Download ModelsOfChange23_2

The Evolution of
Populations: Models of
Change
Chapter 23
What you need to know!


How mutation and sexual reproduction
each produce genetic variation.
How natural selection, genetic drift,
and gene flow contribute to changing
allele frequencies.
Changing Allele Freq.
3.
Gene Flow: migration of fertile
individuals into or out of a population.
This tends to reduce variation
between separate populations
Natural Selection


The only mechanism that consistently
causes adaptive evolution
Fitness represents the reproductive
success of an individual
– Higher fitness means more babies

Natural selection acts directly on the
phenotypes of individuals (indirectly
on genotypes)
Genetic Variation
Genetic variation within a population are
attributable to two sources:
1. Mutation: the ONLY source of new
genes and alleles.

– point mutations change one base in a gene
– Chromosomal mutations are almost certainly
harmful *but not always
2.
Sexual Reproduction: crossing over,
independent assortment (remember the
math 2n), fertilization (2n x 2n)
Changing Allele Freq.
1.
2.
Natural selection: alleles passed in different
proportions based on adaptations and fitness
Genetic drift: random, nonadaptive change in
allelic frequencies in small populations. Two
models include:
A. Founder effect: small piece of a larger population
is isolated in a new region (different gene pool)
B. Bottleneck effect: most of the individuals in a
population die, leaving a small sample of the
original gene pool
Directional Selection
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A single extreme adaptation is good,
shifting the curve in that direction.
Example: peppered moths
Directional Selection
Stabilizing Selection
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Extreme
phenotypes are
bad adaptations
Example:
primate birth
weight
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Stabilizing Selection
Disruptive Selection

Disruptive Selection
Largest and smallest seeds become more common.
Key
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population

Selection against the common phenotype
Selection for both extremes
Example: weeds are either very tall (difficult to
uproot) or very short (survive lawn mowers)
Number of Birds
in Population

Beak Size
Sexual Selection
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Selection for sexual phenotypes:
Male competition: fittest male mates
most (antlers, horns, body size)
Female choice: attractiveness and
mating behavior
Sexual dimorphism: diifferent
appearance of males and females in
certain species
Preservation of Variety


Diploidy: two alleles for every gene
allows for dominant and recessive
allele combinations
Heterozygote advantage: Certain
genes provide advantages for being
heterzygous (i.e. sickle-cell anemia
heterzygotes are resistant to malaria,
mutts)
Imperfection

1.
2.
3.
4.
Natural selection does not fashion the
perfect organism because:
Selection only edits existing variation
Evolution is limited by historical
constraints
Adaptations are often compromises
Chance, natural selection, and the
environment interact