Download Evolution of Populations

EVOLUTION OF
POPULATIONS
Mrs. Williams
Freshman Biology
Variation


Inherited differences between
individuals of a population
Can be
 Physical
characteristic
 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

Mutation
 Creates
new variation by changing parts of the genetic
code

Gene Shuffling
 Creates
new variation by the reshuffling of genes
during sexual reproduction
 Chromosome
segregation
 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
fitness
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
(continued)

Stabilizing Selection
 Both
phenotype extremes are selected against
(average phenotype is selected for)
 Bell curve narrows
Natural Selection of Polygenic Traits
(continued)

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
mating
 Large population
 No immigration or emigration
 No mutation
 No natural selection
Speciation



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
are
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
together
or
 Create a zygote
or
 Create a viable offspring
or
 Create a fertile offspring