Download Chapter 23

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 provides a foundation for
studying evolution
• Microevolution - change in the genetic makeup of a
populations from generation to generation
The Modern Synthesis
• Population genetics - 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
• Population – localized group of individuals capable of
interbreeding and producing fertile offspring
• Gene pool - total of all alleles/genes in a pop at any 1 time
• gene pool has all gene loci in all individuals of the population
MAP
AREA
CANADA
ALASKA
LE 23-3
Beaufort Sea
Porcupine
herd range
Fairbanks
Fortymile
herd range
Whitehorse
The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem describes a population that is
not evolving
• Frequencies of alleles & genotypes in a populations gene pool
remain constant from generation to generation, provided that
only Mendelian segregation & recombination of alleles are at
work
• Mendelian inheritance preserves genetic variation in a
population
LE 23-4
Generation
1
X
CRCR
genotype
Generation
2
Plants mate
CWCW
genotype
All CRCW
(all pink flowers)
50% CW
gametes
50% CR
gametes
come together at random
Generation
3
25% CRCR
50% CRCW
50% CR
gametes
25% CWCW
50% CW
gametes
come together at random
Generation
4
25% CRCR
50% CRCW
25% CWCW
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
Hardy-Weinberg Equilibrium
• HW equilibrium describes a population in which random
mating occurs
• It describes a pop where allele frequencies do not change
• If p & q represent the relative frequencies of the only 2
possible alleles in a pop at a particular locus, then
• p2 + 2pq + q2 = 1
• p2 & q2 represent the frequencies of the homozygous genotypes
& 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
• HW theorem describes a hypothetical population
• In real populations, allele & genotype frequencies change over
time
• 5 conditions for non-evolving populations; rarely met in
nature:
•
•
•
•
•
Extremely large pop size
No gene flow
No mutations
Random mating
No natural selection
Population Genetics and
Human Health
• We can use the HW equation to estimate the % of the human
pop carrying the allele for an inherited disease
• If 1 out of every 10,000 babies born is affected by a recessive
disorder, what is the frequency of each allele?
• What is the frequency of heterozygotes?
Variation
• Variation in populations is produced by:
• mutation
• sexual recombination
Mutation
• Mutations are changes in the nucleotide sequences of DNA
• Mutations cause new genes & alleles to arise
• The original source of genetic diversity
Point Mutations
• A point mutation is a change in 1 base in a gene
• It is usually harmless but may have significant impact on
phenotype
Mutations That Alter Gene
Number or Sequence
• Chromosomal mutations that delete, disrupt, or rearrange
many loci are typically harmful
• Gene duplication is nearly always harmful
Mutation Rates
• Mutation rates are low in animals & plants
• average is ~ 1 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
• 3 factors alter allele frequencies & bring about most
evolutionary change:
• Natural selection
• Genetic drift
• Gene flow
Natural Selection
• Differential success in reproduction results in certain alleles
being passed to the next generation in greater proportions
Genetic Drift
• The smaller a sample, the > the chance of deviation from a
predicted result
• Genetic drift – describes how allele frequencies fluctuate
unpredictably from 1 generation to the next
• Genetic drift can decrease genetic variation by loss of alleles
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
• bottleneck effect - sudden change in environment that may
drastically decrease size of a population
• resulting gene pool may no longer be reflective of original
population’s gene pool
LE 23-8
Original
population
Bottlenecking
event
Surviving
population
The Founder Effect
• founder effect – occurs when a few individuals become
isolated from a larger pop
• I.e. part of a population migrates to a different area, becoming
geographically isolated from the original population. The
individuals of the migrating group are the “founders” of the
new population
• E.g. Polydactyly in Amish population
Gene Flow
• Gene flow – consists of genetic additions or subtractions from
a population, resulting from movement of fertile indivividuals
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 & maintains favorable
genotypes in a population
Genetic Variation
• Genetic variation occurs in individuals in populations of all
species
• It is not always heritable
