Measuring
Evolution of Populations
Gene Variation is Raw Material
 Evolution - change over time
 Evolution is descent with modification
 Darwin
 Through time, species accumulate differences such that ancestral and
descendent species are not identical.
Gene Variation is Raw Material
 Natural selection and evolutionary change
 Some individuals in a population possess certain inherited
characteristics that play a role in producing more surviving
offspring than individuals without those characteristics.
 The population gradually includes more individuals with advantageous
characteristics.
Darwin versus Lamarck
Macroevolution
 It is large
evolutionary change
 Evolution of new
species from a
common ancestor
 Evolution of one
species into two or
more species
Microevolution
 Evolution on a small scale
 Is the change in gene frequencies within
a population over time
 As the changes in populations
accumulate, they can lead to the formation
of new species.
Populations & gene pools
 Concepts
 a population is a localized group of interbreeding individuals
 gene pool is collection of alleles in the population
 remember difference between alleles & genes!
 allele frequency is how common is that allele in the population
 how many A vs. a in whole population
Hardy-Weinberg Principle
 Hardy-Weinberg - original proportions of genotypes in a
population will remain constant from generation to
generation
 Sexual reproduction (meiosis and fertilization) alone will not
change allelic (genotypic) proportions.
Hardy-Weinberg Principle
 Necessary assumptions
Allelic frequencies would remain constant if…
 population size is very large
 random mating
 no mutation
 no gene input from external sources
 no selection occurring
Hardy-Weinberg theorem
 Counting Alleles
 assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q
 frequencies must add to 1 (100%), so:
p+q=1
BB
Bb
bb
Hardy-Weinberg theorem
 Counting Individuals
 frequency of homozygous dominant: p x p = p2
 frequency of homozygous recessive: q x q = q2
 frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
Bb
bb
H-W formulas
 Alleles:
p+q=1
B
b
p2 + 2pq + q2 = 1
 Individuals:
BB
BB
Bb
Bb
bb
bb
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
Must
is in H-W equilibrium!
Whatassume
are thepopulation
genotype frequencies?
Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium
2pq=.48
q2=.16
BB
Bb
bb
p2=.20
=.74
BB
2pq=.64
2pq=.10
Bb
q2=.16
bb
Null hypothesis
Sampled data
How do you
explain the data?
Application of H-W principle
 Sickle cell anemia
 inherit a mutation in gene coding for hemoglobin
 oxygen-carrying blood protein
 recessive allele = HsHs
 normal allele = Hb
 low oxygen levels causes
RBC to sickle
 breakdown of RBC
 clogging small blood vessels
 damage to organs
 often lethal
Sickle cell frequency
 High frequency of heterozygotes
 1 in 5 in Central Africans = HbHs
 unusual for allele with severe
detrimental effects in homozygotes
 1 in 100 = HsHs
 usually die before reproductive age
Why is the Hs allele maintained at such high
levels in African populations?
Suggests some selective advantage of
being heterozygous…
Malaria
Single-celled eukaryote parasite
(Plasmodium) spends part of its
life cycle in red blood cells
1
2
3
Heterozygote Advantage
 In tropical Africa, where malaria is common:
 homozygous dominant (normal) die of malaria: HbHb
 homozygous recessive die of sickle cell anemia: HsHs
 heterozygote carriers are relatively free of both: HbHs
 survive more, more common in population
Hypothesis:
In malaria-infected
cells, the O2 level is
lowered enough to
cause sickling which
kills the cell &
destroys the parasite.
Frequency of sickle cell allele &
distribution of malaria
Hardy-Weinberg equilibrium
 Hypothetical, non-evolving population
 preserves allele frequencies
 Serves as a model (null hypothesis)
 natural populations rarely in H-W equilibrium
 useful model to measure if forces are acting on a population
 measuring evolutionary change
G.H. Hardy
mathematician
W. Weinberg
physician
Evolution of populations
 Evolution = change in allele frequencies in a population
 hypothetical: what conditions would cause allele frequencies to
not 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)
http://nhscience.lonestar.edu/biol/hwe.html
5 Agents
of
evolutionary
change
Mutation
Gene Flow
Non-random mating
Genetic Drift
Selection
5 Agents of evolutionary change
2005-2006
Five Agents of Evolutionary Change
 Mutation
 Mutation rates are generally so low they have little effect on
Hardy-Weinberg proportions of common alleles.
 ultimate source of genetic variation
 Gene flow
 movement of alleles from one population to another
 tend to homogenize allele frequencies
Five Agents of Evolutionary Change
 Nonrandom mating
 assortative mating - phenotypically similar individuals mate
 Causes frequencies of particular genotypes to differ from those predicted
by Hardy-Weinberg.
