Transcript mechanisms of evolution: genetic variation

MECHANISMS OF EVOLUTION:
GENETIC VARIATION
Quantity of DNA and the number of
genes are highly variable among
species
Variation within a species is the
result of the variety & random
recombination of alleles
possessed by individuals
Results in a high degree of genetic
diversity within most populations
MECHANISMS OF EVOLUTION:
GENETIC VARIATION
Recall the following terms:
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Genes
Alleles
Homozygous
Heterozygous
Genome
Genotype
Phenotype
Population
GODFREY HAROLD HARDY
Wilhelm Weinberg
MECHANISMS OF EVOLUTION:
Hardy-Weinburg Principle
Allele frequencies will not change from
generation to generation as long as the
following conditions are met:
1) Large population
2) Mating opportunities are equal
3) No mutations occur
4) No migration occurs
5) No natural selection occurs – all individuals
have an equal chance of reproduction
Hardy-Weinburg Principle
• For a gene with two alleles (A and a), the
Hardy-Weinburg Principle can be
expressed using the equations below:
If p = frequency for allele A and
q = frequency of allele a Then….
p+q=1
(p + q)2 = 12
p2 + 2pq + q2 = 1
MECHANISMS OF EVOLUTION:
HARDY-WEINBURG PRINCIPLE
p2 = frequency of AA genotype
2pq = frequency of Aa genotype
q2 = frequency of aa genotype
MECHANISMS OF EVOLUTION:
HARDY-WEINBURG SQUARE
PROBLEM #1
A population has only two alleles, R and r
for a particular gene. The allele frequency of
R is 20 %
a) What are the frequencies for RR ?
b) What are the frequencies for Rr ?
c) What are the frequencies for rr ?
PROBLEM #1
Got a
problem ?
No problem…
Step 1…..
Write down all
givens
Step #2
What are
you being
asked ?
Step #3
How is what you are
being asked, related to
what you know ? Then
solve !
SOLUTION
GIVENS:
p = 0.20
I KNOW THAT :p2 + 2pq + q2 = 1
q = 0.80
Substitute values for p and q
Congratulations !
You did it
(.20)2 + 2(.20)(.80) + (.80)2 = 1
0.40 + 0.32 + 0.64 = 1
Frequency of RR genotype = 0.04 or 4 %
Frequency of Rr genotype = 0.32 or 32 %
Frequency of rr genotype = 0.64 0r 64 %
PROBLEM # 2
In a moth population of 100 individuals there are 5 individuals
who are recessive (light grey ww) for wing color:
a) How many are homozygous (WW) dominant ?
b) How many are heterozygous (Ww) dominant ?
Write out
your
givens !
SOLUTION
Given:
q2 = 5_
100
p +q=1
q2 = 0.05
q = 0.2236
FIND p2
a) p2 = (0.7764)2
p2 = 0.6028
Therefore 60 individuals are
homozygous dominant
p= 1 - q
p = 1 – 0.2236
p = 0.7764
FIND 2pq
b) 2pq = 2(0.7764) (0.2236)
2pq = 0.3472
Therefore there are 35 individuals who
are heterozygous
Grade 12
Biology
Rocks
MECHANISMS OF EVOLUTION:
RANDOM CHANGE
• Genetic Drift = changes to
allele frequency as a result of
chance: such changes are
much more pronounced in
small populations
• Evolution = defined in genetic
terms as any change in gene
(and allele) frequencies within
a population
Genetic Drift
• In small populations, chance can play a huge
role in altering allele frequencies
• When a severe event results in a drastic
reduction in numbers, a population may
experience a bottle-neck effect
• In this form of genetic drift occurs, a very small
number of sample alleles survives to establish a
new population
• Their relative frequency may differ those in the
original population & additional genetic drift may
result in further changes in the gene pool
MECHANISMS OF EVOLUTION:
Genetic Drift
BOTTLENECK EFFECT
Occurred in the
northern elephant seal
population
Population was reduced
to 20 individuals in
1890’s and has
rebounded to over 20
000 by 1974
Genetic tests show 24
loci tests showed total
homozygosity
Northern Elephant Seal Distribution
FOUNDER EFFECT
• Founder effect =
genetic drift that
results when a small
number of individuals
separate from their
original population
and find a new
population
PATTERNS OF SELECTION
•
3 types of selection
pressures
1) Stabilizing
Selection=
selection against
individuals
exhibiting
variations in a trait
deviate from the
current average
PATTERNS OF SELECTION
2. Directional Selection
Pressure=
Selection that favours
an increase or decrease
in the value of a trait
from the current
population average
PATTERNS OF SELECTION
