Chapter 23

The Evolution of Populations

Western Historical Context

Gregor Mendel (1822-1884)

Austrian monk whose breeding experiments with peas shed light on the rules of inheritance

Mendel was a contem-porary of Darwin, but his work wasoverlooked until the 20th century

Western Historical Context

A conceptual synthesis of Darwinian evolution, Mendelian inheritance, and modern population genetics

The Modern Synthesis (early 1940s)

Potential for rapid population growth when resources

are not limiting

Resource availability generally limits population size

Competition for resources(“struggle for existence”)

Phenotypic variability (morphology, physiology,

behavior, etc.)

Natural Selection: Survival and reproduction

of the “fittest” individuals

Some variabilityresults from heritable genotypic differences

Phenotype vs. Genotype

Phenotype vs. Genotype

Phenotype: all expressed traits of an organism

Phenotype vs. Genotype

Phenotype: all expressed traits of an organism

Genotype: the entire genetic makeup of an individual (i.e., its genome – it’s full complement of genes and the two alleles that comprise each locus), or a subset of an individual’s genes

Evolution

A change in allele frequency in a population (a change in the

gene pool)

Population = all of the individuals of a species in a given area

Potential for rapid population growth when resources

are not limiting

Resource availability generally limits population size

Competition for resources(“struggle for existence”)

Phenotypic variability (morphology, physiology,

behavior, etc.)

Natural Selection: Survival and reproduction of the

“fittest” individuals

Some variabilityresults from heritable genotypic differences

Adaptive evolution: A change in the phenotypic constitution of a population owing to selection on heritable variation

among phenotypes that changes the genotypic constitution of the population

Population Genetics

Examines the frequency, distribution, and inheritance of

alleles within a population

Hardy-Weinberg Equilibrium

The population genetics theorem that states that the frequencies of

alleles and genotypes in a population will remain constant

unless acted upon by non-Mendelian processes (i.e., mechanisms of

evolution)

See Figs. 23.4 & 23.5 – An example

See Figs. 23.4 & 23.5 – An example

See Figs. 23.4 & 23.5 – An example

This means that 80% of sperm & eggs will carry R, and 20% of sperm & eggs will carry r

See Figs. 23.4 & 23.5 – An example

Under strict Mendelian inheritance, allele frequencies would remain constant from one generation to the next

(Hardy-Weinberg Equilibrium)

Allele Frequencies

RRp2=0.64

Rrpq=0.16

rRqp=0.16

rrq2=0.04

RSperm Eggs

Genotype frequencies: p2=0.64 (RR) 2pq=0.32 (Rr) q2=0.04 (rr)

Allele frequencies: p=0.8 (R) q=0.2 (r)

R

rr

80% (p=0.8)80% (p=0.8)

20% (q=0.2)20% (q=0.2)

At a later date, you determine the genotypes of 500 individuals, and find the following:

Allele Frequencies

280 RR

165 Rr

55 rr

Frequency of R (a.k.a. “p”): 280 + 280 + 165 = 725 R alleles in the pop. 725 / 1000 = 0.725

Frequency of r (a.k.a. “q”): 165 + 55 + 55 = 275 r alleles in the pop. 275 / 1000 = 0.275

The frequencies of alleles R and r have changed:

Allele Frequencies

320 RR160 Rr 20 rr

T1:

p=0.8, q=0.2

280 RR165 Rr 55 rr

T2:

p=0.725, q=0.275

The population has

EVOLVED!

For a two-allele locus: Let p = the frequency of one allele in the population (usually the dominant) Let q = the frequency of the other allele

Hardy-Weinberg Equation

p2 + 2pq + q2 = 1

Notice that: p + q = 1 p = 1 – q q = 1 – p

Genotypes should occur in the population according to:

Hardy-Weinberg Equation

p2 + 2pq + q2 = 1

p2 = proportion of population that is homozygous for the first allele

(e.g., RR)

2pq = proportion of population that is heterozygous (e.g., Rr)

q2 = proportion of population that is homozygous for the second

allele (e.g., rr)

Hardy-Weinberg Equation

p2 + 2pq + q2 = 1

Given either p or q, one can solve for the rest of the above equation

What would q be if p = 0.6?

What would 2pq be if p = 0.5?

Hardy-Weinberg Equation

p2 + 2pq + q2 = 1

Given the frequency of either homozygous genotype, the rest of the equation can be solved

What would q be if p2 = 0.49?

Hint: q = q2

Hardy-Weinberg Equilibrium

Is a null model…

like Newton’s first law of motion:

Every object tends to remain in a stateof uniform motion (or stasis), assuming no external

force is applied to it

The Hardy-Weinberg Equation will be satisfied, as long as all the assumptions are met…

Hardy-Weinberg Equilibrium

Assumptions:

1) Infinite population size

Because genetic drift affects smaller populations more than larger pops.

