Transcript Lecture #10 Date

Lecture #10
Date ________
• Chapter 23~
The Evolution of
Populations
Population genetics
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Population:
a localized group of individuals
belonging to the same species
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Species:
a group of populations whose
individuals have the potential to interbreed
and produce fertile offspring
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Gene pool:
the total aggregate of genes in a
population at any one time
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Population genetics:
the study of genetic changes in
populations
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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
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(equilibrium)
5 conditions:
1- Very large population size;
2- No migration;
3- No net mutations;
4- Random mating;
5- No natural selection
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 plus
aA genotype; q2=frequency of aa genotype;
p2 + 2pq + q2 = 1.0
Problem 1 The allele for
black coat is recessive.
We can use the HardyWeinberg equation to
determine the percent of
the pig population that is
heterozygous for white
coat.
1. Calculate q2 :Count the individuals
that are homozygous recessive in
the illustration above. Calculate the
percent of the total population they
represent. This is q2.
2. Find q. Take the square root of q2
to obtain q, the frequency of the
recessive allele.
3. Find p. The sum of the frequencies of
both alleles = 100%, p + q = l. You
know q, so what is p?
4. Find 2pq. The frequency of the
heterozygotes is represented by 2pq.
This gives you the percent of the
population that is heterozygous for
white coat:
Sample Problem 2
In a certain population of 1000 fruit flies,
640 have red eyes while the remainder
have sepia eyes. The sepia eye trait is
recessive to red eyes. How many
individuals would you expect to be
homozygous for red eye color?
Answer:
160
Sample Problem 3
• If 9% of an African population is born with a
severe form of sickle-cell anemia (ss), what
percentage of the population will be more
resistant to malaria because they are
heterozygous(Ss) for the sickle-cell gene?
• 42% are heterozygous
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
• 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
Microevolution, III
• Founder Effect:
a cause of genetic drift
attributable to
colonization by a limited
number of individuals
from a parent
population
Microevolution, IV
• 2- Gene Flow:
genetic exchange due to
the migration of fertile
individuals or gametes
between populations
(reduces differences
between populations)
Microevolution, V
• 3- Mutations:
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:
inbreeding and
assortive mating
(both shift
frequencies of
different genotypes)
Microevolution, VII
• 5- Natural Selection:
differential success in
reproduction;
only form of
microevolution that
adapts a population to
its environment
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
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the next generation
3 types:
A. Directional
B. Diversifying
C. Stabilizing
Sexual selection
• Sexual dimorphism:
secondary sex
characteristic distinction
• Intersexual and
Intrasexual selection
• Sexual selection:
selection towards
secondary sex
characteristics that leads
to sexual dimorphism