Talk:Basic Evolutionary Theory

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Here is what Aschafly wrote

Here is what Mr. Schafly wrote: "Having competing entries seems like a bad precedent to me. If someone creates an entry called "Richard Nixon", then can someone else create an entry called "The Real Richard Nixon"? Real encyclopedias would not have competing entries, so we shouldn't either."--Aschlafly 22:41, 21 March 2007 (EDT)





I have patched this primer together from various sources, including textbooks, journals, and (God-forbid) various other wikis. I have personally edited the content, so it is not just some copy/paste or quote-mine from elsewhere, although most of the content is NOT original. I repeat, this is NOT ORIGINAL WORK, it is only provided to help illuminate. Please restrict comments to talk page.--PalMDtalk 14:28, 8 April 2007 (EDT)

It's not bad. If I had one criticism it's that your definition - the change in populations of organisms over generations - is a bit wooly. I would say something like the change in allele frequency in a population. Chrysogonus 14:54, 8 April 2007 (EDT)

You refer, of course, to a classic distinction/discussion about what the basic unit of evolution is, the gene or the organism. For an article with "Basic" in the title, i thought it was going a bit too far.-- PalMDtalk 15:12, 8 April 2007 (EDT)


summary

This page is a summary of the scientific theory of evolution. For discussions of controversies surrounding the theory, please see, for instance Talk:Evolution. I politely ask to leave this unadulterated as a reference for the other discussions of the controvery. Thanks, --PalMDtalk 14:29, 8 April 2007 (EDT)


Evolution is the name of the scientific theory that describes the ongoing development of life on Earth. It is usually defined as the change in populations of organisms over generations. Offspring differ from their parents in various ways. When these differences are helpful, the offspring have a greater chance of surviving and reproducing, making the differences more common in the next generation. In this way, differences can accumulate over time, leading to major changes in a population. The basic unit of evolution is the population, the basic unit of natural selection is the individual. It is erroneous to speak of any individual "evolving". Lamarckian thinkers, who once dominated biology, and included Charles Darwin's grandfather Erasmus, felt that organisms could adapt to environmental pressures and pass these adaptations to future generations. This is not a valid part of modern evolutionary theory.

Mechanisms

Several processes drive evolution:

  • Random changes in population genotype over time ("genetic drift")
  • Introduction of new genes into a population ("gene flow")
e.g. plasmid transfer between bacteria
  • Changes in selection pressures ("natural selection", discussed below)
  • Genetic recombination with mating
  • Biased mate selection

Evolution occurs through changes in an organism's genes. An organisms genes can change in several ways. These changes can be neutral, can improve survival, or can hinder survival.


Darwin and Natural Selection

Evolution is driven by natural selection. Natural selection and evolution have no "direction", that is, there is no ultimate goal. The only important consequence is that any organism that is fit enough survives and reproduces. Darwin's explanation of the mechanisms of evolution relies on his theory of natural selection. The modern theory of natural selection incorporates five basic ideas:

  1. Organisms will produce more offspring than their habitat can sustain. Only members "fit enough" will survive to reproduce.
  2. Not all the offspring will be identical.
  3. Some of the differences between the offspring will be due to variations in their genetic makeup, the "code" that determines each organism's inherited traits.
  4. Genetic variations that help an organism to survive and reproduce are more likely to be passed on to the next generation than genetic variations that are unhelpful.
  5. Over time, helpful genetic variations will accumulate until a new species results.

Mendel: The Discovery of Genetics

Darwin’s theory of natural selection laid the groundwork for evolutionary theory, although he lacked an explanation for the source of the variations within the population. It was the emergence of the field of genetics, pioneered by Gregor Mendel (1822-1884), that provided this missing information and thus a clearer picture of how evolution works. Mendel’s experiments with peas led him to realize that heredity in sexual reproduction works by the mixing of separate factors (genes), not by the blending of inherited characters. It is the reshuffling and recombining of these factors during sexual reproduction that leads to variations in the offspring, ensuring that no two individuals would be exact copies of each other. This combination of Darwin's theory and our current understanding of heredity led to the birth of the scientific area called "population genetics".


