Objections to evolution

Although the modern synthesis is a major achievement of modern science, some aspects of it are often misunderstood. These misunderstandings have hindered acceptance of the modern synthesis, most notably in the United States. Some of the most common misunderstandings are outlined in this section.

Distinctions between theory and fact

Main article: Evolution as theory and fact

Further information: Scientific Theory

See also: Theory vs. Fact

Some critics of evolution claim that it is merely a theory. This criticism makes two claims: that evolution is a theory and therefore not a fact, and that theories are less established than facts (and less well supported than other beliefs). In other words, laypeople often use the word "theory" to signify "conjecture", "speculation", or "opinion". In science however, a theory is a model of the world (or some portion of it) that makes predictions that can be tested through controlled experiments.

A second linguistic confusion has arisen around the word "fact". Fact is often used by scientists to refer to empirical data, objective verifiable observations. But fact is also used in a wider sense to include any hypothesis for which there is overwhelming evidence. In this usage "the sun is at the center of the solar system" and "objects fall due to gravity" are considered to be facts.

Moreover the word evolution may refer to the observed fact, or it may refer to a theory which explains it. When "evolution" is used to describe a fact, it refers to the observations that populations of one species of organism do, over time, change into new, or several new, species. In this sense, evolution occurs whenever a new strain of bacterium evolves that is resistant to antibodies that had been lethal to prior strains. Another clear case of evolution as fact involves the hawthorn fly.

When "evolution" is used to describe a theory, it refers to an explanation for why and a model of how evolution occurs. An example of evolution as theory is the modern synthesis of Darwin and Wallace's theory of natural selection and Mendel's principles of genetics. As with any scientific theory, the modern synthesis is constantly debated, tested, and refined by scientists. There is an overwhelming consensus in the scientific community that it remains the only robust model the accounts for the known facts concerning evolution.

Gravity	Evolution
Things falling is an observation of the pull of bodies towards each other.	Fruit flies changing generation to generation is an observation of generational organism change.
Bodies pulling towards each other is called gravity.	Organisms changing generation to generation is called evolution.
Gravity is a "fact".	Evolution is a "fact".
The "facts" of gravity require an explanation.	The "facts" of evolution require an explanation.
Aristotle and Galileo created explanations of the "fact" of gravity. These are now obsolete explanations. Newton created an explanation which is substantially correct as far as it goes but turned out to require refinement.	Lamarckism, Transmutationism and Orthogenesis were created as explanations of the "fact" of evolution. These are now obsolete explanations. Darwin created an explanation which is substantially correct as far as it goes, but turned out to require refinement.
Einstein's explanation is a refinement of Newton's explanation of gravity. Einstein's explanation is currently the most accepted explanation of the "fact" of gravity.	The Neo-Darwinist explanation is a refinement of Darwin's explanation of evolution. Neo-Darwinism is currently the most accepted explanation of the "fact" of evolution.
Einstein's explanation of the "fact" of gravity is called the Theory of Gravity.	The Neo-Darwinist explanation of the "fact" of evolution is called the Theory of Evolution.
Gravity is a "fact" and a "theory."	Evolution is a "fact" and a "theory."

Evolution, complexity, and devolution

One of the most common misunderstandings about evolution is that one species can be "more highly evolved" than another; that evolution is necessarily progressive and/or leads to greater "complexity", or that its converse is "devolution". Evolution is a non-directional process in the sense that it does not proceed toward any ultimate goal. Rather, evolution acts to optimize organisms to the conditions of their environment, whatever they might be, through the process of natural selection. Thus, evolution provides no assurance that later generations will be more complex than earlier generations, a fact that applies equally to human complexity and intelligence as it does to all other organisms on Earth today and in the past. The claim that evolution results in directional progress is not part of modern evolutionary theory; it derives from earlier belief systems which were held around the time when Darwin formulated his ideas.

In many cases evolution does proceed in the direction of increasing complexity. The earliest organisms were maximally simple life forms. Evolution caused life to become more complex, since becoming simpler wasn't advantageous; indeed it was likely impossible since organisms of any lower complexity would be too simple to carry out basic life functions. Virtually the only avenues open for evolution in the earliest epoch of life’s history was toward more complex and specialized forms. Since then, evolved complexity is a pattern that has occurred again and again in numerous lineages since it produces more specialized forms that are capable of exploiting environmental niches more effectively than competitors. Indeed, specialization through increased complexity allows an organism to define its own niche and evolve to it, thus also increasing biodiversity.

