Download CHARLES DARWIN AND THE NATURE OF BIOLOGICAL CHANGE

CHARLES DARWIN AND THE NATURE OF BIOLOGICAL CHANGE
DON WHITE, JR.
A. Charles Darwin and the origin of species.
1. In November of 1859, a monumental piece of English
literature was published, "The Origin of Species by Means
of Natural Selection, or the Preservation of Favored Races
in the Struggle for Life," by Charles Darwin.
2. What did Darwin try to say in his influential book?
a. Living things vary. Within a species, no 2 individuals
(except identical siblings) are exactly alike.
b. More individuals are born than survive to reproduce.
c. Individuals compete with one another for the
resources that enable them to survive.
d. Within populations, the characteristics of some individuals make them more able
to survive and reproduce in the face of certain environmental conditions than
others.
e. As a result of this environmentally selected "survival of the fittest," only those
individuals with adaptive traits live long enough to pass these traits on. Over time,
this natural selection can change the characteristics of populations, even molding
new species.
B. Evolution defined.
1. Evolution is the adaptive modification of organisms with time, or as Charles Darwin
put it, "descent with modification."
a. In its simplest sense, evolution is a change in the proportions of individuals in a
population possessing some given hereditary factor, called a gene. That is, if a
population of butterflies shifts through time from 40% blue individuals to 60%
blue individuals, and if the color blue is hereditary, evolution of a simple kind has
occurred.
b. Incidentally, Swiss naturalist Charles Bonnet coined the term “evolution,” in
reference to the appearance of life forms over time, in the 18th century.
c. The ultimate source of the genetic variability important in evolutionary processes
is presumably mutation.
2. There are actually 2 types of evolution.
a. Microevolution: involves change within kind (e.g., the development of all the
varieties of dogs and cats).
b. Macroevolution: involves change between kinds (e.g., 1 kind of organism (say a
reptile) changes into another kind of organism (say a mammal).
3. Biologists have observed microevolution occurring.
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a. The development of resistance to antibiotics in bacteria and the development of
industrial melanism in peppered moths (Biston betularia) are commonly cited
examples in high school and college-level biology textbooks.
b. The mechanism by which evolution occurs is primarily through a process called
natural selection. But what is natural selection?
C. Natural selection defined.
1. In spite of its revolutionary philosophical impact, Darwin's concept of natural
selection is quite easy to understand.
a. I was based on observations of artificial selection, the results of selective breeding
by farmers and animal breeders.
b. Darwin, for example, referred to all the different breeds of pigeons that had been
produced by selective breeding (also called artificial selection).
1) All the different varieties of pigeons can be bred from the wild rock pigeon,
and crossing among the different varieties can lead right back to the wild rock
pigeon.
2) Most everyone is familiar with the results of selective breeding with dogs, cats,
cattle, roses, and so on.
c. "So," Darwin said in effect, "we see what selective breeding by humans can do. I
wonder if selection can also occur in nature?"
d. Selective breeding can be compared to playing a piano.
1) The keys of the piano are like the gene pool of an organism.
2) Not every piece utilizes every key on the piano keyboard.
3) The composer, like the animal or plant breeder, can choose and select which
notes he/she wants to include in the composition.
4) The result can be a wonderful diversity of pieces in vastly different musical
styles.
5) But, and here is the limitation, the composer cannot create any new keys on
the keyboard.
6) The composer is limited to the keys that already exist.
e. Selective breeding can also be compared to the game of football.
1) Just as a football coach has his team try a variety of plays but keeps in the
team’s game plan only those plays that work, a population of organisms keeps
only those changes (mutations) that work.
2) The population doesn’t decide which changes to keep any more than the
football coach does.
3) The pattern of success determines the outcome.
2. By definition, natural selection is the differential survival and reproduction of
organisms that results in an increase in the frequency of those best adapted to the
current environment.
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3. Darwin's argument certainly seems logical. Is there any evidence that he was right?
Can nature select like humans? In fact, there is considerable evidence that Darwin was
correct.
C. Evidence of natural selection.
1. The peppered moths of southern England.
a. Perhaps the best example of natural selection is the one that is in all the biology
textbooks: the peppered moths of southern England.
b. Until the mid-19th century, almost every individual peppered moth that was
captured had light-colored wings.
c. Since about 1850, individuals with dark-colored wings increased in frequency in
the moth population near industrialized centers until they made up almost 98% of
these populations.
d. Biologists soon noticed that in industrialized regions where the dark colored
moths were common, the tree trunks were darkened, blackened by the soot of
pollution, and the dark moths were much less conspicuous resting on them than
were the light colored moths.
e. Can natural selection explain the increase in the frequency of the gene(s) that
control color in the peppered moth? Why was it an advantage for the dark moths
to be less conspicuous?
1) The ecologist H.B.D. Kettlewell hypothesized that birds eat peppered moths
while they rested on the trunks of trees during the day. (See Kettlewell, H.B.D.
1955. Selection experiments on industrial melanism in the lepidoptera.
Heredity 9:323-342.)
2) He tested why it was an advantage for the dark moths to be less conspicuous
by rearing populations of peppered moths in which dark and light individuals
were evenly mixed.
