Encyclopedia of Evolution.pdf - Online Reading Center

macroevolution Macroevolution refers to the evolution of
major new characteristics that make organisms recognizable as
a new species, genus, family, or higher taxon (see speciation).
Divergence of an evolutionary lineage into two or more lineages
has also been called cladogenesis (“origin of branches”).
In contrast, microevolution refers to small changes within an
evolutionary lineage (also called anagenesis). Microevolution
usually occurs by natural selection but can also occur as a
result of other processes such as genetic drift.
Controversy has long surrounded the concept of macroevolution.
To many evolutionary scientists, the distinction
between microevolution and macroevolution is artificial;
macroevolution is simply what happens after microevolution
has occurred long enough. Ever since the modern synthesis,
in which Darwinian natural selection was integrated with
Mendelian genetics, most biologists have assumed that
gradual evolutionary change (microevolution), given enough
time, can produce major evolutionary differences. Other scientists
claim that macroevolutionary patterns cannot be completely
explained by microevolution. Scientists who studied
the fossil record expected to find evidence, in all lineages, of
gradual change over time. As paleontologist Niles Eldredge
explains, he and evolutionary biologist Stephen Jay Gould,
in their independent lines of research (Gould with snails,
Eldredge with trilobites) expected to find a record of gradual
change. Instead they found that species remained essentially
unchanged for millions of years (stasis or equilibrium), then
became extinct. Rapid evolutionary change occurred when
a new species originated, as a punctuation to the history of
life. They called this pattern punctuated equilibria (see
Eldredge, Niles; Gould, Stephen Jay). Microevolution,
in contrast, has seldom been controversial; even creationists
conspicuously accept it (see creationism).
Evolutionary biologist Richard Dawkins pointed out
that there is no reason to expect natural selection to always
proceed at the same rate (see Dawkins, Richard). His most
memorable example of this was the story of the Israelites
M
crossing the desert of Sinai during the Exodus. According to
the biblical story, it took the Israelites 40 years to travel a
couple of hundred miles. This does not mean that they traveled
a few miles every year, or a few feet per day. Instead,
long periods of encampment alternated with a few long expeditions.
Similarly, microevolution proceeds sometimes rapidly,
sometimes slowly; sometimes by directional selection,
sometimes by stabilizing selection. In this way gradualism
could produce a punctuated equilibrium pattern.
Not quite, Eldredge and Gould responded. Punctuated
equilibria were not merely random patterns of fast and slow
evolution. The punctuation of rapid evolution occurred in
most cases at the time a species originated, followed in each
case by a long period of stasis. After a population separates
into two or more populations, evolution would then proceed
more rapidly than at other times. This would occur, they
pointed out, when a population is marginalized (for example,
a small population is driven into a new and unfamiliar
habitat) or when the entire species experiences a major habitat
change. This is why, said Eldredge and Gould, speciation
often occurred in conjunction with major pulses of species
turnover. Species do not respond to minor environmental
changes by evolution; instead they migrate to new locations.
In contrast, major crises of environmental change (e.g., a relatively
sudden cooling or drying of the environment) would
cause the extinction of some species, and quick evolutionary
change in others. Cooling and drying in East Africa about
two and a half million years ago resulted in the evolution of
many new species of mammals, including the first members
of the human genus, Homo.
The origin of complex adaptations has presented a challenge
to evolutionary theory. How could gradual microevolution
have produced the macroevolution of a complex
adaptation such as the vertebrate eye? For natural selection to
work, each of the gradual steps needs to provide a significant
benefit, or else natural selection would actually eliminate it.
This is still one of the favorite arguments of creationists (see