Download Notes 1 Ch 23 Evolution_Pop

Population Genetics - the foundation of evolution
Microevolution: change in the genetic makeup of a
population from generation to generation; individuals
are selected and populations evolve
Grass growing on
toxic heavy metals
only ones that
can tolerate the
metals survive
Chapter 23
The Evolution of Populations
Mrs. MacWilliams
AP Biology
Darwin and Mendel
Darwin: evolution by natural selection but how do
organisms get heritable variations and how do they
transmit them to their offspring?
Blending Hypothesis??? No…this eliminates differences
Mendel: Particulate Theory of Inheritance; pass on
discrete units (genes) that retain identity
Variation Within a Population
• Discrete characters can be classified on an either-or
basis
 Example: EITHER red OR white flowers
• Quantitative characters vary along a continuum within
a population
 Example: Variation in Beak size
Gene Pools and Allele Frequencies
Population: a localized group
of individuals that are
capable of interbreeding
and producing fertile
offspring; some are
isolated, some are not
Gene Pool: the total aggregate
of genes in a population at
any one time
Hardy-Weinberg Theorem
Hardy-Weinberg equation
Predict allele frequencies within a
population.
Its prediction of genetic equilibrium
or no frequency change under a
specific set of conditions also
serves as strong evidence for
microevolution.
As long as a population does not meet
all the prerequisite conditions for
genetic equilibrium, allele
frequencies must change over
time, and this is the primary
process of evolution.
Hardy-Weinberg
Theorem
p2 + 2pq + q2 = 1
AA + Aa + aa = 100%
p+q=1
A + a = 100%
HARDY-WEINBERG & EVOLUTION
• Biologists can determine whether an agent of
evolution is acting on a population by comparing the
population’s genotype frequencies with Hardy–
Weinberg equilibrium frequencies.
• If there is no change in frequencies, there is no
evolution
• Conversely, if there have been changes in the
frequencies, then evolution has occurred.
• Evolution is change of allelic frequencies
H-W Conditions
1. Extremely large population size…small populations have greater
chances of fluctuations
2. No Gene Flow…no transfer of genes between populations
3. No Mutations…introducing, reducing, modifying genes can change the
gene pool
4. Random Mating…if individuals preferentially choose mates then no
random mixing of gametes
5. No Natural Selection…differential survival and reproductive success
will alter allele frequency
Mutation & Sexual Recombination = variation
Only mutations in gametes can be passed to offspring
Point mutations, Deletion…
Sexual Recombination
Shuffling of genes…
Genetic Drift, Gene Flow = variation
Genetic Drift: small population = greater chance of deviation
Genetic Drift, Gene Flow = variation
Genetic Drift --> ex: Bottle Neck Effect
• Northern elephant seals have reduced
genetic variation probably because of
a population bottleneck humans
inflicted on them in the 1890s.
Hunting reduced their population size
to as few as 20 individuals at the end
of the 19th century. Their population
has since rebounded to over 30,000—
but their genes still carry the marks of
this bottleneck: they have much less
genetic variation than a population of
southern elephant seals that was not so
intensely hunted.
Genetic Drift, Gene Flow = variation
• Genetic Drift --> ex: Founder Effect
• The establishment of a new population by a few
original founders (in an extreme case, by a single
fertilized female) which caarry only a small fraction
of the total genetic variation of the parental
population. The result is that a given allele, gene,
chromosome or part of a chromosome found in
members of the population can be traced back to the
one ancestral individual(s).
Genetic Drift, Gene Flow = variation
Genetic Drift --> ex: The Founder Effect
An Example of Founder Affect:
• The Afrikaner population of Dutch
settlers in South Africa is
descended mainly from a few
colonists. Today, the Afrikaner
population has an unusually high
frequency of the gene that causes
Huntington’s disease, because those
original Dutch colonists just
happened to carry that gene with
unusually high frequency. This
effect is easy to recognize in
genetic diseases, but of course, the
frequencies of all sorts of genes are
affected by founder events.
Genetic Drift, Gene Flow = variation
Gene Flow: immigration/emigration ; add/subtract genes
Gene Flow in Humans
• Gene flow has been observed in humans. For example, in
the United States, gene flow was observed between a
white European population and a black West African
population, which were recently brought together. In
West Africa, where malaria is prevalent, the Duffy
antigen provides some resistance to the disease, and this
allele is thus present in nearly all of the West African
population. In contrast, Europeans have either the allele
Fya or Fyb, because malaria is almost non-existent. By
measuring the frequencies of the West African and
European groups, scientists found that the allele
frequencies became mixed in each population because of
movement of individuals. It was also found that this gene
flow between European and West African groups is much
greater in the Northern U.S. than in the South.
Gene Flow
Natural Selection = adaptive evolution
• Of all the factors that can change a gene pool, only
natural selection is likely to adapt a population to its
environment
• Natural selection accumulated & maintains favorable
genotypes in a population
• Variations that are heritable are the raw material for
natural selection
Variation Within a Population
• Polymorphism: 2 or more distinct
morphs (different types) represented
in a population
• Average Heterozygosity-measures
the average percent of loci that
are heterozygous in a population
• Geographic Variation
Modes of Selection
Preserving Genetic Variation
• Diploidy- maintains genetic variation in the form
of hidden recessive alleles
Preserving Genetic Variation
• Balancing Selection- natural selection maintains stable
frequencies of two or more phenotypic forms in a
population
 Heterozygote Advantage-heterozygotes have a
higher fitness than do both homozygotes
• Ex. Sickle Cell and Malaria
• Ex. Cystic Fibrosis and cholera
 Frequency Dependent Selection- fitness of a
phenotype declines if it becomes too common in the
population
• Neutral Variation- genetic variation that appears to
confer no selective advantage or disadvantage
Preserving Genetic Variation
• Sexual Selection
• Intrasexual selection- competition among
individuals of one sex (often males) for mates of
the opposite sex
• Intersexual selection-mate choice, occurs when
individuals of one sex (usually females) are choosy
in selecting their mates
• sexual dimorphism- marked differences between
the sexes in secondary sexual characteristics
Reproductive “handicap” of sex
Sexually reproducing organisms don’t leave as many offspring BUT
offspring varies which may make them more ‘fit’
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