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Ch. 16:Evolution: Evolution of
Section 16-1: Genes & Variation
I. Terms to Know
A. Population- a group of individuals belonging to the
same species in a given area
B. Species- group of populations whose individuals can
interbreed and produce fertile offspring
C. Population Genetics - study of kinds of number of
genes in a populations
D. Evolution- generation to generation change in a
population’s allele frequency
II. Gene Pools - all genes, including all the
different alleles, that are present in a population.
A. Relative Frequency (of an allele) - the # of
times that the allele occurs in a gene pool,
compared with the number of times other
alleles for the same gene occur.
• Relative frequency is often expressed as a %.
• In genetic terms, evolution is the change in the
relative frequency of alleles in a population.
In a total of 50 alleles, 20 alleles are B (black), and 30 are
b (brown). How many of each allele would be present in
a total of 100 alleles?
• If relative frequency of B allele decreased in the gene
pool, what would happen to the relative frequency of
the other allele? Increase or Decrease?
III. Sources of Genetic Variation
A. Mutations- Changes in an organisms DNA due
to error in replication, radiation, or chemicals
1. Some can affect an organisms fitness(ability to
survive & reproduce) others have no effect
2. Heritable
3. Can result in novel alleles (both good and bad)
4. Ultimately change the gene pool
B. Gene Shuffling
1. Occurs during the production of gametes.
2. 23 pairs of chromosomes can produce 8.4 million
combinations of genes
3. Crossing over increases the number of
genotypes that can appear in offspring
4. Sexual reproduction results in many different
phenotypes, but it does not change the relative
frequency of alleles in a population.
IV. Single-Gene & Polygenic Traits
*The number of phenotypes produced for a given trait
depends on how many genes control the trait.
A. Single gene trait - controlled by single gene
1. Single with 2 alleles – only 2 possible phenotypes
2. Ex Widow’s peak (allele is dominant over the
allele for a straight hairline but is less frequent)
3. Compare by Bar graph.
B. Polygenic Traits - controlled by 2 or more
1. Each gene has 2 or more alleles – many possible
genotypes and even more phenotypes.
2. Ex. Height in Humans. It is represented by
symmetrical bell shaped curve.
Section 16-2: Evolution as Genetic Change
• Natural selection does not act on genes.
• It acts on phenotype and decides which
phenotype is suitable to survive & reproduce.
I. Natural selection on single-gene traits
A. Leads to changes in allele frequencies = evolution.
Figure shows a population of brown lizards in which mutation
produces red & black forms. If population lives in the dark soil
then how does a color affect the fitness of the lizards?
What do you predict the lizard population will look like by
generation 50? Explain
II. Natural Selection on Polygenic Traits - action
of multiple alleles on traits such as height produces
a range of phenotypes that often fit a bell curve.
A. Directional Selection - individuals at one end of
the curve have higher fitness than in the middle
or at the other end.
Ex.- increase in the average size of the beaks of finches who
compete for food.
Dotted - original distribution of beak sizes
Solid - changed distribution of beak sizes.
Peak shifts as average beak size increases.
B. Stabilizing Selection – individuals near the center of
the curve have higher fitness than individuals at
either end of the curve.
The smaller babies are less healthy, the larger have difficulty
being born therefore both are less fit than the average. The
average weighing babies are more likely to survive than smaller
or larger.
C. Disruptive Selection - individuals at the upper and
lower ends of the curve have higher fitness than
individuals near the middle
1. The selection acts against an intermediate type &
can cause the single curve to split into two
(creates two phenotypes).
The average-sized seeds become less common than larger
and smaller seeds. The bird population splits into two
subgroups eating larger & smaller seeds.
III. Genetic Drift- Random change in allele
frequency in a population due to chance.
A. Most likely to occur in small population or when a
small group of organisms colonize a new habitat.
B. In small populations, an allele can become more or
less common simply by chance.
1. Individuals that carry a particular allele may
leave more descendants than others, just by
2. Founder Effect - Allele frequencies change as a
result of the migration of a small subgroup of a
Founder Effect
Bottleneck Effect
IV. Hardy-Weinberg Theorem
• allele frequencies in a population will remain
constant unless one or more factors cause
those frequencies to change.
A. Populations that meet these requirements are
said to be in Genetic Equilibrium and are not
B. This is useful for comparison. If frequencies of
alleles deviate from values predicted by the
theorem then the population is evolving.
Following Five conditions are required to
maintain genetic equilibrium:
The population is large
There is no migration
No mutations
Random Mating
No natural selection
If the above conditions are not met, genetic equilibrium
will be disrupted and the population will evolve.
Section 16-3: Process of Speciation
I. Speciation: the development of new species
A. When two populations become
reproductively isolated, new species evolve.
B. What isolates a species? Barriers
Following are Reproductive Isolation - two
populations cannot interbreed and produce
fertile offspring.
A. Geographic Isolation: 2 populations are
separated by geographic barriers such as
rivers, mountains.
1. It does not form new species when it enables to
separate populations.
2. Ex: Birds begin foraging on the ground while the
others forage in the tree tops.
Geographic Isolation
Figure 24.8
A. formosus
A. nuttingi
Atlantic Ocean
Isthmus of Panama
Pacific Ocean
A. panamensis
A. millsae
B. Behavioral Isolation: 2 populations are capable of
interbreeding but have differences in courtship rituals
or other reproductive strategies that involve
1. the song of 2 frog/bird species is different; the
plumage or dance of 2 species is different.
C. Temporal Isolation: 2 or more species
reproduce at different times
1. 2 species that breed at different times of day,
different seasons, or different years cannot mix
their gametes.
2. nocturnal vs. day; seed release during
spring vs. summer