Presentation on theme: "Evolution of Populations"— Presentation transcript:
1 Evolution of Populations Chapter 23.Evolution of Populations
2 Bent Grass on toxic mine site Populations evolveNatural selection acts on individualsdifferential survival“survival of the fittest”differential reproductive successwho bears more offspringPopulations evolvegenetic makeup of population changes over timefavorable traits (greater fitness) become more commonBent Grass growing on mine tailings; only individuals tolerant of toxic heavy metals will grow from the seeds blown in from nearby fieldBent Grass on toxic mine site
4 Not every mutation has a visible effect. Mutation & VariationMutation creates variationnew mutations are constantly appearingMutation changes DNA sequencechanges amino acid sequence?changes protein?change structure?change function?changes in protein may change phenotype & therefore change fitnessEvery individual has hundreds of mutations1 in 100,000 bases copied3 billion bases in human genomeBut most happen in introns, spacers, junk of various kindNot every mutation has a visible effect.Some effects on subtle.May just affect rate of expression of a gene.
5 Sex & Variation Sex spreads variation one ancestor can have many descendantssex causes recombinationoffspring have new combinations of traits = new phenotypesSexual reproduction recombines alleles into new arrangements in every offspring
6 Variation impacts natural selection Natural selection requires a source of variation within the populationthere have to be differencessome individuals must be more fit than others
7 Changes in populations Evolution of populations is really measuring changes in allele frequencyall the genes & alleles in a population = gene poolFactors that alter allele frequencies in a populationnatural selectiongenetic driftfounder effectbottleneck effectgene flow
8 Natural selectionNatural selection adapts a population to its environmenta changing environmentclimate changefood source availabilitynew predators or diseasescombinations of alleles that provide “fitness” increase in the population
9 Genetic drift Effect of chance events founder effect bottleneck small group splinters off & starts a new colonybottlenecksome factor (disaster) reduces population to small number & then population recovers & expands again1 family has a lot of children & grandchildren therefore has a greater impact on the genes in the population than other familiesGenghis Khan tracked through Y chromosome.
10 Founder effectWhen a new population is started by only a few individualssome rare alleles may be at high frequency; others may be missingskew the gene pool of new populationhuman populations that started from small group of colonistsexample: white people colonizing New WorldSmall founder group, less genetic diversity than AfricansAll white people around the world are descended from a small group of ancestors100,000 years ago(Chinese are white people!)
11 Distribution of blood types Distribution of the O type blood allele in native populations of the world reflects original settlementSouth & Central American Indians were nearly 100% type O for the ABO blood system. Since nothing in nature seems to strongly select for or against this trait, it is likely that most of these people are descendants of a small band of closely related "founders" who also shared this blood type
12 Distribution of blood types Distribution of the B type blood allele in native populations of the world reflects original migrationThe global frequency patterns of the type B blood allele: Note that it is highest in central Asia and lowest in the Americas and Australia. However, there are relatively high frequency pockets in Africa as well. Overall in the world, B is the rarest ABO blood allele.
13 Out of Africa Likely migration paths of humans out of Africa According to the "Out of Africa" theory, modern humans appeared as a single African species nearly 100,000 years ago, then spread throughout the world (K.Wong, Is Out of Africa Going Out the Door?, Scientific American 281(2), August 1999).Many patterns of human traits reflect this migration
14 Bottleneck effectWhen large population is drastically reduced by a disasterfamine, natural disaster, loss of habitat…loss of variation by chancealleles lost from gene poolnarrows the gene pool
15 Cheetahs All cheetahs share a small number of alleles 2 bottlenecks less than 1% diversityas if all cheetahs are identical twins2 bottlenecks10,000 years agoIce Agelast 100 yearspoaching & loss of habitat
16 Conservation issuesBottlenecking is an important concept in conservation biology of endangered speciesloss of alleles from gene poolreduces variationreduces ability to adaptat risk populations
17 Gene flow Population spread over large area migrations = individuals move from one area to anothersub-populations may have different allele frequenciesMigrations cause genetic mixing across regions = gene flownew alleles are moving into gene poolreduce differences between populations
18 Are we moving towards a blended world? Human evolution todayGene flow in human populations is increasing todaytransferring alleles between populationsAre we moving towards a blended world?
