Download mechanisms

Mechanisms of Evolution
and their effect on populations
Genetic variation
Is needed for evolution to occur!
Variations that occur are RANDOM
Variation occurs through:
sexual reproduction
mutations
important to know
Individual organisms DO NOT evolve - Populations DO
The gene pool of a population consists of all the alleles of
a all the genes of each individual in that population
The percentage of each allele of a any given gene
present in the population determines the genetic
characteristics of that population.
How does Evolution occur?
Small events accumulate to form a new species.
Changing percentages / frequencies of alleles within a
population leads to evolution in a population.
FIVE common factors can change the allele frequency.
1. mutations
Mutations change the DNA of an individual
Since the DNA is heritable this can affect the gene pool
The more genetic variation the greater the diversity and
the greater chance of a selective advantage
2. GENE FLOW
The movement of alleles from one population to another
due to migration
Ex: pollen being blown across into a new field
Ex: a lone wolf travels long distance to find a
new mate
3. NON-RANDOM MATING
Mating based on selecting a mate that displays a
particular phenotype. Prevents individuals with a certain
phenotype from mating.
Ex: Caribou females choose the male that wins
a fight with another male
Ex: Female sage grouse select males with
colourful feathers
4. GENETIC DRIFT
The change in allele frequency due to chance events.
Occurs more frequently in small populations
Can lead to the elimination of some alleles and thereby
reducing genetic diversity
GENETIC DRIFT
GENETIC DRIFT - THE
FOUNDER EFFECT
A change in a gene pool that occurs when a few
individuals start a new isolated population
Ex: Afrikaner (Dutch) population in South Africa higher amounts of Huntington’s disease
Ex: Amish population in Philadelphia - higher
amounts of polydactylism (p. 355)
GENETIC DRIFT - THE
BOTTLENECK EFFECT
Changes in gene distribution due
to a rapid decrease in the size of
the population
Genetic diversity is reduced
Ex: overhunting of cheetahs and
elephant seals (very low genetic
diversity)
5. NATURAL SELECTION
Selective forces such as competition and predation affect
populations.
Some individuals are more likely to survive and
reproduce than others.
If having one allele provides an advantage, the frequency
of that allele in the population will increase.
NATURAL SELECTION
STABILIZING SELECTION
Natural selection that favours intermediate phenotypes
and acts against extreme variants.
This reduces variation and improves the adaptation of the
population to aspects of the environment that remain
fairly constant
NATURAL SELECTION
DIRECTIONAL SELECTION
Natural selection that favours the phenotypes at one
extreme. Results in a phenotype shift.
Common when there is environmental change.
NATURAL SELECTION
DISRUPTIVE SELECTION
Natural selection that favours the extremes of
phenotypes. Can result in eliminating the intermediate.
NATURAL SELECTION SEXUAL
SELECTION
Choices of females for their mates.
Can result in SEXUAL DIMORPHISM when males and
females species have different characteristics.
Usually involves the competition between males (through
combat or visual displays).