Usually regarded as a slow, gradual process that
describes how life has changed with time on the Earth, ranging
from simple single-celled organisms to the complex biosphere
on the planet today. However, it is better defined as a
sustained change in the genetic makeup of populations over a
period of generations leading to a new species. The field of
evolution was pioneered by Jean-Baptiste Lamarck (1809)
and Charles Darwin in his On the Origin of Species (1859)
and The Descent of Man (1871), and it is a multidisciplinary
science incorporating geology, paleontology, biology, and,
with neo-Darwinism, geneticists.
Darwin sailed on the H.M.S. Beagle (1831–36) when he
made numerous observations of life and fossils from around
the world, leading to the development of his theory of natural
selection, in which races with favorable traits stand a better
chance for survival. The main tenets of his theory are that
species reproduce more than necessary, but populations tend
to remain stable since there is a constant struggle for food
and space, and only the fittest survive. Darwin proposed that
the traits that led to some individuals surviving are passed on
to their descendants, hence propagating the favorable traits.
However, Darwin did not have a good explanation about
why some individuals would have favorable traits that others
would not, and this evidence would not come until much
later with the field of genetics and the recognition that mutations
can cause changes in character traits. Sequential passing
down to younger generations of mutation-induced changes in
character traits can lead to changes in the species, and eventually
the evolution of new species. Darwin’s process of natural
selection worked by eliminating the less successful forms
of species, favoring the others that had favorable mutations.
More modern variations on evolution recognize two
major styles of change. Macroevolution describes changes
above the species level and the origin of major groups, whereas
microevolution is concerned with changes below the species
level and the development of new species. Another major
change in understanding evolution in the past century concerns
the rate of evolutionary changes as preserved in the fossil
record. Darwin thought that evolution was gradual, with
one species gradually changing into a new species, but the fossil
record shows only a few examples of this gradual change
(with notable examples including changes in trilobites in the
Ordovician, and changes in horses in the Cenozoic). Nearly
all species in the fossil record exist with little change for long
durations of geologic time, then suddenly disappear, and are
suddenly replaced by new species. In other cases, new species
suddenly appear without the disappearance of other species.
Some of the apparent rapid change was initially regarded as
an artifact of an incomplete record, but many examples of
complete records show that these rapid changes are real. A
new paradigm of evolution named punctuated equilibrium,
advanced in the 1970s by Steven J. Gould and Niles Eldredge,
explains these sudden evolutionary changes. Physical or geo
graphic isolation of some member of a species, expected during
supercontinent breakup, can separate and decimate the
environment of a species and effectively isolate some members
of a species in conditions in which they can change. This small
group may have a mutation that favors their new environment,
letting them survive. When supercontinents collide,
many species that never encountered each other must compete
for the same food and space, and only the strongest will survive,
leading to extinction of the others.
In other cases, major environmental catastrophes such as
meteorite impacts and flood basalt eruptions can cause
extreme changes to the planetary environment, causing mass
extinction. Relatively minor or threatened species that survive
can suddenly find themselves with traits that favor their explosion
into new niches and their dominance in the fossil record.
See also MASS EXTINCTIONS; PALEONTOLOGY; SUPERCONTINENT
CYCLE.
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