The evolution of populations

• Concept 23.1: Population genetics provides a foundation for studying evolution population from generation to generation... The Hardy-Weinberg Theoremgenotypes in a population’s gene po

PowerPoint Lectures for

Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 23

The Evolution of Populations

• Overview: The Smallest Unit of Evolution

that individual organisms evolve, in the

Darwinian sense, during their lifetimes

populations evolve

• Genetic variations in populations

Figure 23.1

• Concept 23.1: Population genetics provides a

foundation for studying evolution

population from generation to generation

The Modern Synthesis

genetically over time

• The modern synthesis

Darwinian theory of evolution by natural selection

Gene Pools and Allele Frequencies

interbreeding and producing fertile offspring

MAP AREA

NO RT HW

EST

TERRITO RIES

• The gene pool

at any one time

population

The Hardy-Weinberg Theorem

genotypes in a population’s gene pool remain constant from generation to generation

provided that only Mendelian segregation and recombination of alleles are at work

• Mendelian inheritance

– Preserves genetic variation in a population

Generation 1

genotype

genotype Plants mate

Generation 2

Generation 3

25% C R C R 50% C R C W 25% C W C W

50% C R 50% C W

Preservation of Allele Frequencies

contribute to the next generation randomly, allele frequencies will not change

Hardy-Weinberg Equilibrium

mating occurs

frequencies do not change

• A population in Hardy-Weinberg equilibrium

With random mating, these gametes will result in the same

mix of plants in the next generation:

64% C R C R , 32% C R C W and 4% C W C W plants

4% C W from

C W C W homozygotes

• If p and q represent the relative frequencies of

the only two possible alleles in a population at

a particular locus, then

homozygous genotypes and 2pq represents

the frequency of the heterozygous genotype

Conditions for Hardy-Weinberg Equilibrium

over time

• The five conditions for non-evolving

populations are rarely met in nature

Population Genetics and Human Health

population carrying the allele for an inherited disease

• Concept 23.2: Mutation and sexual

recombination produce the variation that

makes evolution possible

recombination

contributes to differences among individuals

Figure 23.6

Point Mutations

impact

Mutations That Alter Gene Number or Sequence

• Gene duplication

Mutation Rates

genes per generation

Sexual Recombination

recombination

producing the genetic differences that make adaptation possible

• Concept 23.3: Natural selection, genetic drift,

and gene flow can alter a population’s genetic composition

bring about most evolutionary change

Natural Selection

next generation in greater proportions

Genetic Drift

predicted result

• Genetic drift

unpredictably from one generation to the next

The Bottleneck Effect

drastically reduce the size of a population

the original population’s gene pool

Original population

Bottlenecking

Figure 23.8 A

(a) Shaking just a few marbles through the

narrow neck of a bottle is analogous to a

drastic reduction in the size of a population

after some environmental disaster By chance,

blue marbles are over-represented in the new

population and gold marbles are absent.

• Understanding the bottleneck effect

activity affects other species

(b) Similarly, bottlenecking a population

of organisms tends to reduce genetic

variation, as in these northern

elephant seals in California that were

once hunted nearly to extinction.

The Founder Effect

isolated from a larger population

Gene Flow

individuals or gametes

populations over time

• Concept 23.4: Natural selection is the primary

mechanism of adaptive evolution

genotypes in a population

Variation Within a Population

• Polymorphism

distinct morphs for a character are each represented in high enough frequencies to be readily noticeable

that occur along a continuum in a population

• Measuring Genetic Variation

population by determining the amount of heterozygosity at the gene level and the molecular level

Variation Between Populations

populations or population subgroups

XX19

13.1710.16

9.128.11

XX15.18

13.1711.12

9.10

Figure 23.10

• Some examples of geographic variation occur

as a cline, which is a graded change in a trait along a geographic axis

EXPERIMENT Researchers observed that the average size

of yarrow plants (Achillea) growing on the slopes of the Sierra

Nevada mountains gradually decreases with increasing

elevation To eliminate the effect of environmental differences

at different elevations, researchers collected seeds

from various altitudes and planted them in a common

garden They then measured the heights of the

resulting plants

RESULTS The average plant sizes in the common

garden were inversely correlated with the altitudes at

which the seeds were collected, although the height

differences were less than in the plants’ natural

A Closer Look at Natural Selection

population

certain genotypes, fitting organisms to their environment over generations

Evolutionary Fitness

“survival of the fittest”

• Fitness

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

generation as compared to the contributions of alternative genotypes for the same locus

Directional, Disruptive, and Stabilizing Selection

phenotypes of certain organisms

• The three modes of selection

Phenotypes (fur color)

Original population

Original population

Evolved population

The Preservation of Genetic Variation

variation in a population

hidden recessive alleles

Balancing Selection

frequencies of two or more phenotypic forms in

a population

• The sickle-cell allele

confers malaria resistance

Figure 23.13

Frequencies of thesickle-cell allele

Plasmodium falciparum

(a protozoan)

• Frequency-Dependent Selection

too common in the population

• An example of frequency-dependent selection

Parental population sample

Experimental group sample

On pecking a moth image the blue jay receives a food reward If the bird does not detect a moth

on either screen, it pecks the green circle to continue

to a new set of images (a new feeding opportunity)

0.060.050.040.03

0.02

Generation numberFrequency-independent control

Neutral Variation

selective advantage

Sexual Selection

differences between the sexes in secondary sexual characteristics

• Intrasexual selection

one sex for mates of the opposite sex

• Intersexual selection

females) are choosy in selecting their mates from individuals of the other sex

appearance

Figure 23.15

The Evolutionary Enigma of Sexual Reproduction

reproduction, a so-called reproductive handicap

• If sexual reproduction is a handicap, why has it

persisted?

disease resistance

Why Natural Selection Cannot Fashion Perfect Organisms

• Chance and natural selection interact