Chapter 22: The Mechanisms of Evolution
22.1 What Facts Form the Base of Our Understanding of Evolution?
•Darwin developed the major features of an explanatory theory for
evolutionary change based on two major propositions:
oSpecies are not immutable; they change over time.
oThe process that produces these changes is natural selection.
•Darwin also observed that, although offspring ten to resemble their parents,
the offspring of most organisms are not identical to one another or to their
•He suggests that slight variations among individuals affect the chances that a
given individual will survive and reproduce natural selection: differential
contribution of offspring to the next generation by various genetic types
belonging to the same population.
•Individuals do no 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
•Refers both to the processes by which characteristic that appear to be useful
to their bearers evolve and to the characteristics themselves.
•With respect to characteristics, an adaptation is a phenotypic characteristic
that has help an organism adjust to conditions in its environment.
Population genetics provides an underpinning for Darwin’s Theory
•We cannot directly observe the genetic composition of organisms; what we
do see in nature are phenotypes, the physical expression of organism’ genes.
•The features of a genotype 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.
•A population evolves when its individuals with different genotypes survive or
reproduce at different rates.
•Population genetics has three main goals:
oTo explain the origin and maintenance of genetic variation
oTo explain the patterns and organization of genetic variation
oTo understand the mechanism that cause changes in allele frequencies
•Different forms of a gene, alleles, 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 constitutes
its gene pool.
•The gene pool contains the genetic variations that produce the phenotypic
traits on which natural selection acts.
Most populations are genetically variable
•Nearly all populations have genetic variation for many characters.
Evolutionary change can be measure by allele and genotype frequencies
•Allele frequencies are usually estimated in locally interbreeding groups,
Mendelian populations, within a geographic population of a species.