Download Ch 23 lecture - D and F: AP Biology

Chapter 23
The Evolution of Populations
Overview: The Smallest Unit of
Evolution
• One misconception is that organisms evolve
during their lifetimes
– Exceptions do exist however
• 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 is change in the genetic
makeup of a population from generation to
generation
• Micro- versus macroevolution is a large
controversy of evolution
The Modern Synthesis
• Population genetics: study of how populations
change genetically over time
– 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 aggregate of genes in a
population at any one time
– The gene pool consists of all genes of all the
individuals in a population
The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem describes
numerically what is occurring in a population that
is not evolving
– Describes a hypothetical population because in real
populations, allele and genotype frequencies do change
over time
• It states that frequencies of alleles and genotypes in a
population’s gene pool remain constant from generation to
generation
Conditions for
Hardy-Weinberg Equilibrium
• The five conditions for non-evolving populations
are rarely met in nature, they include:
1.
2.
3.
4.
5.
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection
Hardy-Weinberg Equations
p2 + 2pq + q2 = 1
p+q=1
• p and q represent the relative frequencies of the
only two possible alleles in a population at a
particular locus
– p2 = frequency of homozygous dominant genotype
– q2 = frequency of homozygous recessive genotype
– 2pq = frequency of the heterozygous genotype
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
Practice Problem
• You have sampled a population in which you
know that the percentage of the homozygous
recessive genotype (aa) is 36%. Calculate the
following:
1.
2.
3.
4.
The frequency of the aa genotype.
The frequency of the a allele.
The frequency of the A allele.
The frequencies of the genotypes AA and Aa.
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
Mutation
• Mutations are changes in the nucleotide
sequence of DNA
– Mutations cause new genes and alleles to arise
• A point mutation is a change in one 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 and plants
– The average is about one mutation in every 100,000
genes per generation
• Mutations are more rapid in microorganisms
Sexual Recombination
• Half of one parent's genes are combined with
half of another other parent's genes within the
offspring
– This results in gene a combination that did not
previously exist
– Sexual recombination is far more important in
producing the genetic differences that make
adaptation possible
Factors alter a population’s genetic
composition
•
Three major factors alter allele frequencies and
bring about most evolutionary change:
1. Natural selection
2. Genetic drift
3. Gene flow
Natural Selection
• Differential success in fitness in offspring results
in certain alleles being passed to the next
generation in greater proportions
• Those alleles that produce less fit offspring die
with the offspring
Genetic Drift
• The smaller a sample, the greater the chance of the
offspring being inbreed
– Genetic drift tends to reduce genetic variation through
losses of alleles
– Can be found seen within the:
1. Bottleneck effect
2. Founder effect
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
Original
population
Bottlenecking
event
Surviving
population
The Founder Effect
• The founder effect occurs when a few
individuals become isolated from a larger
population
• The resulting gene pool may no longer be
reflective of the original population’s gene pool
Gene Flow
• 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
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
Variation Between Populations
• Most species exhibit geographic variation
differences between gene pools of separate
populations or population subgroups
• Some examples of geographic variation occur as
a cline, which is 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
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: 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 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:
1. Directional: favors individuals at one end of the
phenotypic range
2. Disruptive: favors individuals at both extremes of the
phenotypic range
3. Stabilizing: favors intermediate variants and acts
against extreme phenotypes
Frequency of
individuals
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
1.
2.
3.
4.
5.
Diploidy
Balancing selection
Heterozygote advantage
Frequency-dependant selection
Sexual selection
Diploidy
• Diploidy maintains genetic variation in the form
of hidden recessive alleles
Balancing Selection
• 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
Heterozygote Advantage
• Some individuals who are heterozygous at a
particular locus have greater fitness than
homozygotes
• Natural selection will tend to maintain two or
more alleles at that locus
– The sickle-cell allele causes mutations in hemoglobin
but also confers malaria resistance
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
• In frequency-dependent selection, the fitness of
any morph declines if it becomes too common
in the population
Sexual Selection
• Sexual selection is natural selection for mating
success
– It can result in sexual dimorphism, marked
differences between the sexes in secondary sexual
characteristics
• Intrasexual selection is competition among
individuals of one sex for mates of the opposite
sex
The Evolutionary Enigma of Sexual
Reproduction
• Sexual reproduction produces fewer
reproductive offspring than asexual
reproduction, a so-called “reproductive
handicap”
• However, sexual reproduction produces genetic
variation that may aid in disease resistance
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