II. Natural selection.

III. Evolution.

Terms

A. Linnaeus (Carl von Linne, 1707-78) devised a system of classification and nomenclature in which every organism belongs to a species (s. and pl.), the basic unit of classification of life forms.

B. Similar species are grouped into the same genus (pl. genera), similar genera into the same family, etc. The orders of grouping and the names of the groups to which we belong are:

1. Kingdom Animalia
2. Phylum (pl. phyla) Chordata
3. Class Mammalia
4. Order Primates
5. Family Hominidae
6. Genus Homo
7. Species sapiens

C. Members of the same species can interbreed to produce fertile offspring. Members of different closely related species may sometimes produce hybrid offspring, but they are infertile or have reduced fertility. Species are reproductively isolated from each other. Eg: horse × donkey —> mule.

A. A number of scientists, including Darwin's father, proposed that evolution is responsible for the relationships among organisms. However, they were unable to explain why it occurred.

B. Charles Darwin (1809-82) and Alfred Russel Wallace (1823-1913) independently proposed the theory of natural selection, first presented publicly in 1858.

1. Individuals with greater biological fitness leave more offspring on the average than do others. Such traits as robust health, physical fitness, beauty, intelligence, and longevity mean nothing unless they increase the likelihood of leaving offspring.

2. Those better adapted to a particular environment are generally more fit.

3. In modern terms, fitness is measured only by the number of copies of genes left to future generations.

A. Although mutation produces the variation necessary for evolution, mutation itself is random and is too small to be an evolutionary force.

B. Change often occurs due to selection that favors better adapted genotypes and therefore the alleles associated with that genotype.

1. Selection against less favorable dominant alleles is rapid and efficient, since every individual with the dominant allele expresses it in the phenotype where it is subject to selection.

2. Selection against less favorable recessive alleles is less effective. Most of the recessive alleles are in heterozygous combination and are not subject to selection, which can act only against expressed alleles, i.e. phenotype.

C. As alleles are removed by selection, new ones are formed by mutation. Eventually, an equilibrium is reached whereby the two rates are equal.

D. Genetic drift is defined as a change in allele frequencies due to chance. It is especially characteristic of small breeding populations.

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