Download Population Genetics and Speciation

POPULATION GENETICS
AND SPECIATION
The Evolution of Populations
Individuals do not evolve, populations do…Darwin
Lynn English High School Science
Biology/Ms. Mezzetti
Genetic Equilibrium
Genetic Variation
 Population genetics is the study of evolution from
a genetic point of view
 In the early 1900’s after Darwin’s death and the
rediscovery of Mendel’s work, scientists started
studying what caused variations in populations
 When measuring traits in a population a bell
curve shows that most have average traits and few
with extreme traits.
Bell Curve
•A bell curve shows the average of a population for certain traits
•Scientists wanted to look at what caused these variations
Causes of Variation
 Variations in genotype arise by mutation, recombination,
and the random pairing of gametes.
Mutations
Recombination
Random pairing
The Gene Pool
 The total genetic
information
available in a
population is called
the gene pool.
Frequency of alleles
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.
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.
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No mutations occur
Individuals do not enter or leave
The population is large
Individuals mate randomly
Selection does not occur
Disruption of Genetic Equilibrium
 Evolution may take place when populations are subject to
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genetic mutations, gene flow, genetic drift, nonrandom
mating, or natural selection.
Mutations are changes in the DNA
Gene Flow: Emigration and immigration cause gene
flow between populations and can thus affect gene
frequencies.
Genetic drift is a change in allele frequencies due to
random events which greatly affects small pop.
Nonrandom mating : mating is nonrandom whenever
individuals may choose partners, affected by sexual
selection
Natural selection can influence evolution in one of three
general patterns.
Three kinds of Natural Selection
• Directional selection
favors the formation of
more-extreme traits.
• Stabilizing selection
favors the formation of
average traits.
• Disruptive selection
favors extreme traits
rather than average
traits.
Formation of species
 Morphological Concept of Species: scientists
classified species according to their structure and
appearance- Problem: did not account for
phenotypic differences
 Biological Species Concept: s species of a
population of organisms that can successfully
interbreed-Problem: does not account for extinct
organisms
Results: Speciation
One specie may
evolve into two or
more species
Changes take
place over a very
long period of
time.
Isolating mechanisms
 Geographic isolation-physical separation of a
population
Spotted owl subspecies living in different geographic locations show
some genetic and morphological differences. This observation is
consistent with the idea that new species form through geographic
isolation. What separates them?
Geographic Isolation
These two types of squirrels live on
opposite sides of the canyon.
Reproductive Isolation
 Barriers to successful breeding between
population groups in the same area
Reproductive Isolation Animation
Rates of Speciation
• In the gradual model of
speciation (gradualism),
species undergo small
changes at a constant rate.
• Under punctuated
equilibrium, new species
arise abruptly, differ greatly
from their ancestors, and
then change little over long
periods.
Review
 What causes variations in populations?
 What disrupts genetic equilibrium?
 What do you need to be in Hardy Weinberg?
 Name two types of isolation. What happens when
isolation takes place?
 Name 3 types of selection.