3) Kettlewell released these populations into 2 sets of woods: an area near
Birmingham that was heavily polluted; the other, in Dorset, that was relatively
unpolluted.
4) Kettlewell set up rings of traps around the woods to see how many of both
kinds of moths survived. To be able to evaluate his results, he marked the
moths that he had released with a dot of paint on the underside of their wings.
5) In the polluted area near Birmingham, Kettlewell trapped 25% of the light
moths, but 53% of the dark ones, thus providing evidence that the dark moths
had a better chance of surviving in polluted woods where the tree trunks were
dark.
6) In the relatively unpolluted Dorset woods, Kettlewell recovered 14% of the
light moths but only 5% of the dark ones. These results indicated that where
the trunks of the trees were still light-colored, the light moths had a better
change of survival that the dark ones (Table 1).
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Table 1. Comparison of the number of released and recaptured light versus dark peppered
moths in polluted and pollution-free areas in England. From Kettlewell (1955).
Location
Birmingham
Dorset
Number Released
Light
Dark
64
154
393
406
Number Recaptured
Light
Dark
16 (25%)
82 (53.2%)
54 (13.7%)
19 (4.7%)
f. Note 2 things:
1) The moths themselves did not change; there were always dark and light colored
moths from the earliest observations.
2) The environment changed, so the dark moths were better camouflaged (better
adapted to the current environmental conditions). Thus, the dark moths had a
better chance of surviving and leaving more offspring that grew into dark
moths in succeeding generations.
g. Sure enough, just as Darwin would have predicted, the gene frequencies shifted.
The "dark environment" naturally selected for the dark colored moth as more
likely to survive and reproduce.
2. Drug resistance.
a. Indiscriminate use of antibiotics and pesticides has caused pathogenic organisms to
become resistant to these chemicals.
b. It should be realized that the mutation(s) enabling them to survive the
unfavorable environment was already present before exposure; the chemicals are
merely acting as selective agents. Here are 2 specific examples.
1) When bacteria are grown on a medium containing streptomycin, a few survive.
These few can grow on a medium both with and without streptomycin and,
therefore, this new generation of bacteria is now resistant to the antibiotic.
2) The human struggle against malaria.
a) The mosquito, Anopheles, transfers the disease-causing protozoan
Plasmodium from person to person.
b) In the early 1960s, international health authorities thought that malaria
would soon be eradicated. The administration of a new drug, chloroquine,
was more effective than quinine and DDT spraying for mosquito
eradication.
c) But in the mid-1960s, Plasmodium showed signs of chloroquine resistance,
and worse yet, mosquitoes were becoming resistant to DDT.
d) A few drug-resistant Plasmodium and a few DDT-resistant mosquitoes has
survived and multiplied, making the fight against malaria more difficult
than ever.
3. Industrial melanism and drug resistance are good examples of microevolutionary
change: change within kind.
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D. Is natural selection evidence of evolution?
1. The peppered moths and drug resistance examples do seem to provide strong evidence
of natural selection. But is that evidence of macroevolution: change between kinds?
2. Notice I have changed the question. First I asked if there was any evidence that
Darwin was correct about natural selection. The answer quite simply is "Yes, there is."
But now I'm asking a radically different question, "Is natural selection evidence of
macroevolution?"
3. Many people say natural selection and evolution are the same thing. Are they the
same thing? No, not necessarily. It depends upon how you define evolution.
4. Consider the peppered moth example again.
a. What did we start with? Dark and light varieties of the peppered moth, Biston
betularia.
b. After 100 years of natural selection, what did we end up with? Dark and light
varieties of the peppered moth, Biston betularia.
c. All that changed was the percentage of moths in the 2 color categories—that is,
variation within type.
E. The potential connection between natural selection and evolution.
1. Natural selection and evolution are potentially connected by "extrapolation," that is,
the process of following a trend to its logical conclusion.
2. Wildlife biologists, for example, extrapolate wildlife population growth trends to
estimate population size at some time in the future. In order to do that with any
degree of confidence, however, the growth rate must be constant over the time
interval of interest.
3. Similarly, if natural selection continues over very long periods of time, the same
process that changes moths from mostly light to mostly dark forms will gradually
change fish to philosophers and molecules to monkeys: macroevolution.
4. There is nothing wrong with extrapolation in principle. There are, however, things to
watch for in practice.
a. For example, using the present human growth rate, extrapolation shows that there
should be about a quadrillion people on earth by 3000 AD.
b. But, of course, there will come a point when the earth is simply not big enough to
support any more people. In other words, whenever an extrapolation is
conducted, the one performing the extrapolation must realize the logical limits, or
boundary conditions to their extrapolation.
7. The take home points are:
a. Natural selection can only deal with currently available variation and selects for
traits currently adapted to existing environmental conditions.
b. Natural selection is not a source of biological variation.
c. Natural selection does not have the ability to cause 1 kind of organism to evolve
into another kind of organism.
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d. Natural selection is woefully over extrapolated when it is used to explain
macroevolution.
e. Darwin was right about natural selection, it does provide a mechanism for
evolution, but only for microevolution, not macroevolution.