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Evolution

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.

Since the 19th century "evolution" is generally used in reference to biological evolution, changes in allele frequencies in a population from one generation to another. Often it is shorthand for the modern theory of evolution of species 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

Table of contents

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. Some have suggested that life may have had more than one origin, but the high degree of commonality argues strongly against multiple origins.

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 structures 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. Vertebrate limbs are a favorite example of homologous structures, organs on two organisms that share a basic structure that had existed in the last common ancestor of the organisms. The current dominant theory of evolution is known as "the modern synthesis", referring to the synthesis of Darwin's theory of evolution by natural selection and Mendel's theory of the gene. According to this theory, the fundamental event of speciation is the genetic isolation two populations, which allows their gene pools to diverge.

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 reversible. 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.

Microevolution and Macroevolution

Microevolution refers to small-scale changes in gene-frequencies in a population over a few generations (population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution). These changes may be due to a number of processes: mutation, gene flow[?], genetic drift, as well as natural selection. Macroevolution refers to large-scale changes in gene-frequencies in a population over a long period of time (and may culminate in the evolution of new species). The difference between the two is hard to distinguish because, over time, successive tiny mutations like those evidenced in microevolution build up in isolated populations and eventually create entirely new species, which is known as macroevolution. The two terms are not used much by scientists, who see no need to refer to the same process by different names solely because of the degree to which the process has taken place. The two terms are largely used by religious fundamentalists, who claim that microevolution can and does happen but that macroevolution cannot. This is based on the supposition that microevolution may occur with an existing gene pool, whereas macroevolution requires the introduction of newly-evolved genes. Notwithstanding, they are both essentially the same process, with the latter simply taking longer.

The study of macroevolution addresses such 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 leads to speciation?

There are two main ways in which "macroevolution" may occur. The first way is through the extrapolation of microevolutionary processes. Tiny microevolutions, over sufficient time, add up and accumulate in isolated populations and eventually result in new species. The second way in which "macroevolution" occurs is through sudden and rapid changes. This theory, punctuated equilibrium, put forth by Stephen Jay Gould, is based on the fact that there are critical genes (such as the homeobox) in all living organisms, and a small change in them could cause drastic changes in the organism, resulting in a new species quite rapidly.

Single small mutations are sometimes the main difference between one species and another. Scientists have discovered very important genes, such as the homeobox, which regulate the growth of animals in their embryonic state. Scientists have managed to create new species of fly by irradiating the homeobox gene, causing a radical mutation in the development of the segments of the body. The fly may grow an extra thorax, or grow legs out of its eyestalks, all due to a single base pair alteration! The additional information needed for these structures did not arise from the mutation, of course, but existed elsewhere in the animal's DNA and was replicated at the novel location. It has been proposed that centipedes and millipedes originated from insect precursors, but their homeobox gene mutated and they ended up growing dozens of body segments instead of just one. A very small change, and an entire species is formed.

It must be noted that many mutations are common and unexpressed, particularly when it involves toggling of the third base sequence in a codon. Most deleterious mutations are not seen simply because they do not result in viable reproduction.

Mutations of the homeobox and other critical genes are sometimes called macromutations, which cause the addition of body segments among the Arthropoda. 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, which allows it to clearly show slow speciation events that are the result of accumulated mutations over a long time, but records sudden "jumps" in species that are most likely the result of mutations in the critical regulatory genes in only a few generations. Macromutations are probably the best explanation of the Cambrian Explosion that occurred 550 million years ago.

Some proponents of creationism accept that microevolution occurs in the short term, whereas macroevolution, specifically leading to speciation, is expressly rejected. They claim that known sources of variation can only account for variation within species, and can not account for the variation between larger taxonomic groups, thus making macroevolution impossible.

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 a controlled 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 through natural experiments. They confirm 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.

Scientists consider large gaps between taxonomic groups to be explainable by ecological/evolutionary factors, such as extinctions, population bottlenecks, and the emergence of unoccupied ecological niches. Macroevolution is simply the result of microevolution over a longer period of time. According to the modern synthesis, no distinction needs to be drawn between different kinds of evolution because all are caused by the same factors.

