THE EVOLUTION OF POPULATIONS

EVOLUTION AND VARIATION

Microevolution- small scale evolution; change in allele frequencies in a population over generations.

Discrete Characters- classified on an either-or basis

Quantitative Characters- vary along a continuum

Average Heterozygosity- (gene variability) the average percent of loci that are heterozygous.

Nucleotide Variability- comparing DNA sequences of two individuals

Geographic Variation- differences in genetic composition of separate populations.

MUTATION

Mutation- the ultimate source of new alleles Point mutations- a change in one base in a

gene Neutral and Beneficial Mutations Mutations Rates

Plants/Animals- 1/100,000 genes per generation Prokaryotes- fewer mutations, shorter generation

span, more genetic variation Viruses- more mutations, shorter generation

span, RNA genome with fewer repair mechanisms

GENE POOLS AND ALLELE FREQUENCY

Population- a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

Gene pool- all of the alleles for all the loci in all individuals of the population. Fixed- only one allele exists for a particular locus

and all individuals are homozygous for that allele

HARDY-WEINBERG PRINCIPLE

H-W Equilibrium describes a constant frequency of alleles within a gene pool.

p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of the

homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Frequencies of alleles

Alleles in the population

Gametes produced

Each egg: Each sperm:

80%chance

80%chance

20%chance

20%chance

q = frequency of

p = frequency of

CR allele = 0.8

CW allele = 0.2

HARDY-WEINBERG ASSUMPTIONS

1.No mutations2.Random mating3.No natural selection4.Extremely large population size5.No gene flow

*Departure from any of these conditions usually results in evolutionary change.

PRACTICE HARDY-WEINBERG PROBLEM

For a locus with two alleles (A and a) in a population at risk from an infections neurodegenerative disease, 16 people had genotype AA, 92 had genotype Aa, and 12 had genotype aa. Use the Hardy-Weinberg equation to determine whether this population appears to be evolving.

MECHANISMS THAT ALTER ALLELE FREQUENCY

Natural selection Leads to adaptive radiation

Genetic drift Founder Effect Bottleneck Effect

Gene flow

Fig. 23-8-3

Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

Generation 2

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

Fig. 23-9

Originalpopulation

Bottleneckingevent

Survivingpopulation

EFFECTS OF GENETIC DRIFT

1. Genetic drift is significant in small populations

2. Genetic drift causes allele frequencies to change at random

3. Genetic drift can lead to a loss of genetic variation within populations

4. Genetic drift can cause harmful alleles to become fixed

MECHANISMS THAT ALTER ALLELE FREQUENCY

Natural selection Leads to adaptive radiation

Genetic drift Founder Effect Bottleneck Effect

Gene flow the transfer of alleles into or out of a population

due to the movement of fertile individuals or their gametes.

NATURAL SELECTION AND ADAPTIVE EVOLUTION Relative Fitness- the contribution an

individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

Natural selection is the only evolutionary mechanism that continually leads to adaptive evolution.

DIRECTIONAL SELECTION

Occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting the frequency curve for the phenotypic character in one direction or another.

Original population

(a) Directional selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Original population

Evolved population

DISRUPTIVE SELECTION

Occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes.

Fig. 23-13b

Original population

(b) Disruptive selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Evolved population

STABILIZINGSELECTION

Acts against both extreme phenotypes and favors intermediate variants.

Fig. 23-13c

Original population

(c) Stabilizing selection

Phenotypes (fur color)

Fre

qu

enc

y o

f in

div

idu

als

Evolved population

Sexual Selection a form of natural selection in which individuals with

certain inherited characteristics are more likely than other individuals to obtain mates.

Sexual Dimorphism marked differences between the two sexes in

secondary sexual characteristics, which are not directly associated with reproduction or survival.

Intrasexual Selection Selection within the same sex. Individuals of one sex

compete directly for mates of the opposite sex. Intersexual Selection

“mate choice”- individuals of one sex (usually females) are choosy in selecting their mates from the other sex.

PRESERVATION OF GENETIC VARIATION Diploidy

Hides genetic variation from selection in the form of recessive alleles

Balancing Selection Occurs when natural selection maintains two or more

forms in a population. Heterozygote Advantage

Individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes

Frequency-Dependent Selection The fitness of a phenotype declines if it becomes too

common in the population Neutral Variation

Has no selective advantage or disadvantage

WHY NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS

Selection can only act on existing variations. Evolution is limited by historical constraints. Adaptations are often compromises Chance, natural selection, and the

environment interact.

EXIT SLIP

Of all the mutations that occur in a population, why do only a small fraction become widespread among the population’s members?

If a population stopped reproducing sexually (but still reproduced asexually), how would its genetic variation be affected over time? Explain.