Evolution__3rd_Edition

..
Summary
1 Heredity is determined by a molecule called DNA.
The structure and mechanisms of action of DNA are
understood in detail.
2 The DNA molecule can be divided into regions
called genes that encode for proteins. The code in the
DNA is read off to produce a protein in two stages:
transcription and translation. The genetic code has
been deciphered.
3 DNA is physically carried on structures called chromosomes.
Each individual has a double set of chromosomes
(one inherited from its father, the other from
its mother), and therefore two sets of all its genes. An
individual’s particular combination of genes is called
its genotype.
CHAPTER 2 / Molecular and Mendelian Genetics 41
of natural selection was known to Darwin. Darwin was very worried by it and never did
find a wholly satisfactory way round it.
Mendelism was what he needed. In the example just given, the original light green
mutation will be in an Aa heterozygote, and fully half its offspring will be light green
like it a because they are also Aa heterozygotes. There is ample time for natural selection
to increase the proportion of light green individuals, and eventually there would be
enough of them for there to be a chance that two will mate together and produce some
AA homozygotes among their offspring. A population of dark green AA individuals
can now theoretically be produced (Figure 2.11b). Thus natural selection is a powerful
process with Mendelian heredity, because Mendelian genes are preserved over time;
whereas it is at best a weak process with blending inheritance, because potentially
favorable genes are diluted before they can be established.
Further reading
4 New genetic variation originates by mutational
changes in the DNA. Rates of mutation can be estimated
by direct observation.
5 When two individuals, of given genotypes, mate
together, the proportions of genotypes in their offspring
appear in predictable Mendelian ratios. The
exact ratios depend on the genotypes in the cross.
6 Different genes are preserved over the generations
under Mendelian heredity, and this enables natural
selection to operate. Before Darwin, it was generally
(but wrongly) thought that the maternal and paternal
hereditary materials blended in an individual rather
than being preserved. If heredity did blend, natural
selection would be much less powerful than under
Mendelian heredity.
Any genetics text, such as Lewin (2000), Griffiths et al. (2000), or Weaver & Hedrick
(1997), explains the subject in detail. I include an account of mutation rate measurements
in a popular book (Ridley 2001); see also the reviews referred to in Chapter 12
below. The classic statement of why Darwinism requires Mendelism, and does not
work with blending heredity, is in the first chapter of Fisher (1930), which was reprinted,
editorially reduced, in Ridley (1997). Graveley (2001) explains alternative splicing.