Transcript Evolution of Populations

Evolution of Populations
The Smallest Unit of Evolution
• Natural selection acts on individuals, but only
populations evolve
– Genetic variations contribute to evolution
Population genetics
• Population genetics
– study of how populations change genetically
over time
• Mendelian genetics with the Darwinian
theory
• 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
– all gene loci in all individuals of the
population
The Hardy-Weinberg Theorem
• population that is not evolving
• 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
• preservation of genetic variation in a
population
Hardy-Weinberg Equilibrium
• The 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
Hardy-Weinberg Equilibrium
• 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 23-4
Generation
1
X
CRCR
genotype
Generation
2
CWCW
genotype
Plants mate
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
Evolutionary Change
• Three major factors alter allele frequencies
and bring about most evolutionary change:
– Mutations
– Natural selection
– Nonrandom Mating
– Genetic drift
– Gene flow
Variations that make Natural Selection Possible
• Mutation
– changes in the nucleotide sequence of DNA
– new genes and alleles to arise
– Point Mutations
• change in one base in a gene
• usually harmless
• may impact on phenotype
Mutations
• Chromosomal mutations that delete, disrupt,
or rearrange many loci are typically harmful
• Gene duplication is nearly always harmful
Natural Selection
• Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
• 3 conditions for natural selection to occur and
to result in evolutionary change
1. Variation must exist among individuals in a
population
2. Variation among individuals must result in
differences in the number of offspring surviving
in the next generation
3. Variation must be genetically inherited
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Sexual Recombination
• far more important than mutation
• produces the genetic differences that make
adaptation possible
• Nonrandom mating
– Assortative mating
• Phenotypically similar
individuals mate
• Increases proportion of
homozygous individuals
– Disassortative mating
• Phenotypically different
individuals mate
• Produces excess of
heterozygotes
Genetic Drift
• The smaller a sample, the greater the chance
of deviation from a predicted result
• allele frequencies fluctuate unpredictably
from one generation to the next
• reduces genetic variation through losses of
alleles
Genetic Drift
• The Bottleneck Effect
– sudden change in the
environment that may
drastically reduce the
size of a population
– gene pool may no
longer be reflective of
the original
population’s gene pool
Genetic Drift
• The Founder Effect
– a few individuals
become isolated from a
larger population
– affects allele
frequencies
Gene Flow
• genetic additions or subtractions from a
population, resulting from movement of
fertile individuals or gametes
• gain or loss of alleles
• reduce differences between populations over
time
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
• Misleading
– “struggle for existence”
– “survival of the fittest”
• 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:
– Directional
• favors individuals at one end of the phenotypic range
– Disruptive
• favors individuals at both extremes of the phenotypic range
– Stabilizing
• favors intermediate variants and acts against extreme
phenotypes
The Preservation of Genetic Variation
• Diploidy
– maintains genetic variation in the form of hidden
recessive alleles
• Balancing selection
– natural selection maintains stable frequencies of
two or more phenotypic forms
• 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
– Sickle cell and malaria
• Sexual selection
– natural selection for mating success
– sexual dimorphism
• 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 Natural Selection Cannot Fashion
Perfect Organisms
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Evolution is limited by historical constraints
Adaptations are often compromises
Chance and natural selection interact
Selection can only edit existing variations