Evidence of evolution

Many fields of science have provided a wide range of evidence for evolution. The most prominent of these are fossil records, which can show the many life forms that have existed. The fossil itself reveals the organism's structure, and the age of the fossil reveals when its species existed. In addition, studies involving anatomical and genetic comparisons between present day species serve as evidence for evolution.


The fossil record

Paleontology, the study of fossils, supports Darwin's original idea that all living creatures are related. Fossils also provide evidence that accumulated changes over long periods have led to the diverse forms of life we see today.

The fossil serves as a chronological record documenting the emergence of new, more complex species from simpler ancestral forms. The fossil record also provides some examples of transitional species that provide evidence of ancestral links between species that exist today.

Missing links, or Transitional Forms, are undiscovered fossils between two organisms that are supposedly evolutionarily linked. These abrupt changes in the evolutionary line are a cause of doubt to the validity of the evolutionary theory. In 1972, Niles Eldredge and Stephen Jay Gould developed the Theory of Punctuated Equilibrium, stating that evolutionary change occurs in rapid bursts, prolonged by periods of little change.

Comparative anatomy

Taxonomy is the branch of biology that names and classifies all living things. Scientists use morphological similarities to assist them in categorizing life forms. However, the bases of such groupings are the ancestral relationships that link them. For example, orangutans, gorillas, chimpanzees, and humans all belong to the same taxonomic grouping, in this case the family called "Hominidae". These animals are grouped together because of similarities in morphology which they share as a result of common ancestry. A similarity in anatomical features as a result of shared ancestry is called homology.


Humans also have many vestigial structures. Human vestigial structures include the ear muscles, the wisdom teeth, the appendix, the tail bone, body hair (including goose bumps), and the semilunar fold in the corner of the eye.

Anatomical comparisons can also be misleading. Organisms which share similar environments will often develop similar physical features. This is known as "convergent evolution". For example, both sharks and dolphins have similar body forms, yet are only distantly related, the first being a fish and the second a mammal. Such similarities are a result of both populations being exposed to the same selective pressures. Within both groups, random mutations that produce bodies that are more efficient for swimming would be favored. Thus, over time, they develop similar morphology.

As molecular genetics has advanced, it has become more and more the basis for classifying organisms.

Artificial selection

Artificial selection is the controlled breeding of domestic plants and animals. For example, people have produced many different types of dogs by controlled breeding. The differences between the Chihuahua and the Great Dane are the result of artificial selection.

Darwin drew much of his support for natural selection from observing the outcomes of artificial selection. In fact, much of his book On the Origin of Species, was based on his observation of the diversity in domestic pigeons. Darwin proposed that if dramatic changes in domestic plants and animals could be achieved by humans in short periods of time, then natural selection, given billions of years, could account for the differences seen in living things today.

Molecular biology

Geneticists and molecular biologists have been working on mapping evolutionary relationships between organisms based on genetic similarities. Since morphology (the phenotype) is an outward expression of that genetic code (the genotype), then an analysis of the genes themselves should provide an even clearer understanding of the relationships between species.

Comparing both DNA and amino acid sequences has been extremely useful when studying species that are so closely related that there are no obvious anatomical differences. The extent of their relationship can be determined based on the premise that the closer the relationship between species, the greater the similarities between their genetic codes.

Genetic comparisons also allow scientists to draw conclusions about organisms whose common ancestors lived such a long time ago that morphological similarities are not apparent. For example, comparison of the DNA sequences in chimpanzees with those of gorillas and humans has demonstrated that chimpanzees share more genetic similarities with humans than with gorillas. Therefore, this implies that these two species, humans and chimpanzees, share a closer evolutionary relationship as well.


Species

Biologist Ernst Mayr has given one of the most useful definitions of species. He defined a species as a population or group of populations whose members have the potential to interbreed naturally with one another to produce viable, fertile offspring. Also, the members of a species cannot produce viable fertile offspring with members of other species.

Speciation is the process that results in separate species forming from a single common ancestral population. One of the most common forms of speciation relies on geographic isolation. This method of speciation requires the geographic separation of a population. Separation may be due to a variety of geological forces such as the emergence of mountain ranges or the formation of canyons. For speciation to occur, separation must be complete to the point that genetic exchange between the two populations is completely disrupted. In their separate environments, the genetically isolated groups follow their own unique evolutionary pathways. Each group will accumulate different mutations as well as be subjected to different selective pressures. The accumulated genetic changes may result in separated populations no longer being capable of interbreeding should they be reunited. If interbreeding is no longer possible, then they would be considered different species. This form of speciation is also known as ring species.

