Mutationsist and Selectionists • Two schools of thought, late 19th century

– Selectionist

• Continuous variation

• Biometricians, Weldon and Pearson

• Mendelian factors, only exceptional traits

– Mutationist, Mendelians

• Discountinuous Variation

• Bateson and supporters

• Segregation of Mendelian factors

• Large mutations

• Saltation

– Evolution of species with one or few mutational steps

– Quantitative characters

Population Gene Frequencies• Population

– Group of organisms of one species (conspecific) occupying well-defined geographical region and reproductive continuity

• Gene pool

– Gametes a population contributes to the next generation

• Hardy-Weinberg equilibrium

• Population genetics

– Evolution as change in gene (allele) frequencies

– Study of allele frequencies and their changes

The new-Darwinian or Modern Synthesis

• Allele frequency differences

– Random mutations introduce alleles, frequencies change through time due to natural selection and other factors (migration, genetic drift)

• Accumulation of allele frequency differences lead to more pronounced differences among populations

• Gene duplication

• Gene duplicates can change function

• Repeat sequences

• Changes in gene regulation

• Alternate splicing

• RNAi and post-translational modification

Variation and Selection

• Modern Synthesis– Mutation or genetic drift introduce variation

– Subject to selection

– Resulting in adaptive changes in phenotype

– Eventually resulting in organismal diversity

• Questions Provoked by the Modern Synthesis– What are the allele frequencies in populations?

– How do allele frequencies change?

– Why do they change? …

Populations and Allele Frequencies

• Deme– Local group of an interbreeding population

– Geographic features can affect size of group

• Impassable river, landslip

• Allele frequency– Proportion of the different alleles of the genes in a

population

• Gene pool– Sum total of alleles in the reproductive gametes of a

population

Conservation of Allele Frequencies

• Under condition of random mating (panmixia), where all genotypes are equally viable, the allele frequencies depend on allele frequencies of previous generation (not the genotype frequencies)

• Frequencies of different genotypes produced through random mating depend only on the allele frequencies

Table 21-3 pg 547

The Hardy-Weinberg

Equilibrium

• Based on stable allele frequencies and random mating

• p2 +2pq + q2

Fig 21.2 pg 549

Genotypic frequencies at Hardy-Weinberg equilibrium

Fig 21.4 pg 551

Attaining Equilibrium at Two or More Loci

• Speed of approach to equilibrium slows when more gene pairs are involved

Table 21.5 pg 552

Linkage Disequilibrium

• Equilibrium is delayed

– Lower recombination frequency between linked loci

• Epistasis

• Coadapted gene complexes

Fig 21.6 pg 553

Association Mapping and the HapMap Project

• Gene mapping

– Linkage analysis of individuals with known pedigree

– Identifies genes affecting diseases or syndromes

• Association mapping

– Linkage analysis at the population level

• More complete analysis of allele past history

• Finer scale mapping of the allele

• Detection of rare alleles, or with small affects

• HAPMAP

– Consortium from six countries

– Mapping variation of humans from different regions

Sex Linkage• Number of possible

genotypes is increased

– Difference in sex chromosomes

– XX and XY

– (AA, Aa, aa) and (A, a)

• At equilibrium, sex-linked allele frequencies are the same in both sexes, but the genotypes differ

Fig 21.7 pg 555

Equilibria in Natural Populations

• Can estimate allele frequencies when able to score all segregants of a gene at a single locus

• MN Blood group locus (Boyd 1950)

– Two alleles M and N

– 104 American Ute Indians

– 0.59 MM, 0.34 MN, 0.07NN (observed)

0.274 MM, 0.502 MN, 0.224 NN

Table 21.06: Comparison of mating combinations expected according to random mating and those observed in 741 Japanese couples by

Matsunaga and Itoh, 1958

Table 21.07: Genotype frequencies for some human diseases causes by recessive alleles

H-W for 3 alleles

p + q + r = 1

p2 + 2pq + 2pr + q2 + 2qr +r2 = 1

p = 2 (A1A1 + A1A2 + A1A3)/2N

q = 2 (A2A2 + A2A1 + A2A3)/2N

r = 2 (A3A3 + A3A2 + A3A1)/2N

p = 2 (A1A1 + A1A2 + A1A3)/2N

Table 21.08: Comparison of observed acid phosphatase phenotypes and those expected according to Hardy-Weinberg equilibrium

Inbreeding• When related individuals

of similar genotype breed with each other

– Nonrandom breeding interferes with Hardy-Weinberg equilibrium

• Inbreeding Coefficient

– F = 1 when inbreeding is complete

– F = 0 when inbreeding is absent

• Self-fertilization

• Population size

• Inbreeding depression

– Rare deleterious recessives appear with increased homozygous frequency

• 2pq(1-F) = 2pq – 2Fpq

Fig 21.9 pg 558Fig 21.8 pg 557

Mutation Rates• Most mutations do not affect

only one allele, but affect nucleotide sequences

• If A continually mutates to a, chances are that a will increase in frequency.