• Like the butterflies on the following page have the same
genotype
LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white
Variation Within a Population
• Both discrete & quantitative characters contribute to variation
within a population
• Discrete characters can be classified on an either-or basis
• Flowers are either red, pink, or white
• Quantitative characters vary along a continuum within a
population
• Height
• Skin tone
Polymorphism
• Phenotypic polymorphism - population in which 2 or more
distinct morphs for a character are represented in high
enough frequencies to be readily noticeable
• Genetic polymorphisms - heritable components of characters
that occur along a continuum in a population (height)
Measuring Genetic Variation
• Population geneticists measure polymorphisms in a
population by determining the amount of heterozygosity at
the geneetic & molecular levels
• Average heterozygosity measures the average % of loci that
are heterozygous in a population
• For ex: Drosophila is heterzygous at ~14% of its loci (so the avg
hetero is 14%)
• So ~1900 of its genes out of 13,700 are hetero
Variation Between Populations
• Most species exhibit geographic variation differences between
gene pools of separate populations or population subgroups
LE 23-10
• Some examples of geographic variation occur as a cline
• Cline - a graded change in a trait along a geographic axis
LE 23-11
Heights of yarrow plants grown in common garden
Mean height (cm)
100
50
0
3,000
2,000
1,000
Sierra Nevada
Range
0
Seed collection sites
Great Basin
Plateau
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” & “survival of the fittest”
are commonly used to describe natural selection but can be
misleading
• Reproductive success is generally more subtle & depends on
many factors
• Fitness - contribution an individual makes to the gene pool of
the next generation, relative to the contributions of other
individuals
• Relative fitness - contribution of a genotype to the next
generation, compared w/ contributions of alternative
genotypes for the same locus
• More successful genotype is set at a value of 1; other
genotypes are compared to the more successful genotype
Directional, Disruptive, and
Stabilizing Selection
• Selection favors certain genotypes by acting on the
phenotypes of certain organisms
• 3 modes of selection:
• Directional
• Disruptive
• Stabilizing
• Directional selection favors individuals at 1 end of the
phenotypic range
• Disruptive selection favors individuals at both extremes of the
phenotypic range
• Stabilizing selection favors intermediate variants & 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
Balancing Selection
• Balancing selection - natural selection maintains stable freq’s
of 2 or more phenotypic forms in a pop; leads to a state called
balanced polymorphism
• 2 kinds:
• Heterozygote advantage
• Frequency dependent selection
Heterozygote Advantage
• Heterozygote Advantage – individuals who are heterozygous
at a locus have greater fitness than homozygotes
• Natural selection will tend to maintain 2 or more alleles at
that locus
• E.g. Sickle cell allele
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%
Frequency-Dependent
Selection
• frequency-dependent selection – fitness of any morph
declines if it becomes too common in the population
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
Neutral Variation
• Neutral variation – genetic variation that appears to confer no
selective advantage
• Such as mutations in introns etc….
Sexual Selection
• Sexual selection – natural selection for mating success
• Can result in sexual dimorphism - marked differences between
the sexes in secondary sexual characteristics
• Intrasexual selection - competition among individuals of one
sex for mates of the opposite sex
• Intersexual selection - individuals of one sex (usually females)
are choosy in selecting their mates from individuals of the
other sex
Why do you think the female is usually the choosier sex?
The Evolutionary Enigma of
Sexual Reproduction
• Sexual reproduction produces fewer reproductive offspring
than asexual reproduction, a so-called “reproductive
handicap”
LE 23-16
Asexual reproduction
Female
Sexual reproduction
Generation 1
Female
Generation 2
Male
Generation 3
Generation 4
• Sexual reproduction produces genetic variation that may aid in
disease resistance
Why Natural Selection Cannot
Fashion Perfect Organisms
•
•
•
•
•
Why Natural Selection Can’t make Perfect Organisms:
Evolution is limited by historical constraints
Adaptations are often compromises
Chance & natural selection interact
Selection can only edit existing variations