Transcript Mechanisms of Evolution

Mechanisms of Evolution
Biology
Mr. Solis
Populations, Not Individuals Evolve
An organism cannot evolve a new phenotype,
but rather natural selection acts on the range of
phenotypes in a population.
Evolution occurs as a populations genes and
their frequencies change over time.
Gene Pool- All the alleles of a populations
genes.
Allelic Frequency- the percentage of any specific
allele in a gene pool.
Genetic Equilibrium- frequency of alleles
remains the same over generations.
Changes in genetic equilibrium
A population in genetic equilibrium is not
evolving.
Anything that affects the genes in gene
pool and affects the frequency will cause
the population to evolve.
Genetic Drift- the alteration of allelic
frequencies by chance events.
Natural Selection acts on variations
Some variations increase or decrease an
organisms chance of survival in an
environment.
Three types of natural selection that act on
variations.
– Stabilizing
– Directional
– Disruptive
Stabilizing
Natural selection that
favors average
individuals in a
population.
Example: organisms that
range from small,
medium and large.
The large ones are easily
seen and eaten, the small
have difficulty finding
food. Therefore the
average size has a
selective advantage.
Directional Selection
Natural selection favors
one of the extreme
variations.
Example an insect
invades an area
populated with long beak
woodpeckers and short
beak woodpeckers. The
insect lives deep within
the bark only allowing
birds with long beaks to
reach them. Therefore
having the selective
advantage.
Disruptive Selection
Favors both extreme
variations of a trait.
Example: When you
have a light colored,
medium colored and dark
colored organism and the
environment contains
surroundings of light and
dark color areas. This
allows the dark and light
colored organisms to
survive but the medium
color gets eliminated.
The Evolution of Species
Speciation: occurs
when members of
similar populations no
longer interbreed to
produce fertile
offspring with in their
natural environment.
Physical Barriers can prevent
interbreeding
Geographic Isolation:
occurs when a physical
barrier divides a
population.
Example: A single
species of a frog may
live in the forest, within
time a river forms
creating a new trait.
Eventually this new trait
will procreate and
create a sub population
of its own. Eventually
natural selection will
occur and these frogs
will never inter breed
with each other.
Reproductive Isolation
Occurs when formerly interbreeding organisms
can no longer mate and produce fertile offspring.
There are two types of Reproductive Isolation.
– When the genetic material is so different that
fertilization will not occur.
– When reproductive behaviors are different.
Speciation Rates
Gradualism- species
originate through a
gradual change in
adaptation.
Punctuated Equilibrium:
speciation happens
quickly, in a rapid burst,
with long periods of
genetic equilibrium in
between.
Patterns of Evolution
Adaptive radiation:
when an ancestral
species evolves into
an array of species to
fit a number of
diverse habitats.
Divergent Evolution:
When one species
evolves to two
subpopulations and
eventually become
two different species.
Convergent
Evolution: A pattern
of evolution in which
distantly related
organisms evolve
similar traits.