Natural selection

1. In genetic terms, we know that selection
results in alleles being passed to the next
generation in proportions that differ from
those in the present generation.

2. Genetic drift describes random fluctuations in the
numbers of gene variants in a population. Genetic drift
takes place when the occurrence of variant forms of a
gene, called alleles, increases and decreases by chance
over time. These variations in the presence of alleles are
measured as changes in allele frequencies.

4. -When a few individuals become isolated from a larger
population, this smaller group may establish a new
population whose gene pool differs from the source
population.
The Bottleneck Effect
-A severe drop in population size
-a sudden change in the environment, such as fire or
flood, may drastically reduce the size of the population

6.  The transfer of alleles into or out of a population due
to the movement of fertile individuals of their
gametes.

7.  Occurs when conditions favor individuals exhibiting
one extreme of a phenotypic range, thereby shifting a
population’s frequency curve for the phenotypic
character in one direction or the other.

9.  Occurs when conditions favors one individuals at both
extremes phenotypic range over individuals with
intermediate phenotypes.

11.  Acts against both extreme phenotypes and favors
intermediate variants. This mode of selection reduces
variation and tends to maintain the status quo for a
particular phenotypic character.

13.  Diploidy
- In diploid eukaryotes, a considerable amount of genetic
variation is hidden from selection in the form of recessive
alleles.
 Balancing Selection
-occurs when natural selection maintains two or more forms
in a population. This selection includes heterozygote
selection and frequency-dependent selection

14.  Heterozygote Advantage
- If individuals who are heterozygous at a particular locus
have greater fitness than do both kinds of homozygous, they
exhibit heterozygous advantage. In such case, natural
selection tends to maintain two or more alleles at that locus.
Note that heterozygote advantage is defined in terms of
genotype, not phenotype. Thus, whether heterozygote
advantage represents stabilizing or directional selection
depends on the relationship between the genotype and
phenotype.

15.  Frequency-Dependent Selection
- In Frequency-dependent selection, the fitness of a
phenotype depends on how common it is in the population.