Evolution__3rd_Edition

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Mendelian heredity conserves
genetic variation
Natural selection is more powerful
with Mendelian, than with
blending, heredity
CHAPTER 2 / Molecular and Mendelian Genetics 39
heterozygotes, in the analogous cross with blending heredity they do not a all the
grandchildren are light green (see Figure 2.10).
Mendelism is an atomistic theory of heredity. Not only are there discrete genes that
encode discrete proteins, but the genes are also preserved during development and
passed on unaltered to the next generation. In a blending mechanism, the “genes” are
not preserved. The genes that an individual inherits from its parents are physically lost,
as the two parental sets are blended together. In Mendelism, it is perfectly possible for
the phenotypes of the parents to be blended in the offspring (as they are in the initial
AA × aa cross in Figure 2.10), but the genes do not blend. Indeed, the phenotypes of real
mothers and fathers often do blend in their offspring, and it was because they do that
most students of heredity before Mendel thought that inheritance must be controlled
by some blending mechanism. However, the case of heterozygotes that are intermediate
between the two homozygotes (i.e., no dominance) shows that the blending
of phenotypes need not mean blending of genotypes. In fact, the underlying genes are
preserved.
One way of expressing the importance of Mendelism for Darwin’s theory is to say
that it efficiently preserves genetic variation. In blending inheritance, variation is
rapidly lost as extreme types mate together and their various “genes” are blended out of
existence into some general mean form. In Mendelian inheritance, variation is preserved
because the extreme genetic types (even if disguised in heterozygotes) are passed
down from generation to generation.
Why does this preservation of genes matter for Darwinism? Our full discussion of
natural selection comes in later chapters, and some readers may prefer to return to this
point after they have read about natural selection in more detail; but even with only the
elementary account of natural selection in Chapter 1, it is possible to understand why
Darwin, so to speak, needed Mendel. Figure 2.11 illustrates the argument.
Suppose that a population of individuals is white in color, and has the aa genotype,
and heredity blends (in the manner of Figure 2.10). For some reason, it is advantageous
for individuals in this population to be dark green in color: dark green individuals
would survive better and leave more offspring. Moreover, it is better to be a bit green
(i.e., light green) than to be white. Suppose now that a single new light green individual
somehow crops up by mutation, and it has an Aa genotype. This Aa individual will
survive better than its aa fellow members of the population and will produce more offspring.
However, the advantageous gene cannot last long with blending. In the first
generation it produces A′ gametes; these combine with a gametes (because every one
else in the population is white) and produce A′a offspring. We can suppose these individuals
are a bit lighter green in color than the original Aa mutant; they still have an
advantage, but it is lower.
The A′a individual’s genes in turn blend, such that all its gametes will have an A″
gene. When that unites with an a gamete (because still almost everyone else is white) an
A″a offspring results, which is even lighter green in color. It is only a matter of time and
the original favorable mutation will be blended almost out of existence (Figure 2.11a).
The best result possible would be a population that was very slightly less white than it
was to begin with. A population of dark green individuals cannot be produced from the
original mutation. That original mutation, which potentially was able to produce dark
green individuals, will cease to exist after one generation. This objection to the theory