Measuring Evolution of Populations

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Measuring Evolution of Populations. Gene Variation is Raw Material. Evolution - change over time Evolution is descent with modification Darwin Through time, species accumulate differences such that ancestral and descendent species are not identical. Gene Variation is Raw Material. - PowerPoint PPT Presentation


  • Measuring Evolution of Populations

  • Gene Variation is Raw MaterialEvolution - change over timeEvolution is descent with modificationDarwinThrough time, species accumulate differences such that ancestral and descendent species are not identical.

  • Gene Variation is Raw MaterialNatural selection and evolutionary changeSome individuals in a population possess certain inherited characteristics that play a role in producing more surviving offspring than individuals without those characteristics.The population gradually includes more individuals with advantageous characteristics.

  • Darwin versus Lamarck

  • Macroevolution It is large evolutionary change Evolution of new species from a common ancestor Evolution of one species into two or more species

  • Microevolution Evolution on a small scale Is the change in gene frequencies within a population over time As the changes in populations accumulate, they can lead to the formation of new species.

  • Populations & gene poolsConceptsa population is a localized group of interbreeding individualsgene pool is collection of alleles in the populationremember difference between alleles & genes!allele frequency is how common is that allele in the population how many A vs. a in whole population

  • Hardy-Weinberg PrincipleHardy-Weinberg - original proportions of genotypes in a population will remain constant from generation to generationSexual reproduction (meiosis and fertilization) alone will not change allelic (genotypic) proportions.

  • Hardy-Weinberg PrincipleNecessary assumptionsAllelic frequencies would remain constant ifpopulation size is very largerandom matingno mutationno gene input from external sourcesno selection occurring

  • Hardy-Weinberg theoremCounting Allelesassume 2 alleles = B, bfrequency of dominant allele (B) = p frequency of recessive allele (b) = q frequencies must add to 1 (100%), so: p + q = 1bbBbBB

  • Hardy-Weinberg theoremCounting Individualsfrequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pqfrequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1bbBbBB

  • H-W formulasAlleles:p + q = 1

    Individuals:p2 + 2pq + q2 = 1bbBbBB

  • What are the genotype frequencies?Using Hardy-Weinberg equationq2 (bb): 16/100 = .16q (b): .16 = 0.4p (B): 1 - 0.4 = 0.6population: 100 cats84 black, 16 whiteHow many of each genotype?bbBbBBp2=.362pq=.48q2=.16Must assume population is in H-W equilibrium!

  • Using Hardy-Weinberg equationbbBbBBp2=.362pq=.48q2=.16Assuming H-W equilibriumSampled data p2=.742pq=.10q2=.16How do you explain the data? p2=.202pq=.64q2=.16How do you explain the data? Null hypothesis

  • Application of H-W principleSickle cell anemiainherit a mutation in gene coding for hemoglobinoxygen-carrying blood proteinrecessive allele = HsHsnormal allele = Hblow oxygen levels causes RBC to sicklebreakdown of RBCclogging small blood vesselsdamage to organsoften lethal

  • Sickle cell frequencyHigh frequency of heterozygotes 1 in 5 in Central Africans = HbHsunusual for allele with severe detrimental effects in homozygotes1 in 100 = HsHsusually die before reproductive ageWhy is the Hs allele maintained at such high levels in African populations?Suggests some selective advantage of being heterozygous

  • Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells123

  • Heterozygote AdvantageIn tropical Africa, where malaria is common:homozygous dominant (normal) die of malaria: HbHbhomozygous recessive die of sickle cell anemia: HsHsheterozygote carriers are relatively free of both: HbHssurvive more, more common in populationHypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.Frequency of sickle cell allele & distribution of malaria

  • Hardy-Weinberg equilibriumHypothetical, non-evolving populationpreserves allele frequenciesServes as a model (null hypothesis)natural populations rarely in H-W equilibriumuseful model to measure if forces are acting on a populationmeasuring evolutionary changeW. WeinbergphysicianG.H. Hardymathematician