Common Misconceptions

There are many misconceptions about biological evolution. These have been addressed with the following observations:

  • Evolution is non-directional. Whatever is fittest survives. There is no "pinnacle of evolution"
  • Humans have no special biological place in evolution, other than being the only species so far to develop complex language and intelligence.
  • Evolution is not just something that has happened, resulting in the species we see today. It is continuing to happen.
  • Evolution does not attempt to describe the process which brought forth life on earth (such as abiogenesis or some other method).
  • Populations evolve, individuals adapt.
  • The use of the word "theory" in the "theory" of evolution does not imply that evolution is any less well accepted or less supported by evidence than any other scientific theory, including the theory of gravity or the theory of quantum mechanics. A theory is a well-supported explanation for a given set of data, not a mere hypothesis.
  • Evolution does not state that humans are descended from monkeys, or that human ancestors are monkeys. However, evolution implies that humans and present-day monkeys share a common ancestor.
  • It is often claimed that evolution has never been observed. This is incorrect. Evolution has been observed in the laboratory and in the wild. The records of evolution in the past are found in fossils.
  • It is sometimes claimed that there is no evidence for evolution. This is incorrect. Evolution is supported by an immense body of scientific evidence. There is as much or more evidence for evolution as for any other scientific theory.
  • There is no serious disagreement among biological scientists about the validity of evolution. Though some aspects of evolution, such as the mechanisms and processes that drive it, are subject to some professional debate, more than 99.9% of all professional biological scientists support evolution.[1] It is the foundation of the research conducted in all fields of biology.

Summary

Evolution is a scientific theory that provides a mechanism to account for the variety of biological species we observe. Evolution is the result of two basic mechanisms:

  • Evolution requires genetic variation within the population. Offspring are not perfect copies of their parents, or each other. If they were, the only factor determining survival and reproductive success would be random chance.
  • Some offspring might have features that allow them to survive and thrive better than others. The offspring that survive will be more likely to have offspring of their own. Some of these useful features are passed along to new generations.

Therefore, evolution is an inevitable result of imperfectly copying, self-replicating machines reproducing over billions of years under the selection pressure of the environment.

There is a tremendous amount of evidence that is explained by evolution. Evolution has been observed in the laboratory. Domesticated animals evolve as we selectively breed them for certain traits. The records of past evolution are found in fossils as well as in our fundamentally similar genetic codes, demonstrating common ancestry of all organisms, both surviving and extinct.

Evolution is one of the most successful scientific theories ever produced and is widely accepted by biological scientists. An understanding of evolution underlies all biological sciences and much of medicine.

References

  1. Finding the Evolution in Medicine (National Institutes of Health).


Genetics

Further reading

  • Berra, Tim M., Evolution and the myth of creationism : a basic guide to the facts in the evolution debate Stanford, Calif. : Stanford University Press, c1990. ISBN: 0804715483
  • Darwin, Charles, The illustrated Origin of species, abridged & introduced by Richard E. Leakey ; consultants, W.F. Bynum, J.A. Barrett. London : Faber and Faber, 1979. ISBN: 0571114776
  • Dawkins, Richard, River out of Eden : a Darwinian View of Life. New York: Basic Books, 1995. ISBN 0465016065.
  • Dawkins, Richard, The Blind Watchmaker : Why the Evidence of Evolution Reveals a Universe Without Design. New York: Norton, 1996. ISBN 0393315703.
  • Dawkins, Richard, Climbing Mount Improbable. New York: Norton, 1996. ISBN 0393039307.
  • Gould, Stephen Jay, The Panda’s Thumb : More Reflections in Natural History. New York: Norton, 1980. ISBN 0393013804.
  • Gould, Stephen Jay, Wonderful life : the Burgess Shale and the nature of history, New York : W.W. Norton, c1989. ISBN 0393027058
  • Mayr, Ernst, What Evolution Is. New York: Basic Books, 2001. ISBN 0465044255
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