Evolution ❮ 139It is easiest to illustrate natural selection with an example. Let’s revisit our pond before the
tornado came, where short- and long-finned fish inhabit murky waters. A new predator
invades the pond. Fin length determines swimming speed (longer fins allow a fish to swim
faster), and only the fastest fish can escape the predator. How would you expect the allele
frequencies to change under these conditions? Fish with what length fin would be eaten
the most? Because the short-finned fish would be the slowest, they would be featured on
the menu. But the long-finned fish, able to escape this new predator, would survive and
reproduce, and the frequency of the long-fin allele would increase relative to the short-fin
allele. We have created a situation in which allele frequencies change as a result of a non-
random event; the predator’s presence results in a predictable decrease in the short-fin allele
and a consequent increase in the long-fin allele. Remember that allele frequencies always
add up to 100 percent, so the long-finned fish don’t have to do particularly well for the
long-fin allele to increase—they only have to do well relativeto the short-finned fish. The
actual numbers of fish could decrease for both variants of this fish species.
Why aren’t organisms perfectly adapted to their environments? Since natural selection
increases the frequencies of advantageous alleles, why don’t we get to a point where all
individuals have all the best alleles? For one, different alleles confer different advantages in
different environments. Furthermore, remember that the environment—which includes
everything from habitat, to climate, to competitors, to predators, to food resources—is con-
stantly changing. Species are therefore also constantly changing as the traits that give them
an advantage also change. In cases where a trait becomes unconditionally advantageous, we
do in fact see fixed alleles; for example, all spiders have eight legs because the alternatives
just aren’t as good under any circumstances. But where there are heritable characters that
both vary and confer fitness advantages (or disadvantages) on their host organisms, natural
selection can occur.Lamarck and Darwin
The two key figures whose research you should know for the evolution section of the
AP Biology exam are Jean-Baptiste Lamarck and Charles Darwin. Lamarck proposed the
idea that evolution occurs by the inheritance of acquired characters. The classic example is
giraffe necks: Lamarck proposed that giraffes evolved long necks because individuals were
constantly reaching for the leaves at the tops of trees. A giraffe’s neck lengthened during its
lifetime, and then that giraffe’s offspring had a long neck because of all that straining its
parents did. The key here is that change happened within organisms during their lifetimes
and then the change in the trait was passed on.
What’s wrong with Lamarck’s theory? Try explaining to yourself how the changed char-
acter could be passed on to the offspring. The answer is that it couldn’t—the instructions in
the sex chromosomes that direct the production of offspring cannot be changed after they
are created at the birth of an organism. Lamarck confused genetic and environmental
(postconceptive) change, which is not surprising because no one had discovered genes yet.
Darwin had another idea, one that ended up being entirely consistent with Mendelian
genetics (although Mendel had already written his thesis during Darwin’s time, it is
rumored that his book sat on Darwin’s shelf, with the pages still uncut, until Darwin’s
death). Darwin suggested the idea of natural selection described above and coined the phrase
“survival of the fittest.” Although he didn’t call them genes,he proposed a hypothetical unit
of heredity that passed from parent to offspring. Incidentally, a man named Wallace also
came up with the idea of natural selection during the same time, but Darwin got the pub-
lication out first and has become famous as a result.BIG IDEA 1.A.2
Natural selection
acts on phenotypes.
BIG IDEA 1.A.3
Evolutionary
change is driven by
random processes.
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