CK12 Life Science

We will use only two alleles (sop+qmust equal 1), but similar equations can be written
for more alleles.

Allele frequency equals the chance of any particular gamete receiving that allele. There-
fore, when egg and sperm combine, the probability of any genotype is the product of the
probabilities of the alleles in that genotype. So:

Probability of genotypeBB=pXp=p^2 and

Probability of genotypeBb= (pXq) + (qXp) =2pqand

Probability of genotypebb=qXq=q^2

We have included all possible genotypes, so the probabilities must add to 1.0. In our example
0.49 + 2(0.21) + 0.9 = 1. Our equation becomes:

Table 7.1:

p^2 + 2 pq + q^2 = 1

frequency

of geno-

typeBB

frequency

of geno-

typeBb

frequency

of geno-

typebb

This is theHardy-Weinberg equation, which describes the relationship between allele
frequencies and genotype frequencies for a population at equilibrium.

Genetic Drift

Recall that the third requirement for Hardy-Weinberg equilibrium is a very large population
size. This is because variations in allele frequencies that occur by chance are minimal in
large populations. In small populations, random variations in allele frequencies can signifi-
cantly influence the ”survival” of any allele. Random changes in allele frequencies in small
populations is known asgenetic drift. As the population (and therefore the gene pool) is
small, genetic drift could have substantial effects on the traits and diversity of a population.
Many biologists think that genetic drift is a major cause of microevolution.

The Origin of Species

The creation of a new species is calledspeciation. Most new species develop naturally, but
humans have also artificially created new subspecies, breeds, and species for thousands of
years.

Natural selection causes beneficial heritable traits to become more common in a population,
and unfavorable heritable traits become less common. For example, a giraffe’s neck is bene-