Transcript Ch 23 The Evolution of Populations notes

Objective:
Understand how allele frequencies can
show evolution in a population.
 Individuals
have variations but they do not
evolve.
 The smallest scale (microevolution) shows
change in allele frequencies in a population over
time.
 Causes of evolution:



Natural selection (fittest organisms)
Genetic drift (random catastrophes change allele
frequencies)
Gene flow (movement of alleles between populations)
 Genetic

Diverse, inheritable traits set the stage for
evolution


Variation within a population
Variation between populations
 Sources



Variation
of Genetic Variation
Formation of new alleles by mutation
Chromosomal alterations (deletion, duplication,
translocation, etc.)
Sexual reproduction (crossing over, independent
assortment, and fertilization).
A
gene pool is a summative of a population’s
genes.
 Allele frequency is the number of times one
allele appears in the gene pool.

# of time allele is present/total # of alleles
Ex: 500 flowers = 320 Red (RR) + 160 pink (RW) + 20 White (WW)
Total alleles = 1,000 (each individual has 2 alleles)
 640 + 160 = 800 red alleles
 160 + 40 = 200 white alleles
 Frequency of red = 800/1000 = 80%
 Frequency of white = 200/1000 = 20%

A
control to compare evolving populations to.
 H-W Equilibrium
1.
2.
3.
4.
5.
Large population
No movement into/out of population
No mutations
Random mating
No natural selection (no beneficial/lethal
alleles)
p
= dominant allele frequency
 q = recessive allele frequency

p+q=1
 p2
+ 2pq + q2 = 1
 AA + Aa + aA + aa = 1
 Applying the H-W Equation


See if evolution is happening (allele frequencies
change.)
This can be used to calculate the number of
heterozygotes vs homozygous dominant individuals
Natural Selection
 Nonrandom mating


Traits allow you to have more offspring.
Ex: sexual dimorphism: males are elaborately
decorated to attract mates.
Animation: Causes of Evolutionary Change
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Genetic Drift
 Random changes in allele frequencies over time,
reducing variation.
 Bottleneck Effect

An event causes a loss of the majority of a population.
 Founder

Effect
A few individuals leave to start a new population
Gene Flow
 Reduces differences between populations by
sharing of gametes across them.
Modes of Selection
 At any moment, populations show a normal
curve for most traits.
 This curve can change in 3 ways depending
on how the environment selects for a trait.
Original population
Phenotypes (fur color)
Directional Selection
 Common when environment changes.
 One of two extremes is favored.

EX: Increase size of bears during ice ages
Original
population
Evolved
population
Disruptive Selection
 Both extremes are favored while average
disappears.

EX: beak size in finches (large for hard seeds and
small for soft)
Stabilizing Selection
 Extremes disappear increasing the intermediate.

EX: birth weight of babies