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Chapter 23~
• Chapter 23~
The Evolution of
Populations
Population genetics
•
Population:
a localized group of individuals
belonging to the same species
•
Species:
a group of populations whose
individuals have the potential to interbreed
and produce fertile offspring
•
Gene pool:
the total aggregate of genes in a
population at any one time
•
Population genetics:
the study of genetic changes in
populations
•
•
Modern synthesis/neo-Darwinism
“Individuals are selected, but populations
evolve.”
Hardy-Weinberg Theorem
• Serves as a model for the genetic structure of a
nonevolving population (equilibrium)
• Evolution = change in allele frequencies in a population
– hypothetical: what conditions not would cause allele
frequencies to change?
– non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
Hardy-Weinberg Equation
• p=frequency of one allele (A); q=frequency of the
other allele (a);
p+q=1.0 (p=1-q & q=1-p)
• p2=frequency of AA genotype; 2pq=frequency
of Aa genotype; q2=frequency of aa genotype;
•
frequencies of all individuals must add to 1 (100%),
so:
p2 + 2pq + q2 = 1
Using Hardy-Weinberg equation
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.16
bb
What assume
Must
are the population
genotype frequencies?
is in H-W equilibrium!
5 Agents of evolutionary change
Gene Flow
Genetic Drift
Mutation
Non-random mating
Selection
Microevolution, I
• A change in the gene
pool of a population
over a succession of
generations
• 1- Genetic drift:
changes in the gene
pool of a small
population due to
chance (usually
reduces genetic
variability)
Microevolution, II: type of genetic drift
• The Bottleneck Effect:
type of genetic drift
resulting from a reduction
in population (natural
disaster) such that the
surviving population is no
longer genetically
representative of the
original population
Conservation issues
Peregrine Falcon
• Bottlenecking is an important
concept in conservation biology
of endangered species
– loss of alleles from gene pool
– reduces variation
– reduces adaptability
Breeding programs must
consciously outcross
Golden Lion
Tamarin
Microevolution, III type of genetic drift
• Founder Effect:
a cause of genetic drift
attributable to colonization
by a limited number of
individuals from a parent
population
– just by chance some rare
alleles may
be at high frequency;
others may be missing
– skew the gene pool of
new population
• human populations that
started from small
group
of colonists
• example:
colonization of New
World
Microevolution, IV
• 2- Gene Flow:
genetic exchange due to
the migration of fertile
individuals or gametes
between populations
(reduces differences
between populations)
• seed & pollen distribution by
wind & insect
• migration of animals
Microevolution, V
• 3- Mutations:
•
Mutation creates
variation
a change in an organism’s
DNA (gametes; many
generations); original
source of genetic
variation (raw material for
natural selection)
Microevolution, VI
• 4- Nonrandom mating:
• Sexual selection
• inbreeding and
assortive mating
(both shift
frequencies of
different genotypes)
Sexual selection
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
It’s FEMALE CHOICE, baby!
Microevolution, VII
5.NaturalSelection
• differential success
in reproduction;
• climate change
• food source availability
• predators, parasites,
diseases
• toxins
•
only form of microevolution that
adapts a population to its
environment
• combinations of alleles
that provide “fitness”
increase in the population
Natural Selection
• Selection acts on any trait that affects
survival or reproduction
– predation selection
– physiological selection
– sexual selection
Variation & natural selection
• Variation is the raw material for natural
selection
– there have to be differences within population
– some individuals must be more fit than others
Where does Variation come from?
• Mutation
• errors in mitosis & meiosis
• environmental damage
Beak depth
– random changes to DNA
Wet year
Dry year
Dry year
1977
• Sex
Dry year
1980
1982
1984
– mixing of alleles
• recombination of alleles
– new arrangements in every offspring
• new combinations = new phenotypes
– spreads variation
• offspring inherit traits from parent
Beak depth of
offspring (mm)
11
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
Population variation
• Polymorphism:
coexistence of 2 or more
distinct forms of
individuals (morphs)
within the same
population
• Geographical
variation: differences in
genetic structure
between populations
(cline)
Variation preservation
• Prevention of natural selection’s
reduction of variation
• Diploidy
2nd set of chromosomes hides
variation in the heterozygote
• Balanced polymorphism
1- heterozygote advantage
(hybrid vigor; i.e., malaria/sicklecell anemia);
2- frequency dependent
selection (survival &
reproduction of any 1 morph
declines if it becomes too
common; i.e., parasite/host)
Natural selection
• Fitness:
contribution an
individual makes to
the gene pool of the
•
•
•
•
next generation
3 types:
A. Directional
B. Diversifying
C. Stabilizing
•
Effects
of
Selection
Changes in the average trait of a population
DIRECTIONAL
SELECTION
giraffe neck
horse size
STABILIZING
SELECTION
human birth weight
DISRUPTIVE
SELECTION
rock pocket mice
Sexual selection
• Sexual dimorphism:
secondary sex
characteristic distinction
• Sexual selection:
selection towards
secondary sex
characteristics that leads
to sexual dimorphism
Any Questions??