Evolution__3rd_Edition

14 PART 1 / Introduction
Figure 1.7
Early Mendelians and
biometricians. (a) Early
Mendelians studied large
differences between organisms,
and thought that evolution
happened when a new
species evolved from a
“macromutation” in its
ancestor. (b) Biometricians
studied small interindividual
differences, and explained
evolutionary change by the
transition of whole
populations. Mendelians were
less interested in the reasons for
small interindividual variations.
The figure is a simplification a
no historic debate between two
groups of scientists lasting for
three decades can be fully
represented in a single
diagrammatic contrast.
Biometricians rejected Mendelian
theory
Frequency
Frequency
(a) (b)
Main form of
ancestral species
Macromutation
Character (e.g., height)
Character (e.g., height)
suggested that evolution proceeded in big jumps, by macromutations. A macromutation
is a large and genetically inherited change between parent and offspring (Figure 1.7a).
(Chapters 10 and 20 discuss various perspectives on the question of whether evolution
proceeds in small or large steps.)
Mendelism was not universally accepted in the early twentieth century, however.
Members of the other principal school, which rejected Mendelism, called themselves
biometricians; Karl Pearson was one of the leading figures. Biometricians studied
small, rather than large, differences between individuals and developed statistical techniques
to describe how frequency distributions of measurable characters (such as
height) passed from parent to offspring population. They saw evolution more in terms
of the steady shift of a whole population rather than the production of a new type
from a macromutation (Figure 1.7b). Some biometricians were more sympathetic to
Darwin’s theory than were the Mendelians. W.F. Weldon, for instance, was a biometrician,
and he attempted to measure the amount of selection in crab populations on the
seashore.
1.3.4 The modern synthesis
By the second decade of the twentieth century, research on Mendelian genetics had
already become a major enterprise. It was concerned with many problems, most of
which are more to do with genetics than evolutionary biology. But within the theory of
evolution, the main problem was to reconcile the atomistic Mendelian theory of genetics
with the biometrician’s description of continuous variation in real populations.
This reconciliation was achieved by several authors in many stages, but a 1918 paper by
Frequency
Frequency
Small interindividual
differences
Character (e.g., height)
Character (e.g., height)
..