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0 Bates, Henry Walter
advantage. The cystic fibrosis gene is particularly common
in populations of European origin. Scientists have also suggested
that the gene that promotes osteoporosis (calcium loss
from bones in older adults) may confer resistance to the bacteria
that cause tuberculosis.
The possibility has also been raised that blood type proteins
have been selected by disease resistance. The human
ABO blood groups are determined by a gene on chromosome
9. The gene codes for proteins that are in the membranes of
red blood cells. Allele A differs from allele B by seven nucleotides.
Three of these nucleotide differences are neutral in their
effect, while the other four result in amino acid differences in
the blood protein. This is enough to allow the immune system
to distinguish between blood protein A and blood protein B.
Allele O results from a deletion, which prevents the production
of the blood protein. Each person has two alleles for this
group of blood proteins:
• A person who carries two A alleles, or an A and an O
allele, will have blood type A, since their red blood cells
carry protein A.
• A person who carries two B alleles, or a B and an O allele,
will have blood type B, since their red blood cells carry
protein B.
• A person who carries one A and one B allele will have
blood type AB, since their blood cells have both proteins.
• Blood type O results from the absence of these blood
proteins.
This has long been considered an example of neutral
genetic variation since the differences in blood type proteins
are not known to have any adverse health effects, and since
no benefit of one blood type over another has been proven.
Populations in most parts of the world consist of a mixture of
blood types. The only major pattern seems to be that blood
type O is relatively more common in populations of Native
Americans than in other populations. Research has suggested
that people with blood type AB are resistant to the bacterial
disease cholera, while people with blood type O are susceptible
to it. Although the blood proteins are found mainly on
red blood cell membranes, they are also secreted in saliva,
where they might function in preventing cholera. On the
other hand, people with blood type O are more resistant to
malaria and syphilis than those with the A and/or B proteins.
If this is the case, scientists would expect that natural selection
would make blood type AB very common in populations
that frequently encounter cholera. This has not proven to be
the case, perhaps because cholera is a disease of relatively
recent origin. It did not reach Europe until the 18th century,
and it may not have existed in Asia (its place of origin)
very many centuries before that. Scientists would also expect
blood type O to be more common in regions with malaria
and syphilis. However, Native Americans did not encounter
malaria until it was brought from the Old World in the 16th
century, and the hypothesis of the New World origin of syphilis
is controversial.
What looks like neutral variation in blood group proteins
may very well be left over from the past history of natural
selection for disease resistance. As geneticist and science
writer Matt Ridley wrote, “In a sense the genome is a written
record of our pathological past, a medical scripture for each
people and race.” Whether or not this turns out to be the case
with blood group proteins, it has certainly happened with
sickle-cell anemia, which evolutionary scientists still consider
the best example of a balanced polymorphism.
Further Reading
Dean, A. M. “The molecular anatomy of an ancient adaptive event.”
American Scientist 86 (1998): 26–37.
Pier, G. B., et al. “Salmonella typhi uses CTFR to enter intestinal epithelial
cells.” Nature 393 (1998): 79–82.
Ridley, Matt. Genome: The Autobiography of a Species in 23 Chapters.
New York: HarperCollins, 1999.
Bates, Henry Walter (1825–1892) British Naturalist Born
on February 8, 1825, Henry Walter Bates was a British naturalist
who specialized in collecting and studying insects. Bates
befriended another British naturalist (see Wallace, Alfred
Russel). In 1848 Bates and Wallace traveled together to the
Amazon rain forest of South America, where both of them
extensively collected insects and other animals. Wallace
returned to England and then traveled to Indonesia, but Bates
remained in the Amazon rain forest for 14 years.
While studying Amazonian insects, Bates discovered that
certain nonpoisonous species of heliconid butterflies had wing
coloration patterns that resembled those of poisonous heliconid
butterfly species. Since Bates’s initial discovery, numerous
other examples of mimicry have been found. This type of
mimicry is now called Batesian mimicry. Bates’s discovery of
mimicry contributed significantly to Charles Darwin’s work
on evolution. Darwin cited Bates’s discovery in later editions
of Origin of Species (see Darwin, Charles; origin of species
[book]). Bates and Darwin both maintained that such
mimicry could not have resulted from the use and disuse of
the structures involved (see Lamarckism) and must have
resulted from natural selection.
Bates returned to England in 1862. He published an
account of his work in the rain forest, The Naturalist on the
River Amazons, in 1863. Bates continued scientific research
and was elected a fellow of the Royal Society in 1881. His
studies contributed to an understanding of biodiversity and
of evolution, particularly of coevolution. He died February
16, 1892.
Further Reading
Beddall, Barbara G. Wallace and Bates in the Tropics: An Introduction
to the Theory of Natural Selection, Based on the Writings of
Alfred Russel Wallace and Henry Walter Bates. London: Macmillan,
1969.
Beagle, HMS See Darwin, Charles.
behavior, evolution of Behavior is the activity that results
from the voluntary nervous activity of animals. Activities that
are caused by purely metabolic or hormonal events are not
considered behavior. Plants grow toward light. They do so
because a hormone diffuses from the growing tip, primarily