Lecture Six: Natural Selection
Evolution means "change."
When we refer to a change in living populations over time, we are talking about
ORGANIC EVOLUTION.
WHO CAN TELL ME THE NAME OF THE MOST HIGHLY EVOLVED SPECIES?
Early European scientists were creationists who believed that the world was made in
seven days by a Supreme Being. Among these scientists was a young student
named Charles Darwin.
Darwin may well be the most influential scientist of all time. His controversial work,
On the Origin of Species by Means of Natural Selection, (published in 1859) is still
probably the most important biological work ever written, and every branch of biology
is framed in the context of evolution by natural selection.
Who was this man called Darwin? Where did his ideas come from? To understand a
bit more, let's look at the historical context of his work.
As long ago as 500 years before the birth of Christ, Greek philosophers were already
toying with the idea that life was constantly changing. However, the most influential of
these, Plato and his student, Aristotle, believed in an unchanging world in which all
species had been created in a perfect state, with perfect interactions intact. In such a
world, evolution would be counterproductive, and against the plan of The Creator.
The Old Testament account of The Creation fortified this idea, and even as recently a s
the 1700's, biology was done in a framework of natural theology--the idea that
science should be dedicated to studying nature in order to figure out the Grand Plan of
the Creator.
Linnaeus, creator of Systema naturae, wrote that he was studying the diversity of life
"for the greater glory of God." Although Linnaeus was a creationist, his taxonomic
groupings would become a focal point in Darwin's arguments for evolution.
Georges Cuvier (1769-1832) , a French anatomist, was the father of the science we
now call paleontology--the study of fossils. He, too, was a staunch creationist, and
insisted that the various signs of change in the fossil record were merely confirmation
of catastrophic events such as floods and massive destruction wrought by the
Creator.
James Hutton, a Scottish geologist, challenged Cuvier's view in 1795 with his idea of
gradualism, which stated that large changes in the earth's surface could be caused
by slow, constant processes such as erosion.
Charles Lyell added to this by saying that such earth processes had been going on
constantly, and could explain the appearance of the earth.
This theory,
uniformitarianism, was a strong basis for Darwin's later theory of natural selection.
Jean Baptiste Lamarck published his famous theory in 1809, the year Darwin was
born. He stated that characteristics acquired during an organism's lifetime could be
passed on to that organism's offspring. This could result in the gradual change of
one species into another. (Example: giraffe necks)
People already knew about ARTIFICIAL SELECTION (humans breeding animals and
plants for desired characteristics). Why should nature not operate in a similar way?
The stage was set for the Darwininan Revolution!
1809 - Darwin born in Shrewsbury, England
From the very start, he loved bugs n slugs, and spent most of his time outside or
reading nature books.
His dad, a famous physician, thought that no good life could await a naturalist, and s o
sent young Charles, at the age of 16, off to the University of Edinburgh medical school.
HATED IT. Dropped out after making mediocre grades
Enrolled at Christ College at Cambridge University with plans to become a clergyman.
(This isn't as weird as it sounds, since most scientists of his day were members of
the clergy!)
He fell in with the biologists and became the star pupil of Rev John Henslow,
Professor of Botany.
1831 - with the help of Darwin's uncle, Henslow convince both Captain Robert Fitzroy
of H.M. S. Beagle *and* Darwin's dad to let Charles go on the 5-year Beagle voyage
as "unpaid gentleman scholar and naturalist."
So at the age of 22, Darwin set sail. While the Beagle's crew mapped South
American coastlines, he went ashore and collected every living things he could lay his
hands on.
1836 - Darwin returned to England, settled down and got married and wrote up all his
work.
1858 - Darwin was nearly scooped by Alfred Wallace, a young British scientist
studying plants in Malaysia. (Lyell had warned him, but Darwin was too slow on the
draw!)
Lyell and some colleagues presented both Wallace's and Darwin's work at the
meetings of the Linnaean Society of Longond on July 1, 1858. Wallace's and
Darwin's ideas were identical, but Darwin's had been written first, and with much
more completeness than Wallace's. Today, Darwin is given credit as the Father of the
Theory of Evolution by means of Natural Selection.
Darwin made some profound observations, from which he inferred some brilliant
conclusions!
Observation #1. All species have huge potential fertility
Observation #2. Except for seasonal changes, populations tend to maintain a stable size.
Observation #3. Environmental resources are limited.
INFERENCE #1: The production of more individuals than the environment can
support leads to a "struggle for existence," with only a fraction of offspring surviving
in each generation.
Observation #4: No two individuals in a population are exactly alike
Observation #5: Much of the observed variation in a population is heritable
INFERENCE #2: Survival in this "struggle for existence is not random, but depends,
in part, on the hereditary makeup of the survivors. Those individuals who inherit
characteristics that allow them to best exploit their environment are likely to leave
more offspring than individuals who are less well suited to their environment.
INFERENCE #3: Unequal reproduction between suited and unsuited organisms
will eventually cause a gradual change in a population, with characteristics
favorable to that particular environment accumulating over the generations.
SO WHAT IS THIS THEORY OF NATURAL SELECTION, ANYWAY?
It can be broken down into four basic tenets, or ideas…
1. Organisms are capable of producing huge numbers of offspring. (The tenet of
OVERPRODUCTION)
2. Those offspring are variable in appearance and function, and some of those
variations are heritable. (The tenet of HERITABLE VARIABILITY)
3. Environmental resources are limited, and those varied offspring must
compete for their share. (The tenet of COMPETITION)
4. Survival and reproduction of the varied offspring is not random. Those
individuals whose inherited characteristics make them better able to compete
for resources will live longer and leave more offspring than those not as able to
compete for those limited resources.
(The tenet of DIFFERENTIAL
REPRODUCTION)
Note that EVOLUTIONARY FITNESS is nothing more and nothing less than
DIFFERENTIAL REPRODUCTION due to organisms' differing abilities to cope with
environmental limitations.
Evolutionary fitness is defined by the environment, and there is no such thing as a
"most highly evolved species."
When you design and present your Symbiosis posters, keep this in mind. Each
relationship is an example of COEVOLUTION--species being selected by
environmental limitations, including INTERACTIONS WITH OTHER SPECIES.
As we'll see later, NATURAL SELECTION is only one of several ways that populations
can gradually (or not so gradually) change.
Different species are, by definition, REPRODUCTIVELY ISOLATED from one another.
This means that at some time during a species' evolution, some individuals of one
species branch off to become members of a different species.
And scientists who study the processes and mechanisms
speciation events are called EVOLUTIONARY BIOLOGISTS.
that lead to such