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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.
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