Geology Reference
In-Depth Information
In the first half of the nineteenth century, two naturalists — Alfred Wallace and Charles
Darwin — were independently developing the same idea about biological change and
didn't know it. Darwin eventually published his ideas in the topic
On the Origin of Spe-
cies
and has become known for the idea of natural selection, even though Wallace was
also working on the same ideas at the same time.
In Darwin's time, plant and animal breeders were already practicing artificial selection,
much like breeders do today. In
artificial selection,
the breeder chooses an organism
with traits that are desirable and carefully breeds it to produce a specific result.
Examples of desirable traits would be beautiful color in flowers and particular flavors in
apples.
Understanding how artificial selection resulted in offspring with particular traits, Darwin
proposed that a natural process of selection worked similarly in nature. He proposed
that some traits may be more useful than others to an organism's survival and that
these traits would be naturally selected for, leading these traits to be passed on to the
next generation.
To use the example of a giraffe again, Darwin suggested that a giraffe with a longer neck
can reach leaves on higher branches than giraffes with shorter necks. This long-necked
giraffe therefore has access to more food. The success of the long-necked giraffe at find-
ing food increases its chance of survival and its opportunities to mate, reproducing off-
spring with similarly long necks.
Darwin and Wallace did not explain exactly how traits are passed from a parent to its off-
spring, but other scientists at the time were asking that very question.
Mendel's peas please
In the mid-1860s, a monk named Gregor Mendel experimented with pea plants and de-
termined that characteristics (such as flower color) seem to be controlled by a pair of
factors, one from each parent. Mendel determined that if the two factors in the pair are
different, one may be expressed in the offspring while the other remains present in the
cells and may be expressed in future offspring. These days, scientists know that the
“factors” Mendel proposed are actually genes (which I describe next).
For example, consider eye color. Maybe you have a grandparent with blue eyes but both
your parents have brown eyes. It is still possible for you (or your children) to have blue
eyes: an expression of the blue-eyed gene that is present, though perhaps not expressed,
in every generation.
