In the evolutionary struggle for existence, entire organisms, not individual genes, either

1
Natural Selection

• In the evolutionary struggle for existence,
  entire organisms, not individual genes, either
• survive and reproduce or do not.
• Natural selection can operate only on the
  phenotypic variation among individuals.
• An organism's phenotype includes all the physical
  and behavioral characteristics
• produced by the interaction of genotype and
  environment.

2
Evolution as Genetic Change

• A species is defined as a group of
  similar-looking organisms that breed with one
  another and produce fertile offspring in the
  natural environment.
• Evolutionary biologists study groups of organisms
  called populations.
• A population is a collection of individuals of
  the same species in a given area at the same time.

3
Evolution as Genetic Change

• Because all members of a population can
  interbreed, they and their offspring share a
  common group of genes called a gene pool.
• Each gene pool contains a number of alleles for
  each inheritable trait.
• The number of times an allele occurs in a gene
  pool compared with the number of times other
  alleles for the same gene occur is called the
  relative frequency of the allele.

4
Evolution as Genetic Change

• Sexual reproduction alone does not change the
  relative frequency of alleles in a population.
• It just reshuffles the already existing alleles.
• Evolution, therefore, can be viewed another way.
  That is, evolutionary change involves a change in
  the relative frequencies of alleles in the gene
  pool of a population.

5
Fitness and Adaptation

• Fitness is defined as the success an organism has
  in passing on its genes to the next generation.
• An adaptation is any genetically controlled
  characteristic of an organism that increases its
  fitness.

6
The Niche

• The combination of an organism's "profession" and
  the place that it lives is called its niche.
• No two species can occupy the same niche in the
  same location for a long period of time.
• The more efficient species will survive and drive
  the less efficient species to extinction.
• If two species occupy different niches their
  chance of survival is greatly increased.

7
The Process of Speciation

• A common way in which new species form is when
  populations are separated.
• The separation of populations that do not
  interbreed is called reproductive isolation.
• When a species is divided into two isolated
  populations, natural selection can work
  differently on each group.

8
The Process of Speciation

• Because their gene pools are separate,
• adaptations that appear in one group are not
  passed to the other group.
• With enough time, this may result in two distinct
  species.