20 Measuring Evolution of Populations Chapter 23.Measuring Evolution of Populations
21 Populations & gene pools Conceptsa 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 populationhow many A vs. a in whole population
22 Evolution of populations Evolution = change in allele frequencies in a populationhypothetical: what would it be like if allele frequencies didn’t change?non-evolving populationvery large population size (no genetic drift)no migration (movement in or out)no mutation (no genetic change)random mating (no sexual selection)no natural selection (no selection)
23 Hardy-Weinberg equilibrium Hypothetical, non-evolving populationpreserves allele frequenciesServes as a modelnatural populations rarely in H-W equilibriumuseful model to measure if forces are acting on a populationmeasuring evolutionary changeG.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?"G.H. HardymathematicianW. Weinbergphysician
24 Hardy-Weinberg theorem Allelesassume 2 alleles = B, bfrequency of dominant allele (B) = pfrequency of recessive allele (b) = qfrequencies must add to 100%, so:p + q = 1BBBbbb
25 Hardy-Weinberg theorem Individualsfrequency of homozygous dominant: p x p = p2frequency of homozygous recessive: q x q = q2frequency of heterozygotes: (p x q) + (q x p) = 2pqfrequencies of all individuals must add to 100%, so:p2 + 2pq + q2 = 1BBBbbb
26 Using Hardy-Weinberg equation population: 100 cats84 black, 16 whiteHow many of each genotype?q2 (bb): 16/100 = .16q (b): √.16 = 0.4p (B): = 0.6p2=.362pq=.48q2=.16BBBbbbMust assume population is in H-W equilibrium!
27 Using Hardy-Weinberg equation p2=.362pq=.48q2=.16Assuming H-W equilibriumBBBbbbNull hypothesisp2=.20p2=.742pq=.642pq=.10q2=.16q2=.16Sampled data 1:Hybrids are in some way weaker.Immigration in from an external population that is predomiantly homozygous BNot random mating... white cats tend to mate with white cats and black cats tend to mate with black cats.Sampled data 2:Heterozygote advantage.What’s preventing this population from being in equilibrium.bbBbBBSampled dataHow do you explain the data?How do you explain the data?
28 How do allele frequencies change? Think of all the factors that would keep a population out of H-W equilibrium!
29 Real world application of H-W Frequency of allele in human populationExample:What % of human population carries allele for PKU (phenylketonuria)Should you screen prospective parents?~ 1 in 10,000 babies born in the US is born with PKUresults in mental retardation, if untreateddisease is caused by a recessive allelePKU = homozygous recessive (aa)PKU (phenylketonuria) is a rare, inherited metabolic disease that results in mental retardation and other neurological problems when treatment is not started within the first few weeks of life. When a very strict diet is begun early and well-maintained, affected children can expect normal development and a normal life span.The disease arises from the absence of a single enzyme (phenylalanine hydroxylase). This enzyme normally converts the essential amino acid, phenylalanine, to another amino acid, tyrosine. Failure of the conversion to take place results in a buildup of phenylalanine. Through a mechanism that is not well understood, the excess phenylalanine is toxic to the central nervous system and causes the severe problems normally associated with PKU.PKU is carried through an autosomal recessive gene. The incidence of carriers in the general population is approximately one in fifty people, but the chance that two carriers will mate is only one in Carrier tests are available only through PKU treatment programs.
30 H-W & PKU diseasefrequency of homozygous recessive individuals q2 (aa) = 1 in 10,000 =frequency of recessive allele (q): q = √ = 0.01frequency of dominant allele (p):p (A) = 1 – 0.01 = 0.99frequency of carriers, heterozygotes:2pq = 2 x (0.99 x 0.01) = = ~2%~2% of the US population carries the PKU allele 300,000,000 x .02 = 6,000,000 people