Download Chapter 15: Darwin*s Theory of Evolution

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Chapter 16 Section Assessments: Due Fri. 5/2
Chapter 16.1 SA: p. 396 (1-5)
Chapter 16.2 SA: p. 402 (1-5)
Chapter 16.3 SA: p. 410 (1-2)
Chapter 16 Assessment: p. 413 (1-10, 12, 17, 18, 19)
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Gene pool
Relative Frequency
Single-gene trait
Polygenic trait
Directional selection
Stabilizing selection
Disruptive selection
Genetic drift
Founder effect
• Hardy-Weinberg
principle
• Genetic equilibrium
• Speciation
• Reproductive isolation
• Behavioral isolation
• Geographic isolation
• Temporal isolation
• A weakness in Darwin’s theory stemmed from his
lack of understanding in genetic variation.
• In the 1930s, evolutionary biologists combined
Mendel’s work with that of Darwin’s to support
evolutionary change, specifically the process of
natural selection.
• How do you think Mendel would have felt about
that?
• Population: a group of individuals of the same species
that interbreed.
• Gene pool: consists of all genes, including all the
different alleles, that are present in a population.
• Relative frequency: the number of times that the allele
occurs in a gene pool, compared with the number of times
other alleles for the same gene occur. (Fig. 16-2)
• Key Concept: In genetic terms, evolution is any
change in the relative frequency of alleles in a
population.
• Key Concept: The two main sources of genetic
variation are mutations and the genetic
shuffling that results from sexual reproduction.
• Remember mutations are changes in the genetic
code. Some are expressed, some remain silent.
Some mutations can affect an organism’s fitness,
while others have not effect.
• Crossing over and independent assortment
during gamete formation leads to genetic
variation. (23 pairs of chromosomes can have
8.4 million different combinations of genes!)
• Key Concept: Natural selection on single-gene
traits can lead to changes in allele frequencies
and thus to evolution. (Fig. 16-5)
• Remember: Evolution is change over time in the
relative frequencies of alleles in a population,
therefore it is populations that can evolve, not
individual organisms.
• Key Concept: Natural selection can affect the
distributions of phenotypes in any of three
ways:
• Directional Selection: when individuals at one
end of the curve have higher fitness than
individuals in the middle or at the other end.
(Fig. 16-6)
• Stabilizing Selection: center curve has higher
fitness (Fig. 16-7)
• Disruptive Selection: upper and lower ends have
high fitness (Fig. 16-8)
Average favored
# of birds
# of birds
One extreme favored
Birth Mass
Beak Size
# of birds
Both extremes favored
Beak Size
• Key Concept: In small populations, individuals
that carry a particular allele may leave more
descendants that other individuals, just by
chance. Over time, a series of chance
occurrences of this type can cause an allele to
become common in a population. (Fig. 16-9)
• Genetic Drift: Random change in allele
frequencies that occurs in small populations
• Founder Effect: A situation in which allele
frequencies change as a result of the migration
of a small subgroup of a population
• Genetic Equilibrium: A situation in which
allele frequencies remain constant.
• Key Concept: Five conditions are required
to maintain genetic equilibrium from
generation to generation:
1.
2.
3.
4.
5.
Random mating
Large population size
No movement into or out of the population
No mutations
No natural selection
• p+q=1
• p = dominant allele frequency (A)
• q = recessive allele frequency (a)
• p2 + 2pq + q2 = 1
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p2: frequency of AA homozygous
2pq: frequency of Aa heterozygous
q2: frequency of aa homozygous
1: sum of frequencies for all genotypes
(100%)
• Extra Credit Assignment!! Due Fri. 5/2
• Speciation: formation of new species
• Key Concept: A new species evolve, populations
become reproductively isolated from each
other.
• Reproductive Isolation: When members of two
populations cannot interbreed and produce
fertile offspring. They now have separate gene
pools.
• Causes of reproductive isolation:
•Behavioral Isolation: capable of
interbreeding, but have different courtship
rituals or other reproductive behaviors.
•Geographic Isolation: separated by physical
barriers (i.e. rivers, mountains, etc…)
• Temporal Isolation: reproduce at different
times.
Is the definition of species constant?
“Can vs. Will”
• Peter and Rosemary Grant have spent more
than 20 years observing collecting data on
Galapagos finches.
• They found variation in physical characteristics.
• They observed natural selection occurring during
drought seasons.
• Key Concept: Speciation in the Galapagos
finches occurred by founding of a new
population, geographic isolation, changes in the
new population’s gene pool, reproductive
isolation, and ecological competition.
• “New data from genetics, physics, and
biochemistry could have proved him wrong on
many counts. They didn’t.”
• “…while the Grants observed changes in the
size of the finches’ beaks, they did not observe
the formation of a new species.”
• Read: Unanswered Questions (p. 410)
• Why is understanding evolution important?
After Break...Origins of the Earth!