Evolution: Encyclopedia II - Evolution - Overview of evolution

Evolution - Overview of evolution

Evolution - Evidence of evolution

The process of evolution has left behind numerous records which reveal the history of different species. While the best-known of these are the fossils, fossils are only a small part of the overall physical record of evolution. Fossils, taken together with the comparative anatomy of present-day plants and animals, constitute the morphological record. By comparing the anatomies of both modern and extinct species, biologists can reconstruct the lineages of those species with some accuracy. Using fossil evidence, for instance, the connection between dinosaurs and birds has been established by way of so-called "transitional" species such as Archaeopteryx.

The development of genetics has allowed biologists to study the genetic record of evolution as well. Although we cannot obtain the DNA sequences of most extinct species, the degree of similarity and difference among modern species allows geneticists to reconstruct lineages with greater accuracy. It is from genetic comparisons that claims such as the 95% similarity between humans and chimpanzees come from, for instance.^[3]

Other evidence used to demonstrate evolutionary lineages includes the geographical distribution of species. For instance, monotremes and most marsupials are found only in Australia, showing that their common ancestor with placental mammals lived before the submerging of the ancient land bridge between Australia and Asia.

Scientists correlate all of the above evidence – drawn from paleontology, anatomy, genetics, and geography – with other information about the history of the earth. For instance, paleoclimatology attests to periodic ice ages during which the climate was much cooler; and these are found to match up with the spread of species such as the woolly mammoth which are better-equipped to deal with cold.

Fossils are important for estimating when various lineages developed. Since fossilization on an organism is an uncommon occurrence, usually requiring hard parts (like bone) and death near a site where sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Fossil evidence of organisms without hard body parts, such as shell, bone, and teeth, is sparse but exists in the form of ancient microfossils and the fossilization of ancient burrows, (trace fossils), and rarer examples of soft-bodied organisms.

Fossil evidence of prehistoric organisms has been found all over the Earth. The age of fossils are typically sychronized with the geologic context in which they are found; many of their absolute ages can be verified with radiometric dating. Some fossils bear a resemblance to organisms alive today, while others are radically different. Fossils have been used to determine at what time a lineage developed, and transitional fossils can be used to demonstrate continuity between two different lineages. Paleontologists investigate evolution largely through analysis of fossils.

Phylogeny, the study of the ancestry of species, has revealed that structures with similar internal organization may perform divergent functions. Vertebrate limbs are a common example of such homologous structures. Bat wings, for example, are very similar to hands. A vestigial organ or structure may exist with little or no purpose in one organism, though they have a clear purpose in other species. The human wisdom teeth and appendix are common examples.

Comparison of the genetic sequence of organisms reveals that phylogenetically close organisms have a higher degree of sequence similarity than organisms that are phylogenetically distant. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons.^[4] Sequence comparison is considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce.

Further evidence for common descent comes from genetic detritus such as pseudogenes, regions of DNA which are orthologous to a gene in a related organism, but are no longer active and appear to be undergoing a steady process of degeneration.^[5]

Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged when new metabolic processes appeared, and it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of 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 (which is called having "common descent").

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 genetics. For example, every living cell 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. The universality of these traits strongly suggests common ancestry, because the selection of these traits seems somewhat arbitrary.

The evolutionary process can be exceedingly slow. Fossil evidence indicates that the diversity and complexity of modern life has developed over much of the age of the earth. Geological evidence indicates that the Earth is approximately 4.6 billion years old. (See Timeline of evolution.)

Studies on guppies by David Reznick at the University of California, Riverside, however, have shown that the rate of evolution through natural selection can proceed 10 thousand to 10 million times faster than what is indicated in the fossil record.^[6]

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 chemical evolution from self-catalytic chemicals to life (see Origin of life) is not a part of biological evolution.

Not much is known about the earliest developments in life. However, all existing organisms share certain traits, including cellular structure, and genetic code. Most scientists interpret this to mean all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, but there is no scientific consensus on the relationship of the three domains of life (Archaea, Bacteria, Eukaryota) or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of macromolecules, particularly RNA, and the behavior of complex systems.

The emergence of oxygenic photosynthesis (around 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, believed to have emerged around 2 billion years ago.

In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals, the Cambrian explosion (a period of unrivaled and remarkable, but brief, organismal diversity documented in the fossils found at the Burgess Shale) saw the creation of all the major body plans, or phyla, of modern animals. This event is now believed to have been triggered by the development of the Hox genes. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of land ecosystems with which we are familiar.

