Download 3.4-Evolutionary Mechanisms

How do populations evolve?....
Are there any trends?....
Gene pool: all of the genes of a population
Allele frequency: the percentage of any
particular allele in a gene pool
A population in which an allele frequency
remains the same over many generations is
stable and is known as “genetic
equilibrium” and therefore is NOT
evolving
Gene pool: all of the genes of a population
Allele frequency: the percentage of any
particular allele in a gene pool
Any factor affecting the genes in a gene
pool will result in “microevolution”
• Evolution within a population/species
FOUR factors affect evolutionary change…
1.
Natural Selection
2.
Mutation
3.
Gene Flow
4.
Genetic Drift
According to the survival of the fittest,
the best adapted phenotype is
“selected,” favouring that particular set
of alleles

Selective Pressures

• Disease
• Climate conditions
• Food availability
• Predators
• Choice of mate
A. Stabilizing Selection: (most common)
•
•
Limits evolutionary change by
favouring the current population
norm
Examples:
-human birth weights
-ideal bill length in
hummingbirds
B. Directional Selection:
•
•
•
•
Produce evolutionary change by favouring
individuals that differ from the population norm
(in one direction)
Common in artificial breeding
Results from a sudden change
in environment
Example:
-Salmon in B.C.
(net fishing)
C. Disruptive Selection:
•
•
Works the same as directional selection, but
sometimes favours more than one trait (two
directions)
Example:
-Finches in Africa
food = soft and hard seeds
D. Sexual Selection:
•
•
•
Selection favours individuals with variation of a
trait that aids in the success of mating
Leads to “sexual dimorphism”
which is difference in physical
appearance between males and
females
Example:
-Peacock tails
1.
2.
3.
4.
5.
A parrot population has only very large
and very small tail feathers
A population of ducks lays eggs of
intermediate mass
Most individuals in a population of
hummingbirds have long beaks
A population includes only mediumsized spiders
The Peppered Moth
6.
7.
8.
9.
10.
Most birds have extremely light weight
and hollow bones
Trees in windy areas tend NOT to grow
any bigger each year
The brain size of hominids steadily 
The same species of butterflies tends to
have blue stripes in open areas and
orange stripes in forested areas
The average size of salmon  due to
over-fishing in British Columbia
A parrot population has only very large
and very small tail feathers
1.
•
2.
•
3.
•
4.
•
disruptive
A population of ducks lays eggs of
intermediate mass
stabilizing
Most individuals in a population of
hummingbirds have long beaks
directional
A population includes only medium-sized
spiders
stabilizing
The Peppered Moth
5.
•
directional
Most birds have extremely light weight
and hollow bones
6.
•
directional
Trees in windy areas tend NOT to grow
any bigger each year
7.
•
stabilizing
The brain size of hominids steadily 
8.
•
directional
The same species of butterflies tends to
have blue stripes in open areas and
orange stripes in forested areas
9.
•
10.
•
disruptive
The average size of salmon  due to
over-fishing in British Columbia
directional
Occurs when individuals select mates based on
their phenotypes
 Inbreeding: mating between relatives of a
species
 Artificial selection: changes to a population
caused by deliberate, selective breeding by
humans
 Leads to…

•
•
•
•
Decreases () diversity
 the frequency on inheriting recessive abnormalities
 vulnerability to disease and environmental change
Long term outcomes of artificial selection hard to predict

Mutations: a change in the DNA
• A mutation provides new alleles and therefore
produces new variation
• It is necessary for all other mechanisms of evolution
a) Harmful mutations
 MOST common
 nature selects against them
 rare in the gene pool
b) Beneficial mutations
 Occur rarely
 Nature selects for
 Accumulate in gene pool

Mutations: a change in the DNA
• A mutation provides new alleles and there produces
new variation
• It I necessary for all other mechanisms of evolution
a) Neutral mutations
 Nature selects neither for or against

Gene flow: transfer of alleles from one
population to another via migration of
individuals to and from existing populations

Genetic drift: changes of allele frequencies
due to chance events which change
population size; small populations are
impacted more
a) Bottleneck Effect
 Some chance even cause extreme  in populations size
(disease; natural disasters; human interference etc.)
 Result in loss of diversity
 Surviving genotypes are dictated
by chance
 Example: Northern elephant seals
(overhunting in 1890s)

