WEEK 11: MONDAY, NOVEMBER 11TH TO FRIDAY, NOVEMBER 15TH, 2013
CHAPTER 15: HOW POPULATIONS EVOLVE
A localized group of individuals of the same species, considered at the same time
Distinct group of populations that have the ability to interbreed and are evolutionary independent of other
GENE POOL (Fig. 15-2)
All genes present in a population at any given time, all alleles of all genes
Fig. 15-3 Mutations occur spontaneously
How would you know if a population is evolving?
Follow populations gene frequencies over time and look for changes in frequencies in the gene pool
STARTING POINT FOR EXAMINING EVOLUTION ***
HARDY-WEINBERG EQUILIBRIUM represents a non-evolving base-line reference point
Look at genetics of a non-evolving population.
ALLELE FREQUENCIES (MEMORIZE)*** (KNOW FOR EXAM)
Upper Case = Dominant, Lower Case = Recessive
Let p represent allele frequency of R.
Let q represent allele frequency of r.
p2 = probability of producing a RR genotype (Dominant)
q2 = probability of producing a rr genotype (Recessive)
2pq = probability of producing a Rr genotype (Heterozygous)
Sum of these frequencies must equal 100%.
p2 + 2pq + q2 = 1*
Sum of allele frequencies = 1, therefore p+q = 1 *
RR = p2
Rr & rR = 2pq rr = q2
HARDY WEINBERG EQUILIBRIUM EQUATION
p2 + 2pq + q2 =1
Hardy-Weinberg relationship determines allele frequencies for loci with recessive alleles.
1/10, 000 USA babies are born with Phenylketonuria (PKU), this is a homozygous recessive condition
q2 = 0.0001
take square root to find q = 0.01
Then, p= 1- q = 0.99
CALCULATE THE FREQUENCY OF HETEROZYGOUS CARRIERS OF PKU :
2pq = 2(.99 x .01)
2% are carriers
THE HARDY-WEINBERG THEOREM
- allele and genotype frequencies within a population will remain constant from one generation to the next as long as the
following Hardy Weinberg equilibrium conditions are met when:
1) Pop'n size – very large (low genetic drift)
2) No migration (population is isolated from other pop'ns – low gene flow)
3) No met mutations (no accumulative mutations)
4) Mating - ??? Home study task
5) No natural selection
Microevolution is NOT occurring when the aforementioned 5 points are present.
- the values will remain constant from generation to generation if all of these conditions are met
- Hardy-Weinberg equilibrium = null hypothesis (is used to test for microevolution).
Founder Effect * Expand
Fig. 15-5 Genetic Drift Fig. 15-7 Population bottlenecks reduce variation
Fig. 15-8 A human example of the founder effect
- generation to generation change in a population’s genetic structure
- a change in a population’s relative allelic frequencies over time
- small scale evolutionary change
TYPES OF NATURAL SELECTION (Fig. 15-12)
- favours one extreme, shifts frequency curve in one direction towards a rare variant
Ex. Beak sizes in finches
- selects against extreme phenotypes, favours intermediate variants, reduces phenotypic variation with
phenotypes suited to relatively stable environments
Ex. Human birth weight
- opposite phenotype extremes are favoured over intermediate phenotypes
Ex. Black, grey, white rabbits
Examples of different types of selection:*
+ Iclickr questions
1. Suppose that the peyri dishes shown in Step 3 of figure contained both streptomycin and gentanmicin. A small
group of colonies resulted in all three dishes in Step 4. Why?
A) All resulting colonies were resistant to both antibiotics
B) Some resulting colonies were resistant to one antibiotic, but not the other
C)All bacteria in Step 1 Were resistant to one antibiotics
D) all bacteria in Step 1 were resistant to both antibiotics
2. If q = 04, p =0.6 and 2pq = 0.48 for a population, then the Hardy-Weinberg equation must add up too ___.
3. Suppose that 16% of a population exhibits a recessive phenotype. What does this 16% as a decimal, represent in
the Hardy-Weinberg equation?
a) p2 (squared)
c) q2 (sqaured)
4. In an island population of birds, the large birds eat the