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.