CHAPTER 23 THE EVOLUTION OF POPULATIONS

• It is the population, not its individual, that evolve.• Evolution on the scale of populations, called

microevolution, is defined as a change in the allele frequencies in a population.

Population Genetics

• Population- a localized group of individuals that belong to the same species.• Species- a group of populations whose individuals have the

potential to interbreed and produce fertile offspring in a nature.

• Members of a population are far more likely to breed with members of the same population than with members of other populations

Gene pool- the total genes in a population at any one time is called the population

A population’s gene pool is defined by its allele frequencies

• Hardy-Weinberg theorem describes the gene pool of a nonevolving population.

• The frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations • unless acted upon by agents other than Mendelian

segregation and recombination of alleles.

The Hardy-Weinberg Theorem describes a nonevolving population

• Populations at Hardy-Weinberg equilibrium must satisfy five conditions.1) Very large population size.

2) No migrations

3) No net mutations

4) Random mating

5) No natural selection.

Hardy-Weinburg Equationp2 + 2pq + q2 = 1

• p2 = Frequency of homozygous dominant genotype

• 2pq = Frequency of both heterozygous genotypes• q2 = Frequency of homozygous recessive

genotype


• In a population of 500 plants• 20 have white

flowers (rr).

• 480 have red flowers (Rr or RR).

• 320 are RR

• 160 are Rr.


• Genotype frequency- % of population that have that particular genotype


• Allele frequency- % of a particular allele that is circulating in the population


• p + q = 1• If q2 equal to .04

(for rr) then q = 0.2

• If q =0.2, then p = 0.8


• In Drosophilia, the allele for normal length wings is dominant over the allele for vestigial wings. In a population of 1,000 individuals, 160 show the recessive phenotype.


• 160 (nn) / 1000 (population) = 0.16 for genotype frequency for nn

• q2 = 0.16, then q = 0.4• p + q = 1• p + 0.4 = 1, then p = 0.6


• q = 0.4, p = 0.6

Allele frequency• 60% N, 40% n


• q = 0.4, p = 0.6• p2 + 2pq + q2 = 1• (0.6) 2 + 2(0.6*0.4) + (0.4) 2

• 0.36 + 0.48 + .16 = 1• Genotype frequency=• NN 36%• Nn 48%• nn 16%


• Genotype frequency=• NN 36%• Nn 48%• nn 16%• Verify, we know 160

are recessive out of 1000!


• The allele for the hair pattern called "widow's peak" is dominant over the allele for no "widow's peak." In a population of 1,000 individuals, 360 show the dominant phenotype.


• WW + Ww = 360• Therefore ww = 640

• 640 (ww) / 1000 (population) = 0.64 ww

• Genotype frequency = .64 for ww


• Genotype frequency = 0.64 for nn

• If q2 = 0.64, then q= 0.8

• p + q = 1• If q= 0.8, then p = 0.2


• p = 0.2, q= 0.8• (0.2) 2 + 2(0.2*0.8) + (0.8) 2

• 0.04 + 0.32 + 0.64 = 1

Genotype frequency

4% for WW

32% for Ww

64% for ww


Genotype frequency

4% for WW

32% for Ww

64% for ww

Verify: 360 show dominant phenotype

WW (4%) + Ww (32%) = 36%

36% of 1000= 360

• Microevolution- generation-to-generation change in a population’s frequencies of alleles.

Genetic drift -chance events that occur when populations are finite in size.

Microevolution

The two main causes of microevolution are drift and natural selection

• For example, in a small wildflower population with a stable size of only ten plants, genetic drift can completely eliminate some alleles.

• Bottleneck effect- occurs when the numbers of individuals in a larger population are drastically reduced by a disaster.

• Founder effect- occurs when a new population is started by only a few individuals that do not represent the gene pool of the larger source population.

• Natural selection- in a population with variable individuals some individuals to leave more offspring than others.

• Natural selection accumulates and maintains favorable genotypes in a population.

• Gene flow- genetic exchange due to migration of fertile individuals or gametes between populations.

• Mutation- a change in an organism’s DNA.

• Quantitative characters- vary along a continuum within a population. Usually due to polygenic inheritance • Ex: height of humans

• Discrete characters- Usually determined by a single locus with different alleles with distinct impacts on the phenotype.

• Ex: flower color

• Polymorphism occurs when two or more discrete characters are present and noticeable in a population.

• Ex: Freckles, blood types

Genetic variation occurs within and between populations

Gene diversity- measures the average percent of gene loci that are heterozygous.

Nucleotide diversity- measures the level of difference in nucleotide sequences (base pair differences) among individuals in a population.

Geographic variation- differences in genetic structure either between populations that inhabit different areas.

• New alleles originate only by mutation.• Most point mutations, those affecting a single base

of DNA, are probably harmless.• Mutations that alter the structure of a protein

enough to impact its function are more likely to be harmful than beneficial.

Mutation and sexual recombination generate genetic variation

• Diploidy in eukaryotes prevents the elimination of recessive alleles via selection because they do not impact the phenotype in heterozygotes.• Even recessive alleles that are unfavorable can persist in

a population through their propagation by heterozygous individuals.

• Heterozygote protection maintains a huge pool of alleles that may not be suitable under the present conditions but that could be beneficial when the environment changes.

Diploidy and balanced polymorphism preserve variation

• Heterozygote advantage- heterozygous individuals have an advantage and are able to reproduce more than homozygotes.• Ex: sickle-cell disease

• Homozygous recessive causes the disease

• Homozygous dominant are vulnerable to malaria

• Heterzygous are resistant to malaria

• Directional selection- shifts the frequency curve for a phenotypic character in one direction by favoring what had been rare individuals

• Diversifying selection- when environmental conditions favor individuals at both extremes of the phenotypic range over intermediate phenotypes.

• Stabilizing selection favors intermediate variants and acts against extreme phenotypes.• Ex: birth weight

• Sexual dimorphism- differences between the sexes: size, coloration, enlarged or exaggerated features, or other adornments.

• Males are usually the larger and showier sex, at least among vertebrates.

Intrasexual selection is direct competition among individuals of one sex (usually males) for mates of the opposite sex.

Intersexual selection or mate choice occurs when members of one sex (usually females) are choosy in selecting among individuals of the other sex.

Sexual selection may lead to pronounced secondary differences between the sexes

1. Evolution is limited by historical constraints.• Evolution co-opts existing structures and adapts them to

new situations.

2. Adaptations are often compromises.• Organisms are often faced with conflicting situations

that prevent an organism from perfecting any one feature for a particular situation.

Natural selection cannot fashion perfect organisms

3. Not all evolution is adaptive.• Chance affects the genetic structure of

• Founders of new populations may not necessarily be the best individuals, but rather those individuals that enter the open habitat by chance.

4. Selection can only edit existing variations.• Selection favors only the fittest variations from those

phenotypes that are available.

• New alleles do not arise on demand.

CHAPTER 23 THE EVOLUTION OF POPULATIONS

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