Transcript Evolution of Populations

Evolution of
Populations
23.1 – Mutation & sexual reproduction
produce genetic variation that makes
evolution possible
1) Microevolution
Change in the allele frequencies of
a population over generations
Evolution on the smallest scale
2) Mutations
The only source of NEW genes &
NEW alleles
Only mutations in cell lines that
produce gametes can be passed on
to offspring
Types of Mutations
A) Point Mutation
Change in one base in a gene
Can impact phenotype
Sickle cell anemia
B) Chromosomal Mutation
Delete, disrupt, duplicate, or rearrange
many loci at once
Most are harmful, but not always
3) Variations due to sexual
reproduction
Rearranges alleles into new
combinations in every generation
3 mechanisms for this shuffling:
Next slide
1) Crossing over
During Prophase I of meiosis
2) Independent assortment
During meiosis (223 different
combinations possible)
3) Fertilization
223 x 223 for sperm and egg
23.2: The Hardy-Weinberg equation can be
used to test whether a population is
evolving
Population genetics
Study of how populations change
genetically over time
Population
Group of individuals of the same
species that live in the same area
Interbreed & produce fertile offspring
Gene pool
All of the alleles at all loci in all the members
of a population
In diploids, each individual has 2 alleles for a
gene & the individual can be heterozygous or
homozygous
If all are homozygous for an allele, the allele is
FIXED – only one allele exists at the locus in
the population
The greater the # of FIXED alleles, the lower
the species’ diversity
Hardy-Weinberg
Used to describe a population that
is NOT evolving
Frequencies of alleles & genes in a
gene pool will remain constant
over generations
5 Conditions for
Hardy-Weinberg
1) No mutations
2) Random mating
3) No natural selection
4) The population size must be large
(no genetic drift)
5) No gene flow (Emigration,
immigration, transfer of pollen, etc.)
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
where p2 and q2 represent the frequencies
of the homozygous genotypes and 2pq
represents the frequency of the
heterozygous genotype
Practice
Suppose in a plant population that red
flowers (R) are dominant to white
flowers (r). In a population of 500
individuals, 25% show the recessive
phenotype. How many individuals
would you expect to be homozygous
dominant and heterozygous for this
trait?
23.3 – Natural Selection, genetic drift, & gene flow can
alter allele frequencies in a population
Mutations can alter gene frequency, but are rare
3 major factors alter allelic frequencies
1) Natural selection
Alleles are passed to the next generation in proportions
different from their frequencies to the present
generation
Those that are better suited produce more offspring than
those that are not
2) Genetic Drift
Unpredictable fluctuation in frequencies from one
generation to the next
The smaller the population, the greater chance
Random & nonadaptive
A) Founder effect = individuals are isolated and establish
a new population – gene pool is not reflective of the
source population
B) Bottleneck effect = a sudden change in the
environment reduces population size – survivors have a
gene pool that no longer reflects original
1.
Genetic drift is significant in small populations
2.
Genetic drift causes allele frequencies to change at
random
3.
Genetic drift can lead to a loss of genetic variation
within populations
4. Genetic drift can cause harmful alleles to become
fixed
3) Gene Flow
Populations loses or gains alleles by genetic additions
or subtractions
Results from movement of fertile individuals or
gametes
Reduces the genetic differences between populations,
makes populations more similar
23.4 Natural Selection is the only mechanism
that consistently causes adaptive evolution
Relative fitness
The contribution an organism makes to the gene pool
of the next generation relative to the contributions of
the other members
Does NOT indicate strength or size
Measured by reproductive success
Natural selection acts more directly on the
phenotype and indirectly on the genotype
Can alter the frequency distribution of heritable
traits in 3 ways:
1) Directional selection
2) Disruptive selection
3) Stabilizing selection
1) Directional selection
Individuals with one extreme of a phenotypic range
are favored, shifting the curve toward this extreme
Example: Large black bears survived periods of extreme
cold better than small ones, so they became more
common during glacial periods
2) Disruptive Selection
Occurs when conditions favor individuals on both
extremes of a phenotypic range rather than individuals
with intermediate phenotypes
Example: A population has individuals with either large
beaks or small beaks, but few with intermediate –
apparently the intermediate beak size is not efficient in
cracking either the large or small seeds that are available
3) Stabilizing Selection
Acts against both extreme phenotypes and favors
intermediate variations
Example: Birth weights of most humans lie in a narrow
range, as those babies who are very large or very small
have higher mortality rates
How is genetic variation preserved in a population?
Diploidy
Capable of hiding genetic variation (recessive alleles)
from selection
Heterozygote advantage
Individuals that are heterozygous at a certain locus have
an advantage for survival
Sickle cell anemia – homozygous for normal hemoglobin are
more susceptible to malaria, homozygous recessive have
sickle-cell, but those that are heterozygotes are protected
from malaria and sickle-cell
Why Natural Selection cannot produce perfect
organisms:
1) Selection can only edit existing variations
2) Evolution is limited by historical constraints
3) Adaptations are often compromises
4) Chance, natural selection, & the environment
interact