Chapter 23~ The Evolution of Populations
Population genetics
Population: a localized group of individuals belonging to the same species
Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring
Gene pool: the total aggregate of genes in a population at any one time
Population genetics: the study of genetic changes in populations
Modern synthesis/neo-Darwinism
“Individuals are selected, but populations evolve.”
Hardy-Weinberg Theorem
Serves as a model for the genetic structure of a nonevolving population (equilibrium)
5 conditions:
1- Very large population size;
2- No migration;
3- No net mutations;
4- Random mating;
5- No natural selection
Hardy-Weinberg Equation
p=frequency of one allele (A); q=frequency of the other allele (a); p+q=1.0 (p=1-q & q=1-p)
P 2=frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q 2=frequency of aa genotype; p 2 + 2pq + q 2 = 1.0
Hardy-Weinberg Problem 1 (step by step)
P = frequency of 1 allele
Q = frequency of second allele
p + q = 1
p 2 + 2pq + q 2 = 1
Problem: 1 in 1700 US Caucasian newborns have cystic fibrous. C for normal is dominant over c for cystic fibrous.
When counting the phenotypes in a population why is cc the most significant?
2. What percent of the above population have cystic fibrous (cc or q 2)?
ALLELE FREQUENCY CALCULATIONS:
Why calculate "q" first?
c = q = ?
Why is it now easy to find "p"?
C = p = ?
ALLELE FREQ. CALCULATIONS:
Why calculate "q" first?
You are given q 2 (cc). Take the square root of the homozygous recessive.
c = q = ? cc = q 2 = 1/1700 = .00059
q = .024
Why is it now easy to find "p"?
The formula gives it to you p + q = 1
C = p = ?
p + q = 1 p = 1 - q p = 1 - .024 = .976
Now that you know that p =.976 and q = .024, the genotypes can be calculated
GENOTYPE FREQUENCY CALCULATIONS :
CC- Normal homozygous dominate = p 2 = ?
Cc -carriers of cystic fibrous = 2pq = ?
How many of the 1700 of the population are homozygous normal?
How many of the 1700 in the population are heterozygous (carrier)?
It has been found that a carrier is better able to survive diseases with severe diarrhea. What would happen to the frequency of the "c" if there was a epidemic of cholera or other type of diarrhea producing disease?
Would "c" i ncrease or would it d ecrease?
CC- Normal homozygous dominate = p 2 = ?
P2 = (.976)2 = .953 or 95.3%
Cc -carriers of cystic fibrous = 2pq = ?
2pq = 2(.976 X .024) = .0468 or 4.68%
How many of the 1700 of the population are homozygous normal?
95.3% of 1700 = 1620.1 (CC)
How many of the 1700 in the population are heterozygous (carrier)?
4.68% of 1700 = 79.56 (Cc)
It has been found that a carrier is better able to survive diseases with severe diarrhea. What would happen to the frequency of the "c" if there was a epidemic of cholera or other type of diarrhea producing disease?
Would "c" i ncrease or would it d ecrease?
Increase. Cc would survive over CC. This would reduce the frequency of C and increase the frequency of c
Hardy-Weinberg Problem 2
If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous(Ss) for the sickle-cell gene?
Solution to Problem 2
9% =.09 = ss = q2
s = q = Square root of .09 = .3
p = 1 - .3 = .7
2pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers)
Solution to Problem 3
A = (2 * (1469) + (138))/(2 * (1469 + 138 + 5)) = .954 or 95.4%
a = 1 - .954 = .046 or 4.6%
AA = (.954) 2 = .910 or 91%
Aa = 2 (.954)(.046) = .087 or 8.7%
aa = (.046) 2 = .002 or .2%
Hardy-Weinberg Problem 4
After graduation, you and 19 friends build a raft, sail to a deserted island, and start a new population, totally isolated from the world. Two of your friends carry (that is, are heterozygous for) the recessive cf allele, which in homozygotes causes cystic fibrosis.
A. Assuming that the frequency of this allele does not change as the population grows, what will be the instance of cystic fibrosis on your island?
B. Cystic fibrous births on the island is how many times greater than the original mainland. The frequency of births on the mainland is .059%.
Solution to Problem 4
A
There are 40 total alleles of the 20 people of which 2 alleles are cystic fibrous causing.
2/40 = .05 the frequency of the cystic fibrous allele
thus cc or q 2 = (.05)2 =.0025 or .25% of the population will be born with cystic fibrous.
B
0.25/.059 = about 4 times greater occurrence
Microevolution, I
A change in the gene pool of a population over a succession of generations
1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)
Microevolution, II
The Bottleneck Effect : type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population
Microevolution, III
Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population
Microevolution, IV
2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)
Microevolution, V
3- Mutations: a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)
Microevolution, VI
4- Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes)
Microevolution, VII
5- Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment
Population variation
Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population
Geographical variation: differences in genetic structure between populations (cline)
Variation preservation
Prevention of natural selection’s reduction of variation
Diploidy 2nd set of chromosomes hides variation in the heterozygote
Balanced polymorphism
1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia);
2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)
Natural selection
Fitness: contribution an individual makes to the gene pool of the next generation
3 types :
A. Directional
B. Diversifying
C. Stabilizing
Sexual selection
Sexual dimorphism : secondary sex characteristic distinction
Sexual selection : selection towards secondary sex characteristics that leads to sexual dimorphism
