288 Raphael Falk
mental stages of the Drosophila as a model organism; and by coinciding with the
newly developed molecular “chromosome-walking” method, the bithorax-complex
became one of the first to be DNA-sequenced [Benderet al., 1983]. The bithorax
complex was actually a complex of homeotic genes, regulatory genes of develop-
ment, the mutations of which bring about one body part to develop into another
(see Section 7). Within a short time it was shown that genetic analysis can be
applied successfully to a large scale of genes (known and newly discovered ones)
that act at very early stages of embryogeneisis of Drosophila [N ̈usslein-Volhard and
Wieschaus, 1980] and the era of molecular developmental genetics encompassed
an increasing number of animals and plants.
7 POPULATION GENETICS UPHOLDS DARWINISMMendel’s hypothesis of inheritance of discrete factors that are not diluted should
have resolved a major difficulty that Darwin encountered. Shortly after the pub-
lication of hisOrigin of Species, in 1867, Fleeming Jenkins showed that, adopting
Darwin’s theory of inheritance by mixing pangenes, would wash out any achieve-
ment of natural selection (see Hull [1973, 302-350]). Hugo de Vries and especially
William Bateson, considered Mendel’sFaktorenas indicated by his hypothesis
of inheritance to provide a rational basis for the theory of evolution. Although
as early as in 1902 Yule showed that, given small enough steps of variation, the
Mendelian model reduces to the biometric claim [Yule, 1902], this was largely
ignored in the bitter disputes between the Mendelians and the Biometricians
[Provine, 1971], (see Tabery [2004]). Hardy’s [1908] proof that in a large pop-
ulation, the proportion of heterozygotes to homozygotes will reach equilibrium
after one generation of random mating (provided no mutation or selection inter-
fered), developed in the same year by Weinberg [Stern, 1943], became the basic
theorem of population genetics — the Hardy-Weinberg principle. It took, how-
ever, another decade for R. A. Fisher to convince that the continuous phenotypic
biometric variation reduces to the Mendelian model of polygenes [Fisher, 1918].
Thus, finally the way was cleared to examine the Darwinian theory of natural
evolution on the basis of Mendelian genetic analysis, not onlyin vivobut alsoin
papyro. As formulated by Fisher in hisfundamental theorem of natural selection:
“The rate of increase in fitness of any organism at any time is equal to its genetic
variance in fitness at that time” [Fisher, 1930, 37].
Whereas Fisher examined primarily the effects of selection of alleles of single
genes in indefinitely large population under the assumption of differences in geno-
typic fitness, J. B. S. Haldane concentrated on the impact of mutations on the rate
and direction of evolution of one or few genes (and the influence of population
size) [Haldane, 1990]. Sewall Wright in his models of the dynamics of popula-
tions wished to be more “realistic”, and stressed the influence of finite population
size, the limited gene flow between subpopulations, and the heterogeneity of the
habitats in which the population and its subpopulations lived [Wright, 1986].
Experimentally, the main British group, led by E. B. Ford adopted a strict