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Evolution

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Evolution is any process of growth, change or development. The word stems from the Latin evolutio meaning "unfolding" and prior to the late 1800s was confined to referring to goal-directed, pre-programmed processes such as embryological development. A pre-programmed task, as in a military maneuver, using this definition, may be termed an "evolution." One can also speak of stellar evolution, cultural evolution or the evolution of an idea.

In the 19th century the word "evolution" was identified with improvement. 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 improvement (see also social evolution and cultural evolution); Most interpretations of Darwin's account of evolution similarly argue against identifying biological changes with improvement.

Evolution in self-replicating systems involves a 3-stage ratchet process. (1) A mutation (change in the information comprising the original form) occurs. (2) The new form interfaces and interacts with its environment. (3) The result of that interaction is referred to as natural selection, and amounts to either the survival of the new form (the ratchet bites successfully and holds in place) or the destruction of the new form (the ratchet fails and falls back).

Since the 19th century "evolution" is generally used in reference to biological evolution, changes in the characteristics of life. Often it is shorthand for the modern theory of evolution based upon Darwin's idea of natural selection. The remainder of this article addresses biological evolution

The commonly accepted scientific theory about how life has changed since it originated has three major aspects.

  1. The ancestral relationship between organisms, both living and fossilized.
  2. The emergence of novel traits in a lineage
  3. The mechanisms that cause some traits to persist while others perish

Ancestry of organisms

Most biologists believe that all life on Earth is descended from one common ancestor, affectionately called LUCA (Last Universal Common Ancestor). This conclusion is based upon the fact that many traits of living organisms, such as the genetic code, seem arbitrary yet are shared by all organisms.

The study of the ancestry of species is phylogeny. Phylogeny has revealed that organs with radically different internal structures can bear a superficial resemblance and perform similar functions. These examples of analogous evolution show that there are multiple ways to solve most problems and make it difficult to believe that the universal traits of life are all necessary. Likewise other organs with similar internal structures will perform radically different functions. These examples of homologous evolution indicate that two organisms descended from one organism that had developed the basic structure.

Further evidence of the universal ancestry of life is that abiogenesis has never been observed under controlled conditions, indicating that the origin of life from non-life is either very rare or only happens under conditions that are not at all like those of modern earth.

The emergence of novel traits

If life is to change, then new traits must emerge at some point. Geneticists have studied how traits emerge and are passed to succeeding generations. In Darwin's time, there was no widely accepted in-depth mechanism for heritability. However, it is now known that most inherited variation can be traced to discrete, persistent entities called "genes", which are aspects of a linear molecule called DNA. Alterations in DNA, known as mutations, have been observed to alter traits. Furthermore, DNA variants may have little phenotypic effect in isolation but create new traits when combined in an organism through genetic recombination. Genetic recombination is produced both by the fusion of cells of opposite mating types (such as human sex), and by the transfer of material into an intact cell (such as bacterial conjugation and transformation).

Researchers are also investigating heritable variation that is not connected to variations in DNA sequence that influence standard DNA replication. The processes that produce this variation leave the genetic information intact and are often reversable. These are often referred to as epigenetic inheritance and may include phenomenon such as DNA methylation, prions, and structural inheritance. Investigations continue into whether these mechanisms allow for the production of specific beneficial heritable variation in response to environmental signals. If this is shown to be the case, then some instances of evolution would lie outside of the framework that Darwin established, which avoided any connection between environmental signals and the production of heritable variation. In general, Darwin knew little about the nature or source of heritable variation.

Macroevolution

Some researchers point out that known sources of variation can only account for variation within species, and don't account for the variation between larger taxanomic groups. They hypothesize that different processes must be responsible for such large variations. In this context, they refer to the small changes as microevolution and the large changes as macroevolution. However, the majority of evolution researchers consider the large gaps between taxanomic groups to be explainable by ecological factors, such as extinctions, population bottlenecks, and the emergence of unoccupied ecological niches. They refuse to make a distinction between microevolution and macroevolution. This viewpoint of evolution is sometimes termed "the modern synthesis".

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 than 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?

Macroevolution versus Microevolution; Are they the same process?

