Transcript video slide - Chicagoland Jewish High School

Chapter 23
The Evolution of Populations
Overview: The Smallest Unit of
Evolution

One misconception is that organisms evolve, in the Darwinian sense,
during their lifetimes

Natural selection acts on individuals, but only populations evolve

Genetic variations in populations contribute to evolution
: Population genetics:
foundation for studying
evolution

Microevolution is change in the genetic makeup of a population from
generation to generation

Population genetics is the study of how populations change genetically over time

Population genetics integrates Mendelian genetics with the Darwinian theory of
evolution by natural selection

This modern synthesis focuses on populations as units of evolution
Gene Pools and Allele
Frequencies
•
A population is a localized group of individuals capable of interbreeding and
producing fertile offspring

The gene pool is the total aggregate of genes in a population at any one time

The gene pool consists of all gene loci in all individuals of the population
The Hardy-Weinberg Theorem

The Hardy-Weinberg theorem describes a population that is not
evolving

It states that frequencies of alleles and 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

It describes a population where allele frequencies do
not change
populationHardy-Weinberg equilibrium describes a
population in which random mating occurs
Preservation of Allele Frequencies

In a given population where gametes contribute to the next generation
randomly, allele frequencies will not change.

If p and q represent the relative frequencies of the only two possible alleles
in a population at a particular locus, then

p2 + 2pq + q2 = 1

And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq
represents the frequency of the heterozygous genotype
LE 23-5
Gametes for each generation are
drawn at random from the gene pool
of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
CW
(20%)
p2
pq
64%
CRCR
16%
CRCW
(20%)
CR
(80%)
CW
Eggs
(80%)
qp
4%
CWCW
16%
CRCW
q2
Conditions for Hardy-Weinberg
Equilibrium

The Hardy-Weinberg theorem describes a hypothetical population

In real populations, allele and genotype frequencies do change over timeThe
five conditions for non-evolving populations are rarely met in nature:





Extremely large population size
No gene flow
No mutations
Random mating
No natural selection
Population Genetics and Human
Health

We can use the Hardy-Weinberg equation to estimate the percentage of the
human population carrying the allele for an inherited disease
Mutation

Mutation and sexual recombination produce the variation that makes
evolution possible

Two processes, mutation and sexual recombination, produce the variation
in gene pools that contributes to differences among individuals

Mutations are changes in the nucleotide sequence of DNA

Mutations cause new genes and alleles to arise
Animation: Genetic Variation from Sexual Recombination
Mutations

A point mutation is a change in one base in a gene

It is usually harmless but may have significant impact on phenotype

Chromosomal mutations that delete, disrupt, or rearrange many loci are
typically harmful

Gene duplication is nearly always harmful

Mutation rates are low in animals and plants

The average is about one mutation in every 100,000 genes per generation

Mutations are more rapid in microorganisms
Sexual Recombination

Sexual recombination is far more important than mutation in producing
the genetic differences that make adaptation possible
Natural selection, genetic drift, and gene flow can alter
a population’s genetic composition

Three major factors alter allele frequencies and bring about most
evolutionary change:

Natural selection

Genetic drift

Gene flow

Differential success in reproduction results in certain alleles being passed to the
next generation in greater proportions
Genetic Drift

The smaller a sample, the greater the chance of deviation from a predicted
result

Genetic drift describes how allele frequencies fluctuate unpredictably
from one generation to the next

Genetic drift tends to reduce genetic variation through losses of alleles
Animation: Causes of Evolutionary Change
LE 23-7
CWCW
CRCR
CRCR
CRCW
Only 5 of
10 plants
leave
offspring
CRCR
CWCW
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCR
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect

The bottleneck effect is a sudden change in the environment that may
drastically reduce the size of a population

The resulting gene pool may no longer be reflective of the original
population’s gene pool

Understanding the bottleneck effect can increase understanding of how
human activity affects other species
LE 23-8
Original
population
Bottlenecking
event
Surviving
population
The Founder Effect/Gene Flow

The founder effect occurs when a few individuals become isolated from a
larger population

It can affect allele frequencies in a population

Gene flow consists of genetic additions or subtractions from a population,
resulting from movement of fertile individuals or gametes

Gene flow causes a population to gain or lose alleles

It tends to reduce differences between populations over time
Natural selection is the primary
mechanism of adaptive evolution

