Download A View of Life

Process of Evolution
Chapter 18
Mader: Biology 8th Ed.
Outline
•
•
•
•
•
•
Evolution in a Genetic Context
- Microevolution
Hardy-Weinberg
Causes of Microevolution
Natural Selection
– Types of Selection
Maintenance of Variations
Speciation
– Modes of Speciation
Mader: Biology 8th Ed.
Evolution in a Genetic Context
•
Microevolution
– In population genetics, the various alleles
at all the gene loci in all individuals make
up the gene pool of the population.
 To determine the frequency of each
allele, calculate its percentage from the
total number of alleles in the population.
Mader: Biology 8th Ed.
Hardy-Weinberg
•
The Hardy-Weinberg principle states an
equilibrium of allele frequencies in a gene
pool, p2 + 2pq + q2, will remain in effect in
each succeeding generation of a sexually
reproducing population, assuming:
– No Mutations
– No Gene Flow
– Random Mating
– No Genetic Drift
– No Selection
Mader: Biology 8th Ed.
Hardy-Weinberg
•
Under real conditions, these conditions are
rarely, if ever, met, and allele frequencies in
the gene pool of a population change
between generations.
– Evolution has occurred.
 Accumulation of small changes is
referred to as microevolution.
Mader: Biology 8th Ed.
Industrial Melanism
Mader: Biology 8th Ed.
Causes of Microevolution
•
•
Genetic Mutations
– Once alleles have mutated, certain
combinations of alleles might be more
adaptive than others in a particular
environment.
Gene Flow
– Movement of alleles between populations
by migration of breeding individuals.
 Continual gene flow reduces variability
between populations.
Mader: Biology 8th Ed.
Gene Flow
Mader: Biology 8th Ed.
Causes of Microevolution
•
Nonrandom Mating
– Individuals do not choose mates randomly.
 Assortative mating - Individuals tend to
mate with those with the same
phenotype.
 Sexual selection - Males compete for the
right to reproduce and females choose to
mate with males possessing a particular
phenotype.
Mader: Biology 8th Ed.
Causes of Microevolution
•
Genetic Drift
– Changes in allele frequencies of a gene
pool due to chance.
 Larger effect in small populations.
Mader: Biology 8th Ed.
Genetic Drift
Mader: Biology 8th Ed.
Genetic Drift
•
•
Bottleneck Effect
– An event prevents a majority of genotypes
from entering the next generation.
Founder Effect
– Rare alleles occur at a higher frequency in
a population isolated from the general
population.
 The alleles carried by population
founders are dictated by chance.
Mader: Biology 8th Ed.
Natural Selection
•
Natural Selection is the process that results
in adaptation of a population to the biotic
and abiotic environment. Requires:
– Variation
– Inheritance
– Differential Adaptiveness
– Differential Reproduction
 Relative Fitness
Mader: Biology 8th Ed.
Types of Selection
•
Directional Selection
– An extreme phenotype is favored and the
distribution curve shifts in that direction.
 Can occur when a population is
adapting to a changing environment.
Mader: Biology 8th Ed.
Directional Selection
Mader: Biology 8th Ed.
Types of Selection
•
Stabilizing Selection
– Occurs when an intermediate phenotype
is favored.
 Can improve adaptation of the
population to constant conditions.
Mader: Biology 8th Ed.
Stabilizing Selection
Mader: Biology 8th Ed.
Types of Selection
•
Disruptive Selection
– Two or more extreme phenotypes are
favored over any intermediate phenotype.
 Two distinctly different phenotypes are
found in the population.
Mader: Biology 8th Ed.
Disruptive Selection
Mader: Biology 8th Ed.
Maintenance of Variations
•
Maintenance of variation is beneficial
because populations with limited variation
may not be able to adapt to new conditions.
– Only exposed alleles are subject to natural
selection.
 Sickle-Cell Disease
 Homozygote remains in equilibrium in
some regions of Africa because the
heterozygote is protected from sicklecell and malaria.
Mader: Biology 8th Ed.
Sickle-Cell Disease
Mader: Biology 8th Ed.
Speciation
•
Speciation is the splitting of one species into
two or more species, or the transformation
of one species into a new species over time.
– Species Definition
 Morphological
 Biological
 Reproductive Isolation
 Phylogenetic
Mader: Biology 8th Ed.
Reproductive Isolating Mechanisms
•
Prezygotic Isolating Mechanisms
– Prevent reproduction attempts, and make
it unlikely fertilization will be successful.
 Habitat Isolation
 Temporal Isolation
 Behavioral Isolation
 Mechanical Isolation
 Gamete Isolation
Mader: Biology 8th Ed.
Reproductive Isolating Mechanisms
•
Postzygotic Isolating Mechanisms
– Prevent hybrid offspring from developing
or breeding.
 Zygote Mortality
 Hybrid Sterility
 F2 Fitness
Mader: Biology 8th Ed.
Modes of Speciation
•
•
Allopatric Speciation
– Occurs when one population is
geographically isolated from other
populations.
Sympatric Speciation
– A population develops into two or more
reproductively isolated groups without
prior geographic isolation.
Mader: Biology 8th Ed.
Adaptive Radiation
•
Adaptive Radiation is an example of
allopatric speciation.
– Many new species evolve from a single
ancestral species when members of the
species become adapted to different
environments.
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
Review
•
•
•
•
•
•
Evolution in a Genetic Context
- Microevolution
Hardy-Weinberg
Causes of Microevolution
Natural Selection
– Types of Selection
Maintenance of Variations
Speciation
– Modes of Speciation
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.