Chapter 12 – The Mechanisms of the Evolutionary Process
Genetic Variation
Genetic variation can explain how natural selection occurs. Inheritable traits in the form
of DNA can be passed from generations to the next. Variation is made possible by
particular alleles of a gene being passed on to an individual’s offspring. Individuals can
be homozygous dominant/recessive or heterozygous for an allele. DNA varies between
species; species with larger genomes (have more DNA) have more diversity and possible
mutations. Variations in specie populations results from the allele combinations, passed
on through reproduction. Sexual reproduction allows random recombination of alleles,
resulting in genetic variation.
 Allele(s) – forms of a gene, most genes have 2+ alleles.
 Gene – portion of DNA coding for particular polypeptides.
 Homozygous – gene in which alleles are identical, either recessive or dominant.
 Heterozygous – gene in which alleles are non identical, dominant trait will
always be expressed with this genotype.
 Genotype – the genetic makeup of an organism.
 Phenotype – the physical appearance of an organism and subject to artificial
selection.
 Population(s) – all individuals of a species inhabiting the same environment.
Hardy-Weinberg Principle
Exhibits that under certain conditions change within and organism gene pool will occur
over generations. These changes are expressed by allele frequencies in populations. By
looking at allele frequencies evolutionary changes can be observed within populations
over generations.
 Gene pool – all alleles with in a population.
 Allele frequency – the proportion of a given allele in a specific gene pool.
Conditions for Hardy-Weinberg;
 Large population.
 Fair mating opportunities, no natural selection.
 No mutation within the population.
 No migration.
Hardy-Weinberg equation
p^2 + 2pq + q^2 = 1
Where;
p = the frequency of the dominant allele
q = the frequency of the recessive allele
2pq = the frequency of the heterozygous allele combinations
Population Changes
Changes in allele frequencies can lead to evolutionary changes with in a population.
Factors affecting evolution;
 Fluctuations in population size.
 Non equal mating opportunities, natural selection.
 Mutations
 Migration
Genetic Drift(s)
Can affect small populations when individuals are removed from the gene pool, this
disrupts the allele frequencies. It can lead to the fixation of alleles of lead to an
increase/decrease of a certain allele.
Bottleneck Effect
Occurs when a population’s numbers are dramatically decreased causing a severe genetic
drift. It can lead to homozygousity in a population due to the limited alleles left with in
the gene pool.
Founder Effect
Occurs when a small population migrates from an original large population, creating a
new small population in which a limited amount of alleles are found. This results in a
genetic drift from the original population.
Gene Flow
Migrating organisms can cause the appearance/disappearance of alleles within a species.
Gene flow can also result from cross breeding.
Mutation(s)
Mutations result in new genetic information. They’re rare in individuals but are numerous
in large populations. Mutations can be neutral, beneficial or harmful to an organism’s
fitness. Harmful mutations are selected against but occur frequently. Beneficial mutations
are selected for but occur rarely.
 Fitness – the reproductive success of an organism within its life time.
 Neutral – no effect on an individual’s fitness.
 Beneficial – an inheritable change that results in enhanced fitness.
 Harmful – inheritable change that decreases an organism’s fitness, can lead to
death or improper function.
Mutations provide new genetic information which can be expressed in an organism’s
phenotype. Through natural selection variation in phenotypes can occur within a
population and its habitat.
Types of Selection
 Stabilizing selection – a common phenotype within a population is favored by
the organism’s environment.
 Directional selection – an organism’s environment favors individuals with more
extreme variations of a trait. Could be caused by a migration to a new
environment. Can lead to a significant change in the species over generations.
 Disruptive selection - selection that favors 2+ variations with in an organism’s
population. Can favor both extremes in a population. Can lead to different distinct
species.
 Sexual selection – individuals can reproduce frequently contributing more to the
gene pool. Favors inherited traits that enhance mating success but may lower
survival chance. Many mutations occur in sexual reproduction and sexual
dimorphism can occur.
 Cumulative selection – adaptations in species resulting from the accumulation of
small evolutionary changes. Mutations and selective pressures may have started
evolution of complex structures, 2 scenarios; spontaneous vs. accumulation.
Adaptations
An adaptation can be describes as any trait that increases an organisms chance to
survive. It can be attributed to an organism’s behavior, coloration, or bodily functions.
Adaptations are not willed by an individual’s desire to be at an advantage, they arise
from natural mutations in a species. When an adaptation arises in a species, an advantage
is seen. Individuals not possessing the desired trait will be selected against, inhibiting its
chances of reproduction.
Speciation
Selection propels adaptation and the formation of new species. Through natural selection
species can diverge and create new species, this is called speciation. There are 2 types of
speciation, allopatric and sympatric.
 Species – a group of individuals that can breed to produce fertile offspring and
evolve independently, genetically isolated.
 Allopatric – evolution of species as a result of geographical isolation.
 Sympatric – evolution of a species with in the same area into 2 separate species,
gene pool separation occurs.
Reproductive isolating mechanisms
Prezygotic isolating mechanisms
Mechanism
Example
Ecological isolation
Elevation separation
Temporal isolation
Nocturnal vs. non-nocturnal breeders
Behavioral isolation
Sexual Dimorphism
Prevention of mating
Mechanical isolation
Differences in reproductive organs
Gametic isolation
Gametes recognize each other
Postzygotic isolating mechanisms; Prevent hybrid reproduction
Zygotic mortality
No fertilized zygotes develop
Hybrid inviability
Hybrid offspring has reduced fitness
Hybrid infertility
Offspring are infertile but genetically
strong