Darwin`s immense contribution to modern thinking about evolution can be traced to four
insights: that species are not fixed, but rather change over time; that evolution is a
branching process, implying all species descend from a single common ancestor; that
evolution is continuous, with gradual changes; and that evolution is based on natural
Natural selection is based on two premises. First, individuals within a population
show variability in heritable behavioural and physical characteristics. Second, the capacity
of the environment to sustain a population of any species is limited, producing competition.
Darwin and Wallace realized that these two factors meant that those individuals within the
population with characteristics that compete better are most likely to survive and
reproduce. Behavioural adaptations were especially important to survival and therefore an
important part of evolution.
Reproductive Success – the number of viable offspring an individual produces relative to
the number of viable offspring by other members of the same species.
Variation refers to the differences among members of a species, including physical
characteristics such as size, strength, or physiology, and behavioural characteristics such as
intelligence or sociability. Genotype is an organism’s genetic makeup and it differs for all
organisms except for identical twins. As a result to these genetic differences, an individual
organism’s physical characteristics and behaviour, or its phenotype, also vary from
individual to individual.
It is important to recognize that every individual’s phenotype is produced by the
interaction of its genotype with the environment. In essence, the genotype
determines how much the environment can influence an organism’s development and
Although evolution occurs over the long run, natural selection can produce important
changes in the short run- in the space of only a few years. Phenotypic variation can produce
important selective advantages that affect survival.
Competition is the second premise underlying the concept of natural selection. Because
individuals of a given species share the same environment, competition within a species for
food, mates, and territory is inevitable. Competition also occurs between species when
members of different species vie for similar ecological resources, such as food and territory.
Natural selection works because the members of any species have different phenotypes.
Because these phenotypes are caused by different genotypes, successful individuals will