Books and articles on the Extended Evolutionary Synthesis:
Before Darwin, transmutation of species generally meant that a species as a whole changed into a more complex species through some (unspecified) process. Darwin intended no such meaning when he introduced evolutionary theory in On the Origin of Species (1859). First, he realized that change was not necessarily a process of increasing complexity or perfection. Second, he proposed a specific mechanism that explained why new species were different in appearance and behavior — natural selection. Third, and most important, Darwin saw that the origin of a new species did not involve an entire species' undergoing change. He saw that the origination of a new species might occur in only a part of the parental species — in a subpopulation. The remaining populations of the original species could remain unchanged.
Species to Darwin were just permanent varieties. The species he had studied in the wild, and those he read about in the work of others, and most of all his alternately loved and hated barnacles (he spent 9 years cataloguing modern and fossil barnacles before publishing the Origin), all exhibited variation, including what were then called "races", or subspecies. This is the meaning of "races" in the subtitle of the Origin ("The preservation of favoured races in the struggle for life") — varieties within a species.
The fact that speciation occurred in subpopulations implied that species shared common ancestry with others that had split off from the same or related parental stock. So Darwin concluded that there was an "insensible series" of varieties — or what we would speak of today as genetic variants — from geographic differences to fully distinct but interfertile varieties or races, to distinct and mutually infertile species. This conclusion is not generally accepted today. The contemporary view is that species arise not from genetically distinct races, but from local and isolated populations that may initially be much the same genetically as the main populations of a species.
However, Darwin did not use the word "evolution" in the first edition of the Origin (although he used "evolving" in the sense of "unfolding" in the final paragraph). Only later, in the Descent of Man (1871; see Gould 2000), was Darwin forced to adopt the term "evolution", partly because it was in common use, and also because his associate Herbert Spencer had used the term two years before Darwin went public on evolution. The fact that the ideas he outlined are often included in the general understanding of "evolution" causes some confusion, which can be alleviated by carefully distinguishing among the components of Darwin's theory.
The main components of Darwin's theory are
We conclude with some proposed revisions of modernization theory.
Fisher may be historically inexact, but there is a solid point here: natural selection is what many people mean by "evolution". This impression is reinforced by the writings of such luminaries as Richard Dawkins, who makes selection the core of his view of evolution.
But natural selection is also the mechanism by which species do not change. Selection only forces change if a population is not well-suited to competing for resources or overcoming risks in its local environment. If a species is well-adapted, then selection will inhibit change; this is called balancing selection. Thus, we have to distinguish between the mechanism that is sometimes an agent for change from the process of change itself (see Figure 1).
Before Darwin, the prevailing view was that species are types. A type was both a means of identifying and classifying an organism and also a force that caused it to exhibit certain physical characteristics. The type made that organism what it was. Organisms that varied too far from the type were "monsters", degradations of the type (for example, a French biologist contemporary of Linnaeus, Georges Buffon, thought that evolution was a degradation of a number of created types; Lovejoy 1959). Entire species were called monsters, because they were too far removed from the central type of a family or genus of organisms. Such central types were called the archetype by Richard Owen.
Darwin imagined the archetype as an actual historical ancestor — that the variation present in an ancestral population could be the basis for descendant populations both to share important identifying characteristics and to differ significantly in some ways from the ancestor. Furthermore, he saw that neither the archetype nor the type of a single species exerted any influence on the subsequent history of a lineage. For Darwin, types were just the most common form of a species or genus. The type might remain stable, or it might change. In modern terms, we would say that the type is just the mode of the distribution of species' characters. What causes the type to remain stable or to change is another matter. Whether the type changes or remains stable, the cause might be selection or some other process, such as random drift.
Natural selection deals with frequency changes brought about by differences in ecology among heritable phenotypes; evolution includes this as well as random effects and the origin of these variants (Endler 1992: 221).
Understanding Modern Evolutionary Synthesis - …
Weismann's idea was that the relationship between the hereditary material, which he called the (de: Keimplasma), and the rest of the body (the ) was a one-way relationship: the germ-plasm formed the body, but the body did not influence the germ-plasm, except indirectly in its participation in a population subject to natural selection. Weismann was translated into English, and though he was influential, it took many years for the full significance of his work to be appreciated. Later, after the completion of the modern synthesis, the term neo-Darwinism would come to be associated with its core concept of evolution being driven by natural selection acting on variation produced by genetic mutation and recombination (see ).
The modern evolutionary synthesis is about evolution
Mendelism is now seen as an essential part of the theory of evolution. Mendelian analysis does not merely explain the distributive hereditary mechanism: it also, together with selection, explains the progressive mechanism of evolution (1942: 26)