Evolution, 4th Edition

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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.


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

  2. 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.

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

  4. 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.”

  5. 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.

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

  7. 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

01_EVOL4E_CH01.indd 18 3/23/17 8:43 AM

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