Download BIO152 Summer Evolutionary processes

BIO152 Summer
Lectures 17&18
Evolutionary processes
(Hardy-Weinberg principle)
Evolutionary processes
1
Announcements
Lab 5-Agents of evolution online –quiz
at the beginning of lab
Lab 3 & 4 bacteria results& discussion
due
Test marks and papers- Thursday
No help sessions (lab or lecture) today
Final exam!
Tuesday, June 28th 9 – 12 (morning)
Format-multiple choice and written
Covers all labs, lectures, and readings
Help session Monday June 27th 12-2pm
2
• Evolution = change in
allele frequency over time
• Natural selection acts on
individuals
• Evolutionary change
occurs in populations
Mechanisms for shifting allele
frequencies in populations
1. Natural selection increases the
freq of alleles which
contribute to improved
reproductive success
2. Gene flow (individuals move
into or out of a population)
changes the proportion of
alleles
3. Mutation introduces new
alleles-beneficial, detrimental,
or no effect on reproductive
survival
4. Genetic drift -random change
in allele frequencies
3
Hardy-Weinberg principle
Claim 1—about the frequency = 1
• Frequency of alleles for a trait
p is the frequency of allele A1
q is the frequency of allele A2
p +q=1
• Frequency of genotypes for a
trait A1A1, A1A2, A2A2
2
p + 2pq + q2 = 1
Hardy-Weinberg Fig 24-1
4
Hardy-Weinberg claim 2
When alleles are transmitted
according to the rules of
Mendelian inheritance,
allele frequencies do not
change over time.
For evolution to happen,
something must happen to
change the allele
frequencies
HW = null hypothesis for evolution
If allele frequencies remain
the same, then evolution
does not happen
If allele frequencies are
changing, then one of the
processes (natural
selection, drift, mutation,
gene flow) is affecting the
change and evolution is
happening
5
Natural selection affects allele freq
• Directional selection-allele
freq change in one
direction (higher or lower)
Directional selection fig 24-2
6
Stabilizing selection Fig 24.3
Disruptive selection 24.4
7
Sexual selection
z
Individuals in a population differ in
their ability to attract mates
A special case of natural selection
Another form of nonrandom mating
Sexual selection cont
Bateman-Rivers theory of sexual selection:
Basic asymmetry of sex
Pattern: sexual selection acts on males more
strongly than females, traits affecting
sexual selection are more elaborated
(exaggerated) in males than females.
8
Sexual selection-process
Mechanism
Eggs are expensive, but sperm is cheap
►females usually invest more in their
offspring
So females are choosy & males
compete
More to follow!
Thursday lecture
z Lab 6
z Judson’s book
z Text book section in chapters 12 & 24
z
9
Genetic drift
† Change in allele frequencies in a
population due to chance
† Random, not adaptive change
† Most pronounced in small populations
† Can lead to loss of alleles
Genetic drift-founder effect
† New population established, may
effect allele freq depending on
‘sample’ founding new population
† E.g., inhabiting islands
10
Genetic drift-bottleneck effect
† Drastic drop in a population►change
in allele frequencies
† E.g., disease, disasters
Gene flow (migration)
† Movement of alleles from one
population to another
Tends to equalize allele frequencies
between populations (Fig 24.11)
Improved fitness? Depends…
Increased genetic diversity may provide
better solutions in the new population
11
Mutation
† Does not occur often enough (plus
many are either bad or neutral), so
not strong evolutionary force on its
own
† Mutation rate per allele versus per
genome—mutation introduces new
alleles into every individual in every
population in every generation
Random/nonrandom mating
† Inbreeding = matings between
relatives
† Relatives share alleles inherited from
common ancesters
12
Nonrandom mating vs
sexual selection
† Inbreeding affects all genes (in
contrast to sexual selection affecting
genes responsible for mate selection
and competition)
† Inbreeding changes genotype
freqency
Sexual selection changes allele and
genotype frequency
13