Encyclopedia of Evolution.pdf - Online Reading Center

win, Hooker could not pay his own way. Like Huxley, Hooker
was hired as ship surgeon. On board HMS Erebus, Hooker
sailed in 1839 for the southern seas, exploring islands around
Antarctica during the summers and visiting New Zealand and
Van Diemen’s Land (now called Tasmania) during the winters.
When he returned to Great Britain in 1843, Hooker
assisted his father, who had become the director of Kew
Gardens. Under the directorship of the elder Hooker, Kew
became one of the world’s leading centers of botanical
research, which it remains. Joseph Hooker had no regular
employment but managed to obtain money to pay for the
publication of books that documented the numerous plants
that he discovered on the islands of the southern seas.
Joseph Hooker obtained a government grant to cover
the expenses of an expedition to the Himalayas, from 1847
to 1849. He also visited India from 1850 to 1851. Hooker
accompanied a mapmaking expedition. The expedition
crossed the border into Tibet, which they were not authorized
to do, and Hooker was arrested along with the others.
The prisoners were released only when the British government
threatened force. While a botanist may not have seemed
much of a threat to local Asian governments, Hooker’s
research actually helped to advance British colonial interests.
His father William Hooker had supervised the illegal transfer
of Cinchona feverbark trees (the source of quinine) from Brazil
to British plantations in India. Joseph Hooker supervised a
similar illegal transfer from Brazil to India, this time of Hevea
rubber trees. The new kinds of rhododendron Joseph Hooker
brought back from the Himalayas became a profitable commodity
for the gardens of England.
Joseph Hooker’s botanical research caught the attention
of his hero, Charles Darwin. Because of his irregular employment
(he was mainly occupied with publishing his discoveries
from the Himalayas and India), Joseph Hooker could spend
a lot of time with Darwin and answer Darwin’s many questions
about the distribution of plants. Charles Darwin very
much appreciated Hooker’s assistance, and Emma Darwin
made him feel welcome at their home.
Hooker and Darwin did not always agree. The biogeography
of southern hemisphere plants was puzzling. Hooker
suggested that it could be explained if the separate lands of
the southern hemisphere, such as New Zealand, Australia,
and Tasmania, had once been connected together in a single
continent which had subsequently sunk beneath the sea. Darwin
rejected Hooker’s speculation, preferring to say that the
various species of plants had floated across the seas to get to
their present locations. This discussion prompted Darwin to
begin a landmark series of experiments regarding how long
various kinds of seeds could tolerate soaking in saltwater.
Darwin was surprised and pleased to report that many kinds
of seeds could germinate quite well after long periods of inundation
in brine. Today scientists know that Hooker was right,
not Darwin. The southern islands and continents were once
part of a southern supercontinent that broke up and moved
to its current locations (see continental drift).
Sir Joseph Hooker was one of the few people (along
with geologist Sir Charles Lyell and American botanist Asa
horizontal gene transfer
Gray; see Lyell, Charles; Gray, Asa) with whom Darwin
dared to discuss his theory of evolution by means of natural
selection. It was to Hooker that Darwin said that his belief in
transmutation of species was almost like confessing a murder.
In the midst of their discussions, Hooker interviewed
for a professorship in Edinburgh. Darwin was relieved when
Hooker did not get the job and had to stay around and help
him. Joseph Hooker became assistant director of Kew Gardens
in 1855. Upon the death of his father in 1865, Hooker
became the director. Darwin no longer needed to fear the
departure of his friend. Darwin later said that Hooker was
“the one living soul from whom I have constantly received
sympathy.” Darwin and Hooker both experienced the tragic
loss of a young daughter.
Joseph Hooker married the daughter of John Stevens
Henslow, the botany professor from whom Darwin learned
so much. Hooker died December 10, 1911.
Further Reading
Endersby, Jim. Sir Joseph Dalton Hooker. Available online. URL:
http://www.jdhooker.org.uk. Accessed April 11, 2005.
horizontal gene transfer Most genes are passed vertically
from one generation to another by sexual reproduction (see
reproductive systems), within a species or during hybridization
between closely related species. Genetic information
can also move horizontally among different species of organisms
by means other than sexual reproduction. Though much
rarer than vertical gene transmission, horizontal gene transfer
has occurred many times. It does not alter the process of
natural selection, which acts upon genetic variability in
populations, whatever its source, but it does alter the scientific
understanding of where genetic variability comes from.
Most evolutionary explanations assume that genetic variability
is enhanced mostly by mutations occurring in the genes
of individuals in a population, or by immigrants from outside
the population but from the same species (see population
genetics). With horizontal gene transfer, a whole new
source of genetic variation appears: injection of genes from
entirely different species.
For several decades scientists have known that bacteria can
exchange segments of DNA with one another (see bacteria,
evolution of). In a process called conjugation, one bacterium
grows a tube that connects with another bacterium, and segments
of DNA (usually small circles of genes called plasmids)
travel through the tube. Bacteria can also absorb intact chunks
of DNA from their fluid environments, although usually this
requires some sort of temperature or chemical shock to make
their cell membranes receptive to the transfer. Both conjugation
and the absorption of DNA segments increase under conditions
of environmental stress. This allows populations of bacteria to
increase their variability of genetic combinations.
The surprise came when microbiologists realized that conjugation
and other forms of gene transfer could occur between
bacterial species. At first this was considered an interesting
but unimportant aberration, but since antibiotic resistance
genes are often on plasmids, interspecific conjugation turned