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Mutation

Generates random genetic variability.

Mutations are random, permanent changes to the genetic code; only those that occur within the gametes, sperm and ova, are heritable.
Mutations can lead to advantageous, deleterious, or neutral changes in phenotypes.

For example:

In a population of blue birds, a mutation in feather color changes the alleles and allele frequencies in that population; in other words, the mutation introduced genetic variability into the population. However, because mutations are relatively rare, they, alone, are not sufficient for evolution.

Genetic drift

Occurs when chance events cause "drifts," aka, fluctuations, in allele frequencies.
The founder effect occurs when individuals (and their alleles) leave their original group and populate a new location. As a result, the new, isolated population will have different allele frequencies from the original population.

Notice that the difference between the founder effect and gene flow is that gene flow occurs as result of interbreeding between populations, whereas the founder effect occurs in reproductively isolated populations.
The founder effect is particularly discernible in small, reproductively isolated populations when rare and/or deleterious mutations become unusually frequent.

For example, a type of dwarfism called Ellis-van Creveld syndrome is disproportionately frequent within the Amish population of eastern Pennsylvania; researchers have traced the mutation responsible for Ellis-van Creveld syndrome to an early founder of the community (Samuel King).

The bottleneck effect, which can be thought of as "survival of the luckiest," occurs when a random event extinguishes specific alleles from a population by chance.
For example imagine that a portion of the population, which just so happens to include the pink-feathered birds, lives in a tree that gets stuck by lightening – all of the pink birds die, not because of their color, but because they were in the wrong place at the wrong time.

Consequently, the alleles for pink feathers are removed from the gene pool, and future generations comprise only blue birds.

Notice that both mechanisms of genetic drift reduce genetic variation within a population.

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Evolutionary Forces

Forces of Evolution

Evolution is commonly defined as genetic change through time, or, more specifically, from generation to generation.
In order for evolution to occur, genetic variation, the so-called "raw material for evolutionary change," must exist.
Variation can arise from:

Mutation
Sexual reproduction, via: errors in meiosis and sexual recombination, which produces new genotypic combinations.

Forces act on genetic variation to shape allele frequencies:

Genetic drift
Gene flow
Natural selection

The forces of evolution operate simultaneously in real life.

Evolutionary Forces

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