Standard Set 7. Evolution (Population genetics)
This discussion applies to Standards Sets 7 and 8. Students in grades nine through twelve should be
readyto explore and understand the concept of biological evolution from its basis in genetics. The
synthesis of genetics, and later of molecular biology, with the Darwin-Wallace theory of natural selection
validated the mechanism of evolution and extended its scientific impact. Students need to understand that
the same evolutionary mechanisms that have affected the rest of the living world have also affected the
Students need to understand that a theory in science is not merely a hypothesis or a guess, but a unifying
explanation of observed phenomena. Charles Darwinâ€™s theory of the origin of species by natural
selection is such an explanation. Even though biologists continue to test the boundaries of this theory
today, their investigations have not found credible evidence to refute the theory. Scientists have also had
many opportunities to demonstrate the gradual evolution of populations in the wild and in controlled
laboratory settings. As more populations of organisms are studied at the level of DNA sequence and as
the fossil record improves, the under-standing of species divergence has become clearer.
7. The frequency of an allele in a gene poll of a population depends on many
factors and may be stable or unstable over time. As a basis for understanding
7. a. Students know why natural selection act on the phenotype rather that the genotype of an
Natural selection works directly on the expression or appearance of an inherited trait, the phenotype,
rather than on the gene combination that produces that trait, the genotype. The influence of a dominant
allele for a trait over a recessive one in the genotype determines the resulting phenotype on which natural
7. b. Students know why alleles that are lethal in a homozygous individual may be carried in a
heterozygote and thus maintained in a gene pool.
Two types of allele pairings can occur in the genotype: homozygous (pairing two of the same alleles,
whether dominant, codominant, or recessive) and heterozygous (pairing of two different alleles).
Recessive lethal alleles (e.g., Tay-Sachs disease) will, by definition, cause the death of only the
homozygous recessive individual. Healthy heterozygous individuals will also contribute the masked
recessive gene to the populationâ€™s gene pool, allowing the gene to persist.
7. c. Students know new mutations are constantly being generated in a gene pool.
Mutation is an important source of genetic variation within a gene pool. These random changes take the
form of additions, deletions, and substitutions of nucleotides and of rearrangements of chromosomes. The
effect of many mutations is minor and neutral, being neither favorable nor unfavorable to survival and
reproduction. Other mutations may be beneficial or harmful. The important principle is that culling, or
selective breeding, cannot eliminate genetic diseases or unwanted traits from a population. The trait
constantly reappears in the population in the form of new, spontaneous mutations.
7. d. Students know variation within a species increases the likelihood that at least some
members of a species will survive under changed environmental conditions.
As environmental factors change, natural selection of adaptive traits must also be realigned. Variation
within a species stemming either from mutation or from genetic recombination broadens the opportunity
for that species to adapt to change, increasing the probability that at least some members of the species
will be suitably adapted to the new conditions. Genetic diversity promotes survival of a species should the
environment change significantly, and sameness can mean vulnerability that could lead to extinction.
7. e.* Students know the conditions for Hardy-Weinberg equilibrium in a population and why
these conditions are not likely to appear in nature.
The principle of Hardy-Weinberg equilibrium, derived in 1908, asserts that the genetic structure of a
nonevolving population remains constant over the generations. If mating in a large population occurs
randomly without the influence of natural selection, the migration of genes from neighboring populations,
or the occurrence of mutations, the frequency of alleles and of genotypes will remain constant over time.
Such conditions are so restrictive that they are not likely to occur in nature precisely as predicted, but the
Hardy-Weinberg equilibrium equation often gives an excellent approximation for a limited number of
generations in sizeable, randomly mating populations. Even though genetic recombination is taken into
account, mutations, gene flow between populations, and environmental changes influencing pressures of
selection on a population do not cease to occur in the natural world.
7. f.* Students know how to solve the Hardy-Weinberg equation to predict the frequency of
genotypes in a population, given the frequency of phenotypes.
The Hardy-Weinberg equilibrium equation can be used to calculate the frequency of alleles and
genotypes in a populationâ€™s gene pool. When only two alleles for a trait occur in a population, the
letter p is used to represent the frequency of one allele, and the letter q is used to represent the frequency
of the other. Students should agree first that the sum of the frequencies of the two alleles is 1, and this
equation is written p + q = 1. That is, the combined frequencies of the alleles account for all the genes for
a given trait.
Students should then consider the possible combinations of alleles in a diploid organism (the genome of a
diploid organism consists of two copies of each chromosome). An individual could be homozygous for
one allele (pp) or homozygous for the other (qq) or heterozygous (either pq or qp). These diploid
genotypes will apÂ¬pear at frequencies that are the product of the allele frequencies (e.g., the frequency
of a diploid pp individual is p2, and the frequency of a diploid qq individual is q 2).
The heterozygotes are of two varieties, pq and qp (because the p allele might have been inherited from
either parent), but the products of frequency pq and qp are the same. Therefore, the frequency of
heterozygotes can simply be expressed as 2pq. The sum of the frequencies of the homozygous and
heterozygous individuals must equal 1, since all individuals have been accounted for. These principles are
usually expressed as the equation p2 + 2pq + q2 = 1. Both equations represent different statements. The
first (p + q = 1) is an accounting of the two types of alleles in the population, and the second (p2 + 2pq +
q2 = 1) is an accounting of the three distinguishable genotypes.
If the allele frequencies are known (e.g., if p = 0.1 and q = 0.9) and Hardy-Weinberg equilibrium is
assumed, then the frequencies p2, 2pq, and q2 are respectively 0.01, 0.18, and 0.81. That is, 81 percent
of individuals would be homozygous qq. If p were a dominant (but nonselective) allele, then p2 + 2pq, or
19 percent of the population, would express the dominant phenotype of the p allele.
The calculation can be used in reverse as well. If Hardy-Weinberg equilibrium conditions exist and 81
percent of the population expresses the qq recessive phenotype, then the allele frequency q is the square
root of 0.81, and the rest of the terms can be calculated in a straightforward fashion.
Students can convince themselves of the state of equilibrium by constructing a Punnett Square that
assumes random mating. The scenario might be a mass spawning of fish, in which 100,000 eggs and
sperm are mixed in a stream and meet with each other randomly to form zygotes. Students can calculate
the fraction of p and q type gametes in the stream by thinking through the types of gametes produced by
heterozygous and homozygous adult fish. (For this exercise to work, the genotype distribution of adults
must agree with Hardy-Weinberg equilibrium.) With the frequencies or numbers of each type of zygote
calculated in the cells of a Punnett Square, students will see that equilibrium is preserved. Frequencies of
alleles and genotypes, which are the genetic structure of the study population, would remain constant for
generations under the premise of Hardy-Weinberg equilibrium.