Introduction to Natural Selection : Natural selection is the differential survival and/or reproduction of organisms as a function of their physical attributes. Because of their phenotypes, which are due to the amalgam of traits that make up an individual, some individuals do better than others. The concept of selection is central to Darwin's theory of evolution, and forms the conerstone of many theories in the field of animal behavior. Selection is defined as some sort of functional relationship between fitness and phenotype.

Natural Selection of Genes : Natural selection would be occurring if, in a population of jungle fowl (the wild progenitors of chickens), single-comb genotypes were more reproductively successful than pea-comb genotypes. Note that the emphasis is not on survival but on reproduction. Thus while selection can occur because some individuals do not survive long enough to reproduce, sterile individuals also lack "fitness" in an evolutionary sense, as do individuals unable to find mates.

Natural Selection : For natural selection to occur, two requirements are essential: (1) There must be heritable variation for some trait. Examples: beak size, color pattern, thickness of skin, fleetness. (2) There must be differential survival and reproduction associated with the possession of that trait. Unless both these requirements are met, adaptation by natural selection cannot occur.

Synthetic Theory of Evolution: Natural Selection : Nicely illustrated with graphs to illustrate the differential survival of alleles in a population. There is a discussion of the sickle-cell trait as one example.

The original description of Natural Selection is found in Charles Darwin's "Origin of Species" as well as in the much briefer papers jointly published by Darwin and Alfred Russel Wallace in 1858.
On the Origin of Species (1st. Ed., HTML) and Gutenberg .txt & .zip files
On the Origin of Species (6th. Ed., HTML) and Gutenberg .txt & .zip files
Darwin-Wallace 1858 Papers : includes 1855 article by Wallace, and 1857 Darwin letter to Asa Gray.

Sexual Selection

Introduction to Sexual Selection : Charles Darwin distinguished sexual selection as variance in the number of mates. Sexual selection acts to refine secondary sexual characters of the phenotype such as morphological differences between males and females, or differences between male types. Primary sexual characters are the basic differences between male and female reproductive genital systems.

Mating Success : Darwin was impressed by the fact that qualities of sexual attractiveness were often the reverse of qualities leading to individual survival. He thought that gaining a higher chance to win mates was worth the risk conferred by such characters. Bright colors, long tails, plumes, antlers and horns threaten the survival of the animal but they also give them an advantage in fighting other males or attracting females.

The Pheasant Project -- An Example Case : In males a significant difference was found in spur length between different genotypes. There were fewer than expected MHC homozygous individuals and different survival of different MHC genotypes. These are the first data that directly support the "good genes" hypothesis by Hamilton and Zuk predicting that females discriminate among males on the basis of secondary sexual characters in order to pass on genes for disease resistance that improve fitness in their offspring.

Sexual Selection in Birds : Consider the reversal of normal sexual differences in the ornamentation of some polyandrous birds. There, the male must choose among females, which, in turn, must be as alluring as possible -- it is her secondary sexual characteristics that are enhanced. This fooled even Audubon, who confused the sexes when labeling his paintings of phalaropes. Female phalaropes compete for the plain-colored males, and the latter incubate the eggs and tend the young.

Darwin wrote about Sexual Selection in Origins of Species but he perhaps wrote more fully on the subject in The Descent of Man and Selection in Relation to Sex. That book may be downloaded here:
Descent of Man : HTML and Gutenberg .txt & .zip files

Genetic Drift

Random Genetic Drift : Of course random genetic drift is not limited to species that have few offspring, such as humans. In the case of flowering plants, for example, the stochastic element is the probabilty of a given seed falling on fertile ground while in the case of some fish and frogs it is the result of chance events which determine whether a newly hatched individual will survive.

What is a Species, and What is Not? : by Ernst Mayr

Stephen Jay Gould on Facts in Science : In science "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional consent."

Nothing in Biology Makes Sense Except in the Light of Evolution : Essay by Theodosius Dobzhansky (1900-1975). Transcribed from The American Biology Teacher, March 1973.

Evolution -- Fact or Theory? : Evolution is both a fact and a theory. In science all facts are provisional and subject to revision. There is no ultimate truth. This is the greatest strength of science, the humility to know that all things are not known.

