1. The phenotype (the observed characteristics of an individual) is different from the genotype (the
set of genes in an individual’s DNA). Phenotypic differences among individual organisms
may be due partly to genetic differences and partly to direct effects of the environment.
2. Environmental effects on an individual’s phenotype do not alter the genes passed on to its off-
spring. In other words, acquired characteristics are not inherited. (See Chapter 9.)
3. Hereditary variations are based on particles—genes—that retain their identity as they pass
through the generations; they do not blend with other genes. This is true of both discretely
varying traits (e.g., brown vs. blue eyes) and continuously varying traits (e.g., body size,
intensity of pigmentation). Variation in continuously varying traits is based on several
or many discrete, particulate genes, each of which affects the trait slightly (POLYGENIC IN-
4. Genetic variation arises by random
mutation and recombination. Genes
mutate, usually at a fairly low rate,
to equally stable alternative forms,
known as ALLELES. The phenotypic
effects of such mutations can range
from undetectable to very great. The
variation that arises by mutation is
amplified by recombination among
alleles at different loci.
5. Evolutionary change is a populational
process. It entails, in its most basic
form, a change in the relative abun-
dances (proportions, or frequencies)
of individual organisms with differ-
ent genotypes (hence, often, with
different phenotypes) within a popu-lation. (A population is a group of individuals of the same species that occupy a speci-
fied geographic space and may interact with each other.) One genotype may gradually
replace other genotypes over the course of generations. Replacement may occur in only
certain populations or in all the populations that make up a species.
6. Change in genotype frequencies may be random or nonrandom. The rate of mutation is too
low for mutation by itself to shift a population from one genotype to another. Instead,
the change in genotype frequencies within a population can occur by either of two
principal processes: random fluctuations (GENETIC DRIFT) or nonrandom changes due
to the superior survival or reproduction of some genotypes compared with others (i.e.,
natural selection). Natural selection and random genetic drift can operate simultaneously.
7. Natural selection can account for both slight and great differences among species. Even a low
intensity of natural selection can (under certain circumstances) bring about substantial
evolutionary change over time. Adaptations are traits that have been shaped by natural
8. Natural selection can alter populations beyond the original range of variation by increasing
the frequency of alleles that, by forming new combinations with other genes, give rise
to new phenotypes.
9. Populations can accumulate considerable genetic variation. Mutations can accumulate in
natural populations. Hence many populations contain enough genetic variation to
evolve rapidly when environmental conditions change, rather than having to wait for
new favorable mutations.
10. Populations of a species in different geographic regions differ in characteristics that have a ge-
netic basis. These differences are often adaptive and must therefore be the consequence
of natural selection.
11. The differences between different species, and between different populations of the same spe-
cies, are often based on differences at several or many genes, many of which have a small
phenotypic effect. This pattern supports the hypothesis that the differences between species evolve by rather small steps.
12. Species are groups of interbreeding or potentially interbreeding individuals that do not exchange
genes with other such groups. Species are not defined simply by phenotypic differences.
Rather, different species of sexually reproducing organisms represent distinct, separately
evolving “gene pools.”(See Chapter 17.)
13. Speciation (the origin of two or more species from a single ancestor species) usually occurs
by the genetic differentiation of geographically segregated populations. Because of the geo-
graphic segregation, interbreeding does not prevent incipient genetic differences from
developing. Fully distinct species have genetic differences that prevent free interbreeding
even if they are no longer geographically separated. (See Chapter 18.)
14. Higher taxa arise by the prolonged, sequential accumulation of small differences, rather than by
the sudden mutational origin of drastically new “types.” This principle is supported by the
observation that there are often gradations in phenotypic characteristics among species
assigned to the same genus, to different genera, and to different families or other higher
taxa. (See Chapters 4 and 22.)
15. All organisms form a great “tree of life” (or PHYLOGENY) that has developed by the branching
of common ancestors into diverse lineages, chiefly through speciation. (See Chapter 2.) All
forms of life appear to have descended from a single common ancestor in the remote past.