Chapter 23: The Evolution of a Population. Population Group of individuals in a geographic location, capable of interbreeding and producing fertile offspring.

Chapter 23: The Evolution of a Population

PopulationGroup of individuals in a geographic location, capable of interbreeding and producing fertile offspring

Natural selection acts on individuals, but only populations evolve

Galapagos Islands• Population of medium ground

finches on Daphne Major Island

– During a drought, large-beaked birds were more likely to crack large seeds and survive

– Evolution by natural selection

Types of Evolution

• Microevolution= change in the gene pool of a population over many generations

• 4 Methods of Microevolution– Mutations– Natural Selection– Genetic Drift= chance events cause genetic changes

from one population to the next– Gene Flow= individuals or gametes move to a

different population

Genetic Drift

• Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next– Bottleneck Effect= event kills a large number of

individuals and only a small subset of population is left– Founder Effect= Small number of individuals colonize

new location

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken

• Loss of prairie habitat= severe reduction in the population of greater prairie chickens in Illinois – Low levels of genetic variation, only 50% of their eggs hatched

Pre-bottleneck(Illinois, 1820)

Post-bottleneck(Illinois, 1993)

Greater prairie chicken

Range of greater prairie chicken

• Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck

• Results showed a loss of alleles at several loci

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken

Figure 23.11b

Location Population size

Number of alleles per locus

Percentage of eggs hatched

93<50

5.23.7

5.8

5.8

99

96

1,000–25,000 <50

750,000

75,000–200,000

Nebraska, 1998 (no bottleneck)

Kansas, 1998 (no bottleneck)

Illinois 1930–1960s 1993

• Researchers introduced greater prairie chickens from population in other states

• Introduced new alleles into population• Increased the egg hatch rate to 90%

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken

Genetic Drift: Summary

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

Gene Flow

• Gene flow = movement of alleles among populations

• Alleles can be transferred through:– Movement of fertile individuals– Gametes

• Gene flow tends to reduce variation among populations over time

• Barriers to dispersal can limit gene flow between populations

Geographic Variation

• Most species exhibit geographic variation– Differences between gene pools of separate populations

• Mice in Madeira– Island in Atlantic Ocean– Several isolated populations of non-native mice

• Mountain range prevents gene flow– Fusion of chromosomes

Geographic Variation

Variation among populations is due to drift, not natural selection

Geographic Variation

Cline= graded change in a trait along a geographic axis

Mummichog Fish and Cold-Adapted Allele

Effect of natural selection

Natural Selection

• Evolution by natural selection involves both change and “sorting”

– New genetic variations arise by chance– Beneficial alleles are “sorted” and favored by natural

selection• Only natural selection consistently results in adaptive

evolution

Evolution

• Evolution by natural selection is possible because of genetic variation in population– Gene pool= all the alleles for all loci in a population

• Genetic Variation = differences in DNA sequences– Gene variability

• Average heterozygosity= average percent of loci that are heterozygous in a population

• Fixed loci= all individuals in a population have same allele– Nucleotide variability

• Measured by comparing the DNA sequences of pairs of individuals

Evolution

• Phenotype= product of inherited genotype + environmental influences– Discrete characters= classified on an either-or basis

• Flower color: red or white

– Quantitative characters= vary along a continuum within a population

• Skin color in humans

• Natural selection can only act on variation with a genetic component

Evolution

• Genetic variation primarily comes from 2 sources:1. Mutation and Gene Duplication

• Original source of new alleles or genes• May be neutral before it becomes an advantage• “raw material” of evolution

2. Sexual Reproduction= unique combination of genes following crossing over, independent assortment of chromosomes, and random fertilization

Evolution

• Genetic drift and gene flow do not consistently lead to adaptive evolution – Can increase or decrease the match between an

organism and its environment• Natural selection increases the frequencies of

alleles that enhance survival and reproduction– Adaptive evolution occurs as the match between an

organism and its environment increases• Because the environment can change, adaptive

evolution is a continuous process

Evolution of a Population

• Types of Natural Selection– Directional Selection

• Highest reproduction in one extreme phenotype

– Stabilizing Selection• Highest reproduction of

intermediate phenotypes

– Disruptive Selection• Highest reproduction of two

extreme phenotypes

Generations1 2 3

Figure 23.13

Original population

Phenotypes (fur color)F

req

uen

cy

of

ind

ivid

ual

sOriginal population

Evolved population

(a) Directional selection (b) Disruptive selection (c) Stabilizing selection

Sexual Selection• Form of natural selection • Individuals with certain traits are more likely to mate• Sexual Dimorphism= differences in appearance of

males and females• Vertebrates= males usually “showier” of sexes or

engage in competition for females

• Characteristics may be disadvantage – Male birds with bright feathers

more obvious to predators

Sexual Selection• Intrasexual selection=

competition among individuals of one sex (often males) for mates of the opposite sex

• Intersexual selection (mate choice)= individuals of one sex (usually females) are choosy in selecting their mates

Sexual Selection

• How do female preferences evolve?• Good genes hypothesis= if a trait is related to

male health, both the male trait and female preference for that trait should increase in frequency

Example of Sexual Selection

• Females select males based on traits indicating defenses against parasites and pathogens– Bird, Mammal, and Fish Species

• Female stickleback fish and Major Histocompatibility Complex (MHC)

• Higher reproductivesuccess increases frequency of defense trait in next generation

Genetic Variation in Populations

• Neutral variation= genetic variation that does not confer a selective advantage or disadvantage