3. Disruptive Selection
Pressure=
Selection pressure
that favours two or more
variations or traits that
differ from the current
average
PATTERNS OF SELECTION
4. Sexual Selection
Pressure:
Different reproductive
success that results from
variation in the ability to
obtain mates: results in
sexual dimorphism and
mating courtship behaviours
PATTERNS OF SELECTION
• Not all species show
obvious sexual
dimorphism
• In some species of
penguin, males & females
look the same and they
have a hard time telling
each other apart
• A male picks up a stone &
drops it at the feet of a
would-be-mate. If the
other penguin is a male,
the offering is rejected
ALTRUISM
Altruism =
behaviour that
decreases the fitness of
an individual that is
assisting or cooperating
with a recipient whose
individual fitness is
increased
SPECIATION:
The formation of a new species
• Species = members or breeding groups or
populations that are reproductively
isolated from other groups and evolve
independently
• Speciation evolutionary formation of a new
species
• Reproductive Isolating Mechanisms are
any behavioral, structural or biochemical
traits that prevent individuals from different
species from reproducing together
REPRODUCTIVE ISOLATING MECHAMISMS:
Prezygotic Isolating Mechanisms
Pre-zygotic mechanisms include:
1. Ecological isolation: species
occupy different habitats or
separate niches
Lion
Ex) lions = savannah
animals
tigers = forest
animals
Could mate and hybridize if found
together and produce ligars
Tiger
Liger
REPRODUCTIVE ISOLATING MECHAMISMS:
Prezygotic Isolating Mechanisms
2. Temporal Isolation =
similar plants bloom at
different times of the
day, night,
month/seasons.
This leaves no
change of hybridization
since reproductive
cycles or mating occurs
at different times
Spring Iris
Summer Iris
REPRODUCTIVE ISOLATING MECHAMISMS:
Prezygotic Isolating Mechanisms
3. Behavioural Isolation =
Each species may use
different signals for
attracting a mate. Use the
wrong dance,
misinterpret the dance,
and you are breakfast,
lunch, or dinner
REPRODUCTIVE ISOLATING MECHAMISMS
• Ecological isolation, temporal isolation &
behavioural isolation are all pre- zygotic
isolating mechanisms
• These mechanisms prevent interspecies
mating
• Another prezygotic isolating mechanism is
the prevention of fertilization. There are
two mechanisms that prevent fertilization:
REPRODUCTIVE ISOLATING MECHAMISMS:
Prevention of Fertilization
1. Mechanical Isolation =
structural differences
in reproductive organs
can prevent
copulation.
Square Peg/Round Hole ?
Who Is He
Kidding ?
REPRODUCTIVE ISOLATING MECHAMISMS:
Prevention of Fertilization
Shapes of floral
features in plants can
affect the transfer of
pollen is also an
example of mechanical
isolation
The pink lady’s slipper
orchid exhibits a
mechanical isolating
mechanism
This orchid looks like a wasp
REPRODUCTIVE ISOLATING MECHAMISMS:
Gametic Isolation
• In coral reefs, many
species release sperm &
eggs into the water at the
same time.
• Sperm & egg of the
SAME species recognize
each other by molecular
markers
• In many species sperm
cannot survive inside the
female
Coral spawning a cloud of sperm
REPRODUCTIVE ISOLATING MECHAMISMS:
Post Zygotic mechanisms preventing hybrids from mating
Prevents maturation
& reproduction in
offspring from
interspecies
reproduction
A spotted mule is sterile, resulting
from horse & donkey mating
Hybrid trilliums are sterile
REPRODUCTIVE ISOLATING MECHAMISMS:
Post Zygotic Isolating Mechanisms
• Zygote mortality &
Hybrid inviability
ensure reproductive
isolation of a gene
pool by preventing the
exchange of alleles
between the parent
species
MODES OF SPECIATION
1. Allopatric Speciation
is the evolution of
populations into
separate species as a
result of geographic
isolation
The canyon is a barrier to
dispersal by small mammals,
(Grand Canyon Squirrels) and
as a consequence the isolated
populations can diverge.
Mountain range is the barrier
MODES OF SPECIATION
2. Sympatric Speciation
is the evolution of
populations within the
same geographic area
into separate species.
Largely due to genetic
drift.
Good Bye
Chapter 12
Evidence suggests stickleback fish
species evolved by sympatric speciation