The Nature of Natural Selection
Although the Theory of Evolution by Natural Selection can be stated concisely, tested rigorously in
natural populations, and validated, it can be difficult to understand thoroughly.
o One reason: Darwin’s theory descent with modification is essentially a statistical
process: a change in the trait distributions of populations.
Natural Selection Acts on Individual, but Its Consequences Occur in Populations
o When HIV strains were selected by exposure to AZT, or finch populations were selected
by changes in seed availability; none of the selected individuals changed in any way.
o They simply lived through the selection event while others died or reproduce more than
the competing virions or birds
o What changed after the selection process was the characteristics of the populations of
virions and finches, not the individuals themselves.
o A higher frequency of HIV virions in the population were able to replicate in the
presence of AZT and a higher proportion of finches had deep beaks.
Natural Selection Acts on Phenotypes but Evolution Consists of Changes in Allele Frequencies
o Selection would have altered the frequencies of the phenotypes in the population, but
in the next generation the phenotype distribution might have gone back to what it was
before selection occurred.
o Only when the survivors of selection pass their successful phenotypes to their offspring,
via genotypes that help determine phenotypes, does natural selection cause
populations to change from one generation to the next.
Natural Selection Is Not Forward Looking
o Natural selection adapts populations to conditions that prevailed in the past, not
conditions that might occur in the future
o Adapting for the future is impossible and is often a misconception
New Traits Can Evolve, Even Though Natural Selection Acts on Existing Traits
o The evolution of new traits is possible for two reasons:
During reproduction in sexual species, meiosis and fertilization recombine
existing alleles into new genotypes.
Mutation and recombination yield new suites of traits for selection to sort
Read example on page 91.
o Persistent natural selections can lead to the evolution of entirely new functions for
existing behaviour (e.g. the giant panda’s thumb; read on page 92).
o A trait that is used in a novel way and is eventually elaborated by selection into a
completely new structure is known as a preadaption.
Preadaption improves an individual’s fitness fortuitously –not because natural
selection is conscious or forward looking.
Natural Selection Does Not Lead to Perfection
o Evolution does not result in organisms that are perfect
o Each population evolves a phenotypes that strikes a compromise between opposing
agents of selection
o Natural selection cannot simultaneously optimize; it leads to adaptation, not perfection
Natural Selection is Nonrandom, but It Is Not Progressive
o Mutation and recombination are random with respect to the changes they produce in
phenotypes. But natural selection, the mechanism that sorts among var