Chapter 22: The Mechanisms of Evolution
22.1 What Facts Form the Base of Our Understanding of Evolution?
Darwin went on a 5-year-long voyage and made a lot of observations. When he returned to
England, in 1836, he tried to understand and analyze these observations. He developed the major
features of an explanatory theory for evolutionary change based on two major propositions:
• Species are not immutable; they change over time.
• The process that produces these changes is natural selection.
Darwin observed that although offspring tend to resemble their parents, the offspring of most
organisms are not identical to one another or to their parents. He suggests slight variations
among individuals affect the chance that a given individuals will survive and reproduce
natural selection: differential contribution of offspring to the next generation by various genetic
types belonging to the same population.
It is important to remember that individuals do not evolve; populations do. A population is a
group of individuals of a single species that live and interbreed in a particular geographic area at
the same time.
• Adaptation has two meanings
Adaptation refers both to the processes by which characteristics that appear to be useful
to their bearers evolve and to the characteristics themselves. In other words, an adaptation
is a phenotypic characteristic that has helped an organism adjust to conditions in its
An organism is adapted to a particular environment when they can demonstrate that a
slightly different organism reproduces and survives less well in that environment.
• Population genetics provides an underpinning for Darwin’s theory
For a population to evolve, its members must possess heritable genetic variation. The
physical expressions of an organism’s genes are what one sees. The features of a
phenotype are its characters (e.g. eye colour). The specific form of a character (e.g. brown
eyes) is a trait. A heritable trait is a characteristic of an organism that is at least partly
determined by its genes.
The genetic constitution that governs a character is called a genotype. A population
evolves when individuals with different genotypes survive or reproduce at different rates.
Gregor Mendel’s publications paved the way for the development of the field of
population genetics. It has three main goals:
1. To explain the origin and maintenance of genetic variation
2. To explain the patterns and organization of genetic variation
3. To understand the mechanisms that cause changes in allele frequencies in
Different forms of a gene are called alleles and may exist at a particular locus.
At any particular locus, a single individual has only some of the alleles found in the
population to which it belongs. The sum of all copies of all alleles at all loci found in a
population makes up the gene pool; which produces the phenotypic traits on which
natural selection acts.
• Most populations are genetically variable
Nearly all populations have genetic variation for many characters. The study of the genetic
basis of natural selection is difficult because genotypes alone do not uniquely determine all
phenotypes. Different phenotypes can be produced by a given genotype depending on the
environment encountered during development.
• Evolutionary change can be measured by allele and genotype frequencies
Allele frequencies are usually estimated in locally interbreeding groups within a geographic
population Mendelian population.
An allele’s frequency is calculated using the following formula:
p =numberof copiesof theallelein the population Refer to
Page 491 & 492 sumof allelesin the population
• A locally interbreeding group within a geographic population is called a Mendelian
population. • Genetic variation: the relative proportions, or frequencies, of all alleles in a population
• Biologists can estimate allele frequencies for a given locus by measuring numbers of
alleles in a sample of individuals from a population.
• Sum of all allele frequencies at a locus is equal to 1, so measures of allele frequency range
from 0 to 1
• If there is only one allele at a locus, its frequency = 1. The population is monomorphic at
that locus; the allele is said to be fixed. The population is said to be polymorphic at a
locus, if there are more than one allele at that locus.
• The frequencies of different alleles at each locus and the frequencies of different genotypes
in a Mendelian population describe that population’s genetic structure
o Allele frequencies measure the amount of genetic variation in a population;
genotype frequencies show how a population’s genetic variation is distributed
among its members
The genetic structure of a population does not change over time if certain conditions
• If an allele is not advantageous, its frequency remains constant from generation to
generation, its frequency will not increase even if the allele is dominant
• A population of sexually reproducing organisms in which allele and genotype frequencies
do not change from generation to generation is said to be at Hardy–Weinberg
equilibrium. Genotype frequencies can be predicted from allele frequencies.
• Five assumptions must be made in order to meet Hardy–Weinberg equilibrium.
o Mating is random.
o Population size is very large. (larger the population, the smaller will be the effect of
genetic drift-random(chance) fluctuations in allele frequencies)
o There is no migration either into or out of the populations.
o There is no mutation. No change to alleles A and a, and no new alleles are added to
change the gene pool.
o Natural selection does not affect the survival of particular genotypes. There is no
differential survival of individuals with different genotypes.
• If the conditions of the Hardy–Weinberg equilibrium are met, two results follow.
o The frequencies of alleles at a locus will remain constant from generation to
o After one generation of random mating, the genotype frequencies will not change.
• The Hardy–Weinberg equation: p + 2pq + q = 1.2
Genotype: AA Aa aa
Frequency: p 2pq q
• Populations in nature never fit the conditions for Hardy-Weinberg equilibrium. Two reasons
why this model is considered important for the study of evolution:
o It is useful in predicting genotype frequencies from allele frequencies; and
o Since the model describes conditions that would result in no evolution,
patterns of deviation from the model help identify specific mechanisms of
22.2- What Are the Mechanisms of Evolutionary Change?
• Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are
• Known evolutionary mechanisms:
• Mutation • Nonrandom mating
• Gene flow • Natural selection
• Genetic drift
Mutations Generate Genetic Variation
• Origin of genetic variation is mutation; mutation is any change in an organism’s DNA
• Most mutations are harmful to their bearers or are neutral, but if environmental conditions
change, previously harmful or neutral alleles may become advantageous
• Mutations can restore to populations alleles that other evolutionary processes have
• Most mutations appear to be random and are harmful or neutral to their bearers. • Some mutations can be advantageous.
• Mutation rates are low; one out of a million loci is typical.
• Although mutation rates are low, they are sufficient to create considerable genetic
• Rates as high as one mutation per locus in a thousand zygotes per generation are rare; one
in a million is more typical
• One condition for Hardy–Weinberg equilibrium is that there is no mutation.
• Although this condition is never met, the rate at which mutations arise at single loci is
usually so low that mutations result in only very small deviations from Hardy–Weinberg
• If large deviations (from H-W expectations) are found, it is appropriate to dismiss mutation
as the cause and look for evidence of other evolutionary agents.
Gene flow may change allele frequencies
• Gene flow results when individuals migrate to another population and breed in new
• No immigration is allowed for a population to be in Hardy–Weinberg equilibrium.
Genetic drift may cause large changes in small populations
• Genetic drift is the random loss of individuals (and their alleles)-may produce large
changes in allele frequencies from one generation to the next
• In very small populations, genetic drift may be strong enough to influence the direction of
change of allele frequencies even when other evolutionary agents are pushing the
frequencies in a different direction.
• Organisms that normally have large populations may pass through occasional periods
when only a small number of individuals survive (a population bottleneck). Genetic
variation can be reduced by genetic drift.