Evolution

Evolution is another word for change; since the 20th century usually refers to biological evolution, the change in the bodyplans of species over time and the appearance of new species. Often it is shorthand for Darwin's theory of evolution of species through natural selection.

In the 19th century the word "evolution" was also identified with progress. It was clear to European thinkers at that time -- in the wake of the Enlightenment and the French Revolution -- that human societies evolved; many people have claimed the same about the evolution of biological species. In the 20th century, most social scientists came to reject the strict identification of social and cultural change with progress (see also social evolution and cultural evolution); Most interpretations of Darwin's account of evolution similarly argues against identifying biological changes with progress.

Biological evolution and natural selection

This section discusses biological evolution, and the commonly accepted scientific theory about the origin and development of living things, including plants, humans, and other animals.

The word "evolution" may refer to many types of change. In this context, "evolution" refers to inheritable changes in organisms. Biological evolution's basis is that populations of organisms (species) have changed over time from the forms we see in the fossil record to the forms we see today. In this way, biological evolution is distuingished from Darwinism, which makes specific claims about the method by which this change is accomplished.

Darwinism, and its descendent theories, state that biological evolution results through natural selection. Since natural selection is so important to Darwinism and modern theories of evolution, a very short summary of its main points follows:

Organisms have children which inherit similar characteristics to their parents. In plain english, kids are like mom and dad.
Organisms are differentially sexually successful based on their traits in a given environment. In plain english, animals (or plants!) that are good at what they do have more kids.
Therefore, over time, the types of organisms which have traits better adapted to their environment will tend to become the dominant ones in an environment. In plain english, there tend to be more of the kinds that are better at having more kids.

In Darwin's time, there was no widely accepted mechanism for heritability. In modern times, the mechanism for heritability is known to be DNA. There is also the interesting possibility that proteins are responsible for some heritability. (See The Central Dogma for more details). Currently, the most widely accepted theory for the origin of life is called RNA World, in which RNA by process of natural selection slowly recruits DNA and proteins into what we now think of as life.

Natural selection also provides for a mechanism by which life can sustain itself over time. Since, in the long run, environments always change, if successive generations did not develop adaptations which allowed them to survive and reproduce, species would simply die out as their biological niche dies out. Therefore life is allowed to persist over great spans of time, in the form of evolving species.

Scientists generally regard evolution via natural selection as an established theory, on the grounds that it is so overwhelmingly well supported by evidence that there is no serious doubt of its occurrence. One might compare it with the legal requirement of "beyond a reasonable doubt". Though there has been dispute among scientists as to what specific mechanisms in the natural world cause it, virtually all scientists agree that natural selection plays an important role in evolution. There still exist various viewpoints on which other mechanisms are most important.

Microevolution

Microevolution can be thought of as the explanation for characteristics observed in organisms. This veiwpoint of evolution is sometimes termed 'the modern synthesis. The synthesis states that inheritable changes in the characteristics of an individual can arise in a population. This can be through a number of processes;

Recombination
Mutation
Gene flow

Differential survival of characteristics that arise in the population mean that some will become more frequent while others may be lost. Two processes are generally thought to contribute to the survival of a characteristic;

Genetic drift (basically random processes)
Natural selection, (some characteristics will confer a reproductive benefit, resulting in an increase in their frequency.)

Natural selection is often cited as an explanation for apparent design in nature, (otherwise known as adaptation. However Stephen Jay Gould criticised many scientists for inappropriate invocation of this explanation, suggesting that many adaptive explanations amount to little more then "Just So Stories" without any real scientific evidence. Drift is an alternative explanation for the occurrence of many characteristics. Equally drift may be used to explain apparent maladaptations.

Macroevolutionary Forces

Macroevolution, on the other hand, refers to large-scale changes in gene-frequencies in a population over a long period of time, usually with wider ranging consequences then simple adaptations. This may cover many areas of biology that are difficult to explain using microevolutionary process. These include questions such as;

Why did the major groups of animals suddenly appear in the fossil record (known as the Cambrian Explosion)?
Why have no new major groups of living things appeared in the fossil record for a long time?
Why does evolution apparently occur in spurts, with many species undergoing long periods of stasis with little evolutionary change (punctuated equilibrium,)?
What process lead to speciation?

