Evolution, 4th Edition

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.

1. 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.


2. Acquired characteristics are not inherited.*


3. 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).


4. 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.


5. 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.


6. 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.


7. 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.

8. Natural selection can alter populations beyond the
   original range of variation when changes in allele
   frequencies generate new combinations of genes.


9. 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.


10. The differences between species evolve by
    rather small steps, and are often based on differ-
    ences at many genes that accumulated over many
    generations.*


11. 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.”


12. 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.


13. Higher taxa arise by the sequential accumulation
    of small differences, rather than by the sudden ap-
    pearance of drastically new types by mutation.


14. 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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