Chapter 11
11.1 & 11.3
Starter
Write & Answer
1.
Name and graph the three ways natural selection can
change the distribution of traits.
2.
How might the overfishing of large pink salmon affect
salmon size in future generations? Which pathway of
natural selection is this?
3.
Woodpeckers with extremely large beaks are unable to fit
their beaks into small holes and retrieve bugs.
Woodpeckers with very small beaks are unable to reach
bugs that have crawled to the back of holes and tunnels.
What will happen to the woodpecker population? What
pathway of natural selection is this?
Genetic Variation
in Populations
• A population shares a common gene pool
• gene pool - the combined alleles of all the
individuals in a population
Genetic Variation increases the probability
that some individuals will survive
• Genetic variation leads to phenotypic variation
• Phenotypic variation is necessary for natural selection
• Genetic variation is stored in the population’s gene pool
Allele Frequencies measure
genetic variation
• measures how common an allele is in a
population
• can be calculated for every allele in a gene pool
Genetic variation has several
sources:
• Mutation (a random change in DNA) – can form a new
allele, can be passed on to offspring
• Recombination (parents’ alleles recombined in
offspring)
Natural Selection is not the only
mechanism through which populations
evolve
• Gene Flow : the movement of alleles between populations
• Occurs when individuals join other populations and
reproduce
• Gene flow keeps neighboring populations similar
• Low Gene Flow increases the chances that two populations
will evolve into different species
Genetic Drift
• Genetic drift is a change in allele frequency due
to chance
• Bottleneck Effect: a random/destructive event
reduces a population’s size
• The Founder Effect: genetic drift that occurs after a
small number of individuals colonize a new area
Genetic Drift
The illustration to the right represents the
bottleneck effect. An event reduces the
population size, reducing the variation. Here,
the “red” alleles are eliminated.
The illustration to the left represents
the founder effect – a new population
of flowers is founded, without the
“orange” allele.
Genetic Drift has negative effects on
populations
• less variation, less likely that individuals can adapt
• harmful alleles may be more common in the population
Sexual
Selection
• Sexual Selection occurs when certain traits increase
mating success
• Occurs due to the higher cost of reproduction for
females than males
• Males produce sperm continuously
• Females are more limited in number of potential offspring
Two types of
Sexual
Selection
• Intrasexual selection: competition among males
• Intersexual selection: males display certain traits
to females