Evolution - History of evolutionary thought

The idea of biological evolution has existed since ancient times, notably among Hellenists such as Epicurus and Anaximander, but the modern theory was not established until the 18th and 19th centuries, by scientists such as Jean-Baptiste Lamarck and Charles Darwin. While transmutation of species was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species by Means of Natural Selection which provided the first cogent mechanism by which evolutionary change could occur: his theory of natural selection. Darwin was motivated to publish his work on evolution after receiving a letter from Alfred Russel Wallace, in which Wallace revealed his own discovery of natural selection. As such, Wallace is sometimes given shared credit for the theory of evolution.

Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life, could not explain the source of variation in traits within a species, and Darwin's proposal of a hereditary mechanism (pangenesis) was not compelling to most biologists. It was not until the late 19th and early 20th centuries that these mechanisms were established.

When Gregor Mendel's work regarding the nature of inheritance in the late 19th century was "rediscovered" in 1900, it led to a storm of conflict between Mendelians (Charles Benedict Davenport) and biometricians (Walter Frank Raphael Weldon and Karl Pearson), who insisted that the great majority of traits important to evolution must show continuous variation that was not explainable by Mendelian analysis. Eventually, the two models were reconciled and merged, primarily through the work of the biologist and statistician R.A. Fisher. This combined approach, applying a rigorous statistical model to Mendel's theories of inheritance via genes, became known in the 1930s and 1940s as the modern evolutionary synthesis.

In the 1940s, following up on Griffith's experiment, Avery, McCleod and McCarty definitively identified deoxyribonucleic acid (DNA) as the "transforming principle" responsible for transmitting genetic information. In 1953, Francis Crick and James Watson published their famous paper on the structure of DNA, based on the research of Rosalind Franklin and Maurice Wilkins. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process: the mutation of segments of DNA (see molecular evolution).

George C. Williams' 1966 Adaptation and natural selection: A Critique of some Current Evolutionary Thought marked a departure from the idea of group selection towards the modern notion of the gene as the unit of selection. In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, firmly establishing the importance of genetic drift as a major mechanism of evolution.

Debates have continued within the field. One of the most prominent public debates was over the theory of punctuated equilibrium, proposed in 1972 by paleontologists Niles Eldredge and Stephen Jay Gould to explain the paucity of transitional forms between phyla in the fossil record.

"While horizontal gene transfer is well-known among bacteria, it is only within the past 10 years that its occurrence has become recognized among higher plants and animals. The scope for horizontal gene transfer is essentially the entire biosphere, with bacteria and viruses serving both as intermediaries for gene trafficking and as reservoirs for gene multiplication and recombination (the process of making new combinations of genetic material)." [7].

Evolution - Misconceptions about modern evolutionary biology

Many critics of evolution claim that the theory robs life and the universe of any transcendental meaning. Indeed, one of the great strengths of the theory of evolution is that it has no need for a supernatural intelligence or any intelligent design. As Louis Menand has pointed out, what was radical about Darwin's theory of speciation through natural selection was not the notion of evolution — a concept people espoused before Darwin, and a word that does not appear in On the Origin of Species — but his materialism: "Darwin wanted to establish ... that the species — including human beings — were created by, and evolve according to, processes that are entirely natural, chance-generated, and blind" (Menand 2001: 121).

Nevertheless, many critiques of the modern theory of evolution involve misunderstandings of the theory itself, or of science in general. One of the most common misunderstandings of the theory is that one species can be "more highly evolved" than another, that evolution is necessarily progressive, or that its converse is "devolution". Evolution provides no assurance that later generations are more intelligent, complex, or morally worthy than earlier generations. This claim was often made in pursuit of Social Darwinism, a 19th-20th century political ideology which held that the subjection of the poor and minority groups was favored by evolution.

Another misunderstanding is the claim that speciation – the origin of new species – has never been directly observed. This is a misunderstanding of both science and evolution. First, scientific discovery does not occur solely through reproducible experiments; the principle of uniformitarianism allows natural scientists to infer causes through their empirical effects. Second, Darwin provided a compellingly large amount of evidence to support his theory. Moreover, since the publication of On the Origin of Species scientists have confirmed Darwin's hypothesis by data gathered from sources that did not exist in his day, such as DNA similarity among species and new fossil discoveries.

