27.3 The Process of Evolution
Learning Goals:
1. List the five conditions necessary to maintain Hardy-Weinberg equilibrium
2. Describe the agents of evolutionary change
3. Compare and contrast the three types of natural selection
1. Population Genetics
a. Individuals CANNOT evolve – evolution happens at population level
b. Microevolution – change in gene frequencies within a population over time
c. Population – all members of a single species that occupy a certain area and breed
together
i. Give an example of two different populations:
1.
2.
ii. Gene pool – (following Mendel’s laws) the total number of alleles at all the gene
loci in all members (that are capable of sexual reproduction)
d. Hardy-Weinberg Equilibrium
i. p2 + 2pq + q2 = 1
1. p2 = frequency of DD genotype (homozygous dominant)
2. 2pq = frequency of Dd genotype (heterozygous)
3. q2 = frequency of dd genotype (homozygous recessive)
ii. See Figure 27.12 (p.551) for an example.
iii. We also did an example with our “Platypapyrus foursuitii” simulation. Check
the notes we took on that for details.
iv. Uses Punnett squares to determine genotype frequencies
v. “allele frequencies in a gene pool will remain at equilibrium, and thus constant
after one generation of random mating in a large, sexually reproducing
population as long as five conditions are met”
1. No mutations
2. No genetic drift
3. No gene flow
4. Random mating
5. No selection
vi. Why is it that in real life these conditions are almost NEVER met?
Check Your Understanding p.552 #1-2
2. Five Agents of Evolutionary Change
Essentially the opposite of Hardy-Weinberg equilibrium conditions.
a. Mutations
i. Mutations are random
ii. May be “silent” due to:
1. No amino acid change
2. Masked by dominant allele in diploid organism (diploid = 2 sets of
chromosomes; humans have 2 x 23 chromosomes)
iii. Mutations may be harmful
1. Lower chance of reproductive success
2. May cause syndromes and diseases (e.g. hemophilia or PKU)
iv. Mutations may be beneficial
1. May give organism an advantage in a changing environment
2. Importance of recessive alleles can increase when environment changes
v. Asexually reproducing prokaryotes show mutations in a population much faster
than diploid organisms
b. Genetic Drift
i. Refers to changes in the allele frequencies of a gene pool due to chance
ii. Impact more profound in smaller populations
iii. “founder effect” occurs when a few individuals start a new colony
1. Only a fraction of the total genetic diversity of the original gene pool
present
2. Gene make-up of founders is random
3. Frequencies of certain rare alleles can be much more common in the
new colony than the original population
4. E.g. polydactylism (extra fingers) and dwarfism (only affects lower arms
and legs) in Amish
c. Gene Flow
i. The movement of alleles among populations
ii. Keeps gene pools of nearby populations similar
iii. Prevents close adaptation to a local environment
d. Nonrandom Mating
i. Individuals pair up by genotype or phenotype instead of randomly
ii. Inbreeding is an example
1. Causes increased proportion of homozygotes at all gene loci
2. Causes increased recessive abnormalities among humans (e.g. TaySachs disease)
e. Natural Selection
i. The process by which some individuals produce more offspring than others
ii. Evolution by natural selection requires:
1. Individual variation
2. Inheritance
3. Overproduction
4. Differential reproductive success
iii. “fitness” in biology = the number of fertile offspring an individual produces in its
lifetime
1. Mutations may increase, decrease, or have no effect on “fitness”
2. Crossing over and independent assortment is also a potential source of
genetic variation, and therefore potential increased/decreased “fitness”
iv. Most traits where natural selection acts are polygenic inheritance (more than
one allele pair codes for the trait)
v. Three types of natural selection:
1. Stabilizing selection
a. Extreme phenotypes are selected against
b. individuals near the average are favored
2. Directional selection
a. Occurs when an extreme phenotype is favored
b. the distribution curve shifts in that direction of the favored
phenotype
3. Disruptive selection
a. Two or more extreme phenotypes are favored over any
intermediate phenotype
Check Your Progress p.557 #1-3
3. Maintenance of Variation
a. Genotypic Variation
i. Beneficial for adaptation to new conditions and/or a changing environment
ii. Ways genetic variation is maintained, despite selection working to reduce it:
1. Mutation generates new alleles
2. Recombination and independent assortment shuffle alleles during
gamete formation
3. Fertilization creates new combinations of alleles in a gene pool
4. Gene flow may occur
5. Natural selection favors certain phenotypes, but other types may still
remain in reduced frequency (polymorphism even results from
disruptive selection)
b. Diploidy and the Heterozygote
i. Only expressed alleles are subject to natural selection
ii. In diploid organisms, recessive alleles can be protected from being eliminated in
heterozygotes
iii. Sickle-cell disease is an example of balanced polymorphism where
heterozygotes can have an advantage over both homozygous conditions
(normal or sickle cell disease) due to its protection against malaria
Check Your Understanding p.558 #1-2