Marco Trujillo
Human Origins 1020
8/7/13
Professor Teri Potter
Charles Darwin’s Natural Selection
One of the most important contributions made to the science of evolution was made
by Charles Darwin and the concept of natural selection. The idea that members of a species
compete with each other for resources and that individuals that are better adapted to their
lifestyle have a better chance of surviving to reproduce revolutionized the field of evolution.
Though it was not accepted until several decades after Darwin first proposed it. Today
natural selection forms the basis for our understanding of how species change over time.
Darwin’s process of natural selection has four components.
1.
Variation. Organisms exhibit individual variation in appearance and behavior. These
variations may involve body size, hair color, facial markings, voice properties, or number of
offspring. On the other hand, some traits show little to no variation among individuals for
example, number of eyes in vertebrates.
2.
Inheritance. Some traits are consistently passed on from parent to offspring. Such traits are
heritable and other traits are strongly influenced by environmental conditions and show
weak heritability.
3.
High rate of population growth. Most populations have more offspring each year and there
are more offspring than local resources can support. Leading to a struggle for
resources. Each generation experiences substantial mortality.
4.
Differential survival and reproduction. Individuals possessing traits well suited for the
struggle for local resources will contribute more offspring to the next generation.
From one generation to the next, the struggle for resources will favor individuals
with some variations over others and change the frequency of traits within the
population. This process is natural selection. The traits that give an advantage to those
individuals who leave more offspring are called adaptations. (Bishop)
In order for natural selection to operate on a trait, the trait must possess
reproductive fitness and must have an advantage in the competition for resources. If one of
these requirements does not occur, then the trait does not experience natural selection.
Natural selection operates by close competive advantage, not an absolute standard of
design.
“…as natural selection acts by competition for resources, it adapts the inhabitants of each
country only in relation to the degree of perfection of their associates” (Charles Darwin).
During the twentieth century, genetics was integrated with Darwin’s mechanism,
allowing us to evaluate natural selection as the differential survival and reproduction of
genotypes, corresponding to particular phenotypes. Natural selection can only work on
existing variation within a population. Such variations arise by mutation, a change in some
part of the genetic code for a trait. Mutations arise by chance and without warning for the
potential advantage or disadvantage of the mutation. In other words, variations do not
arise because they are needed. (Zimmer)
Darwin observed in the natural world around him. He saw that organisms produce
more offspring than is required to replace themselves, so population sizes should increase
rapidly. He figured out resources such as food, water or places to sleep or mate, are limited.
A major influence on Darwin observing this fact was his reading the work of Thomas
Malthus who published a paper stating that the human population was increasing at a rapid
pace and would soon run out of food, water and space. These are three simple facts which
Darwin put together to draw a simple conclusion. Individuals compete with each other for
scarce resources and those with the favorable traits become more common up until a new
species has evolved. (F. Darwin)
Galapagos finches are the famous example from Darwin's Natural Selection. Each
island of the Galapagos that Darwin visited had its own kind of finch (14 in all), found
nowhere else in the world. Some had beaks adapted for eating large seeds, others for small
seeds, some had parrot-like beaks for feeding on buds and fruits, and some had slender
beaks for feeding on small insects. One
used a thorn to probe for insect larvae in
wood, like some woodpeckers do. To
Darwin, it appeared that each was slightly
modified from an original colonist,
probably the finch on the mainland of
South America, some 600 miles to the east.
It is probable that adaptive radiation led to
the formation of so many species because other birds were few or absent, leaving empty
niches to fill; and because the numerous islands of the Galapagos provided ample
opportunity for geographic isolation. (Oxford Journal)
Darwin's finches are an excellent example of the way in which species' gene pools
have adapted in order for long term survival via their offspring. Their beaks have evolved
over time to be best suited to their function. For example, the finches who eat grubs have a
thin extended beak to poke into holes in the ground and extract the grubs. Finches who eat
buds and fruit would be less successful at doing this, while their claw like beaks can grind
down their food and thus give them a selective advantage in circumstances where buds are
the only real food source for finches. (Oxford Journal)
Darwin added to his bulk of evidence throughout his lifetime, for example with
studies on humans. Since 1859 the scientific community has been busy testing his theories,
and alternatives, to see what best holds up. The wealth and diversity of evidence is now vast
and includes evidence from the DNA record, fossil record, and from case studies section.
(Bishop)
Works Cited
1. Evolution By Natural Selection, Beth A. Bishop and Charles W. Henderson, The
Institute For Research of Michigan University. June 1986, July 2013
2. The Autobiography of Charles Darwin, Charles Darwin(Edited by Francis Darwin)
The Thinkers Library. July 2013
3. Evolution, Carl Zimmer, Arrow. April 2003, July 2013
4. Oxford Journals, On The Origin of Darwin’s Finches. Web July 2013
mbe.oxfordjournals.org
5. The Origin of Species, Charles Darwin 1859. Web July 2013
6. En.wikipedia.org/images/finches