A variation of this assertion is that "microevolution" has been observed and "macroevolution" has not been observed. Some creationists redefine macroevolution as a change from one "kind" to another. One of Darwin's key insights was to view species statistically — that is, a "species" is not a homogeneous and immutable thing; rather, it consists of a mass of individuals that vary in form from one another and from their offspring. This view was substantiated with the development of Mendelian genetics, which distinguishes different species in terms of differences in the frequencies of particular genes. "Microevolution" and "macroevolution" both refer fundamentally to the same thing, changes in gene frequencies. The difference between them is primarily one of scale; that is, qualitative differences between species is the result of quantitative differences in gene frequencies. Commonly, macroevolution is defined as microevolution over a longer timescale. Some scientists, such as Stephen Jay Gould, use the term macroevolution to instead describe evolutionary processes that occur at the level of species or above.

Evidence of the mechanisms for the larger scales of time comes from evidence of the mechanisms for the smaller scales of time. The differences between macroevolution and microevolution are a result of this change of scale and do not necessitate mechanisms of change other than that found in microevolution.

Another misconception is the claim that evolution violates the second law of thermodynamics. The second law holds that in a closed system, entropy will tend to increase or stay the same. The misconception is that entropy means "disorder" and evolution means an increase in order (thus, a decrease in entropy). This is a misunderstanding of both entropy and evolution. "Entropy" does not mean "disorder" in a generic sense. For one thing, there is no such thing as "order" in a generic sense (any set of objects may be ordered in any number of ways; disorder from one perspective may be order from another). Secondly, entropy refers specifically to differences in useable energy; an example of which is temperature differences. (See entropy for a more precise discussion.)

What appears to be a violation of the second law is not evolution (meaning, the development of new species of life) but rather life itself. But the existence of life does not violate the second law of thermodynamics for two reasons. First, the second law of thermodynamics applies only to a closed system. Earth is not a closed system because it receives an energy input from the sun. However much life may proliferate on earth, the energy of the sun does dissipate over time.

Second, as James Clerk Maxwell argued, the second law is not deterministic, it is probabalistic (see Statistical mechanics). For example, molecules within a container move at different velocities; the temperature of the contents is an average. The more time passes, the greater the probability that differences in temperature within the chamber will even out. This fact does not mean that at any given moment there is a small chance that differences in temperature will increase. As Louis Menand has observed, Darwin's theory of natural selection operates in an analogous fashion: at any given moment most of the members of a species vary little from the average form. Nevertheless, at any given moment there are deviations from the average, and it is the natural selection of specific deviations that leads to a new species. In other words, Darwin applied the same statistical approach to biology that Maxwell applied to physics (Menand 2001: 197-199).

When they consider rocks that just sit there, some people may think it is obvious that matter cannot organize itself. Matter, in fact, organizes itself in numerous ways. Crystals such as diamonds and snowflakes can and do self-organize. Likewise proteins fold in very specific ways based on their chemical makeup. Amino acids are the building blocks of proteins. While the chemical conditions on the relatively young Earth 3.5 billion years ago, when life evolved, are still being debated, the spontaneous synthesis of amino acids has been shown for a wide range of conditions, in such settings as the Miller-Urey experiment.

Misunderstanding the nature of information, some assert that evolution cannot create information, that information is a manifestation of intelligence. Physical information exists regardless of the presence of an intelligence, and evolution allows for new information whenever a novel mutation or gene duplication occurs and is kept. It does not need to be beneficial nor visually apparent to be "information." However, even if those were requirements they would be satisfied with the appearance of nylon-eating bacteria, which required new enzymes to digest a material that never existed until the modern age.

Evolution - Social and religious controversies

Main articles: Social implications of the theory of evolution and Creation-evolution controversy

There has been constant controversy surrounding the ideas presented by The Origin of Species since it was first printed in 1859. Since the early twentieth century, however, the idea that biological evolution of some form occurred and is responsible for speciation has been almost completely uncontested within the scientific community.

Most controversy over the theory has come because of its philosophical, cosmological, and religious implications, and supporters as well as detractors have interpreted it as generally indicating that human beings are, like all animals, evolved, and that this account of the origins of humankind is squarely at odds with many religious interpretations. The idea that humans are "merely" animals, and are genetically very closely related to other primates, has been independently argued as repellent notions by generations of detractors.

Others also interpreted the truth of the theory to imply varying types of social changes — one prominent example is the idea of eugenics, formulated by Darwin's cousin Francis Galton, which argues for the improvement of human heredity by means of political policies. Others have found different political interpretations which have been used as arguments both for and against the theory.

The questions raised about the relation of evolution to the origins of humans has made it an especially tenacious issue with some religious traditions. It has prominently been seen as opposing a "literal" interpretation of the account of the origins of humankind as described in Genesis, the first book of the Bible. In many countries — notably in the United States — this has led to what has been called the Creation-evolution controversy, which has focused primarily on struggles over teaching curriculum.

Adapted from the Wikipedia article "Overview of evolution", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki