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
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 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 allele’s frequency is calculated using the following formula:
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•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
population in the alleles of sum
population in the allele theof copies ofnumber