Genetic drift = allele frequency change due to chance

Genetic drift reduces genetic variability

See Fig. 23.7 Genetic drift in a small population of wildflowers

See Fig. 23.7 Genetic drift in a small population of wildflowers

See Fig. 23.7 Genetic drift in a small population of wildflowers

Genetic drift often results from populations passing through a population bottleneck

Genetic drift often results from populations passing through a population bottleneck

The founder effect is an example of a population bottle neck

Mainlandpopulation

Mainlandpopulation

Colonists from themainland colonize

an island

The founder effect is an example of a population bottle neck

Mainlandpopulation

Colonists from themainland colonize

an island

Island gene poolis not as variable

as the mainland’s

The founder effect is an example of a population bottle neck

Hardy-Weinberg Equilibrium

Assumptions:

1) Infinite population size (no genetic drift) 2) No gene flow among populations

Gene flow = transfer of alleles among populations

Emigration transfers alleles out of a population and immigration transfers them in

Gene flow connects populations

Populationat t1

Island gene poolis not as variable

as the mainland’s

Population at t2

(after immigration)

time

Gene flow connects populations

Populationat t1

Island gene poolis not as variable

as the mainland’s

Gene flow connects populations

Populationat t1

Island gene poolis not as variable

as the mainland’s

Population at t2

(after immigration)

time

Hardy-Weinberg Equilibrium

Assumptions:

1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations

Populationat t1

Island gene poolis not as variable

as the mainland’s

Population at t2

(after immigration)

time

Mutations generally boost genetic diversity

Populationat t1

Island gene poolis not as variable

as the mainland’s

Population at t2

(after a mutation event)

time

Mutations generally boost genetic diversity

Hardy-Weinberg Equilibrium

Assumptions:

1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations4) Random mating with respect to genotypes

E.g., imagine what would happen if RR males mated only with rr females

Those particular matings would result in no RR or rr offspring, thereby altering population-wide genotype frequencies

Hardy-Weinberg Equilibrium

Assumptions:

1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations 4) Random mating with respect to genotypes 5) No natural selection

E.g., imagine what would happen if rr flowers were the only ones that ever attracted pollinators (even though the population contains RR and Rr individuals as well)

Hardy-Weinberg Equilibrium

Assumptions:

1) Infinite population size (no genetic drift) 2) No gene flow among populations 3) No mutations4) Random mating with respect to genotypes5) No natural selection

Adaptive evolution: A change in the phenotypic constitution of a population owing to selection on heritable variation

among phenotypes that changes the genotypic constitution of the population

Variation within Populations

Let’s briefly review…

Variation within Populations

Since selection acts on phenotypes, yet evolution requires population-level genotypic

change, it is important to understand intraspecific variation

Note: If all individuals were phenotypically identical, there would be no opportunity for

selection

Note: If all individuals were genotypically identical, there would be no opportunity for

evolution

Variation within Populations

Phenotypic variation results from both environmental and genetic influences

Consider identical vs. fraternal twins

Variation within Populations

Phenotypic variation results from both environmental and genetic influences

Phenotypic variation within populations is either discrete or quantitative/continuous

Discrete variation: polymorphism = mutiple phenotypes that are readily

placed in distinct categories co-occur

(e.g., our red and white flowers result from a polymorphic locus)

E.g., a “bar graph” trait like ABO blood type

Variation within Populations

Phenotypic variation results from both environmental and genetic influences

Phenotypic variation within populations is either discrete or quantitative/continuous

Continuous variation: quantitative characters = multiple loci produce a trait (e.g., flower size), and the trait varies

continuously in the population

E.g., a “bell curve” trait like human height

Variation within Populations

Phenotypic variation results from both environmental and genetic influences

Phenotypic variation within populations is either discrete or quantitative/continuous

Phenotypic variation also exists among populations

E.g., geographic variation

Heliconius species A

Heliconius species B

How is genetic variation maintained?

Variation within Populations

1) Diploidy provides heterozygote protection

2) Balanced polymorphism Heterozygote advantage

E.g., A locus for one chain of hemoglobin in humans has a recessive allele that causes sickle-cell anemia in homozygotes, but provides resistance to malaria in heterozygotes

How is genetic variation maintained?

Variation within Populations

1) Diploidy provides heterozygote protection

2) Balanced polymorphism Heterozygote advantageFrequency-dependent selection

3) Neutrality

Fitness

Darwinian fitness = an individual’s reproductive success (genetic contribution to subsequent generations)

Relative fitness = a genotype’s contribution to subsequent generations compared to the contributions of alternative genotypes at the same locus

Effects of Selection

See Fig. 23.12

Coat color

Directional selection consistently favors phenotypes at one extreme

Effects of Selection

See Fig. 23.12

Coat color

Coat color

Stabilizing selection favorsintermediate phenotypes

Effects of Selection

See Fig. 23.12

Coat color

Coat color

Diversifying (disruptive) selection simultaneously favors both phenotypic extremes

Effects of Selection

See Fig. 23.12

Coat color

Coat color

Effects of Selection

Directional, diversifying (disruptive), and stabilizing selection

See Fig. 23.12

Coat color

Coat color Coat color Coat color

Sexual Selection

Intrasexual selection, usually male-male competition

Sexual Selection

Dynastes tityus

Often leads to sexual dimorphism & exaggerated traits

Intrasexual selection, usually male-male competition

Sexual Selection

Dynastes hercules

Intrasexual selection, usually male-male competition

Often leads to sexual dimorphism & exaggerated traits

Sexual Selection

Lucanus elaphus

Intrasexual selection, usually male-male competition

Often leads to sexual dimorphism & exaggerated traits

Sexual Selection

Intersexual selection, usually female mate choice

Sexual Selection

Intersexual selection, usually female mate choice

Often leads to sexual dimorphism & exaggerated traits

Sexual Selection

Intersexual selection, usually female mate choice

Often leads to sexual dimorphism & exaggerated traits

Sexual Selection

Intersexual selection, usually female mate choice

Often leads to sexual dimorphism & exaggerated traits