Download Natural Selection

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Natural Selection
Natural selection is how species evolve by adapting to their environment. Natural selection explains
design in nature. Charles Darwin studied beak variation of finches on the Galapagos Islands as evidence of
natural selection. (Illustration from BSCS, Biological Science: Molecules to Man, 1963.)
Natural selection is the process by which species adapt to their environment. Natural selection leads to
evolutionary change when individuals with certain characteristics have a greater survival or reproductive rate
than other individuals in a population and pass on these inheritable genetic characteristics to their offspring.
Simply put, natural selection is a consistent difference in survival and reproduction between different
genotypes, or even different genes, in what we could call reproductive success. [A genotype is a group of
organisms sharing a specific genetic makeup.]
The reason that natural selection is important is that it’s the central idea, stemming from Charles
Darwin and Alfred Russel Wallace, that explains design in nature. It is the one process that is responsible for
the evolution of adaptations of organisms to their environment.
Darwin’s book On the Origin of Species by Means of Natural Selection caused quite a stir when it
appeared in 1859. Evidence to support evolution and natural selection, of course, has accumulated over time,
and now science accepts that evolution is a fact and that natural selection explains very well how adaptive
evolution takes place.
Natural Selection is not the only mechanism of Evolution. There cannot be evolution without genetic
variation in the first place. So there must be mutation and often recombination to generate the different
genotypes or the different versions of the genes, known as alleles, which then may or may not make a
difference in the ability of an organism to survive and reproduce. You can’t have any evolutionary change
whatever without mutation, and perhaps recombination, giving rise to genetic variation. But once you have
genetic variation, there are basically two major possibilities:
Natural selection is more consistent, adaptive change.
 First, there is simply no difference between the different genotypes or different genes in their impact on survival
or reproduction, and in that case, you can have random changes of one versus the other type in a population or
a species until eventually one replaces the other. That is an evolutionary change. It happens entirely by chance,
by random fluctuations. That is what we call the process of genetic drift.

Genetic drift is very different from possibility number two, natural selection, which is a much more consistent,
predictable, dependable change in the proportion of one gene vs. another, one genotype vs. another. Why?
Simply because there is some consistent superiority, shall we way, of one genotype vs. another in some feature
that affects its survival or some feature affecting its reproductive capabilities.
Natural selection plays a role in the origin of new species. Natural selection has a great deal to do with
evolution but natural selection can happen without the formation of new species. Natural selection is only the
process of adaptation within species, and we see many examples of that. Under some circumstances natural
selection does play a role in the origin of new species, by which I mean a splitting of one species lineage into
two different lineages that do not interbreed with one another — for example, the splitting of one ancestral
primate lineage into one that became today’s chimpanzee and the other that became the hominid line
resulting in our own species. The process of splitting and becoming reproductively isolated, that is, incapable
of breeding with one another, can often involve natural selection but perhaps not always.
You may have heard the phrase, “Survival of the fittest.” This is a slogan that is really very misleading. First
of all, it’s not an adequate description of what really goes on in nature for two reasons:
“Survival of the fittest” is a misleading term.
 Sometimes there isn’t a “fittest” type. There may be several different types that are equally fit for different
reasons. Perhaps they’re adapted to different facets of the environment. One is not going to replace the other
because each has its proper place in the environment.
 Moreover, it’s not just a matter of survival. Natural selection is a difference in reproductive success that involves
both the ability to survive until reproductive age and then the capacity to reproduce.
The notion of the survival of the fittest is also unfortunate because it has been viewed as a kind of
tautology, a kind of empty statement for those who say that the fittest are those that survive and so there’s
no real predictive content to the notion of natural selection. That is simply false.
Chance plays a role in the process of natural selection. Philosophers and scientists use “chance” only in
the sense of unpredictability. Chance means essentially that you cannot predict the outcome of a particular
event. For example, you cannot predict whether your next child will be a son or a daughter, even though you
can specify the probability or likelihood. “Chance” does not mean lack of purpose or goal in science. If it did,
we could say that absolutely everything in the natural world is by chance because we don’t see any purpose or
goal in storms, in ocean currents, or anything else. Evolution certainly does involve randomness; it does
involve unpredictable chance. For example, the origin of new genetic variation by mutation is a process that
involves a great deal of chance. Genetic drift, which is related to mutation, is a matter of chance.
However, natural selection itself is the single process in evolution that is the antithesis of chance. It is
predictable. It says that, within a specific environmental context, one genotype will be better than another
genotype in survival or reproduction for certain reasons having to do with the way its particular features relate
to the environment or relate to other organisms within the population. That provides predictability and
consistency. So, if you have different populations with the same opportunity for evolution, you would get the
same outcome.
Natural Selection cannot select for the future needs of a species. This is because natural selection is
not like Mother Nature watching over us. Since natural selection is totally an impersonal process that is
nothing more than a difference, generation by generation, in the reproductive success of one genome over
another, there’s no way that it can look forward to the future or guard against the possibility of extinction.
What individuals have right now that gives them superior adaptation may lead to disaster tomorrow.
There are many recent examples that show natural selection. One example is the apple maggot fly.
About 100 years ago it started to become a serious pest of apple orchards in New England and New York
State. It’s now a threat throughout most of northern United States. It originally fed just on hawthorn fruits,
but then it adapted to apple and it’s become a serious threat to the industry. That is a genetic change
propagated by natural selection. Perhaps much more crucial is an issue that agriculture has to deal with all the
time: the evolution in hundreds of species of insects of resistance to various chemical insecticides. The insects
then become more and more difficult to control.
Closer to home, and more serious, is the single greatest crisis in medicine: antibiotic resistance. The
fact is that enormous numbers of the most dangerous bacteria and viruses have evolved to be resistant to the
antibiotics or other drugs that used to be effective against them. An obvious example is the HIV virus, which,
as we know, is capable of rapidly evolving resistance to drugs that once were effective against it.
Natural selection poses a threat philosophically to some people. The philosopher Daniel Dennett called
natural selection “Darwin’s dangerous idea” for a good reason: it is a very simple natural mechanism that
explains the appearance of design in living things. Before Darwin, the adaptations and exquisite complexity of
organisms were ascribed to creation by an omnipotent, beneficent designer, namely God, and indeed were
among the major arguments for the existence of such a designer. Darwin’s (and Wallace’s) concept of natural
selection made this “argument from design” completely superfluous. It accomplished for biology what Newton
and his successors had accomplished in physics: it provided a purely natural explanation for order and the
appearance of design. It made the features of organisms explicable by processes that can be studied by
science instead of ascribing them to miracles. The contemporary “intelligent design” movement is simply a
repetition of the predarwinian argument, and of course it cannot be taken seriously as a scientific explanation
of the properties of living things.
Directions:
1. Number the paragraphs (1, 2, 3, …)
2. Identify and circle the key words. These are words that are repeated, defined by the author, or are significant
to the understanding of the article.
3. On a separate sheet of paper (which you will staple to this article) write and define each of the key words you
have circled in the article.
4. Go back to read the article – possibly out loud with a partner or the class.
5. While you read, underline at 1 or 2 main ideas or key claims from each paragraph.
6. Next to each picture, write a description of what is going on in the picture or why you think the author
included that picture.
7. Lastly, on a separate sheet of paper which you will attach with the vocabulary definitions and this article)
write a detailed paragraph answering this question: Summarize the impact of Natural Selection on Evolution.