Post by Irma Pince on Aug 22, 2005 23:27:06 GMT -5
In biology, evolution is a process by which organisms acquire and pass on novel traits from generation to generation. One of the first theories of biological evolution was proposed in the early 19th century by Jean-Baptiste Lamarck, though his idea that individual organisms acquire traits during their lifetimes that they pass on to offspring was fundamentally flawed. With the publication of Charles Darwin and Alfred Russel Wallace's joint paper in 1858 followed by Darwin's book Origin of Species in 1859, the theory of evolution by natural selection became firmly established within the scientific community. In the 1930s, work by a number of scientists combined Darwinian natural selection with the re-discovered theory of heredity proposed by Gregor Mendel to create the modern evolutionary synthesis. In the modern synthesis, "evolution" means a change in the frequency of an allele within a gene pool from one generation to the next. This change may be caused by a number of different mechanisms: natural selection, genetic drift or changes in population structure (gene flow).
The theory underlying the modern synthesis has three major aspects:
1. The common descent of all organisms from a single ancestor.
2. The manifestation of novel traits in a lineage.
3. The mechanisms that cause some traits to persist while others perish.
The modern synthesis, like its Mendelian and Darwinian antecedents, is a scientific theory. In plain English, people use the word "theory" to signify "conjecture", "speculation", or "opinion". In contrast, a scientific theory is a model of the world (or some portion of it) from which falsifiable hypotheses can be generated and be verified through empirical observation. In this sense, "theory" and "fact" do not stand in opposition, but rather exist in a reciprocal relationship — for example, it is a "fact" that an apple dropped on earth will fall towards the center of the planet in a straight line, and the "theory" which explains it is the current theory of gravitation. Currently, the modern synthesis is the most powerful theory explaining variation and speciation, and within the science of biology, it has completely replaced earlier accepted explanations for the origin of species, including creationism and Lamarckism.
A group of organisms is said to have common descent if they have a common ancestor. In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool.
Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds — even those which do not fly — have wings. Today, the theory of evolution has been strongly confirmed by the science of DNA genetics. For example, every living thing makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. Because the selection of these traits is somewhat arbitrary, their universality strongly suggests common ancestry.
In addition, abiogenesis — the generation of life from non-living matter — has never been observed, indicating that the origin of life from non-life is either extremely rare or only happens under conditions very unlike those of modern Earth. The 1953 Miller-Urey experiment suggests that conditions on the ancient earth may have permitted abiogenesis.
Since abiogenesis is rare or impossible under modern conditions and the evolutionary process is exceedingly slow, the diversity and complexity of modern life requires that the Earth be very old, on the order of billions of years. This is compatible with geological evidence that the Earth is approximately 4.6 billion years old. (See Timeline of evolution.)
Information about the early development of life includes input from the fields of geology and planetary science. These sciences provide information about the history of the Earth and the changes produced by life. A great deal of information about the early Earth has been destroyed by geological processes over the course of time.
The theory underlying the modern synthesis has three major aspects:
1. The common descent of all organisms from a single ancestor.
2. The manifestation of novel traits in a lineage.
3. The mechanisms that cause some traits to persist while others perish.
The modern synthesis, like its Mendelian and Darwinian antecedents, is a scientific theory. In plain English, people use the word "theory" to signify "conjecture", "speculation", or "opinion". In contrast, a scientific theory is a model of the world (or some portion of it) from which falsifiable hypotheses can be generated and be verified through empirical observation. In this sense, "theory" and "fact" do not stand in opposition, but rather exist in a reciprocal relationship — for example, it is a "fact" that an apple dropped on earth will fall towards the center of the planet in a straight line, and the "theory" which explains it is the current theory of gravitation. Currently, the modern synthesis is the most powerful theory explaining variation and speciation, and within the science of biology, it has completely replaced earlier accepted explanations for the origin of species, including creationism and Lamarckism.
A group of organisms is said to have common descent if they have a common ancestor. In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool.
Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds — even those which do not fly — have wings. Today, the theory of evolution has been strongly confirmed by the science of DNA genetics. For example, every living thing makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. Because the selection of these traits is somewhat arbitrary, their universality strongly suggests common ancestry.
In addition, abiogenesis — the generation of life from non-living matter — has never been observed, indicating that the origin of life from non-life is either extremely rare or only happens under conditions very unlike those of modern Earth. The 1953 Miller-Urey experiment suggests that conditions on the ancient earth may have permitted abiogenesis.
Since abiogenesis is rare or impossible under modern conditions and the evolutionary process is exceedingly slow, the diversity and complexity of modern life requires that the Earth be very old, on the order of billions of years. This is compatible with geological evidence that the Earth is approximately 4.6 billion years old. (See Timeline of evolution.)
Information about the early development of life includes input from the fields of geology and planetary science. These sciences provide information about the history of the Earth and the changes produced by life. A great deal of information about the early Earth has been destroyed by geological processes over the course of time.