Don't forget that chapter 24 vocabulary is due tomorrow!
- Natural selection acts on individuals
• Their relative fitness determines if they will survive to reproduce
- Evolution acts on populations
• Population is the unit of evolution
• A species is a group of populations of individuals that can interbreed successfully and produce a viable offspring. Populations of the same species may be geographically isolated and only exchange genetic material rarely
• The gene pool is the total aggregate of genes in a population at any one time
• Made up of all the alleles at all loci in the members of the population
• In diploid species, each individual has two alleles, and it may be either heterozygous or homozygous
• If all members of a population are homozygous for the same copy of an allele, the allele is said to be fixed
• The Hardy-Weinberg theory is used to describe a population that is not evolving
• The original proportion of genotypes in a population remains constant if • Population size is large • Random mating is occurring • No mutations • No genes are introduced or lost • No selection occurs
• All genotypes can survive and reproduce equally well
• States that the frequencies of alleles and genes in a population’s gene pool will remain constant over the course of generations unless they are acted upon by forces other than Mendelian segregation and the recombination of alleles.
- Hardy-Weinberg Equilibrium
• The situation in which the allele frequencies within a population are not changing
• Allele Frequency Equation:
• Allele Frequency:
• p + q = 1 (p is dominant, q is recessive)
• Hardy-Weinberg Equation:
• Genotype Frequency:
• p2 + 2pq + q2 = 1 (p2 is homozygous dominant, 2pq is heterozygous and q2 is homozygous recessive)
• Mutation and sexual reproduction produce the genetic variation that makes evolution possible.
• Genetic variation includes variation among individuals within a population in discrete and quantitative characters, as well as geogrphaic variation between polymers.
• New alleles ultimately originate by mutation. Most mutations are harmful or have no effect, but a few may be beneficial.
• In sexually reproducing organisms, most of the genetic differences among individuals reslt from crossing over, the independent assortment of chromosomes, and fertilization.
• The Hardy-Weinberg equation can be used to test whether a population is evolving.
• A population, a localized group of organisms belonging to one species, is united by its gene pool, the aggregate of all the alleles in the population.
• The Hardy-Weinberg principle states that the allele and genotype frequencies of a population will remain constant if the population is large, mating is random, mutation is negligible, there is no gene flow, and there is no natural selection. For such a population if p and q represent the frequencies of the only two possible alleles at a particular locus, then p2 is the frequency of one kind of homozygote, q2 is the frequency of the other kind of homozygote, and 2pq is the frequency of the heterozygous genotype.
Sample Hardy-Weinberg problems: (answers are given in red)
- 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?
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 heterozygotes (carriers)
(a) I have given you information on the frequency of the homozygous recessive (or q2). So start by determining q2 and then solving for q.
q2 = (98/200) = 0.49 (or 49%)
q = square root of 0.49 = 0.7 (70%)
(b) Now that you have q, you can solve for p. Remember there are only two alleles in the population, so if you add the frequency of the two alleles, you have accounted for all possibilities and it must equal 1. So p + q = 1.
p = 1-q
p = 1 - 0.7 = 0.3 (30%)
(c) Now what is the formula for heterozygotes? Think back to the Hardy-Weinberg equation -- it is dealing with the genotypes of individuals in the population.
p2 + 2pq + q2 = 1
frequency of homozygous dominant + frequency of heterozygotes + frequency of homozygous recessive = 1
so.....2pq = frequency of heterozygotes
frequency of heterozygotes = 2 (0.3)(0.7) = 0.42 or 42%
(d) Now that you have figured out the % of heterozygotes, can you figure out the % of homozygous dominant? Does the % of homozygous dominant, heterozygotes and homozygous recessive individuals add up to 100%? If not, you have made an error. Those are the only three genotypes possible with only two alleles and a simple dominant and recessive relationship.
p2 = (0.3)(0.3) = 0.09 (or 9%)
p2 + 2pq + q2 = 1
0.09 + 0.42 + 0.49 = 1.0
3. Your original population of 200 was hit by a tidal wave and 100 organisms were wiped out, leaving 36 homozygous recessive out of the 100 survivors. If we assume that all individuals were equally likely to be wiped out, how did the tidal wave affect the predicted frequencies of the alleles in the population?
Again, start with the frequency you know -- homozygous recessive. Follow the same step-by-step procedure as above.
What is the frequency of homozygous recessive?
q2 = (36/100) = 0.36
q = square root of 0.36 = 0.6
What is the predicted frequency of heterozygotes?
frequency of heterozygotes = 2pq
p = 1 - 0.6 = 0.4
frequency of heterozygotes = 2 (0.4)(0.6) = 0.48
What is the predicted frequency of homozygous dominant?
p2 = (0.4)(0.4) = 0.16
p2 + 2pq + q2 = 1
0.16 + 0.48 + 0.36 = 1.0
Given that the allele frequencies did change as the result of the tidal wave, we would say that microevolution has occurred. What do we call the phenomenon that caused this evolution?
the drastic reduction in size of a population due to some chance event is called a bottleneck event - particularly when the original gene pool (allele frequencies) is no longer represented in the surviving population
since there is now a small population, chances are likely that it will be subjected to genetic drift and continue to shift away from the original allele frequency (pre-tidal wave)
Multiple Choice Questions:
- A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of the gametes produced in the population contain the A1 allele. If the population is in Hardy-Weinberg equidlibrium, what proportion of hte flies carry both A1 and A2?
- There are 40 individuals in population 1, all of which have genotype A1A1, and there are 25 individuals in population 2, all of genotype A2A2. Assume that these populations are located far from one another and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation is mostly likely an example of
a) genetic drift
b) gene flow
c) disruptive selection
d) discrete variation
e) directional seletion
- Natural selection changes allele frequencies because some _______________ survive and reproduce more successfully than others.
c) gene pools