NATURAL SELECTION

By Edward L. Crisp, Ph.D.

Professor of Geology

INTRODUCTION

Organisms must survive the biotic and abiotic factors within their environment if they are going to survive to sexual maturity. But evolutionary success is not just dependent on survival, evolutionary fitness is known to be associated with reproductive success. The more offspring that an individual leaves for the next generation, the more fit the individual. At the level of the gene, fitness is measured by the success of one genotype (or individual gene - allele) over another genotype (or individual gene - allele).

SELECTION WORKS ON THE PHENOTYPE

The biotic and abiotic factors within the environment apply selection pressure to the phenotype, not the genotype. However, the environmental factors working on the phenotype will result in certain phenotypes having greater reproductive success than other phenotypes, thus the genotype ultimately changes. Therefore, the gene pool changes - thus evolution will occur as a result of selection pressures.

ARTIFICIAL SELECTION VERSUS NATURAL SELECTION

Artificial Selection

Darwin studied the artificial selection practiced by breeders of animals and plants to determine if and how particular traits (admired by humans, for agricultural purposes, beauty, etc.) were passed on to offspring (inherited). Of course, as animal and plant breeders had demonstrated, certain desirable traits of animals and plants (as perceived by humans) were heritable and passed on to offspring, thus undesirable traits could be weeded out. "This weeding out of organisms by humans for human purposes is artificial selection." (Kardong, 2005).

Dogs and Cats
Agriculture - corn, cabbage family, etc.
Flowers (like the tulip, rose, etc.)

Natural Selection

"The weeding out of organisms by biological processes, without deliberate or directed human intervention, is natural selection." (Kardong, 2005). Generally, natural selection is an average process that works on the phenotypes in the population, ultimately resulting in the survival and reproductive success of phenotypes that are more fit for their environment. Thus natural selection leads to a change in the genotypic frequencies in a population over time. By human standards, natural selection is a very slow process, acting over hundreds of generations. However, viruses and bacteria (because of very rapid reproduction) may evolve significantly in human lifetimes (examples, the AIDs virus, antibiotic resistance in certain strains of bacteria). There are also examples of more complex organisms that have recently evolved by natural selection, such as the following:

Darwin's Finches (a classic example): Darwin's sketch of some of the finches from the Galapagos Islands (from: http://pages.britishlibrary.net/charles.darwin2/beagle_images/pl77.jpg; originally from: Darwin, Charles, 1913, Journal of researches into the natural history and geology of the countries visited during the voyage round the world of H.M.S. Beagle, 11th ed., London, John Murray, 1913. [first published London, Henry Colburn, 1839].
Darwin reasoned that the finches of the Galapagos Islands had evolved from an ancestral species from the mainland of South America and with the passage of time the descendants on each of the islands had been modified by natural selection to occupy distinctive niches. In particular, the beaks of the various finches are specialized for a specific diet, such as soft or hard seeds, insects, etc.
Adaptive radiation of Darwin's Finches on the Galapagos Islands (from: http://www.pbs.org/wgbh/evolution/library/01/6/l_016_02.html
A modern study of the natural selection of beak size and beak depth of one of Darwin's Finches, Geospiza fortis, by Peter and Rosemary Grant of Princeton University on a small island, Daphne Major, in the Galapagos Archipelgo: "Evidence that natural selection alters beak size in Geospiza fortis. In dry years, when only large, tough seeds are available, the mean beak size increases. In wet years, when many small seeds are available, smaller beaks become more common. " (from: http://www.txtwriter.com/Backgrounders/Evolution/EVpage06.html)
Go here to see some actual photos of Darwin's Finches: http://www.rit.edu/~rhrsbi/GalapagosPages/DarwinFinch.html
Go to this site to see a study of mtDNA of Darwin's Finches that is strong evidence that they are a monophyletic group and arose from a common ancestor that was most likely a founder population from the mainland of South America: http://peptide.ncsa.uiuc.edu/tutorials_current/Darwin/proc_natl_acad_sci_v96_apr99.pdf#search='Darwin's%20Finches'

The Peppered Moth (Biston betularia) and Industrial Melanism - see discussion in Kardong, 2005.
(From: http://www.txtwriter.com/Backgrounders/Evolution/EVpage07.html)

Cliff Swallows - see discussion in Kardong, 2005.

Land Snails (Cepaea) - see discussion in Kardong, 2005.

Water Snakes (Nerodia sipedon) - see discussion in Kardong, 2005.

TYPES OF NATURAL SELECTION

Stabilizing Selection:

Directional Selection:

Disruptive Selection:

Sexual Selection: PBS Video on Evolution - Why Sex?