0089d17f9160f/stable.gif • 2Disruptive selection - either side of bell curve favored - leads to 2 different peaks in distribution of population. • 3Directional selection - change in average for trait in population occurs. • Giraffes live in area where vegetation high up - favor those with longer necks - drives evolution of longer necks. • Kin selection - individuals with many of same alleles live with other members with same alleles. • Lions live with many female relatives, help raise young, even if it is not his. • Increases fitness of mother, even if it doesn’t increase fitness of male. http://www.anth.ucsb.edu/faculty/gurven/images/orangatan.jpg • Species - group of organisms able to interbreed productively with rest of group, not with other organisms. • Reproductive isolation – 2 populations of birds live in different areas never mix - not necessarily separate species. http://www.lakelandwildlife.co.uk/images/species2.jpg • If they breed and produce fertile offspring when placed together same species. • Separation of species can promote evolution into 2 separate species no gene flow possible between 2 species. • Result of separation of gene pools over period of time. • Cladogenesis - formation of 2 species from 1 ancestor species. • Occurs when populations occupy same area or when separated geographically from each other. • 1Allopatric speciation - populations separated by geographic barrier followed by reproductive isolation. http://taxonomy.zoology.gla.ac.uk/~rdmp1c/teaching/L1/Evolution/l6/grandcanyon.gif • Evolution of many diversely adapted species from common ancestor adaptive radiation. • 2Sympatric speciation - speciation by populations that occupy same region. • New species arise within range of parent populations. • Can occur through sudden dramatic genetic change - result of polyploidy of genome. • Can occur within species if individual spontaneously (through mistake in meiosis) produces offspring with 2X normal chromosomal number. • Also result when cross between 2 related species produces hybrid with chromosomal average of both parents. http://en.wikipedia.org/wiki/Hybrid_animals A sheep goat • Tetraploid cannot produce fertile offspring if it mates with diploid, could fertilize itself through selfpollination if plant. • Polyploidy more common in plants has selective advantage over diploid parents. • Mechanisms of reproductive isolation that cause speciation divided into 2 groups. • 1Prezygotic barriers prevent 2 species from mating. • Occurs several ways. http://www.sci.uidaho.edu/bionet/biol115/t9_species/images/L9_Mating-Barriers.jpg • AHabitat isolation: species separated because of where they live. • BBehavioral isolation: species have different mating habits - never interact. • CTemporal isolation: breed at different times of the year. • DMechanical isolation: not anatomically correct for each other. • 2Postzygotic barriers do not prevent mating - prevent formation of fertile offspring. • Examples of this - reduced hybrid viability (hybrid aborts spontaneously), reduced hybrid fertility (hybrids infertile), hybrid breakdown (1st generation hybrids viable and fertile, next generation feeble or sterile). http://www.kyhorsepark.com/imh/bw/images/dmconf.jpg • Rate of speciation and relationship between speciation and evolution still controversy. • Punctuated equilibrium - evolution occurs rapidly in association with speciation in small isolated populations followed by long periods where species changes little.