Transcript A Population

Mechanisms of Evolution
Microevolution
Population Genetics
• A Population is a group of interbreeding organisms living
together in space and time. (This means they are
necessarily all the same species)
• A Population is The Smallest Unit of Evolution
• Individual organisms DO NOT evolve (in the Darwinian
sense)
• Natural selection acts on individuals, but populations
evolve
• What changes is the gene pool of the population, from
generation to generation
• Genetic variations in populations
– Contribute to Natural Selection and are what
is changed by natural selection
Cuban Tree Snails
Figure 23.1
Key Concepts
• 23.1: Population genetics provides a foundation
for studying evolution
• 23.2: Mutation and sexual recombination
produce the variation that makes evolution
possible
• 23.3: Natural selection, genetic drift, and gene
flow can alter a population’s genetic composition
• 23.4: Natural selection is the primary
mechanism of adaptive evolution
• Concept 23.1: Population genetics
provides a foundation for studying
evolution
• Microevolution
– Is change in the genetic makeup of a
population from generation to generation
Figure 23.2
The Modern Synthesis
• Integrates Darwinian selection and
Medelian inheritance and focuses on
population genetics
• Population genetics (began in 1930’s)
– Is the study of how populations change
genetically over time
– Reconciled Darwin’s and Mendel’s ideas
• At the time The Origin of Species was published,
little was known about inheritance.
• Darwin did not know how variation appeared or
how it was transmitted
• His raw material for selection was variation in
quantitative characters (vary along a
continuum)
• Mendel’s characters were discrete
• Mendel’s inheritance was rediscovered in the
early 1900’s, but it wasn’t until the 1930’s that
scientists recognized that Darwin’s quantitative
characters are genetically inherited
• The Modern Synthesis was formulated in
the 1940’s by many scientists.
• Ernst Mayr, biogeographer and
systematist emphasized:
– The population as the unit of evolution
– Natural selection as the primary mechanism
– Gradualism as an explanation of large
changes resulting from the accumulation of
small changes over long periods of time
Evolutionary science
continues to develop
• Current debate focuses on the rate of
evolution and on the importance of
evolutionary mechanisms other than
natural selection
Gene Pools and Allele
Frequencies
• The genetic structure of a population is defined
by its allele and genotype frequencies
• A population
– Is a group of individuals of the same species living
together in space and time
• A Species
– is a group of populations whose individuals have
the potential to interbreed and produce fertile
offspring in nature.
• Most species are not evenly distributed over
their geographic range
• They may have several localized population
centers
MAP
AREA
• The population
centers may be
more or less isolated
• Even when centers
are less isolated,
individuals are still
more likely to mate
with others from their
population center, so
gene flow is reduced
by the intermediate
range
•
Fairbanks
Fortymile
herd range
•
Whitehorse
Figure 23.3
Population Gene Pool
• Is the total aggregate of genes in a population at
any one time
• Consists of all gene loci in all individuals of the
population
• Is made up of alleles that are combined to form
the next generation
– An allele is said to be “fixed” if all members of the
population are homozygous for that gene
– Normally there will be two or more alleles for each
locus, each having a relative frequency in the gene
pool
Gene Pool
• In diploid species, each individual will be
homozygous or heterozygous for each
locus because each locus is represented
twice
The Hardy-Weinberg Theorem
• Describes a population that is not evolving
• States that the 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
Mendelian inheritance
• Preserves genetic variation in a
population
• Does not
alter the
frequency of
alleles or
genotypes
Generation
1
CW CW
CRCR
genotype
genotype
Plants mate
Generation
2
All CRCW
(all pink flowers)
50% CR
gametes
50% CW
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
Figure 23.4
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
Example
• 500 Wildflower plants with two alleles for
flower color (CR and CW)
• 320 Homozygotes CRCR are red
• 160 Heterozygotes CRCW are pink
• 20 Homozygotes CWCW are white
• This means there are 800 CR alleles and
200 CW alleles
Example
500 diploid wildflowers have 1000 alleles
320 CRCR have 640 CR alleles
160 CRCW have 160 CR alleles for a total of 800
20 CWCW have 40 CWalleles
160 CRCW have 160 CW alleles for a total of 200
The frequency of the CR allele is 0.8 and the
frequency of the CW allele is 0.2
Hardy-Weinberg Equilibrium
• Describes a population in which random
mating occurs
• Describes a population where allele
frequencies do not change
• Describes a population that is NOT
evolving
Hardy-Weinberg equilibrium
• p is the frequency
of one allele and q
is the frequency of
the other allele
Gametes for each generation are drawn at random from
the gene pool of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
p2
CR
(80%)
p2
64%
CRCR
CW
(20%)
Eggs
• If there are only
two alleles then
p+q=1
pq
16%
CRCW
16%
CRCW
qp
4%
CW CW
q2
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium:
• Genotype
frequencies are
calculated using
allele frequencies
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of the next generation:
16% CR from
64% CR from
+
CRCW heterozygotes
CRCR homozygotes
4% CW from
CWCW homozygotes
+
16% CW from
CRCW heterozygotes
=
80% CR = 0.8 = p
=
20% CW = 0.2 = q
With random mating, these gametes will result in the same
mix of plants in the next generation:
Figure 23.5
64% CRCR, 32% CRCW and 4% CWCW plants
• 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
• In cases of complete dominance p is usually
reserved to represent the dominant allele
and q represents the recessive allele
• If p is 0.8 and q
is 0.2
Gametes for each generation are drawn at random from
the gene pool of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CW
(20%)
pq
CR
(80%)
Eggs
p2
p2
64%
CRCR
CW
(20%)
– Then p2 is 0.64
– q2 is 0.04
– 2pq is 0.32
CR
(80%)
16%
CRCW
16%
CRCW
qp
4%
CW CW
q2
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium:
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of the next generation:
16% CR from
64% CR from
+
CRCW heterozygotes
CRCR homozygotes
4% CW from
CWCW homozygotes
+
16% CW from
CRCW heterozygotes
=
80% CR = 0.8 = p
=
20% CW = 0.2 = q
With random mating, these gametes will result in the same
mix of plants in the next generation:
Figure 23.5
64% CRCR, 32% CRCW and 4% CWCW plants
Conditions for HardyWeinberg Equilibrium
• The Hardy-Weinberg theorem
– Describes a hypothetical population
• In real populations
– Allele and genotype frequencies do change
over time
• 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
Population Genetics and
Human Health
• Using the Hardy-Weinberg equation to estimate
the percentage of the human population carrying
the allele for an inherited disease
• 1/400 African Americans have sickle-cell disease
• The frequency of the homozygous recessive
genotype is 0.0025 (q2)
• q = 0.05 is the frequency of the recessive allele
• p = 0.95 is the frequency of the dominant allele
• 2pq = 0.095 is the frequency of the
heterozygous carrier genotype in the African
American population of the U.S.