The Creationism v. Evolution Controversy

Religious critics of biological evolution claim that only God, sometimes called the "Intelligent Designer" was responsible for creation of different species. Many point to the Bible as evidence that all living things were created as separate kinds.

In 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.

Some Christian fundamentalist scientists, such as Henry Morris[?] of the Creation Research Institute[?], have offered what they believe to be proofs of the impossibility of macroevolution. So far all of these proofs have been rejected as flawed by the mainstream scientific community. Many scientific criticisms of such proofs accuse the authors of deliberate fraud. In response these authors would claim that scientists are enforcing their territory and are unwilling to entertain a more radical thesis.

More recently, the intelligent design movement has emerged. This theory allows for macroevolution but denies the theory of natural selection, arguing that God has guided the evolution. It has been pointed out that this theory begs the question of whether God had or lacked the competency to create a system in which natural selection could operate.

The most general challenge to Darwinian evolution at present has been articulated by Phillip E. Johnson, Professor of Law at the University of California, Berkeley, who argues that the entire issue of biological origins has been framed in terms of Naturalism, and that natural science per se is not identical with Naturalism. For example, the statement, "Science has nothing to say about whether or not there exists a supernatural realm" is true and based on the fact that rigorous physical science is naturalistic, but the statement, "Science holds that there is no supernatural realm" is false because it is beyond the scope of natural science to make such an assertion, but is instead a philosophical position. According to Johnson, this distinction opens the possibility of natural science and Creationism being non-contradictory.

The "Missing Link" Argument

One of the criticisms of the model of natural selection is raised by the lack of smooth transitions between species in the fossil record, resulting in so-called "missing links".

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 whatever geographic 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. For one thing, the vast majority of fossils involved deposition in an aqueous environment where they are then covered by sediment in a progressive way so that they are not re-exposed to the elements. It's known from observation that this is a rare process, especially considering that most organisms become food for other organisms.

Another observation is that "missing links" are being found all the time, as new fossils are discovered. Fossil finds are generally restricted only to the extremely small amount of sedimentary rock that is exposed on the surface of the Earth at any one time. The vast majority of actual fossils remain concealed within the rock strata.

Common arguments against evolution usually discuss such "missing links." One conundrum created by this kind of argument is that for every "missing link" that the scientists discover, two more "missing links" are going to be created on either side of it, by definition. In reality, scientists have uncovered millions of fossils that all fit together in a coherent evolutionary tree, with a few exceptions that are as yet incertae sedis. It would be scientifically simplistic to think that every living subspecies left behind good, clean fossils that have all already been found by scientists.

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

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 genes from their parents. This genes code for different characteristics in a person. Genetically, a child has 50% the DNA of each parent. Depending on how the genotypes are inherited though, the phenotypes may be manifested in different ways. The genotype is the basic code of the gene, and the phenotype is what is expressed in the individual. Two brown-eyed parents may be heterozygous for the eye color allele and end up having a child with the blue eyed phenotype. In plain English, kids are like mom and dad, though the mechanisms through which this occurs can get very complicated.
  • 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. The central role of natural selection in evolutionary theory has created a strong connection between that field and the study of ecology.

Genetic drift

Genetic drift describes changes in gene frequency that cannot be ascribed to selective pressures, but are due instead to events that are unrelated to inherited traits. This is especially important in small mating populations, which 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). Rare sporadic events (volcanic explosion, meteor impact, etc.) might contribute to genetic drift by altering the gene frequency outside of "normal" selective pressures.

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.

Post-Darwinian theories of evolution

There is currently a renewal of theoretical thought in evolutionary biology, a movement collectively called postdarwinism[?], that incorporates ideas from cybernetics and systems theory, and that emphasizes self-organized[?] processes over natural selection --- some variants consider natural selection as the result of biological evolution and not its cause. Expectably, this line of inquiry is anathema to classic Darwinists such as Richard Dawkins and Daniel Dennett, who rightly perceive the intrusion of any kind of ordering in an evolutionary scenario a fatal step away from strict materialism based on random, non-directed processes to a philosophically unacceptable intrusion by Deity or other external intelligence.

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.

Related articles

Famous evolution researchers and popularizers:

Bibliography

External links


Evolution is also the title of a 1979 rock and roll album and a 2001 movie[?]. There is an email client called Ximian Evolution.



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