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Evolution__3rd_Edition

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14 PART 1 / Introduction<br />

Figure 1.7<br />

Early Mendelians and<br />

biometricians. (a) Early<br />

Mendelians studied large<br />

differences between organisms,<br />

and thought that evolution<br />

happened when a new<br />

species evolved from a<br />

“macromutation” in its<br />

ancestor. (b) Biometricians<br />

studied small interindividual<br />

differences, and explained<br />

evolutionary change by the<br />

transition of whole<br />

populations. Mendelians were<br />

less interested in the reasons for<br />

small interindividual variations.<br />

The figure is a simplification a<br />

no historic debate between two<br />

groups of scientists lasting for<br />

three decades can be fully<br />

represented in a single<br />

diagrammatic contrast.<br />

Biometricians rejected Mendelian<br />

theory<br />

Frequency<br />

Frequency<br />

(a) (b)<br />

Main form of<br />

ancestral species<br />

Macromutation<br />

Character (e.g., height)<br />

Character (e.g., height)<br />

suggested that evolution proceeded in big jumps, by macromutations. A macromutation<br />

is a large and genetically inherited change between parent and offspring (Figure 1.7a).<br />

(Chapters 10 and 20 discuss various perspectives on the question of whether evolution<br />

proceeds in small or large steps.)<br />

Mendelism was not universally accepted in the early twentieth century, however.<br />

Members of the other principal school, which rejected Mendelism, called themselves<br />

biometricians; Karl Pearson was one of the leading figures. Biometricians studied<br />

small, rather than large, differences between individuals and developed statistical techniques<br />

to describe how frequency distributions of measurable characters (such as<br />

height) passed from parent to offspring population. They saw evolution more in terms<br />

of the steady shift of a whole population rather than the production of a new type<br />

from a macromutation (Figure 1.7b). Some biometricians were more sympathetic to<br />

Darwin’s theory than were the Mendelians. W.F. Weldon, for instance, was a biometrician,<br />

and he attempted to measure the amount of selection in crab populations on the<br />

seashore.<br />

1.3.4 The modern synthesis<br />

By the second decade of the twentieth century, research on Mendelian genetics had<br />

already become a major enterprise. It was concerned with many problems, most of<br />

which are more to do with genetics than evolutionary biology. But within the theory of<br />

evolution, the main problem was to reconcile the atomistic Mendelian theory of genetics<br />

with the biometrician’s description of continuous variation in real populations.<br />

This reconciliation was achieved by several authors in many stages, but a 1918 paper by<br />

Frequency<br />

Frequency<br />

Small interindividual<br />

differences<br />

Character (e.g., height)<br />

Character (e.g., height)<br />

..

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