Transcript Genetics Selection and Genetic Drift

Chapter 16 Notes
Spring 2006
Mr. Holmes
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Darwin’s problem was that he did
not understand inheritance.
Although Mendel’s work was
published during Darwin’s lifetime,
the work was not accepted at that
time.
Phenotype (height)
Frequency of Phenotype
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All the genes in an organism makes up its
total genetic makeup. A gene pool is a
combination of all the genetic information
of all the members of a particular
population. It usually contains two or
more alleles (or forms) of a gene. The
relative frequency is the number of times
an allele appears in a population.
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
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The variations in genetic pools are
due to mutations and genetic
shuffling. Variations would lead to
asking the question: Why aren’t all
gene pools the same? Crossing
over is a process that can lead to
variation.
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A phenotype, or trait, can be due to
the result of a single gene or it can
be the result of more than one gene
working together (polygenic)
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
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
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Genetic drift is the idea that a
certain trait disappears or appears
in a population by chance and
becomes common over time. The
founder effect occurs when allele
frequencies change as a result of
the migration of a small subgroup of
a population.
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
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Hardy-Weinberg equilibrium states that the allele
frequencies in a population remains constant
unless one or more factors causes a change. If
this happens the situation is said to be at genetic
equilibrium. The five conditions of HardyWeinberg equilibrium are:
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random mating
large population
no immigration or emigration
no mutations
no natural selection
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The formation of a new specie is caused
speciation. What causes speciation?
The main reason is reproductive isolation
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behavioral isolation (differences in courtship
rituals)
geographic isolation (two species separation by
geographic barriers)
temporal isolation (different times of fertility)
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species