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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
Evolution as Genetic Change
A species is defined as a group of similarlooking 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
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.
Evolution as Genetic Change
Sexual reproduction alone does not change
the relative frequency of alleles in a
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.
Fitness and Adaptation
Fitness is defined as the success an organism
has in passing on its genes to the next
An adaptation is any genetically controlled
characteristic of an organism that increases
its fitness.
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.
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.
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.