NATURAL SELECTION

Jane Choi

B IB Biology HL

Natural Selection: the non-random process by which bi-

ologic traits become either more or less common in a

population as a function of differential reproduction

of their bearers.

Natural Selection= A key mechanism of Evolution

CHARLES DARWIN’S ORIGIN OF SPECIES

Some kinds of organisms survive better in certain conditions

than others do; such organisms leave more progeny and

so become more common with time. The environment

thus “selects” those organisms best adapted to present

conditions. If environmental conditions change, organ-

isms that happen to possess the most adaptive characteris-

tics for those new conditions will come to predominate. 

Observation Deduction

Organisms vary- there are differences between in-dividual organisms even if they are members of the

same species. These differences affect how well suited or fitted an organism is to its environment and mode of existence. This is called adaption.

Some individuals are better adapted than others be-cause they have the favorable variations.

In the struggle for existence, the less well-adapted individuals will tend to die and the better adapted will tend to survive. This is

natural selection

Much of the variation between individuals can be passed on to offspring – it is inheritable.

Because the better-adapted individuals sur-vive, they can reproduce and pass on their

characteristics to their offspring. The greater survival and reproductive success of these individuals leads to an increase in the proportion of individuals in the population that have the favorable variations. Over the generations, the characteristics of the popu-lation gradually change- the evolution by

natural selection.

As a result of the natural selection, the genetic variation

within a population of organisms may cause some indi-

viduals to survive and reproduce more successfully than

others.

Natural selection acts on the phenotype, but the genetic

basis of a phenotype that gives a reproductive advantage

will become more common in a population.

5.4.7 EXPLAIN HOW NATURAL SELECTION LEADS TO EVOLUTION

Natural selection is a process (not a thing) which requires: production of variation. (the random part) the actual selection (non-random)

When a population evolves there is a cumulative change in the heritable characteristics of the population.

Natural selection can act on a population without speciation occurring.

In effect the genetic profile of the population is adapting to changes in local conditions.

Every phase in the process of evolution is affected by varia-tion and by selection.