  • Evolution of populationsEvolution = change in allele frequencies in a populationhypothetical: what conditions would cause allele frequencies to not change?non-evolving populationREMOVE all agents of evolutionary changevery large population size (no genetic drift)no migration (no gene flow in or out)no mutation (no genetic change)random mating (no sexual selection)no natural selection (everyone is equally fit)

  • 5 Agents of evolutionary change

  • 2005-20065 Agents of evolutionary change

  • Five Agents of Evolutionary ChangeMutationMutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.ultimate source of genetic variationGene flowmovement of alleles from one population to anothertend to homogenize allele frequencies

  • Five Agents of Evolutionary ChangeNonrandom matingassortative mating - phenotypically similar individuals mateCauses frequencies of particular genotypes to differ from those predicted by Hardy-Weinberg.

  • Five Agents of Evolutionary ChangeGenetic drift statistical accidents.Frequencies of particular alleles may change by chance alone.important in small populationsfounder effect - few individuals found new population (small allelic pool)bottleneck effect - drastic reduction in population, and gene pool size

  • Genetic Drift - Bottleneck Effect

  • Five Agents of Evolutionary ChangeSelection Only agent that produces adaptiveevolutionary changeartificial - breeders exert selection natural - nature exerts selectionvariation must exist among individualsvariation must result in differences in numbers of viable offspring producedvariation must be genetically inheritednatural selection is a process, and evolution is an outcome

  • Five Agents of Evolutionary ChangeSelection pressures:avoiding predatorsmatching climatic conditionpesticide resistance

  • Measuring FitnessFitness is defined by evolutionary biologists as the number of surviving offspring left in the next generation.relative measureSelection favors phenotypes with the greatest fitness.

  • Interactions Among Evolutionary ForcesLevels of variation retained in a population may be determined by the relative strength of different evolutionary processes.Gene flow versus natural selectionGene flow can be either a constructive or a constraining force.Allelic frequencies reflect a balance between gene flow and natural selection.

  • Natural Selection Can Maintain VariationFrequency-dependent selectionPhenotype fitness depends on its frequency within the population.Negative frequency-dependent selection favors rare phenotypes.Positive frequency-dependent selection eliminates variation.Oscillating selectionSelection favors different phenotypes at different times.

  • Forms of SelectionDisruptive selectionSelection eliminates intermediate types.Directional selectionSelection eliminates one extreme from a phenotypic array.Stabilizing selectionSelection acts to eliminate both extremes from an array of phenotypes.

    Sexual Selection

  • Kinds of Selection

  • Sexual selection Favours the selection of any trait that confers an advantage in terms of the mating success of the individualThis is associated with sexual dimorphism: which is the physical (often extreme) differences in the appearance of males and femalesThe most common forms of sexual selection are the results of female mate choice and male to male competition

  • Sexual selectionFemales can chose based on physical traits, colouration, or behavioural traits such as courtship displays and songsSometimes males develop features that enable them to establish and defend a territory from other males=sometimes detaining the femalesHow would you be able to tell these are not envtal selective pressures?Both sexes would possess the features.

  • Sexual selectionSome features are a compromise between mating and remaining conspicuous to predators==bright colours and song.

  • What about plants?Sexual diversity is not limited to just animalsPlants do not select mates but they do need to attract suitors to assist in pollinationFlowers and scents are the most obvious examples of sexual features that have evolved==maximize pollination

  • Selection on Color in GuppiesGuppies are found in small northeastern streams in South America and in nearby mountainous streams in Trinidad.Due to dispersal barriers, guppies can be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.

  • Evolution of Coloration in Guppies

  • Selection on Color in GuppiesHigh predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.In the absence of predators, larger, more colorful fish may produce more offspring.

  • Evolutionary Change in Spot Number

  • Limits to SelectionGenes have multiple effectsPleiotropy - ex. PKUEvolution requires genetic variationIntense selection may remove variation from a population at a rate greater than mutation can replenish.thoroughbred horsesGene interactions affect allelic fitnessepistatic interactions ex. You may have a widows peak but if