BRIEF HISTORY OF EVOLUTIONARY THINKING

Charles Darwin (1809-1882)

did

not invent the concept of evolution (or “descent with modification,” to use his term).

To

explain the similarity of extant species and the fossils of extinct species, many pre-Darwinian

scientists had argued that species had evolved over time, in opposition to the Genesis-based theory that all species were created

by

God “in the beginning.” Darwin’s major contribution was to provide a plausible mecha- nism for evolution, natural selection, as well as abundant documentation.

(Evolution

by

natural selection was simultaneously discovered

by

Alfred Russel Wallace [1823-19131, although it was Darwin who provided most of the ammunition for the revolution in biological thinking that the discov- ery would create.)

The most influential of the pre-Darwinian evolutionists was French naturalist Jean Baptiste Lamarck (1744-1829). The essential features of Lamarck’s theory were as follows: (a) different environments required differ- ent types of adaptations;

(b)

adaptations are related to the behavior of animals; (c) as a result of use or disuse of an organ, bodily changes are brought about; and

(d)

these bodily changes are inherited.

It

is for the latter idea of the inheritance of acquired characteristics that Lamarck is best remembered (and derided) today. Lamarck’s emphasis on behavioral adapt- ability recognized the importance of the modifiability of behavior in develop- ment for evolution, and Darwin and later evolutionists adopted his notion of the inheritance of acquired characteristics (see Gottlieb, 1992;

S. J.

Gould,

1977;

Schwartz, 1999).

A

major sticking point for Darwin

(1859)

and all other 19th-century evolutionists was that he

did

not have a solid theory of inheritance. Natural selection worked on heritable variation in physical or behavioral characteris- tics, but how those characteristics were inherited was not known. In addition to advocating the inheritance of acquired characteristics, Darwin believed, as

did

most of his contemporaries, that traits of the parents blended to produce an “average” offspring.

Thus,

a male

bird

with a short beak and a female bird with a long beak would produce offspring with a medium-sized beak.

Of

course, Darwin also recognized that two parents produced offspring who varied both from the parents and from their siblings (variation was the necessary condition for natural selection); but exactly how this occurred escaped him and others of his age. It was not until the rediscovery in

1900

of Austrian botanist Gregor Mendel’s (1822-1884) research with pea plants that a plausible mechanism for passing on traits from one generation to the next was available.

Mendel postulated that traits do not blend but rather that heredity is particulate, that is, a particular trait would be represented by discrete alterna- tives, what today we call different alleles of a gene. For example, the shape of a seed may be round or wrinkled. One of these characteristics may be dominant relative to the other, so that if an allele for a round seed is paired with an allele for a wrinkled seed, the phenotype would not be an average, or blending, of the two characteristics, but would be determined

by

the dominance relation of the alleles (in this case, round is dominant). Moreover,

46 THE ORIGINS OF HUMAN NATURE

when forming gametes, these alleles segregate so that half of the sex cells of an individual would possess one allele and half the other. The seven traits Mendel identified in his pea plants all had only two levels (e.g., round vs. wrinkled seed; yellow vs. green unripe pod), and all showed a simple dominance relationship. Most characteristics are influenced

by

many differ- ent genes, and traits are not necessarily dichotomous in their distribution.

Nonetheless, the genetic age was born, and evolutionary theory has not been the same since.

Another important figure at this time was German biologist August Weissman (1834-1914), an ardent opponent of the inheritance of acquired characteristics who may have been the first to understand the significance of Mendel’s discoveries for evolutionary theory. Weissman articulated the distinction between somatic (body) cells and reproductive cells (gametes, or the germ plasma). Whatever happens to an individual during development

will

affect only its somatic cells. The germ plasma, being independent of somatic cells, is not affected. The same genes that individuals received from their parents are passed along to their offspring (in different combinations, of course), unaffected by life experiences.

Exceptions to this were found in the form of mutations, variations in genes in the germ line that could occur spontaneously or be induced by abnormal environmental conditions (e.g., heat shock to fruit fly larvae).

Research on fruit flies (Drosophila) led

by

American geneticist Thomas Hunt Morgan’s (

1866-1 945)

lab at Columbia University illustrated dramatically the phenomenon of mutation and genetic inheritance more generally.

Initially, the new field of genetics was seen as providing an alternative to Darwin’s ideas of evolution

by

natural selection: Species change as a result of mutations, with natural selection playing only a minor role (see Schwartz,

1999).

But researchers from a variety of disciplines within biology began to merge on a common view of evolution, one that incorporated the new science of genetics with Darwin’s idea of natural selection. This new perspective is termed the modern synthesis, or neo-Darwinism, and it became the dogma of evolutionary theory (Dobzhansky,

1937;

Mayr, 1942; Simpson, 1944; see also Gottlieb, 1992; Gould, 1977; Schwartz, 1999). The modern synthesis adopted Weissman’s idea of the separation of the gametes and somatic cells. Inheritance, and thus genetic variation, is found only within the germ line and is not influenced

by

experience. Evolution takes place gradually over eons

by

the accumulation of random mutations, each of which is maintained or excluded in the genome

by

the process of natural selection. A n important new insight of the modern synthesis concerned the focus on populations. Evolution must be considered in terms of changes in the frequencies of individual genes in populations of organisms.

This

should not be confused with group selection, in which natural selection operates for the “good of the group.’’ Individuals possess genes, but it is populations of

individuals that reproduce and thus evolve. Mutations existing in individu- als would not have any consequences to evolution unless the resulting trait is expressed in populations of similar organisms

(G. C.

Williams,

1966).

Evolution, then, becomes the change in genetic composition within populations.

The modem synthesis’s emphasis on the separation of the somatic and germ line essentially afforded no role for development in evolution.

Development may result from the differential expression of genes in interac- tion with the environment, but such effects are governed directly

by

the genome, and variations in development, brought about by variations in the environment, cannot affect the germ line and thus cannot exert any influence on evolution (see Gottlieb, 1992).

The modern synthesis remains intact today, although evolutionary theory has not stood still. For example, the theorizing of evolutionary biolo- gist William Hamilton (1964) and the concept of inclusive fitness (see chapter

2),

and the theories of evolutionary biologist Robert Trivers (1971, 1972,

1974;

see chapters

8

and

9),

among others, have changed the focus of evolutionary theory. The advent of sociobiology (Dawkins,

1976; E. 0.

Wilson,

1975),

with its emphasis on explaining complex social behavior, such as altruism, in terms of evolutionary principles, focused attention on the evolution of behavior, something that many developmental psychologists found appealing (MacDonald,

1988).

But the basic tenets of the modern synthesis (other than the possibility that evolution is not as gradual as Darwin originally postulated, see discussion of punctuated equilibrium below) have not been seriously questioned, and development has not been given a promi- nent role in evolutionary explication. Evolutionary psychology has been no exception (see Barkow, et al., 1992; Buss,

1999).