Why didn't Darwin discover Mendel's laws?

Abstract

Darwin's focus on small quantitative variations as the raw material of evolution may have prevented him from discovering the laws of inheritance.

Figure 1

Gregor Mendel: a commemorative stamp issued by the Free City of Danzig in 1939. The texts above the picture read "Healthy children" and "Happy future".

Figure 2

Reversion to the ancestral type. Two non-allelic, recessive white mutants in domestic fowl (top) complement each other to generate perfect wild-type male (bottom right) and female (bottom left) progeny. Reversion was of polemical value for Darwin in indicating that highly divergent domesticated stocks had been derived from a single ancestral species, but contributed to the widespread confusion over the laws of inheritance. From [16] plate 4 between pages 100 and 101.

Figure 3

Fragmentation of a qualitative character in Primula sinensis. The F1 (3) of a cross between the two parental varieties (1 and 2) is constant and somewhat intermediate in character, although white. The F2 progeny (bottom three rows) are highly variegated, atomizing the overall phenotypic difference between the two parents into a remarkable number of distinct traits. Phenomena like this bedeviled the efforts of many breeders, until Mendel, to find simple laws of inheritance. From [16] plate 7 between pages 298 and 299.

Figure 4

Wild-type and peloric flowers of Antirrhinum majus. Darwin noted the near-complete dominance of the wild type and the segregation of the recessive allele in the F2. This is a floral trait determined by a single recessive allele and could have given Darwin the Mendelian ratios if he had pursued his breeding experiments. Photograph kindly provided by and copyright Enrico Coen. Reproduced with permission.

Figure 5

Drawings taken from Darwin's The Different Forms of Flowers on Plants of the Same Species [5], illustrating the clear morphological distinction between the two floral types of Primula flowers. The 'thrum' (short-styled) form is determined by a dominant 'supergene' in the heterozygous state, the 'pin' (long-styled) form by the homozygous recessive. Thrum and pin forms are maintained in roughly equal numbers by the high fertility of thrum × pin crosses and near infertility of the two illegitimate thrum × thrum and pin × pin crosses. Although Darwin worked extensively on this beautiful qualitative trait, his interest was focused on the quantitative deficit of illegitimate progeny rather than the genetic control of the trait itself.

Figure 6

Darwin's table summarizing the results of Primula crosses. Note especially the last line referring to the Primula auricula data, where the recessive and dominant forms respectively are cited as occurring in a 1:3 ratio in crosses between heterozygous dominant (short-styled) individuals. Reproduced from [5].

Figure 7

Francis Galton, the anthropometrician, in 1893. Reproduced from an anthropometric study cited in [17].