Fossil species appear and disappear throughout the stratigraphic record. The Geologic Time Scale is based on these appearances and disappearances. Each of the Eras ends with a mass extinction. Period boundaries coincide with smaller extinction events, followed by appearances of new species.
Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.
The better adapted individuals will survive to reproduce. This passes along favorable traits to their offspring.
As the less-well-adapted individuals die off before reproducing, and the more tolerant, better adapted individuals reproduce, favorable traits will become dominant in the population. There will be a shift in the gene pool.
The environment "selects" the best adapted individuals, hence the term "natural selection".
Gene pool - the sum of all of the genetic components in a population.
Species - The fundamental unit of biological classification.
A group of individuals that are similar in structure, function, and development,
with the potential to interbreed and produce fertile offspring.
Reproductive barriers between species prevent interbreeding.
Closely related (but different) species, such as the horse and the donkey CAN interbreed, but (generally) do not produce FERTILE offspring (mule).
Liger. Offspring of a male lion and a female tiger. Wild Animal Safari, Pine Mountain, GA.
Zedonk. Offspring of a zebra and a donkey. Wild Animal Safari, Pine Mountain, GA.
Within the nucleus of each of our cells are chromosomes. In a human cell there are 23 pairs of chromosomes. (One of these pairs determines the sex.) Chromosomes consist of long DNA molecules, highly folded and coiled and combined with a variety of protein molecules. DNA stands for deoxyribonucleic acid. The general form of the DNA molecule is described as a "double helix", which resembles a twisted ladder (Levin, 8th edition, p. 130). The long part of the ladder is made of phosphate and sugar compounds, and the rungs on the ladder are made of nitrogenous bases (adenine, thimine, guanine, and cytosine).
The structure of the DNA molecule was discovered by Watson and Crick in 1953.
The part of the DNA molecule responsible for the transmission of inheritable traits is called a gene. Chromosomes are composed of genes. There are approximately 100,000 genes in a human cell, only a small number of which have been identified and roughly located on the various chromosomes.
DNA carries chemically coded information from generation to generation, providing instructions for growth, development, and functioning.
Through sexual reproduction, new combinations of chromosomes result. One member of each pair of chromosomes is inherited from each parent. This sexual genetic recombination leads to variability within the species.
Genetic mutations also produce alterations in genes and DNA. Mutations are simply chemical changes to the DNA molecule. Mutations can be caused by a chemical substance, or by exposure to radiation (which includes cosmic radiation and ultraviolet light. Mutations produce much of the variability on which natural selection operates.
Variability is the raw material for natural selection.
Human embryos also have a well-defined tail by the fourth week of development, which reaches maximum length when the embryo is six weeks old. Similar embryonic tails are also found in other mammals, such as dogs, horses, and monkeys. In humans, the tail eventually shortens, persisting only as a vestighial feature in the adult coccyx.
The most familiar vestigial organ in humans is the appendix. This wormlike structure attaches to a short section of intestine called the cecum, which is located at the point where the large and small intestines join. The human appendix is a functionless vestige of a fully developed organ in other mammals, such as the rabbit and other herbivores, where a large cecum and appendix store vegetable cellulose to enable its digestion with the help of bacteria.
Organic evolution refers to changes in populations.
Phylogeny = the sequence of organisms placed in evolutionary order.
All organisms are controlled by a combination of:
Phenotype - the appearance of an organism; the observable expression of the genotype. It is influenced by both genetics and environment.
All organisms go through changes in appearance during their lifespan. This is called ontogeny. Examples?
There is no change in genes from tadpole (fish-like water dweller) to frog (has legs, breathes air, can live on land). It is reasonable to apply this as a modern analogy to the geologic record to aid in explaining the transition from fish to amphibian. The genotype remains the same but the phenotype changes as a result in a change in a developmental timing gene (either activation of a gene that was always there, or a mutation to a gene). After observing a tadpole "grow up" into a frog, it is not that difficult to understand how a fish in the Devonian could "evolve" into an amphibian.
In each case, rapid and dramatic shifts in appearance have occurred. Indeed, the casual observer might conclude (erroneously) that a caterpillar and a butterfly represented completely different species. The cause of these variations relates to developmental timing. (Note that a mutation causing a slight shift in developmental timing can lead to mature individuals which retain juvenile forms, or juveniles which take on some of the characteristics of adults - such as accelerated sexual development. This is known as paedomorphosis.) It has been stated by Ernst Haeckel (1834-1919) that as a general rule "ontogeny recapitulates phylogeny". In other words, the series of changes that an individual goes through during the course of its lifetime mimics the stages that the organism has passed through during evolution. The early stages of descendants resemble the adult stages of the ancestors.
The "law of recapitulation" has been discredited since the beginning of the twentieth century. Experimental morphologists and biologists have shown that there is not a one-to-one correspondence between phylogeny and ontogeny. Although a strong form of recapitulation is not correct, phylogeny and ontogeny are intertwined, and many biologists are beginning to both explore and understand the basis for this connection. Ernst Haeckel, U. California Berkeley page.
These changes which we know so well are examples of microevolution or evolution at or below the species level.
Changes or evolution at or above the species level are termed macroevolution.
Sources: (1) Discover magazine, March 1988, and (2) National Geographic magazine November 1985.
The question is not whether evolution occurs, but rather, exactly how it occurs. What is the mechanism?
Several theories about how evolution occurs:
In general, a stimulus like an environmental change causes the evolution into a new species. The organism must either:
When the barriers are removed, the isolated population may have changes so much that it is no longer able to interbreed with the remaining populations of the species. Changes may be in morphology, in function, in developmental timing, or in behavior.
The removal of the barriers and the migration of the isolated population to the larger area will look like the sudden appearance of a new species.
Fossil taxa appear and disappear in succession
The pattern provides strong evidence for evolution .
This page created by Pamela J. W. Gore
Georgia Perimeter College, Georgia
Created Fall 1995
Modified November 6, 1996
Last modified February 3, 1999
Links updated to facstaff, email updated August 28, 2008