• Various mechanisms help to preserve genetic variation in a population

• Diploidy= maintains genetic variation in the form of hidden recessive alleles– Heterozygotes can carry recessive alleles that are

hidden from the effects of selection

Genetic Variation in Populations

• Balancing selection= natural selection maintains stable frequencies of two or more phenotypic forms in a population

• Balancing selection includes– Heterozygote advantage– Frequency-dependent selection

Heterozygote Advantage

• Malaria caused by protist, Plasmodium

• 1-2 million people die/yr from disease

• Modifies red blood cells to obtain nutrients, escape destruction by spleen

Heterozygote Advantage

• Sickle-Cell Allele: recessive allele– Produces abnormal hemoglobin

proteinsNormal Blood Cell

Sickle-Cell

Heterozygote Advantage• Homozygous Dominant

– No sickle cell allele– Susceptible to malaria

• Homozygous Recessive– Sickle-cell disease– Resistant to malaria

• Heterozygotes= co-dominant alleles, both types of blood cells– Heterozygotes have decreased

symptoms of malaria and decreased symptoms of sickle-cell disease

– Higher survival

Frequency-Dependent Selection

• Fitness of a phenotype declines if it becomes too common in the population

• Selection can favor whichever phenotype is less common in a population

• Example: Predators can form a “search image” of their prey– Most common phenotype– Rare phenotypes may avoid detection by

predators, increasing survival and reproduction

Limits of Natural Selection• Selection can only act on existing variation in a population.

– New alleles do not appear when needed• Evolution is limited by historical constraints.

– Ancestral structures are adapted to new situations• Adaptations are usually compromises.

– One characteristic may be an adaptation in one situation, a disadvantage in another

• Natural selection interacts with chance/random events and the environment.– Chance events can alter allele frequencies in population– Environment can change

Evolution of Populations

• Measured by calculating changes in gene pool over time– Frequency of an allele in a population

• Diploid organisms: total number of alleles at a locus is the total number of individuals times 2– Homozygous Dominant= 2 dominant alleles– Heterozygous= 1 dominant, 1 recessive allele– Homozygous Recessive= 2 recessive alleles

Frequency of Alleles in Population

• For a characteristic with 2 alleles, we can use p and q to represent their frequencies

• Frequency of Alleles: – p= frequency of “A” allele (dominant)

• Total number of “A” alleles/total number of alleles

– q= frequency of “a” allele (recessive) • Total number of “a” alleles/total number of alleles

– Frequency of Alleles= p+ q =1

• Population of wildflowers that is incompletely dominant for color

– 320 red flowers (CRCR)– 160 pink flowers (CRCW)– 20 white flowers (CWCW)

• Calculate the number of copies of each allele:– CR (number of homozygotes for CR X 2) + number of

heterozygotes• (320 2) 160 800

– CW (number of homozygotes for CW x 2) + number of heterozygotes• (20 2) 160 200

Frequency of Alleles in Population

• To calculate the frequency of each allele:• p freq CR number of CR alleles/total

number of alleles– p = 800 / (800 200) 0.8

• q freq CW number of CW alleles/total number of alleles– q= 200 / (800 200) 0.2

• The sum of alleles is always 1– 0.8 0.2 1

Frequency of Alleles in Population

Hardy-Weinberg Principle

• The Hardy-Weinberg principle describes a population that is not evolving– frequency of alleles will not change from

generation to generation– A “null hypothesis” to check for evidence of

evolution• If a population does not meet the criteria of

the Hardy-Weinberg principle, it can be concluded that the population is evolving

Figure 23.7

Alleles in the population

Gametes produced

Each egg: Each sperm:

80%chance

20%chance

80%chance

20%chance

Frequencies of alleles

p = frequency of

q = frequency ofCW allele  = 0.2

CR allele  = 0.8

Hardy-Weinberg Principle

In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

Mendelian inheritance preserves genetic variation in a population

Hardy-Weinberg Principle

• When allele frequencies remain constant from generation to generation, the population is in Hardy-Weinberg equilibrium

• Assumptions:– No natural selection– No mutation– No gene flow– Random mating– Large population

Hardy-Weinberg Equilibrium

• Hardy-Weinberg Equilibrium Equations• Frequency of Alleles: p + q = 1• Frequency of Genotypes: p2 + 2pq + q2 = 1

Hardy-Weinberg Equilibrium

• The variables “p” and “q” come from Punnett Square for populations

p= frequency of “A” allele (dominant)

q= frequency of “a” allele (recessive)

Hardy-Weinberg Equilibrium

• Hardy-Weinberg Equilibrium Equations• Frequency of Alleles: p + q = 1

– p= 0.7– q=0.3

• Frequency of Genotypes: p2 + 2pq + q2 = 1– p2 = frequency of “AA” genotype

• Ex: 0.72 = 0.49

– 2pq = frequency of “Aa” genotype • Ex: 2(0.7)(0.3)= 0.42

– q2 = frequency of “aa” genotype • Ex: 0.32 = 0.09

– Frequency of Genotypes= 0.49 + 0.42 + 0.09 = 1

Population Variables: Hardy-Weinberg Equilibrium

• Once scientists know what p and q are in the population, they can track the population through time and see if population is at equilibrium or changing

• If p and q change through time, one of the Hardy-Weinberg Equilibrium assumptions are not being met– Evolution is occurring

• Populations can be at equilibrium at some loci, but not at other loci

Real-World Example: Hardy-Weinberg Equilibrium and Fisheries Management

• Kelp Grouper (Epinephelus bruneus)• Commercial fish species in Korea• Recent declines in landings

– 2007: IUCN Red List- Vulnerable• Study by An et al. 2012

– Genotyped 12 gene loci from 30 fish– 3 of 12 loci showed deviations from

Hardy-Weinberg Equilibrium