Simply stated, evolution = change.
Organic evolution refers to changes in populations.
In biology, evolution is the "great unifying theory" for understanding the history of life.
As a result of evolution, plants and animals living today are different from their ancestors. They differ in appearance, genetic characteristics, body chemistry, and in the way they function. These differences appear to be a response to changes in the environment and competition for food.
The idea of evolution had developed by 2,600 years ago in the 7th century BC, as seen in the writings of Greek philosopher, Anaximander.
Jean Baptiste Lamarck (1744-1829) observed lines of descent from older fossils to more recent ones, and to living forms. He correctly concluded that all species are descended from other species.
In trying to explain his observations, Lamarck assumed that new structures in an organism appear because of the needs or "inner want" of the organisms. Structures acquired in this way were thought to be somehow inherited by later generations (i.e., inheritance of acquired traits). The idea was challenged because there was no way to test for the presence of an "inner want."
Lamarck also suggested that unused body parts would not be inherited by succeeding generations. The hypothesis was tested and rejected after an experiment in which the tails were cut from mice for twenty generations. The offspring still had tails. Similarly, circumcision has been practiced for more than 4,000 years with no change among newborn males.
Charles Darwin and Alfred Wallace were the first scientists to assemble a large body of convincing observational evidence in support of evolution. They proposed a mechanism for evolution which Darwin called natural selection.
Natural selection is based on the following observations:
Darwin's theory was unable to explain WHY offspring exhibited variability. This was to come many years later, when scientists determined that genetics is the cause of these variations.
This principle can be simply stated as "the survival of the fittest."
Charles Darwin published his book, On the Origin of Species by Means of Natural Selection in 1859.
Gregor Mendel (1822-1884) demonstrated the mechanism by which traits are passed to offspring through his experiments with garden peas. His findings were published in an obscure journal and not recognized by the scientific community until 1900.
Mendel discovered that heredity in plants is determined by what we now call genes. Genes in the plant pollen and ovules are recombined during fertilization. Genes are linked together to form chromosomes.
Genetics is the branch of biology that deals with the study of heredity or inheritance.
Mendel is known as the "Father of Genetics."
Within the nucleus of each of our cells are chromosomes. Chromosomes consist of long DNA molecules (deoxyribonucleic acid). The DNA molecule consists of two parallel strands, which resemble a twisted ladder. The twisted strands are phosphate and sugar compounds, linked with nitrogenous bases (adenine, thimine, guanine, and cytosine). Genes are the parts of the DNA molecule that transmit hereditary traits.
The structure of the DNA molecule was discovered by Watson and Crick in 1953.
DNA carries chemically coded information from generation to generation, providing instructions for growth, development, and functioning.
Reproduction in organisms may be:
All reproductive methods involve cell division.
New combinations of chromosomes result through sexual reproduction.
One member of each pair of chromosomes is inherited from each parent.
This sexual genetic recombination leads to variability within the species.
In a human cell there are 23 pairs of chromosomes.
One of these pairs determines the sex of the individual.
Cells with paired chromosomes are called diploid cells.
Sex cells (or gametes) have only one half of a pair of chromosomes; they are called haploid cells. Example: egg cells or sperm cells.
Asexual reproduction can occur through:
When two gametes (such as an egg and a sperm) combine, a fertilized egg forms. The fertilized egg has paired chromosomes.
Variation occurs because of the sexual recombination of genes.
Each individual has its own set of genes. The sum of all of the genes in the breeding population is the gene pool. Genes are recombined in each successive generation.
Genetic mutations produce changes in genes and DNA. Mutations are chemical changes to the DNA molecule. Mutations can be caused by a chemical substance (including certain drugs), or by exposure to radiation (which includes cosmic radiation, ultraviolet light, and gamma rays). Mutations may also occur spontaneously without a specific causative agent.
Mutations may occur in any cell, but mutations in sex cells will be passed on to succeeding generations.
Mutations produce much of the variability on which natural selection operates.
Evolution may involve change from three different sources:
September 21, 2005