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 Darwins theory
For a population to evolve, its members must possess heritable genetic variation. The
physical expressions of an organisms 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
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 Mendels 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 populations
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
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 alleles frequency is calculated using the following formula:
number of copies of the allelein the population
p = Refer to
Page 491 & 492 sum of allelesin the population
A locally interbreeding group within a geographic population is called a Mendelian
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 populations genetic structure
o Allele frequencies measure the amount of genetic variation in a population;
genotype frequencies show how a populations 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 HardyWeinberg equilibrium.
Genotype frequencies can be predicted from allele frequencies.
Five assumptions must be made in order to meet HardyWeinberg 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