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Haldane and R. A. Fisher and by the biologists Theodosius Dobzhansky , Julian

Huxley, Ernst Mayr, George Gaylord SIMPSON, Sewall Wright, Berhard Rensch, and G.

Ledyard Stebbins. According to the theory, variability among individuals in a

population of sexually reproducing organisms is produced by mutation and genetic

recombination. The resulting genetic variability is subject to natural selection

in the environment.

POPULATION GENETICS

The word population is used in a special sense to describe evolution.

The study of single individuals provides few clues as to the possible outcomes

of evolution because single individuals cannot evolve in their lifetime. An

individual represents a store of genes that participates in evolution only when

those genes are passed on to further generations, or populations. The gene is

the basic unit in the cell for transmitting hereditary characteristics to

offspring. Individuals are units upon which natural selection operates, but the

trend of evolution can be traced through time only for groups of interbreeding

individuals, populations can be analyzed statistically and their evolution

predicted in terms of average numbers.

The Hardy-Weinberg law, which was discovered independently in 1908

by a British mathematician, Godfrey H. Hardy, and a German physician, Wilhelm

Weinberg, provides a standard for quantitatively measuring the extent of

evolutionary change in a population. The law states that the gene frequencies,

or ratios of different genes in a population, will remain constant unless they

are changed by outside forces, such as selective reproduction and mutation. This

discovery reestablished natural selection as an evolutionary force. Comparing

the actual gene frequencies observed in a population with the frequencies

predicted, by the Hardy-Weinberg law gives a numerical measure of how far the

population deviates from a nonevolving state called the Hardy-Weinberg

equilibrium. Given a large, randomly breeding population, the Hardy-Weinberg

equilibrium will hold true, because it depends on the laws of probability.

Changes are produced in the gene pool through mutations, gene flow, genetic

drift, and natural selection.

Mutation

A mutation is an inheritable change in the character of a gene.

Mutations most often occur spontaneously, but they may be induced by some

external stimulus, such as irradiation or certain chemicals. The rate of

mutation in humans is extremely low; nevertheless, the number of genes in every

sex cell, is so large that the probability is high for at least one gene to

carry a mutation.

Gene Flow

New genes can be introduced into a population through new breeding

organisms or gametes from another population, as in plant pollen. Gene flow can

work against the processes of natural selection.

Genetic Drift

A change in the gene pool due to chance is called genetic drift. The

frequency of loss is greater the smaller the population. Thus, in small

populations there is a tendency for less variation because mates are more

similar genetically.

Natural Selection

Over a period of time natural selection will result in changes in

the frequency of alleles in the gene pool, or greater deviation from the

nonevolving state, represented by the Hardy-Weinberg equilibrium.

NEW SPECIES

New species may evolve either by the change of one species to

another or by the splitting of one species into two or more new species.

Splitting, the predominant mode of species formation, results from the

geographical isolation of populations of species. Isolated populations undergo

different mutations, and selection pressures and may evolve along different

lines. If the isolation is sufficient to prevent interbreeding with other

populations, these differences may become extensive enough to establish a new

species. The evolutionary changes brought about by isolation include differences

in the reproductive systems of the group. When a single group of organisms

diversifies over time into several subgroups by expanding into the available

niches of a new environment, it is said to undergo Adaptive Radiation .

Darwin’s Finches, in the Galapagos Islands, west of Ecuador,

illustrate adaptive radiation. They were probably the first land birds to reach

the islands, and, in the absence of competition, they occupied several

ecological habitats and diverged along several different lines. Such patterns of

divergence are reflected in the biologists’ scheme of classification of

organisms, which groups together animals that have common characteristics. An

adaptive radiation followed the first conquest of land by vertebrates.

Natural selection can also lead populations of different species

living in similar environments or having similar ways of life to evolve similar

characteristics. This is called convergent evolution and reflects the similar

selective pressure of similar environments. Examples of convergent evolution are

the eye in cephalod mollusks, such as the octopus, and in vertebrates; wings in

insects, extinct flying reptiles, birds, and bats; and the flipperlike

appendages of the sea turtle (reptile), penguin (bird), and walrus (mammal).

MOLECULAR EVOLUTION

An outpouring of new evidence supporting evolution has come in the

20th century from molecular

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