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Chapter 16
Evolution of Populations
What Darwin Did Not Know
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He did not know how traits were passed on
from one generation to the next.
He did not know how variations appeared.
Genetic Variation
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Remember: genes have at least 2 forms
(alleles)
Sources of genetic variation:
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1. Mutation – change in a sequence of DNA
2. Gene Shuffling – genes move about during
the production of gametes (sex cells)
Variation and Gene Pools
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Remember – a population is a group of
individuals of the same species that
interbreed.
Because they interbreed, the group of
genes is called the gene pool
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Gene pool – all the genes in a population
Relative Frequency
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The number of times that the allele occurs in
a gene pool
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Expressed in percentage
Ex. Mouse population – relative frequency of the
dominant allele (black fur) is 40%
*In genetic terms – evolution is any change
in the relative frequency of alleles in a
population.
Single-Gene and Polygenic Traits
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The number of phenotypes produced for
a given trait depends on how many
genes control the trait
Single-Gene: one gene, two alleles
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Ex. Widow’s Peak
Polygenic Traits: traits controlled by two
or more genes
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Ex. Height
Genetic Drift
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More common in small populations
(small gene pool)
Individuals that have a certain trait may
have more offspring
Over time, the trait becomes more
common in the population
Genetic Drift
http://evolution.berkeley.edu/evosite/evo1
01/IVBMechanisms.shtml
16-3: Speciation
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Speciation – formation of new species
As new species evolve, populations
become reproductively isolated from
each other
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Reproductive isolation
Other Types of Isolation
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Geographic Isolation – two populations
are separated by geographic barriers
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Ex. Rivers, Mountains
Abert squirrel & Kaibab squirrel: Grand
Canyon
Temporal Isolation – two or more
species reproduce at different times
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Ex. 3 species of orchids release pollen on a
different day
Galapagos Finches
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Darwin thought they were completely
different types of birds.
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Natural selection shaped the beaks of
different bird populations as they adapted
to eat different foods.
Since Darwin, the Grant’s have studied
the finch populations for years.
Galapagos Finches
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Grants’ work showed that finch beak size
can be changed by natural selection.
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Birds with different sized beaks had different
chances of survival during a drought.
Scarce food sources = birds with largest
beaks survive
Next generation – birds with larger beaks
Process of Speciation
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1.
2.
3.
4.
5.
6.
Founders Arrive
Geographic Isolation
Changes in the Gene Pool
Reproductive Isolation
Ecological Competition
Continued Evolution