Many biologists, (notably Richard Dawkins, feel that the explanation to this answers can be reached through extrapolation of microevolutionary processes. However other biologists (most notably Stephen Jay Gould), believe that other processes then those invoked in microevolutionary explanations exist. However few possibilities for these hypothetical forces have been suggested. One major problem are the scales of resolution offered by biological techniques. The fossil record cannot record events that happened in less then a million years, while experiments at best may only cover changes that occur across a few generations.

Some proponents of creationism accept that microevolution occurs in the short term, whereas macroevolution, specifically leading to speciation, is expressly rejected. Microevolution can easily be demonstrated in the laboratory to the satisfaction of most. Whilst speciation events have been demonstrated in the laboratory and observed in the field, really dramatic differences between species do not usually occur in directly observable timescales, (it occurs too quickly for the process to be shown in the fossil record.) Some creationists have argued that since macroevolution can not be confirmed by experiment, it cannot be considered to be part of a scientific theory. However, evolutionists counter, astronomy, geology, archaeology and the other historical sciences, like macroevolution, have to check hypotheses by finding out if they conform or fit with the physical or observational evidence and can make valid predictions. In this way, macroevolution is testable and falsifiable.

Among laymen, the very question whether biological evolution has taken place is controversial, at least in the United States. According to a Gallup poll, nearly half of Americans disbelieve in any sort of biological evolution.

As science has uncovered more and more information about the basic operations of life, such as genetics and molecular biology, theories of evolution have changed. The general trend has been not to overturn well-supported theories, but to supplant them with more detailed and therefore more complex ones.

While transmutation was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species which provided the first cogent mechanism by which evolutionary change could persist: his mechanism of natural selection. The evolutionary timeline outlines the major steps of evolution on Earth as expounded by this theory's proponents.

Following the dawn of molecular biology, it became clear that a major mechanism for variation within a population is the mutagenesis of DNA. An essential component to evolutionary theory is that during the cell cycle, DNA is copied fairly, but not entirely, faithfully. When these rare copying errors occur, they are said to introduce genetic mutations of three general consequences relative to the current environment: good, bad, or neutral. By definition, individuals with "good" mutations will have an a stronger propensity to propagate, individuals with "bad" mutations will have less of a chance at successful reproduction, and those carrying "neutral" mutations will have neither an advantage nor a disadvantage. These definitions assume that the environment remains stable. Considered at the level of a single gene, these variations just described represent different genetic alleles. Following environmental change, alleles may retain their classification of good, bad, or neutral, or may shift into one of the other categories. Individuals carrying alleles formerly classified as neutral may now be "good" as they bear favourably adaptive mutations. Since neutral alleles can accumulate in the population without consequence while an environment is stable, they create a considerable reservoir for adaptability.

Although evolution depends on genetic variation over time, it is not purely restricted in cause to DNA mutation: bacteria can exchange genetic material via small DNA structures called plasmids. Recombination during sexual reproduction can also result variation. Variations in DNA are also not always the result of random mutations: modification of the structure of DNA without rearranging the base-sequence also has phenotypical effects. When these modifications (such as methylation) are inherited by offspring, epigenetic inheritance is said to occur. This differs from Darwinistic evolution in the sense that epigenetic inheritance allows for adaptive variations.

Genetic variation is the first step in evolution cycle; complementary to some form of selection which acts to determine which genes are passed on.

One criticism of the model of natural selection is raised by the lack of smooth transitions between species in the fossil record. One theory about why transitional forms are sometimes missing (although they are also sometimes found) is called punctuated equilibrium. Punctuated equilibrium is the theory that speciation happens in small populations which are cut off, possibly geographically, from others of their species, and which develop independently. Evolution in these small groups is believed to occur relatively quickly, perhaps in only a few thousands of years. Later the isolated population reenters the wider geographical area and supplants its closest relatives. Many scientists support this view, but it is still somewhat controversial.

Another even more controversial, and not widely supported, view of how evolution can occur is provided by quantum evolution. Simply described, this theory relies on DNA acting almost as a computer that is able to perceive the quantum multiverse (which contains all possibile universes) and choose a mutation that is beneficial to the organism. Variations range from extremely simple mutations all the way to large jumps as seen in the fossil record.

Evolutionary programming

Evolutionary processes have recently been put to use in computer science through genetic programming which uses the gene transmission and mutation mechanism as an optimization technique, and through evolutionary programming, which allows one to parameterize computer programs to find optimal solutions according to a goal function.

Books:

Famous evolution researchers and popularizers: