Transcript Evolution as Genetic Change

Evolution as Genetic Change
Evolution as Genetic Change
• Natural selection acts on phenotypes, survival
and reproduction determine which alleles are
inherited, changing relative frequencies of
alleles in a population over time.
• Thus evolution is any change in the relative
frequencies of alleles in a population’s gene
pool and acts on populations, not individuals.
Evolution of Single-Gene Traits
• Natural selection on single-gene traits can
lead to changes in allele frequencies and thus
to evolution.
– One of the two phenotypes may make an
organisms better fit, thus under pressure from
natural selection and its relative frequency will
increase
Single Allele
Selection
• Which phenotype has
higher fitness?
– Orange
• Which allele’s frequency
will decrease?
– Green
Evolution of Polygenic Traits
• Natural selection can affect the distributions of
phenotypes in any of three ways:
– Directional selection
– Stabilizing selection
– Disruptive selection
Graph of Directional Selection
Section 16-2
• Directional Selection: When the entire bell moves
left/right because there’s a higher fitness and increase in
Directional Selection
the number of individuals
with the trait at one end of the
curve.
– Traits at one end or the other are selected for
Key
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Directional Selection
Section 16-2
Stabilizing Selection
• Stabilizing selection:
When the bell becomes
more narrow, because
there’s a higher fitness
and increase in the
number of individuals
with the trait in the
center of the curve
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
• The average trait is selected
for
Birth Weight
Selection against
both extremes
keep curve
narrow and in
same place.
Evolution of Clutch Size
Section 16-2
Disruptive Selection
• Disruptive selection: The bell can split into two,
because there’s a higher fitness and increase in the
number of individuals at both ends of the curve
• Traits at both “extremes” are selected for
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Disruptive Selection
Genetic Drift
• Populations can also evolve without selection pressure
through the process of genetic drift.
• Genetic drift = random change in allele frequencies
• In small populations, individuals that carry a particular allele
may leave more descendants than other individual, just by
chance. Over time, a series of chance occurrences can cause
an allele to become common in a population.
• Genetic drift can happen when a small group of individuals
colonize a new habitat carrying different relative frequencies
that the larger population.
• 2 special Cases:
• Founder effect = allele frequencies change as a result of the migration
of a small subgroup of a population
• Bottle Neck= a population experiences a great reduction in the gene
pool, leaving only a small subset of alleles behind. Results in
inbreeding.
Genetic Drift- Chance
Genetic Drift- Founder Effect
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift- Founder Effect
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift- Founder Effect
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift Bottleneck
Genetic Equilibrium
• Hardy-Weinberg principle states that allele frequencies
in a population will remain constant unless one or
more of a set of factors causes the population to
change.
• The following conditions that must be met to avoid
evolution:
– Random mating- no mate preferences, or choice (rare)
– Large population- lots of diversity (less chance of genetic
drift)
– No movement into or out of the population- individuals
don’t move between populations, carrying new alleles
– No mutations- mutations change the DNA
– No natural selection- all individuals have an equal chance
of surviving and reproducing
Hardy-Weinberg
(p2) + (2pq) + (q2) = 1