BIOL 1407
Instructor: Mr. Sanregret
Review Sheet Chapter 23 and Exercise 23 in the lab manual
1) The modern theory of evolution, the “synthetic theory of evolution,” developed in the early 20th
century when advances in our understanding of genetics helped explain the role of heredity in
evolution.
2) A discrete trait comes in a limited number of forms (e.g. blood type, eye color). The different forms
are called morphs. If there are two or more morphs of a particular trait, it is called a polymorphism.
E.g.,. eye color is a polymorphism. In a population where all the members are blue eyed (i.e. the gene
is fixed with only one type of allele), eye color would not be a polymorphism in that situation.
Polymorphisms are usually controlled by only a few genes (perhaps 1-4).
3) Traits that vary along a continuum (e.g. size, skin complexion), are called quantitative traits. These
traits are usually controlled by many different genes, each one of which may be “on” or “off.”
4) Population: group of individuals of the same species in a localized area (capable of interacting and
interbreeding).
5) Species: for purposes of population genetics, a species is best defined as a population or group of
populations that are potentially capable of interbreeding an producing viable, fertile offspring, and also
not able to interbreed and produce viable fertile offspring with other organisms. We will talk more
about the definition of species in Ch24.
6) Gene pool: total aggregate of genes; i.e. all alleles at all loci. In a diploid population, there will
usually be two versions of each gene for each individual. We often restrict our discussion of a gene
pool to a specific gene or genes (such as when figure our allele frequencies).
7) Allele frequency: the proportion of the genes in the gene pool that are a particular allele. (i.e. p and q)
If there are 500 individuals in a population, then there are 1000 alleles for any given autosomal gene.
If 600 of those alleles are dominant, then p=0.6 and q=0.4. If there are only two kinds of alleles, then
p + q =1 will always hold true.
8) Genotypic frequency: The frequency of one genotype (either homozygous dominant, heterozygous,
or homozygous recessive in the case we are discussing). Each individual has one genotype per trait, so
the total number of genotypes in a population of 500 is 500.
9) Hardy-Weinberg equilibrium: A population that is evolutionarily static. That is, allele frequencies
do not change as generations pass. If a population is in Hardy-Weinberg equilibrium, genotypic
frequencies can be predicted by p2 + 2pq + q2 = 1.
10) Phenotypic frequency: The frequency of a phenotype (the visible trait). In the cases we have used,
the frequency of one phenotype is equal to the sum of the homozygous dominant and heterozygous
genotypic frequencies. The other phenotypic frequency is equal to the frequency of the homozygous
recessive genotype (q2 if in Hardy-Weinberg equilibrium). Examples: free earlobes and attached
earlobes; hairs on 2nd phalange and no hairs on 2nd phalange; normal pigmentation and albino; smooth
pea and wrinkled pea.
11) When you are given phenotypic information (e.g. 670 free earlobe people and 330 attached earlobe
people) and asked to use the Hardy-Weinberg equation calculate genotypic frequencies and or allele
frequencies, you always start by figuring out the frequency of the recessive phenotype. This frequency
is equal to the frequency of the homozygous recessive genotype, q 2. You then take the square root of
q2 to get q. You then figure p as 1-q. Using p and q you can then calculate p2 and 2pg.
12) According to the Hardy-Weinberg Theorem, allele frequencies will not change unless one or more of
the following five conditions is active: 1) Genetic drift (may occur in small populations, much less
likely in large populations); 2) migration in or out of the population; 3) mutation; 4) Non-random
mating (or sexual selection); 5) natural selection. This means that when microevolution occurs, it
must be caused by one of these factors.
13) Genetic drift usually occurs in actual populations under two circumstances; 1) the Bottleneck Effect,
where a large population experiences a die-off reducing it to a small population; 2) the Founder
Effect, when a small group breaks off from a larger population and colonizes a new habitat where gene
flow with the original population is no longer likely.
14) Unlike other forces that cause changes in allele frequencies, natural selection can change allele
frequencies in an adaptive way.
15) Gene flow (migration) mixes alleles from different populations, and thus tends to homogenize the
allele frequencies of different populations.
16) Mutation does not usually cause large changes in allele frequencies by itself, but mutation is unique in
that it can generate alleles that did not previously exist in the population. If such an allele is adaptive,
it will tend to increase in frequency.
17) Natural selection needs genetic variation (and heritable traits) in order to function.
18) Mutations are usually neutral or harmful, but on rare occasions beneficial. Mutations are more likely
to be beneficial in a changing environment.
19) Fitness is the likelihood that an individual will survive and reproduce. Fitness can also means the
likelihood that a particular gene will be passed on.
20) Sexual selection is a form of non-random mating and occurs when organisms prefer mates with certain
heritable traits. Frequently, these traits are not adaptive and may even reduce fitness. It is generally
believed that these sexually selected traits demonstrate to mates that the organism has “fitness to
spare.” E.g. a male peacock that can’t maintain a showy tail probably has difficulties in finding food
or avoiding predators, or some other limitation that makes them less fit.