There are two leading schools of thought on the issue of macro and micro evolution. Many biologists, (notably Richard Dawkins), feel that the explanation of all evolutionary phenomena can be reached through the extrapolation of microevolutionary processes. However, other biologists (most notably Stephen Jay Gould), believe that other processes than those invoked in microevolutionary explanations exist, (i.e. macroevolutionary forces).

The distinction made by people between micro and macroevolution frequently annoys leading scientists of the first school. Their argument is that macroevolution is simply a greater degree of microevolution; there is no absolute difference. Microevolution is very simple single mutations. Over time, in isolated populations, this small mutations add up to large changes that eventually result in "macroevolution", or the forming of a new species. Macroevolution is really just what happens in a longer time period, when a lot of "microevolutions" add up.

Scientists of the second school suggest that other forces occur in evolution then the gradual accumulation of simple mutations. A suggested example could be the importance of macromutations such as the addition of body segments among the Arthropoda. Unfortunately, few possibilities for testing the existence of hypothetical macroevolutionary forces have been suggested. One major problem lies in the scales of resolution offered by biological techniques. The fossil record cannot record events that happened in less than 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 observers. 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 that 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.

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 more prosaic explanation is simply that the transitional forms are missing only because, for some reason, they failed to be fossilized. Considering that fossilization of organisms is actually the incredibly rare and exceptional event rather than the norm, this is a likely explanation. Common arguments against evolution usually discuss such "missing links". In reality, we have uncovered millions of fossils that all fit together in a nice big evolutionary bush. Of course there are going to be "missing links". It would be naive to think that every living subspecies left behind good, clean fossils that have all already been found by scientists. Think about it: for every "missing link" that the scientists discover, two more "missing links" are going to be created, by definition. That's why the missing link argument really only exists in the uniformed public eye and has no scientific validity.

Another more controversial, barely 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 possible 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. While clearly appealing on an aesthetic level, a major problem with this theory, as pointed out at [1], is the lack of a mechanism which would allow quantum coherence to overcome opposing thermodynamic effects. Without such a mechanism, any relevant coherent quantum states at the molecular level would have extremely short lifetimes - on the order of 10-13 seconds - which would be insufficient to allow for a role in selection of mutations. Also, such "quantum evolution" would rely on many physical manipulations that simply don't exist yet. Whereas evolution is a well-established theory that relies only on physical phenomenon that scientists have thorougly verified, "quantum evolution" is little more than a mind experiment. It is not a real alternative to classical Darwinian evolution.

Differential Survival of Traits

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;

  • Natural selection
  • Genetic drift

Natural Selection

Darwinism, and its descendant 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 have differing reproductive (sexual) success based on their traits in a given environment. In plain English, animals (or plants) that are good at what they do are more likely to survive and have kids.
  • Therefore, over time, the types of organisms that have traits better adapted to their environment will tend to become the dominant ones in an environment, while organisms poorly adapted to their environment will become extinct.

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 niches die out. Therefore, life is allowed to persist over great spans of time, in the form of evolving species.

Genetic Drift

Genetic drift describes changes in gene frequency that cannot be ascribed to selective pressures, but are due instead to random processes or accident. This is especially the case in small populations, where a mating population simply cannot have enough offspring to maintain the same gene distribution as the parental generation. Such fluctuations in gene frequency between successive generations may result in some genes disappearing from the population. Two separate populations that begin with the same gene frequency might, therefore, "drift" by random fluctuation into two divergent populations with different gene sets (i.e. genes that are present in one have been lost in the other). Random events (volcanic explosion, meteor impact, etc.) might contribute to genetic drift by altering the gene frequency outside of "normal" selective pressures.

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 than "Just-So Stories" without any real scientific evidence. Genetic drift is an alternative explanation for the occurrence of many characteristics. Genetic drift may also be used to explain apparent maladaptations.

Evolution as a Source of Cultural Controversy

In some countries, such as the US, the very question whether biological evolution has taken place is controversial among laymen. According to a Gallup poll, nearly half of Americans disbelieve in any sort of biological evolution.

Development of Evolutionary Theories

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.

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.

What is a theory?

Common misconceptions about scientific theories.

Further Reading

Famous evolution researchers and popularizers


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