• Rate of forward mutation, u

• Rate of backward mutation, v

• Frequencies of alleles, p and q

• Mutational equilibrium

– Point where frequencies balanced in relationship to mutation rates

– Δq = 0 = up - vq

• Typical observed mutation rates of 5 x 10-5

– Mutational equilibrium rarely if ever reached

Figure 22.01: Selection acting on the life stages of an organism

(Adapted from Christiansen.)

Selection and Fitness• Genotype

– Maternal genes + Epigenetic influences, direct and indirect

• Selection

– Sum of the survival and fertility mechanisms acting on the phenotype, that affect reproductive success of genotypes

– On diploid or haploid stages

• Selection coefficient, s

– Strong, resulting in lethality: s = 1

– Weak, slight reductions in fitness: s = 0.01

• Relative Fitness

– Extent to which a genotype contributes to the offspring of the next generation, relative to other genotypes in the environment

Selection and Fitness

Table 22.2 pg 565

Selection and Fitness

Table 22.3 pg 566

Table 22.04: Single-generation changes in allele frequency for diploid genotypes subject to given selection coefficients under different conditions

of dominance

Heterozygous Advantage• Overdominance

– Heterozygote has superior reproductive fitness to both homozygotes

• Heterosis, hybrid vigor

– Longevity, fecundity, resistance to disease

Fig 22.2 pg 567

Selection and Polymorphism

• Balanced polymorphism

– Two alleles present

– Frequency of second most frequent allele at least 1%

– Overdominance of sickle cell gene

• Protection against malaria

Figure 22.03a: Distribution of falciparum malaria in Eurasia and Africa before 1930

(From Strickberger, adapted from Allison.)

Figure 22.03b: Distribution of the gene for sickle cell anemia

Figure 22.03c: Distribution of the gene for beta-thalassemia

Figure 22.03d: Distribution of the sex-linked gene for glucose-6 phosphate dehydrogenase deficiency

(From Strickberger, adapted from Allison.) Fig 22.3 pg 568

Mimicry• Frequency dependent selection

– Less frequent the mimic compared to model, greater the chances for mimic protection

Fig 22.5 pg 569Batesian mimicry

Müllerian Mimicry

Self-Sterility Genes

• Frequency dependent

– Once allele becomes common, its frequency is reduce by many sterile mating combination

• Considerable numbers of alleles can become established

– 200 alleles for red clover

Polymorphism and Industrial Melanism

• Polymorphism can be established when selection coefficients vary from one environment to another

• Various (superior) phenotypes when population inhabits many environments

Industrial melanism

Fig 22.6 pg 570

Kinds of Selection• Stabilizing selection

– Reduction in extreme phenotypes

– Birth weight

• Directional selection

– Toward an extreme phenotype

– Animal and plant breeders

• Disruptive selection

– Changing conditions

Fig 22.7 pg 572

Figure 22.08abc: Modes and effects of selection

Variation and Variability• "The greater the genetic variability upon

which selection for fitness may act, the greater the expected improvement in fitness" Fisher (1930)