Genetic drift: changes of allele frequencies
due to chance events which change
population size; small populations
are impacted more
b) Founder Effect
 Several individuals establish a new colony
 The new population is small with limited diversity
 The founder genotypes are dictated by chance
 Example: a few organisms migrate to a NEW location

1.
2.
3.
4.
The Hardy-Weinberg principle can be used
to identify factors causing allele frequencies
to change, leading to evolutionary change…
Natural Selection (favours some allele)
Small population size
Mutation (introduces new alleles)
Migration (adds/removes alleles)
Mathematically, a gene pool can be
described by the frequency of each of the
alleles within the population
 This relationship between allele frequencies
and the chance that they remain constant can
be represented by an equation:

A2 + 2Aa + a2 = 1
A=dominant
a = recessive
A population of sunflower plants has only 2
alleles for the “height” gene.
The allele frequency T is 20%.
Determine the genotype frequencies (TT),
(Tt), (tt) in this population.
In a sample of population of 500 peppered
moths, determine the allele frequencies for
the sampled counts listed below…
Genotype
# of moths (total = 500)
Genotype frequency
# of alleles in gene pool (total
= 1000)
Allele frequencies
BB
Bb
bb
20
160
320
1) A large population consists of
400 individuals, of which 289 are
homozygous Dominant, 102 are
heterozygous, and 9 are
Homozygous Recessive.
Determine the allele frequencies
of M and m?
2)The gene pool of a large
population of fruit flies contains
only two eye-colour alleles; the
dominant red eyes (W) and the
recessive white alleles (w). Only
1% of the population has white
eyes. Determine the allele and
genotype frequencies of this
population.
In a certain population of 1000 fruit flies,
640 have red eyes while the remainder
have sepia eyes. The sepia eye trait is
recessive to red eyes. How many
individuals would you expect to be
homozygous for red eye color?
R2 + 2Rr + r2 = 1
1) r2 for this population is 360/1000 = 0.36
2) r = √0.36 = 0.6
3) If r = 0.6, then R = 0.4
4) The homozygous dominant frequency =
R2 = (0.4)(0.4) = 0.16.
Therefore, you can expect 16% of 1000, or
160 individuals, to be homozygous
dominant.
 Speciation:
the formation of a new
species
• but how do brand new species evolve in the first
place? (“The Blind Watchmaker)
• Recall the definitions of a “species”
• Requires the evolution of distinct, complex
features
CHANCE
+ CHOICE
=
(random change) (natural selection)
Development of
new complex
features
Reproductive isolating mechanism:
any biological factor preventing 2
species from exchanging genes
1.
•
Prezygotic mechanisms - prevents hybrid
offspring formation
o
o
o
o
o
Habitat isolation
Temporal isolation
Behavioural isolation
Mechanical isolation
Gamete isolation
Reproductive isolating mechanism:
any biological factor preventing 2
species from exchanging genes
1.
•
Postzygotic mechanisms - prevents hybrid
offspring surviving and reproducing
o
o
o
Hybrid inviability
Hybrid sterility
Hybrid breakdown
 Whenever
reproductive isolation
develops, separate species have formed
and speciation has occurred
2.
3.
Allopatric speciation: populations
evolve into separate species as a result
of geographic isolation (occurs
gradually)
Sympatric speciation: populations
evolve into separate species while in
the same geographic area (gradual or
sudden)

Example: Hawthorn flies +apples
 VIDEO

Predictable outcomes of natural
selection lead to recognizable patterns
Divergent evolution:
A.
•
•
Populations that were once similar evolve into
many different species
Occurs as a result of adapting to different
environmental conditions

Predictable outcomes of natural
selection lead to recognizable patterns
Adaptive evolution:
•
•
A type of divergent evolution in which a single
species evolves into many new species; each
filling an empty ecological niche
Occurs because new resources become
available or competition

Predictable outcomes of natural
selection lead to recognizable patterns
Convergent evolution:
B.
•
•
Unrelated species evolve similar traits
Occurs because they occupy similar niches in
different geological locations

Predictable outcomes of natural
selection lead to recognizable patterns
Coevolution:
C.
•
•
One species evolves in response to the
evolution of another species
Occurs because they are dependent on one
another for survival
Example: Flowers and pollinators, parasites and host