AP Biology Chapter 23 The Evolution of Populations Campbell and Reece 10th Edition

Individuals do not evolve, populations do over time Medium Ground Finch from island of Daphne Major in Galápagos Islands Long period of drought altered their food supply to mostly larger nuts & over the years those individuals with larger beaks were more successful

Overall size of bird b/4 & after drought years

Average beak size & size of individual birds larger after the drought so…. The medium ground finch population had evolved by natural selection

Genetic Variation as Cause of Evolution Darwin reasoned that natural selection acted on genetic variation of populations He knew nothing about genes Few yrs later: Mendel’s paper on inheritance in pea plants: stage set for understanding variation

Genetic Variation Genotype inheritable, phenotypes are not Example: moth, Nemoria arizonaria, appears very different eating oak flowers vs. oak leaves

Phenotype is result of : genotype + environmental influence

In general, only the genetically determined part of a phenotype can affect evolution Discrete characters = “either/or” ( Mendel’s pea) = single gene Most heritable variations involve quantitative characters: vary along a continuum ≥ 2 genes

Average Heterozygosity way to quantify gene variability average % of loci that are heterozygous can calculate average: turns out if the average heterozygosity is 14% there is enough genetic variation for natural selection to act  evolutionary change

Gel Electrophoresis used to calculate heterozygosity

Gel Electrophoresis does not show silent mutations (DNA changes but still codes for same a.a.)

Gene Variation Between Populations Geographic Variation differences in genetic composition of separate populations

Cline: a graded change in a character along a geographic axis

Sources of Genetic Variation Mutation Gene Duplication Sexual Reproduction Other process that results in new alleles or new genes

Sources of genetic variation Organisms with short life spans new genetic variants arise fairly rapidly

Formation of New Alleles Mutations can’t predict where in genome or what type mutation for multicellular organisms only mutations in gametes cell line passed to new generations (most are in somatic cell line) most point mutations silent or only slightly harmful, rarely are they beneficial

Altering Gene # or Position chromosomal changes that delete, disrupt, or rearrange usually lethal or harmful if genes left intact they may be neutral changes Translocation: Part of 1 chromosome breaks off & attaches to another chromosome

Translocations

Duplications of Chromosomes if large segments duplicated usually harmful duplications of small pieces may be beneficial mutations accumulate over time eventually that duplication takes on new role end result: expanded genome

Rapid Reproduction average mutation rate in plants & animals is considered low ~ 1 mutation in every 100,000 genes / generation

Prokaryotic Mutation Rates shorter generation spans allows for generation of genetic variation in a population virus populations, especially retroviruses process is fastest

HIV single stranded RNA: less complicated to duplicate fewer RNA repair mechanisms in host cells

HIV most effective treatment for a quickly mutating retrovirus has been combination protocols

Sexual Reproduction most of genetic variation due to crossing over and independent assortment of chromosomes in meiosis and fertilization

Hardy-Weinberg Equation can tell you if a Population is Evolving 1 of 3 factors that cause evolution must be at work in a population Population: group of same species in same area that interbreed, with fertile offspring presence of genetic variation does not guarantee that population is evolving

Populations examples of isolated populations: Islands Lakes even populations not strictly isolated members tend to breed with own population so are genetically closer to them than other groups

Gene Pools consists of all copies of every allele at every locus in all members of a population

Fixed Genes if there is only 1 allele for a locus that allele is said to be fixed in the gene pool; entire population is homozygous for that gene if there are 2 or more alleles for a locus then individuals may be homozygous or heterozygous

Each allele has a frequency in the population

The Hardy-Weinberg Principle to test whether natural selection is acting on a particular locus: Determine what the frequency would be if it were not evolving Then compare that calculation with what you measure in the population No difference: not evolving difference: evolving

Hardy-Weinberg Principle 1908

Hardy-Weinberg Principle states that the frequencies of alleles & genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation & recombination of alleles are at work If that is true the population is said to be in HARDY-WEINBERG EQUILIBRIUM

Hardy-Weinberg Equilibrium

Hardy-Weinberg Equilibrium

assumes random mating

Hardy-Weinberg Problems http://nhscience.lonestar.edu/biol/hwe/q1d.html http://www.phschool.com/science/biology_place/labb ench/lab8/intro.html Problem 2: If 9% of an African population is born with a severe form of sickle cell anemia (ss) what % of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

Answer to problem 2 2pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers)

Conditions for Hardy-Weinberg Equilibrium No Mutations Random Mating No Natural Selection Extremely Large Populations No Gene Flow

Departure from any of the 5 conditions usually results in evolutionary changes A population may be evolving at some gene loci and in Hardy-Weinberg Equilibrium at other loci

Applying the Hardy-Weinberg Principle Can be used to estimate the frequency of a gene causing inherited disease in a population Must assume: No new mutations Random mating Ignore any effects of differential survival & reproductive success No genetic drift