Five Agents of Evolutionary Change
 Genetic drift – statistical accidents.
 Frequencies of particular alleles may change by chance alone.
 important in small populations
 founder effect - few individuals found new population (small allelic pool)
 bottleneck effect - drastic reduction in population, and gene pool size
Genetic Drift - Bottleneck Effect
Five Agents of Evolutionary Change
 Selection – Only agent that produces adaptive
evolutionary change
 artificial - breeders exert selection
 natural - nature exerts selection
 variation must exist among individuals
 variation must result in differences in numbers of viable offspring produced
 variation must be genetically inherited
 natural selection is a process, and evolution is an outcome
Five Agents of Evolutionary Change
 Selection pressures:
 avoiding predators
 matching climatic condition
 pesticide resistance
Measuring Fitness
 Fitness is defined by evolutionary biologists as the number of
surviving offspring left in the next generation.
 relative measure
 Selection favors phenotypes with the greatest fitness.
Interactions Among Evolutionary
Forces
 Levels of variation retained in a population may be
determined by the relative strength of different evolutionary
processes.
 Gene flow versus natural selection
 Gene flow can be either a constructive or a constraining force.
 Allelic frequencies reflect a balance between gene flow and natural
selection.
Natural Selection Can Maintain
Variation
 Frequency-dependent selection
 Phenotype fitness depends on its frequency within the population.
 Negative frequency-dependent selection favors rare phenotypes.
 Positive frequency-dependent selection eliminates variation.
 Oscillating selection
 Selection favors different phenotypes at different times.
Forms of Selection
 Disruptive selection
 Selection eliminates intermediate types.
 Directional selection
 Selection eliminates one extreme from a phenotypic
array.
 Stabilizing selection
 Selection acts to eliminate both extremes from an
array of phenotypes.
 Sexual Selection
Kinds of Selection
Sexual selection
 Favours the selection of any trait that
confers an advantage in terms of the mating
success of the individual
 This is associated with sexual
dimorphism: which is the physical (often
extreme) differences in the appearance of
males and females
 The most common forms of sexual
selection are the results of female mate
choice and male to male competition
Sexual selection
 Females can chose based on physical traits,
colouration, or behavioural traits such as
courtship displays and songs
 Sometimes males develop features that
enable them to establish and defend a
territory from other males=sometimes
detaining the females
 How would you be able to tell these are
not env’tal selective pressures?
 Both sexes would possess the features.
Sexual selection
 Some features are a compromise between mating and
remaining conspicuous to predators==bright colours and
song.
What about plants?
 Sexual diversity is not limited to just animals
 Plants do not select mates but they do need to attract
suitors to assist in pollination
 Flowers and scents are the most obvious examples of
sexual features that have evolved==maximize pollination
Selection on Color in Guppies
 Guppies are found in small northeastern streams in
South America and in nearby mountainous streams in
Trinidad.
 Due to dispersal barriers, guppies can be found in
pools below waterfalls with high predation risk, or
pools above waterfalls with low predation risk.
Evolution of Coloration in Guppies
Selection on Color in Guppies
 High predation environment - Males exhibit drab
coloration and tend to be relatively small and reproduce
at a younger age.
 Low predation environment - Males display bright
coloration, a larger number of spots, and tend to be
more successful at defending territories.
 In the absence of predators, larger, more colorful fish
may produce more offspring.
Evolutionary Change in Spot Number
Limits to Selection
 Genes have multiple effects
 Pleiotropy - ex. PKU
 Evolution requires genetic variation
 Intense selection may remove variation from a
population at a rate greater than mutation can
replenish.
 thoroughbred horses
 Gene interactions affect allelic fitness
 epistatic interactions –ex.You may have a widows peak
but if you have the baldness gene you will not see your
widows peak.
Any Questions??
2005-2006