18 CHAPTER 1
These are fundamental principles of evolution that
emerged from the modern synthesis. Much of the rest of
this book is devoted to explaining and building on them.
Some statements, marked by an asterisk (*), have to be
qualified to some degree, in light of later research.
- An individual’s phenotype (its observed traits) is
distinct from its genotype (its DNA). Phenotypic differ-
ences among individuals are caused by both genetic
differences and environmental effects. - Acquired characteristics are not inherited.*
- Hereditary variations are based on particles—the
genes.* This is true for traits with continuous variation
(e.g., body size) as well as those with discrete variation
(e.g., eye color). - Genetic variation arises by random mutation. Muta-
tions do not arise in response to need. Variation that
arises by mutation is amplified by recombination of
alleles at different loci. - Evolution is a change of a population, not of an
individual. The elementary process of evolution is a
change across generations in the frequencies of al-
leles or genotypes, which can change the frequencies
of phenotypes. - Changes in allele frequencies may be random or
nonrandom. Natural selection results from differences
among individuals in survival and reproduction, and
causes nonrandom changes. Genetic drift causes
random changes. - Natural selection can account for both slight and
great differences among species. Even a low intensity
of natural selection can cause substantial evolution
ary change over time. Adaptations are traits that
have been shaped by natural selection.
- Natural selection can alter populations beyond the
original range of variation when changes in allele
frequencies generate new combinations of genes. - Populations usually have considerable genetic
variation. Many populations evolve rapidly, to some
degree, when environmental conditions change,
and do not have to wait for new favorable mutations. - The differences between species evolve by
rather small steps, and are often based on differ-
ences at many genes that accumulated over many
generations.* - Species are groups of interbreeding or potentially
interbreeding individuals that do not exchange
genes with other such groups.* Species are not de-
fined simply by phenotypic differences. Rather, they
represent separately evolving “gene pools.” - Speciation (the origin of two species from a single
ancestor species) usually occurs by the genetic
differentiation of geographically isolated popula-
tions.* Species have genetic differences that pre-
vent interbreeding if they are no longer geographi-
cally separated. - Higher taxa arise by the sequential accumulation
of small differences, rather than by the sudden ap-
pearance of drastically new types by mutation. - All organisms form a great Tree of Life (or phylog-
eny) that evolved by the branching of common an-
cestors into diverse lineages, chiefly by speciation.
All forms of life descended from a single common
ancestor that lived in the remote past.
BOX 1A
Fundamental Principles of Biological Evolution
does not equal Darwinism, and any antievolutionary critiques of Darwin that do
not take into account modern research are irrelevant to our understanding of
evolution today.
How Evolution Is Studied
Evolutionary biology is a more historical science than most other biological dis-
ciplines, for one of its goals is to determine what the history of life has been and
what has caused those historical events.
Occasionally we can document an evolutionary change as it occurs or piece
together records to reconstruct a recent change, just as we do when studying
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