The Scientific Method : Through experiments and careful observations the results of this approach are universal in the sense that they can be reproduced by any skeptic. It is from these ideas that the scientific method was developed. When consistency is obtained the hypothesis becomes a theory and provides a coherent set of propositions which explain a class of phenomena. A theory is then a framework within which observations are explained and predictions are made.

Do You Believe in Evolution? : by R. J. Riggins.
In my part of South Texas students ask that question a lot. Fundamentalism-Creationism is endemic here, and somehow that noisy minority has convinced the indifferent majority that to be a Christian of any sort, one must reject evolution.

SVP Statement on Evolution : A statement by The Society of Vertebrate Paleontology

Five Major Misconceptions about Evolution : If you hear anyone using these arguments, chances are he doesn't know enough about the real theory of evolution to make informed opinions about it.

A History of Evolutionary Thought : Celebrated figures and biographies

Darwin and Evolution Overview : from National Univ. of Singapore. A history of evolutionary thought.

Classics in Biogeography, Distribution, and Diversity : Biogeography provides a heap of compelling evidence for evolution and natural selection. Scientists and historians curious about the genesis of this field might want to burrow into this bibliography of works dating from the 1700s to 1950.

See the separate pages dealing with Ring Species and Living Transitionals for many examples of evolution at work today. If you are interested in the fossil evidence there are a series of pages demonstrating fine sequences of transitionals or intermediates in the Transitional Fossil Series.

American Scientist Article: The Origin of Animal Body Plans : For 3 billion years on earth, life was no larger or more sophisticated than a single cell. All of that changed when almost 600 million years ago new, multicellular life forms appear in the fossil record. Starting with simple soft-bodied creatures this innovation culminates in the "Cambrian explosion," a burst of biological creativity unprecedented in earth's history.

The Dawn of Animal Life Exhibit : While most people know of the dinosaurs from a mere 70 million years ago, very few are aware that the Earth's fossil record stretches back over 3 billion years. Using exclusively Canadian rocks and fossils, this exhibit highlights those three billion years.

First Appearances of Metazoans : This chart shows the oldest undoubted fossil occurences of each of the living major groups of animals. Note how many date back to the Cambrian period, over 500 million years ago.

Lightning Provides Vital Spark for Evolution: Bolts of lightning that strike the ground may help bacteria adapt and evolve. Experiments suggest that electrical currents help soil bacteria exchange DNA.

Climate Boosts Evolution: The first land plants and fungi changed earth's climate, paving the way for explosive evolution of land animals. Researchers found that land plants had evolved on Earth by about 700 million years ago and land fungi by about 1,300 million years ago.

Phylum Level Evolution: Phyla were defined more than a century ago based upon descriptions of modern fauna. Using this schema forces ancient animals to be included in one of the modern phyla and ignores the possibility of phyla-level evolution.

Please see the separate pages on Hominid Evolution which covers all the fossil evidence regarding transitional species in the primate lineage leading to modern humans, and/or the page dealing with Human Culture, Anthropology and Archaeology.

Human Evolution : This is an interesting website worth visiting, especially if you are interesting in combining a great deal of philosophy in your study of evolution. Much of the site is devoted to the discussion of such things as morality and human cultural influences.

See the separate page covering Evolution Education. There are many links, some according to age groups, to assist both teachers and students in aquiring information.

Teaching About Evolution and the Nature of  Science : Especially for teachers, but common sense for anyone. This is a well-written online book from the National Academy of Sciences.

The Eonic Effect : Theories in opposition to modern evolutionary synthesis.

Towards a New Theory of Evolution : The ghost of Jean Baptiste Lamark is back, casting a shadow over recent experiments arguably the most important to ever affect the theory of evolution.

Cosmic Ancestry : The modern version of panspermia. by Brig Klyce

Aquatic Ape Hypotheses of Human Evolution : At the fringe of science?

Access Research Network : Essays and other works by individuals who have helped give the "Intelligent Design" movement its form and structure. Currently featured are: Michael Behe, William Dembski, David K. DeWolf, Mark Hartwig, Phillip Johnson, Stephen Meyer, Paul Nelson, Robert C. Newman, Nancy Pearcey, Charles Thaxton, Jonathan Wells. Why list the new creationism here? So that you might become better informed as to what the anti-evolution movement is really about. Creationism is not going to go away.