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Chapter 16
Population Genetics and Speciation
Table of Contents
Section 1 Genetic Equilibrium
Section 2 Disruption of Genetic Equilibrium
Section 3 Formation of Species
Variation of Traits
• Population genetics - The study of evolution from a
genetic point of view.
– Also known as microevolution - a change in the
collective genetic material of a population.
• Remember a population is the smallest unit in which
evolution can occur.
Chapter 16
Section 1 Genetic Equilibrium
Variation of Traits Within a Population
• Population biologists study many different traits in
populations, such as size and color.
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Chapter 16
Section 1 Genetic Equilibrium
Variation of Traits Within a Population, cont.
• Causes of Variation
– Traits vary and can be mapped along a
bell curve, which shows that most
individuals have average traits,
whereas a few individuals have
extreme traits.
– Variations in genotype arise by
mutation (random change in a gene that is
passes on), recombination (reshuffling of
genes during meiosis due to crossing-over),
and the random pairing of gametes (due
to the large amounts if gametes).
Chapter 16
Section 1 Genetic Equilibrium
The Gene Pool
• The total genetic information available in a population
is called the gene pool. The number of alleles and
frequency of those alleles in a gene.
• Allele frequency is determined by dividing the total
number of a certain allele by the total number of
alleles of all types in the population.
• Predicting Phenotype
– Phenotype frequency is equal to the number of
individuals with a particular phenotype divided by
the total number of individuals in the population.
• Generation 1
– Phenotypic ratios
• 4 red (out of 8) frequency = .5,
• 4 pink (out of 8) frequency =.5,
• no white frequency = 0.
– Genotypic ratios
• 12 R (out of 16) frequency = .75
• 4 r (out of 16) frequency = .25
What about generation 2?
Chapter 16
Section 1 Genetic Equilibrium
The Hardy-Weinberg Genetic Equilibrium
• Allele frequencies in the gene pool do not change
unless acted upon by certain forces.
• Hardy-Weinberg genetic equilibrium is a
theoretical model of a population in which no
evolution occurs and the gene pool of the population
is stable.
5 Assumptions for Equilibrium
– q+p=1
– q2+2pq+p2=1
Chapter 16
Section 2 Disruption of Genetic
Equilibrium
Causes of Evolution
• Evolution may take place when populations are
subject to genetic mutations, gene flow, genetic drift,
nonrandom mating, or natural selection.
• Mutations are changes in the DNA.
– Some are harmful and difficult to eliminate, some
are helpful, and increase survival.
Chapter 16
Section 2 Disruption of Genetic
Equilibrium
Gene Flow
• Emigration and immigration cause gene flow
between populations and can thus affect gene
frequencies.
– Immigration - Individuals moving into
population.
– Emigration - Individuals moving out of a
population
• Gene Flow - the process of genes moving from one
population to another through migration or dispersal
of seeds and spores.
Chapter 16
Section 2 Disruption of Genetic
Equilibrium
Genetic Drift
• Genetic drift is a change in allele frequencies due to
random events.
• Genetic drift operates most strongly in small
populations, but genetic equilibrium happens in large
populations because a change is less effective
– Small population, one organism doesn’t mate it
could be catastrophic. In large populations, one
organism doesn’t mate, the population will still go
on.
Chapter 16
Section 2 Disruption of Genetic
Equilibrium
Nonrandom Mating
• Mating is nonrandom whenever individuals may
choose partners.
– Assortative mating - picking a mate that has
similar genes to oneself.
• Sexual Selection
– Sexual selection occurs when certain traits
increase an individual’s success at mating.
– Sexual selection explains the development of
traits that improve reproductive success but that
may harm the individual.
• Ex. Peacock mating
Chapter 16
Section 2 Disruption of Genetic
Equilibrium
Natural Selection
• Natural selection can influence evolution in one of
three general patterns.
– Stabilizing selection favors the formation of
average traits.
– Disruptive selection favors extreme traits rather
than average traits.
– Directional selection favors the formation of
more-extreme traits.
Chapter 16
Three
Kinds of
Selection
Section 2 Disruption of Genetic
Equilibrium
Chapter 16
Section 3 Formation of Species
The Concept of Species
• According to the biological species concept, a
species is a population of organisms that can
successfully interbreed but cannot breed with other
groups.
– Ex. Dogs cannot breed with cats
• Speciation – The development of new species
• Morphology – the internal and external structure and
appearance of an organism, used to classify it as a
species.
Modern Definition of Species/Speciation
• Species – A single kind of organism. Members are
morphologically similar and can interbreed to produce
fully fertile offspring.
• The many species alive today diverged from a
smaller group of species.
Chapter 16
Section 3 Formation of Species
Isolation and Speciation
• Geographic Isolation
– Geographic isolation the physical separation of
members of a population by geographic barriers.
• Ex. Canyons or Mountains forming.
• Allopatric Speciation - the arrival of new species
due to geographic isolation.
– Happens more often in small populations with a
small gene pool because they are more greatly
affected by the environment.
Chapter 16
Section 3 Formation of Species
Geographic Isolation
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Chapter 16
Section 3 Formation of Species
Isolation and Speciation, continued
• Reproductive Isolation results from the separation of
population subgroups by barriers to successful breeding.
– Disruptive Selection causes this to happen, 2 types of
isolation from this.
• Prezygotic – before fertilization. (different mating times
and calls of frogs.)
• Postzygotic – after fertilization. (horse and donkey.)
• Sympatric Speciation - Reproductive isolation within
the same geographic area.
– Ex. If all of a species lives on one type of plant and
then some individuals start to live on another plant,
they may not longer be able to interbreed with their
original population.
Chapter 16
Section 3 Formation of Species
Reproductive Isolation
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Chapter 16
Section 3 Formation of Species
Rates of Speciation
• Slow over Millions of years
– In the gradual model of
speciation (gradualism),
species undergo small
changes at a constant rate.
• Quick Bursts of Change
– Under punctuated
equilibrium, new species
arise abruptly, differ greatly
from their ancestors, and
then change little over long
periods.
Chapter 16
Section 3 Formation of Species
Comparing Punctuated Equilibrium and Gradualism
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