Encyclopedia of Evolution.pdf - Online Reading Center

Fossil Hominin Skulls Intermediate between
Major Hominin Species
Age Cranial capacity
(thousand (cubic cm)
Specimen years ago) (estimate)
Transitional to Homo heidelbergensis:
Gran Dolina, Spain 780 1,000
Ceprano, Italy 800 1,000
Between Homo heidelbergensis and Neandertals:
Arago, France 450 1,150
Sima de los Huesos, Spain 300 1,150
Petralona, Greece 200 1,220
Between Homo heidelbergensis and modern
humans:
Bodo, Ethiopia 600 1,100
Ndutu, Tanzania 400 1,100
Sale, Morocco 400 1,000
Florisbad, South Africa 250 1,280
Kabwe, Zambia 175 1,280
Laetoli, Tanzania 150 1,350
genetics that characterizes the modern understanding of
evolution. The phrase comes from the title of evolutionary
biologist Julian Huxley’s 1942 book Evolution: The Modern
Synthesis (see Huxley, Julian S.).
When Charles Darwin proposed natural selection as
the mechanism of evolutionary change (see origin of species
[book]), neither he nor any other scientist understood
the mechanism of heredity. Scientists generally held a vague
notion called blending inheritance in which offspring displayed
a blend of traits from the parents. This blend, however,
was considered inseparable: As engineer Fleeming Jenkin
pointed out, to Darwin’s dismay, if a new trait appeared, natural
selection would never have a chance to favor it, because
it would get blended in with the other traits and vanish. This
argument would not be true, however, if hereditary traits
were particulate in nature; that is, if they were genes. Genes
could be hidden but could never be blended in; rather than
being like drops of paint, genes are like beads, which can
reappear. If their distinct identity is maintained, then natural
selection can favor them. The particulate nature of genes
was being investigated by an Austrian monk (see Mendel,
Gregor). Mendel published his work shortly after Darwin
published the Origin; but since most scientists who studied
inheritance patterns clung to the blending theory, Mendel’s
work was ignored and Darwin apparently never heard of it.
It was not at all obvious to the scientists who rediscovered
Mendel’s work that Mendelian genetics was compatible
with natural selection. Hugo DeVries (see DeVries, Hugo)
was a botanist who studied inheritance patterns in primroses,
modern synthesis
and William Bateson a zoologist who studied population
variation in molluscs. During background research, DeVries
(as well as geneticists Carl Correns and Erich von Tschermak
von Seysenegg) discovered the obscure publications of Gregor
Mendel about 1900. To DeVries and Bateson, however, Mendel’s
work seemed to indicate that heritable differences produced
large discontinuities: big differences between one breed
of primrose and another, and big differences between mollusks
that inhabited different regions of the Aral Sea, adapted
to different salinity levels. New genes, called mutations, had
big effects that were immediately visible, they believed. This
belief was inconsistent with the slow, gradual changes that
Darwin said would be produced by natural selection acting
upon a population. Meanwhile, scientists who believed that
evolutionary changes were gradual rejected Mendelian principles.
This was because most traits really did look like blending
inheritance rather than Mendelian discontinuities. Mendel
crossed plants that produced green peas with those that produced
yellow peas; most offspring produced yellow peas, a
few produced green peas, nothing in between. But with most
traits, offspring were intermediate between the parents.
To resolve this situation, it was first necessary to demonstrate
that there was someplace that genes could be located.
Microscopists had discovered chromosomes in the nuclei of
cells. Geneticist Thomas Hunt Morgan studied genetic traits
of fruit flies (which just happened to have some cells with
giant chromosomes) and was able to associate the inheritance
of these traits with specific chromosomes. Geneticist Herman
J. Muller bombarded cells with X-rays and induced mutations,
which were heritable. Next, it was necessary to demonstrate
(mathematically) that many apparently continuous
(seemingly blended) traits could result from the concurrent
action of many genes. Ronald A. Fisher (see Fisher, R. A.)
in England did this, in a complex paper that was at first misunderstood
and rejected but, upon publication, was seen to
bridge the gulf between distinct genes and continuous traits.
He published his ideas in The Genetical Theory of Natural
Selection. Further work in England (see Haldane, J. B. S.)
and America (see Wright, Sewall) expanded on Fisher’s
ideas. Theoretical concepts were united with laboratory and
field data by zoologists (see Dobzhansky, Theodosius;
Mayr, Ernst), paleontologists (see Simpson, George Gaylord),
and botanists (see Stebbins, G. Ledyard).
Assumptions that accompanied the modern synthesis were:
• Evolution occurs when natural selection acts upon small
genetic variations within populations; therefore evolutionary
changes (microevolution) are small.
• Large evolutionary changes (macroevolution) can be
explained by microevolution occurring over long periods
of time.
These assumptions, together called gradualism, appear to
be a direct consequence of the modern synthesis. Most scientists
further believed that gradualism would reflect not
just the process of evolution but also the pattern of evolution
over long periods of time. Disagreement over this point
emerged with the proposal of punctuated equilibria in
1972 (see Eldredge, Niles; Gould, Stephen Jay). While