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Mrs. Williams
Freshman Biology
Inherited differences between
individuals of a population
Can be
 Physical
 Biochemical characteristic
 Behavioral characteristic
If there is no variation for a
trait, it is said to be fixed
Gene Pool
All of genes found within a population
Relative frequency of alleles- proportion of gene
pool that the allele makes up
Sources of Variation
 Creates
new variation by changing parts of the genetic
Gene Shuffling
 Creates
new variation by the reshuffling of genes
during sexual reproduction
 Chromosome
 Crossing-over
Single Gene Traits
Traits are coded for by a single gene
If trait has simple Mendelian (dominant/recessive)
inheritance, there are 2 phenotypes possible.
If trait has incomplete dominance or codominance,
there are 3 phenotypes possible.
If trait has multiple alleles, # of phenotypes
depends on # of alleles
For example: ABO blood type have 3 alleles with 4
phenotypes possible
Natural Selection on Single Trait Genes
Occurs if the phenotypes are not equal in their
Relative frequencies within the gene pool change as
some phenotypes are selected for (or some are
selected against)
This is evolution (a change in allele frequencies
within a population over time)
Natural Selection Refresher
Polygenic Traits
Trait is coded for by more than one gene
Various phenotypes possible
Phenotypes form a bell curve
Natural Selection of Polygenic Traits
Directional Selection
 One
phenotype extreme is selected for (or one against)
 Bell curve is shifted to the left or right
Natural Selection of Polygenic Traits
Stabilizing Selection
 Both
phenotype extremes are selected against
(average phenotype is selected for)
 Bell curve narrows
Natural Selection of Polygenic Traits
Disruptive Selection
 Average
phenotype is selected against (extremes are
selected for)
 Bell curve splits into two peaks
Genetic Drift
Change in allelic frequencies due to random effects
Effects are seen more in smaller populations
Genetic Drift: Bottleneck Effect
Event randomly removes large numbers of
individuals from a population
Many variations can be lost
Genetic Drift: Founder Effect
Small part of the population removes itself (or is
removed) from the larger population
Genetic Equilibrium
Allelic Frequencies remain the same
Also referred to as “Hardy-Weinberg Equilibrium”
No evolution
 Random
 Large population
 No immigration or emigration
 No mutation
 No natural selection
Species - group of individuals that can breed
together and produce a fertile offspring
Speciation is the process of forming new species
from existing species
To occur:
 Populations
of one species must be isolated from each
other long enough to accumulate enough changes to
become two species
Types of Isolation
Geographical Isolation
 Populations
separated by a
geographical barrier
and cannot mate and
share genes
Types of Isolation
Behavioral Isolation
 Populations
are separated by behavioral differences
and don’t mate with each other to share genes
Temporal Isolation
 Populations
reproduce at different times so they cannot
mate together and share genes
This leads to…
Reproductive Isolation
 Cannot
mate and produce a fertile offspring
 Occurs because individuals cannot
 Mate
 Create a zygote
 Create a viable offspring
 Create a fertile offspring