Natural selection accumulates and maintains favorable genotypes in a
population
Genetic Variation

Genetic variation occurs in individuals in populations of all species

It is not always heritable

Both discrete and quantitative characters contribute to variation within a
population

Discrete characters can be classified on an either-or basis

Quantitative characters vary along a continuum within a population
LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white
Polymorphism

Phenotypic polymorphism describes a population in which two or more
distinct morphs for a character are represented in high enough frequencies
to be readily noticeable

Genetic polymorphisms are the heritable components of characters that
occur along a continuum in a population
Measuring Genetic Variation

Population geneticists measure polymorphisms in a population by
determining the amount of heterozygosity at the gene and molecular levels

Average heterozygosity measures the average percent of loci that are
heterozygous in a population.

Most species exhibit geographic variation differences between gene pools
of separate populations or population subgroups
LE 23-10
1
2.4
3.14
8.11
9.12
10.16
5.18
6
13.17
19
1
2.19
3.8
4.16
9.10
11.12
13.17
15.18
5.14
7.15
XX
6.7
XX
•
Some examples of geographic variation occur as a cline, which is a graded
change in a trait along a geographic axis
Heights of yarrow plants grown in
common garden
Seed collection sites
A Closer Look at Natural
Selection

From the range of variations available in a population, natural selection
increases frequencies of certain genotypes, fitting organisms to their
environment over generations
Evolutionary Fitness

The phrases “struggle for existence” and “survival of the fittest” are
commonly used to describe natural selection but can be misleading

Reproductive success is generally more subtle and depends on many
factors

Fitness is the contribution an individual makes to the gene pool of the next
generation, relative to the contributions of other individuals

Relative fitness is the contribution of a genotype to the next generation,
compared with contributions of alternative genotypes for the same locus
Directional, Disruptive, and
Stabilizing Selection

Selection favors certain genotypes by acting on the phenotypes of certain
organisms

Three modes of selection:

Directional=Directional selection favors individuals at one end of the
phenotypic range

Disruptive-Disruptive selection favors individuals at both extremes of the
phenotypic range

Stabilizing-Stabilizing selection favors intermediate variants and acts against
extreme phenotypes
Frequency of
individuals
LE 23-12
Original
population
Evolved
population
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
The Preservation of Genetic Variation

Various mechanisms help to preserve genetic variation in a population
 Diploidy maintains genetic variation in the form of hidden recessive alleles
 Balancing selection occurs when natural selection maintains stable
frequencies of two or more phenotypic forms in a population
 Balancing selection leads to a state called balanced polymorphism
 Some individuals who are heterozygous at a particular locus have greater
fitness than homozygotes=HETEROZYGUS ADVANTAGE-SICKLE
CELL AMEMIA
 Natural selection will tend to maintain two or more alleles at that locus
LE 23-13
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
7.5–10.0%
10.0–12.5%
>12.5%
Selection

In frequency-dependent selection, the fitness of any morph declines if it
becomes too common in the population

Neutral variation is genetic variation that appears to confer no selective
advantage

Sexual selection is natural selection for mating success

It can result in sexual dimorphism, marked differences between the sexes in
secondary sexual characteristics
LE 23-14
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 a new set
of images (a new
feeding opportunity).
Parental population sample
0.6
Phenotypic
variation
Experimental group sample
0.5
0.4
Frequencyindependent control
0.3
0.2
0
Plain background
Patterned background
20
40
60
Generation number
80
100

Intrasexual selection is competition among individuals of one sex for
mates of the opposite sex

Intersexual selection occurs when individuals of one sex (usually females)
are choosy in selecting their mates from individuals of the other sex

Selection may depend on the showiness of the male’s appearance
The Evolutionary Enigma of Sexual
Reproduction

Sexual reproduction produces fewer reproductive offspring than asexual
reproduction, a so-called “reproductive handicap”

Sexual reproduction produces genetic variation that
may aid in disease resistance
LE 23-16
Asexual reproduction
Female
Sexual reproduction
Generation 1
Female
Generation 2
Male
Generation 3
Generation 4
Why Natural Selection Cannot
Fashion Perfect Organisms

Evolution is limited by historical constraints

Adaptations are often compromises

Chance and natural selection interact

Selection can only edit existing variations