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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Introduction

The modern theory of evolution is so completely identified with the name of Charles Darwin (1809–1882) that many people think that the concept of organic evolution was first proposed by Darwin, but that is certainly not the case. Most scholars had abandoned the notion of fixed species, unchanged since their origin in a grand creation of life, long before publication of Darwin’s The Origin of Species in 1859. By that time, most biologists agreed that new species arise through some process of evolution from older species; the problem was to explain how this evolution could occur.

Darwin’s theory of the mechanism of evolution begins with the variation that exists among organisms within a species. Individuals of one generation are qualitatively different from one another. Evolution of the species as a whole results from the differential rates of survival and reproduction of the various types, so the relative frequencies of the types change over time. Evolution, in this view, is a sorting process.

For Darwin, evolution of the group resulted from the differential survival and reproduction of individual variants already existing in the group—variants arising in a way unrelated to the environment but whose survival and reproduction do depend on the environment.

MESSAGE

Darwin proposed a new explanation to account for the accepted phenomenon of evolution. He argued that the population of a given species at a given time includes individuals of varying characteristics. The population of the next generation will contain a higher frequency of those types that most successfully survive and reproduce under the existing environmental conditions. Thus, the frequencies of various types within the species will change over time.

There is an obvious similarity between the process of evolution as Darwin described it and the process by which the plant or animal breeder improves a domestic stock. The plant breeder selects the highest-yielding plants from the current population and (as far as possible) uses them as the parents of the next generation. If the characteristics causing the higher yield are heritable, then the next generation should produce a higher yield. It was no accident that Darwin chose the term natural selectionto describe his model of evolution through differential rates of reproduction of different variants in the population. As a model for this evolutionary process, he had in mind the selection that breeders exercise on successive generations of domestic plants and animals.

We can summarize Darwin’s theory of evolution through natural selection in three principles:

1.

Principle of variation.Among individuals within any population, there is variation in morphology, physiology, and behavior.

2.

Principle of heredity.Offspring resemble their parents more than they resemble unrelated individuals.

3.

Principle of selection.Some forms are more successful at surviving and reproducing than other forms in a given environment.

Clearly, a selective process can produce change in the population composition only if there are some variations among which to select. If all individuals are identical, no amount of differential reproduction of individuals can affect the composition of the population. Furthermore, the variation must be in some part heritable if differential reproduction is to alter the population’s genetic composition. If large animals within a population have more offspring than do small ones but their offspring are no larger on average than those of small animals, then no change in population composition can occur from one generation to another. Finally, if all variant types leave, on average, the same number of offspring, then we can expect the population to remain unchanged.

MESSAGE

Darwin’s principles of variation, heredity, and selection must hold true if there is to be evolution by a variational mechanism.

The Darwinian explanation of evolution must apply to two different aspects of the history of life. One is the successive change of form and function that occurs in a single continuous line of descent time,phyletic evolution.Figure 26-1 shows such a continuous change over a period of 40 million years in the size and curvature of the left shell of the oyster,Gryphea.The other is thediversificationthat occurs among species: in the history of life on earth, there are many different contemporaneous species having quite different forms and living in different ways. Figure 26-2 shows some of the variety of bivalve mollusc forms that existed at various times in the past 130 million years. Every species eventually becomes extinct and more than 99.9 percent of all the species that have ever existed are already extinct, yet the number of species and the diversity of their forms and functions have increased in the past billion years. Thus species not only must be changing, but must give rise to new and different species in the course of evolution. Both of these processes are the consequences of heritable variation within populations. Heritable variation provides the raw material for successive changes within a species and for the multiplication of new species. The basic mechanisms of those changes (as discussed in Chapter 24) are the origin of new variation by various kinds of mutational mechanisms, the change in frequency of alleles by selective and random processes, the possibility of divergence of isolated local populations because the selective forces are different or because of random drift, and the reduction of variation between populations by migration. From those basic mechanisms, population genetics, as discussed in Chapter 24, derives a set of principles governing changes in the genetic composition of populations. The application of these principles of population genetics provides an articulated theory of evolution.

Figure 26-1. Changes in shell size and curvature in the bivalve mollusc Gryphaea during its phyletic evolution in the early Jurassic.

Figure 26-1

Changes in shell size and curvature in the bivalve mollusc Gryphaea during its phyletic evolution in the early Jurassic. Only the left shell is shown. In each case, the shell back and a longitudinal section through it are illustrated. (After A. Hallam, (more...)

Figure 26-2. A variety of bivalve mollusc shell forms that have appeared in the past 300 million years of evolution.

Figure 26-2

A variety of bivalve mollusc shell forms that have appeared in the past 300 million years of evolution. (After C. L. Fenton and M. A. Fenton, The Fossil Book, Doubleday, 1958.)

MESSAGE

Evolution, under the Darwinian scheme, is the conversion of heritable variation between individuals within populations into heritable differences between populations in time and in space, by population genetic mechanisms.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21875

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