How Allele Frequencies are Altered in a Population

Conditions Necessary for H-W Equilibrium No Mutations: not usually significant unless mutation produces new alleles that have a strong influence in a (+) or (-) way Random Mating: not usually significant No Natural Selection cause most Extremely Large Populations evolutionary No Gene Flow change

NATURAL SELECTION is based on differential success in survival & reproduction if NS consistently favoring some alleles over others, NS can cause adaptive evolution (dfn: evolution that results in a better match between organisms & their environment)

GENETIC DRIFT process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next the smaller the population the more pronounced the effect

GENETIC DRIFT

2 Examples of Genetic Drift Founder Effect genetic drift that occurs when a few individuals become isolated from a larger population & form a new population whose gene pool composition is not reflective of original population

Founder Effect

Founder Effect Tristan da Cunha 15 British colonist in 1814

Tristan da Cunha 1 colonist carried recessive allele for retinitis pigmentosa

Tristan da Cunha By late 1960’s, there were 240 descendants of the original founders 4 had retinitis pigmentosa This frequency is 10x higher than frequency of retinitis pigmentosa in England

2nd Example of Genetic Drift 2. Bottleneck Effect: occurs when the size of a population is reduced, as by a natural disaster or human actions. The resulting population is genetically different than original population.

Bottleneck Effect

Summarizing Effects of Genetic Drift is significant in small populations can cause allele frequencies to change at random can lead to a loss of genetic variation w/in populations can cause harmful alleles to become fixed

GENE FLOW the transfer of alleles from one population to another as result of movement of fertile individuals or their gametes

Gene Flow

Gene Flow transferred alleles may increase a population’s ability to adapt to local conditions Culex pipiens spread of insecticide- resistant alleles used to treat mosquitoes to prevent spread of West Nile

Natural Selection is only mechanism that consistently causes adaptive evolution outcome of NS is not random NS increases frequencies of alleles that provide reproductive advantage so, leads to adaptive evolution NS acts directly on the phenotype & indirectly on the genotype

Relative Fitness the contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individuals in the population

Types of Selection DIRECTIONAL SELECTION conditions favor individuals favoring one extreme of a phenotype shifts curve in one direction or other

Types of Selection 2. DISRUPTIVE SELECTION conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes

Types of Selection 3. STABILIZING SELECTION conditions favor the intermediate phenotype and act against both extremes reduces variation

ADAPTIVE EVOLUTION Natural selection will increase the frequencies of alleles that enhances survival & reproduction so… over time adaptations arise genetic drift & gene flow may cause changes that are either advantageous or disastrous

Sexual Selection individuals with certain inherited characteristics are more likely to obtain mates can result in sexual dimorphism: marked differences between the 2• sex characteristics

Sexual Dimorphism: House Finches

Mechanisms of Sexual Selection Intrasexual Selection selection w/in same sex Alpha male Intersexual Selection aka mate choice females choosy about their mate (often depends on male showiness)

Intrasexual Selection

Intersexual Selection: Sandpiper male

How do female preferences devlop? 1 hypothesis: females have linked “good genes” with trait study: gray tree frog 2nd hypothesis females have linked “good health” with these traits study: birds

The Preservation of Genetic Variation neutral variation: differences in DNA that do not confer an advantage or disadvantage Why don’t all genes move toward neutrality? tendency for directional or stabilizing selection countered by mechanisms that preserve or restore variation

Diploidy recessive alleles hidden and carried forward in heterozygotes heterozygote protection maintains a huge pool of alleles that might not be favored under present conditions, but could bring benefits in environment changes

Advantages determined by Alleles

Balancing Selection occurs when natural selection maintains 2 or more forms in a population 2 types: heterozygote advantage frequency-dependent selection

Heterozygote Advantage Heterozygotes have survival advantage If phenotype of a heterozygote is intermediate between the 2 homozygotes then this advantage is: stabilizing selection If phenotype of heterozygote same as dominant homozygote this advantage is directional selection

Heterozygote Advantage Example: Sickle Cell

Sickle Cell Allele SS Ss ss homozygous dominant no protection against malaria Ss heterozygous protection against malaria few sickle cells but not harmful ss homozygous recessive die young of sickle cell

Frequency-Dependent Selection the fitness of a phenotype depends on how common it is in the population

Scale-Eating Fish in Lake Tanganyika eat scales off flank of prey some left- mouthed some right-mouthed right-mouthed dominant to left- mouthed

Scale-eating Fish selection favors whichever mouth phenotype is least common (prey fish learn to avoid attacks from more common

Why Natural Selection does not Result in a “Perfect” Organism 1. Selection can only act on existing variations NS favors only the fittest available phenotypes 2. Evolution is limited by historical constraints NS has to work with existing structures

3. Adaptations are often compromises each organisms must do many things: some structures are a compromise (Walrus fins great for swim, not so good for walking on rocks) 4. Chance, natural selection, & the environment interact founding population may not carry “best” alleles for new environment; environments can change