Encyclopedia of Evolution.pdf - Online Reading Center

Malthus, Thomas
intelligent design). Further investigation has shown that
either the intermediate forms could have been functional, or
that the complex adaptation may have arisen as an exaptation
(see adaptation):
• Functional intermediate forms. In one famous scientific
paper, a computer model simulated the origin of the eye.
Starting from a simple optically sensitive surface, the computer
program generated random mutations and selected
the ones that improved visual acuity at each step. The simulation
produced the model of an eye not strikingly different
from the vertebrate eye.
• Exaptation. Complex flight feathers could not have
evolved by gradual steps. But the first feathers might have
functioned to hold in body heat, a function for which simple
feathers are adequate. If these feathers were brightly
colored in one sex, a gradual increase in their complexity
might have resulted from sexual selection. As the feathers
became even more complex, they might have allowed
a male bird to jump and glide in sexual courtship, even if
it could not fly. From these intermediate stages of complexity,
fully complex flight feathers could have gradually
evolved. This might explain why the earliest bird fossils
(see arcHaeopteryx) have complex feathers without having
other adaptations for flight.
Some macroevolutionary patterns may have resulted
from processes that would not be apparent on a microevolutionary
scale. Niles Eldredge and Stephen Jay Gould have
proposed such a mechanism, called species sorting. Some
species have a tendency to produce more new species than
do others, perhaps because natural selection favors mating
or cross-pollination with near neighbors. Many extinctions,
particularly mass extinctions, occur by chance rather
than as a result of natural selection (see extinction; mass
extinctions). The more “speciose” species, those that left
more descendant species, would survive mass extinctions
more often than the less speciose species. Natural selection
has not in this case favored speciation, but enhanced speciation
is the result.
Thus macroevolution may not be simply microevolution
acting over a long period of time but may result from punctuated
equilibria and species sorting.
Further Reading
Dawkins, Richard. The Blind Watchmaker: Why the Evidence of
Evolution Reveals a Universe Without Design. New York: Norton,
1986.
Eldredge, Niles. Reinventing Darwin: The Great Debate at the High
Table of Evolutionary Theory. New York: John Wiley, 1995.
Nilsson, Dan-Eric, and Suzanne Pilger. “A pessimistic estimate of
the time required for an eye to evolve.” Proceedings of the Royal
Society of London B 256 (1994): 53–58.
Malthus, Thomas (1766–1834) British Economist Thomas
Robert Malthus, economist, philosopher, and clergyman, practically
invented the modern study of population, on which all
of evolutionary biology depends. Charles Darwin (see Darwin,
Charles) read Malthus’s Essay on the Principle of Population
which was published in 1798. Malthus’s interest was only in
humans, but it was from Malthus’s essay that Darwin got an
insight that was essential to natural selection.
Born February 13, 1766, Thomas Malthus studied at
Cambridge. He was ordained an Anglican minister, and
was also Professor of History and Political Economy at
the East India College in Hertfordshire. Malthus’s main
argument was that human populations grow in an exponential
fashion (for example by doubling), while resources
increase in a linear fashion (at a constant rate) if at all.
Therefore, Malthus concluded, human populations will
always exceed resources such as food and shelter. This
was not good news to social reformers. Malthus’s doctrine
indicated that all the efforts of social reformers to relieve
the misery of the poor are doomed to failure, unless population
growth is limited. Malthus was notorious for his
opposition to the Poor Laws, which provided assistance to
poor families, and his support of the Corn Laws, which
caused food to be more expensive. He reasoned that welfare
to the poor was only a temporary solution to the
problem of privation. Malthus’s doctrine also directly
contradicted the religious concept of divine providence,
which maintains that God created a world that could sustain
humans. Instead, Malthus claimed (in the first edition
of his Essay, an argument dropped from later editions)
that suffering and privation were part of a divine plan to
continually improve the world. In a world with unlimited
resources, everyone would succeed. But in a Malthusian
world of population growth amid limited resources, those
individuals that had the opportunities, abilities, and motivation
would be the ones to succeed, while the others
would perish.
Darwin realized that Malthus’s doctrine also applied
to animals and plants. In a crowded world, only those individuals
with superior abilities would survive and reproduce.
Because traits are inherited, the next generation would
resemble those superior individuals. This is natural selection.
But the world remains crowded, and only the superior
among the superior individuals would survive and reproduce.
Therefore, natural selection does not produce adaptations
that are simply good enough, but selects for continual
improvement. With this line of reasoning, Darwin realized
that natural selection could cause even the simplest organisms
of the past to eventually develop into the complex
organisms on the Earth today. Another evolutionary scientist
(see Wallace, Alfred Russel) also reached the same
conclusion, independently of Darwin, after reading Malthus’s
essay.
Malthus’s argument has proven true in nonhuman species
but incorrect in human populations. Recent experience
has shown that providing assistance to nations with high levels
of poverty is associated with a decrease, not an increase,
in population growth rate; and that economic success is not
associated with genetic differences.
Malthus died December 23, 1834.
Further Reading
Literary Encyclopedia. “Malthus, Thomas.” Available online. URL:
http://www.litencyc.com/php/speople.php?rec=true&UID=2902.
Accessed April 30, 2005.