• Variation itself is subject to selection

– Propensity to vary (variability) is an important attribute of organisms

• Red Queen Hypothesis

Equilibrium Between Mutation

and Selection

• Mutation-Selection equilibrium frequency

– Mutation frequency and selection coefficient

Fig 22.10 pg 574

Migration• Gene flow

– A source other than mutation for source of new alleles

• 1) Difference in frequencies

• 2 )Proportion of migrant genes incorporated

• 3) Pattern of gene flow

– Once or multiple times

Genetic Drift• When there is no predictable constancy to changes

in allele frequencies

• Genetic drift– Variable sampling of the gene pool of each generation

– Increases variation within population, but in no particular direction

• Rate of fixation, 1/2N

• Effective population size, Ne

Fig 22.11 pg 577

Fig 22.12 pg 577

The Founder Effect• Reduced size

populations

• Limitations in gene frequency variation

• Opportunity for fixation

• Bottlenecks– Unique gene frequencies

explained by founder events

Fig 22.14 pg 579

Ecological Aspects of Population Growth

• Rates

– Numeric change is equal to difference between birth and death

N = (b-d)N

– Intrinsic rate of natural increase, rm

• Growth of population not limited

• Carrying capacity, K

– Number of individual an environment can support

• Logical growth curve

N = rN(K – N)/K Fig 23.1 pg 583

Population Parameters• Density independent

– Independent of population size

– E.g. climate

• Density dependent

– Result of population size, crowding

– E.g. resource depletion

• Age Structure

– Survivorship, lx

• Proportion survive from age 0 to age x

– Fecundity, mx

• Average number of offspring at age x

– Net Reproductive rate, R0

• R0 = lxmx

Fig 23.2 pg 584

Reproductive Strategies: r-

and K-Selection

• Breeding frequency

– Semelparous (once a lifetime)

– Iteroparous (repeatedly

• Number of offspring produced

• r –Selected

– Rapid rate of increase, large number of offspring

– Very little (or no) parental care

• K-Selected

– Few offspring

– Offspring raised, ability to compete

– Density dependent

Fig 23.3 pg 586

Type I

Type II

Type III

Many mammals

Many birds,small mammals,lizards, turtles

Many invertebrates

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Time

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Genetic Load and Genetic Death• Genetic load

• Extent a population departs from a perfect genetic constitution

– Mutational load, sq2N

– Segregation load, balanced load

– Recombinational load

• Genetic death

– Loss of individuals

• Death, sterility, reduced reproductive ability

Cost of Evolution and the Neutralist Position

• Cost of Evolution

– Complete replacement of a deleterious allele

• DN, where D = ln p0, and p is initial frequency of new allele

– D of 10

• Cost is ten times average number of individuals in single generation

• Neutral hypothesis

– To explain persistence of polymorphisms

– Neutral Mutations

• Due to high cost of selection, most amino acid changes are neutral

– Selection does not act

– Fixation incurs no genetic cost

The Selectionist Position• To explain persistence of

polymorphisms

• Frequency-dependent selection

– Genetic load when rare allele is changing, no genetic load when allele at equilibrium

• Truncation Selection

– Threshold number of polymorphic loci, fit and unfit

• Protein Polymorphisms and Ecological Conditions

– Allozymes

• Nonrandom Allelic Frequencies in Enzyme Polymorphisms

– Allozyme polymorphisms correlate with factors such as life habit and climate

• Enzyme Function and Degree of Polymorphism

– Restricted function, less polymorphism

• Polymorphisms for DNA Coding Sequences

– Greater number in non-coding regions

Some Genetic Attributes of Populations

• Populations have evolutionary characteristics distinct from evolutionary characteristics of individuals

• Sex and Sexual Reproduction

– Provides capacity for single individuals to share beneficial mutations

• Mutation

– If not neutral, not likely to provide benefit

– Environmental change more powerful environmental effect

• Linkage

– Recombination between linked alleles can markedly affect response to selection

Fig 23.4 pg 594

Fig 23.5 pg 595© Frank B. Yuwono/ShutterStock, Inc.

(Adapted from Muller.)

The Adaptive Landscape• Adaptive Peak

– Situations of highest fitness for a specific environment

• Population adaptive peaks can vary from individual genomic peaks

– Altruism, parasitism

• Adaptive Landscapes

– Not all peaks are equal

– Fitness values, peaks and valleys

• Shifting Balance

– Network of demes (local populations)

– Demes competing within a general environment

Fig 23.6 pg 597

Fig 23.7 pg 598

(Adapted from Wright 1963.)

Group Selection

• Individuals– Selection within a population

moves up a single adaptive peak

• Populations– Selection among populations

leads to occupation of higher peaks, replacement, extinction or colonization of lower peaks

• Populations evolve, not individuals

• Kin Selection– Social insects, ants

• Diploid females, haploid males

• Sisters more closely related than mothers are to daughters

• Reciprocal Altruism– Cooperation with individuals

who also cooperate

– Success of group depends upon group property

Fig 23.8 pg 600

Group Interaction• Interactions among groups can be positive, neutral or negative

• Competition

– Limited resource shared between groups

• Resource Partitioning

• Character displacement

• Competitive Exclusion

Fig 23.9 pg 603(Adapted from Krebs, based on MacArthur.)

Fig 23.10 pg 605

Fig 23.11 pg 605(Adapted from Gause.)

(Adapted from Krebs, based on MacArthur.)

Group Interaction• Predation

– Predators can reduce capacity of dominant species from reaching carrying capacity

– Parasitism

• Coevolution

– Possible among groups

• Prey-predator

• Pollinators and plants

Fig 23.12 pg 606(Adapted from Pianka, from Huffaker.)