Download Evolution

How Populations Evolve
Voyage of the Beagle
Jean Baptiste Lamarck
Charles Lyell
Artificial Selection
Artificial Selection
Natural Selection
• Darwin saw natural selection as the basic
mechanism of evolution
– As a result, the proportion of individuals with
favorable characteristics increases
– Allele frequencies (and therefore phenotypes
within a population) gradually change over time
 These five canine species evolved from a common
ancestor through natural selection
African wild
dog
Coyote
Fox
Wolf
Jackal
Thousands to
millions of years
of natural selection
Ancestral canine
Figure 13.4C
 The evolution of insecticide resistance is an example of
natural selection in action
Insecticide
application
Chromosome with gene
conferring resistance
to insecticide
Additional
applications of the
same insecticide will
be less effective, and
the frequency of
resistant insects in
the population
will grow
Survivor
Figure 13.5B
Fossils provide strong support for
evolution
Ammonite casts; Fossilized leaf
 Other evidence
for evolution
comes from
 Comparative
anatomy (e.g.
homologous
structures)
Biogeography
Comparative Embryology
Comparative Embryology
Molecular biology
Examples of
natural selection
– camouflage
adaptations in
different
environments
Population Genetics and Modern
Evolutionary Synthesis Theory
 Population genetics
Studies how populations change genetically over
time
 The modern evolutionary synthesis theory

Connects Mendelian Inheritance, Darwin’s theory
with population genetics
 Gene pool

Is the total collection of genes in a population at
any one time
 Microevolution

Is a change in the relative frequencies of alleles in
a given gene pool

Gene pool in a
nonevolving
population
remains
constant over
generations.
Hardy-Weinberg Principle
 States that allele and genotype frequency in a
population remain constant (equilibrium) –
from generation to generation unless an
outside influence disturbs the balance.
Hardy-Weinberg Principle
 Must satisfy five conditions

The population is very large

The population is isolated

Mutations do not alter the gene pool

Mating is random

All individuals are equal in reproductive success
Hardy-Weinberg Equation
 p2 + 2pq + q2 = 1
p+q=1
Where
 p is frequency of the dominant allele; q is the frequency of
the recessive allele
 p2 is the frequency of the homozygous dominant genotype
 2pq is the frequency of the heterozygous genotype
 q2 is the frequency of the homozygous recessive genotype
 The Hardy-Weinberg equation is useful in
public health science
 Public health scientists use the HardyWeinberg equation
 To estimate frequencies of diseasecausing alleles in the human
population
Other contributors to evolution
include
 Genetic Drift – change in the relative frequency of an
allele
 Bottleneck effect
 Founder effect
 Gene Flow – movement of alleles from one population
to another
 Emigration/immigration
Bottleneck Effect
Founder Effect
 Loss of genetic
variability when a new
population is established
by a very small number
of individuals from a
larger population.
Founder Effect
Island of Krakotoa, 1883
Polydactylism
VARIATION AND NATURAL
SELECTION
Variation is extensive in most populations
 Many populations exhibit polymorphism


Different forms of phenotypic characteristics
Functions to retain variety of morphs in a population
living in a varied environment
Polymorphism
 Populations may also
exhibit geographic
variation

Variation of an
inherited
characteristic
along a
geographic
continuum
Sexual recombination
Evolutionary Fitness?
Natural selection can alter variation
in a population in three ways
 Stabilizing selection
 Directional selection
 Disruptive selection
Modes of Selection
Sexual selection may produce sexual
dimorphism
• Sexual selection leads to the evolution of
secondary sexual characteristics
• Which may give individuals an
advantage in mating
Figure 13.17A
Figure 13.17B
Processes and Mechanisms of
Evolution
 Adaptation
 Genetic drift
 Gene flow
 Mutations
 Natural selection
 Speciation