Biol 1441- Principles of Biology II

Evolution by Natural Selection
Historical Perspective & Hardy-Weinberg

Historical Perspective
Darwin and Wallace were not the first to conceive of  organisms changing by the process of evolution.  Concepts about evolution can be traced back to ancient Greek writings.  Others, in history, had theories about evolution.  One theory that especially caught Darwin's and Wallace's interest was Lamarck's theory of Evolution based on Acquired Characteristics.  Darwin's Origin of Species outlines with detailed examples a mechanism for Evolution which is Natural Selection.   Independently, both Darwin and Wallace conceived of natural selection as the mechanism for evolution by observing species in isolation.  The concept of natural selection as "survival of the fittest" refers to individuals best adapted to their environment will have a better chance of survival and reproduction

Evolutionary biologists continue to test the theory of evolution by natural selection and, though refined, no one has put forth compelling evidence to reject natural selection.  It was not until the rise of modem alleletics that this process was understood as the cause for certain alleles remaining in population while others disappeared.  Further reading on natural selection in Anole lizards of the Carribean by J. B. Losos and K. deQueiroz, 1997, Darwin's Lizards, Natural History 106 (11):38.

Hardy-Weinberg Equilibrium
If individuals in a population mate randomly, without regard to genetic constitution, the offspring can have any combination of genes or alleles from the gene pool. That is, whatever alleles are present in the population will have equal chance of appearing from generation to generation. The population is said to be in equilibrium. This generalization is known as the Hardy-Weinberg principle. It operates only when there is random mating, no introduction of new alleles by mutation or migration of alleles into the population (gene flow), and no natural selection or chance change in allele frequency (genetic drift). If any of these conditions are not present, inheritable changes result and evolution occurs.

Allele frequencies are expressed mathematically from 0 to 1. An allele frequency of 1 means that all individuals in the population express that allele. Conversely, an allele frequency of 0 means that no individual expresses the allele. More commonly, alleles will have different frequencies. Within a population the allele frequencies can differ between dominant alleles and recessive alleles.  The Hardy-Weinberg equation for the frequency of alleles in a gene pool is:

(p + q)² = p²+ 2pq + q² = 1, where,
p= the frequency of a dominant allele, A
q= the frequency of a recessive allele, a

Alleles in a gene pool of a population, for example, might have an allele frequency (p) of 0.80 for A and allele frequency (q) of 0.20 for a. Since A has a frequency of 0.80, when it combines with another A of 0.80, the frequency of the genotype for AA will be 0.64. See Punnett square below.

The frequency of genotypes will be:

p²+ 2pq + q² or 0.64 + 2 (0.16) + 0.40 = 1.00.

If an allele is deleterious (lethal) its frequency will gradually decrease although it will never disappear from the population.