(a) There is competition for resources between individuals.
(b) Those individuals that are most capable of obtaining the resources, and can
survive and reproduce, are those that will leave the most progeny. The next gen-
eration will contain a greater proportion of those types.
The selectioncomes from the relative success of the different types in leaving progeny.
The process of natural selection is the replacement of types (or morphs) that pro-
duce fewer successful offspring with those that are more successful. The more suc-
cessful types are described as fitterthan less successful types. Fitnessis defined as
“the relative reproductive success of an individual in the long term” where “repro-
ductive success” includes births, survival, and reproduction of the offspring, “long
term” means over several generations, and “relative” means in comparison with other
members of the population. Fitness is measured by the comparison of reproductive
rates and survival rates between types. Indirect indicators of fitness can be morpho-
logical, physiological, or behavioral traits that are correlated with these rates.
This is the theory of natural selection at its simplest. It carries the following
corollaries:
1 Natural selection results in adaptationto the environment because the types
leaving more progeny are by definition better at surviving and reproducing in that
environment. The most successful types are the fittestindividuals.
2 Since no population has all possible varieties, natural selection cannot produce
perfect adaptation – only the best among those available, and these best may be quite
imperfectly adapted.
3 Natural selection results in adaptation to past and present conditions, not to future
conditions. It cannot anticipate future conditions or select for individuals preadapted
to them. If the changing conditions suit a currently rare individual type, it is through
chance alone and does not indicate predetermined design.
4 Natural selection acts only on the inherited components of an individual, namely
the genes. For these purposes genes are those elements of the chromosome that
segregate independently, and therefore may include several DNA groups if they
are linked. Natural selection cannot maintain either whole phenotypesor whole
genotypes. The genotype is the total complement of genes in the individual. The
phenotype is the individual organism, which is a product of the genotype interact-
ing with the environment during development. Phenotypic variation is reflective of
genotypic and environmental variation.
5 A favorable gene can have both advantageous and disadvantageous effects within
the same individual due to pleiotropyand polygenic effects. Pleiotropy describes
a gene affecting more than one character in the individual, and some effects may be
beneficial while others are disadvantageous. Polygenic effects implies that a character
is affected by several genes, some good some bad. All that is required is that the beneficial
effects outweigh the detrimental ones.
6 Natural selection does not guard against the extinction of species. Many adapta-
tions do indeed promote the continued existence of a species but there are also many
that result in extreme specialization to unusual environments, restricted habitats, or
isolated areas. These species are vulnerable to environmental change. On the island
of Hawaii in the Pacific Ocean the extinction of many species of the Hawaiian hon-
eycreepers has resulted in the extinction or near extinction of all species in the plant
genus Hibiscadelphus. The honeycreepers, with their long, curved bills (see Fig. 3.1),
were the pollinators of the curved, tubular flowers of the Hibiscadelphus(Diamond
and Case 1986).20 Chapter 3