But complexity is not an ultimate goal of evolutionary processes, and the evidence supporting this abounds. Bacterial forms have remained comparatively simple over 3.8 billion years of evolution even though numerous forms have evolved and specialized to a variety of environments using diverse metabolic pathways. In these cases, complexity was not selected for even though substantial evolutionary change occurred. Simple organisms that rapidly metabolize energy rich nutrients available in the environment are optimal forms that dominate the biosphere and the cycling of elements in it. Among eukarya much greater levels of complexity have evolved, but these organisms represent the smaller portion of the biomass on earth. Thus, evolution toward complexity is the exception and not the norm.

There are numerous examples of organisms evolving towards simpler forms. Vestigial structures constitute one line of evidence of lessening complexity evolving among eukarya. Pythons have a vestigial pelvis that is detached from the vertebra and essentially floats in the abdominal cavity. This structure is functionless and degenerate from a functioning pelvis, with legs, in a previous ancestor. The Mexican tetra (Astyanax mexicanus) has evolved from a previous sighted ancestor into a blind form. While the animal retains a rudimentary eye, the nerves and retina are degenerate and functionless. The human appendix is a degenerate form of the caecum found in a wide variety of herbivorous vertebrates. In these animals, the caecum is a significant organ that functions to digest cellulosic plant material. In humans and hominoid apes, changes in diet have favored its degeneration. In each instance, natural selection has acted to produce decreasing complexity over time because the maintenance of useless organs represents a drain on the organism’s energy resources and is thus subject to negative selective pressure.

Similarly, horses evolved from morphologically more complex ancestors with five toes on their feet. Over time, the fossil record shows a progression to reduced complexity that has produced the single toe (hoof) observed today. In the case of this animal, a simpler design is superior for the conditions under which it must survive. Many parasitic and symbiotic species have evolved simpler forms from more complex to the point that they are incapable of existing outside the bodies of their hosts or symbionts. In these cases, reduced complexity offers a selective advantage but comes with the cost of surrendering independence.

There is no guarantee that any particular organism existing today will become more complex in the future. In fact, natural selection will only favor increasing complexity if it increases an organism’s chance of survival and ability to produce viable offspring that live long enough to themselves reach sexual maturity. The same mechanism will also favor lower complexity in traits if that confers a selective advantage in the organism's environment.

Speciation

Main article: Speciation

The existence of several different, but related, finches on the Galápagos Islands is evidence of the occurrence of speciation.

It is sometimes claimed that speciation — the origin of new species — has never been directly observed, and thus evolution cannot be called sound science. 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. 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. Finally, speciation has actually been directly observed. (See the hawthorn fly example.) Further, there are a number of examples of speciation in plants, and differences in ectodysplasin alleles in stickleback fish speciation has developed as a supermodel for studying gene alterations and speciation.

A variation of this assertion, that microevolution has been directly observed and macroevolution has not, is subject to the same counterarguments. However, it is generally accepted that macroevolution uses the same mechanisms of change as those already observed in microevolution.

Entropy and life

Main articles: Entropy and life and Self-organization

It is claimed that evolution, by increasing complexity, violates the second law of thermodynamics. This law posits that in an idealised isolated system, entropy will tend to increase or stay the same. Entropy is a measure of the dispersal of energy in a physical system so that it is not available to do mechanical work. The claim ignores the fact that biological systems are not isolated systems. Life inherently involves open systems, not isolated systems, as all organisms exchange energy and matter with their environment, and similarly the Earth receives energy from the Sun and emits energy back into space. Simple calculations show that the Sun-Earth-space system does not violate the second law because the enormous increase in entropy due to the Sun and Earth radiating into space dwarfs the small decrease in entropy caused by the evolution of life.

Information

Some assert that evolution cannot create information, or that information can only be created by an 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 or 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 efficiently digest a material that never existed until the modern age.

Japanese researchers demonstrated that nylon degrading ability can be obtained de novo in laboratory cultures of Pseudomonas aeruginosa strain POA, which initially had no enzymes capable of degrading nylon oligomers. This indicates that the ability of bacteria to digest nylon can evolve if proper artificial selection is applied. Recently, the same group found X-ray crystal structure of the newly evolved nylon-digesting enzyme. Using the structural results, the authors propose "that the amino acid replacements in the catalytic cleft of a preexisting esterase resulted in the evolution of the" nylon-digesting enzyme. This hypothesis still needs to be confirmed by detailed genetic studies.

Social and religious controversies

Main articles: Social effect of evolutionary theory and Creation-evolution controversy

A satirical 1871 image of Charles Darwin as a quadrupedal ape reflects part of the social controversy over whether humans and other apes share a common lineage.

Ever since the publication of The Origin of Species in 1859, the modern science of evolution has been a source of nearly constant controversy. In general, controversy has centered on the philosophical, cosmological, social, and religious implications of evolution, not on the science of evolution itself. The proposition that biological evolution occurs through the mechanism of natural selection has been almost completely uncontested within the scientific community for much of the 20th century.

As Darwin recognized early on, perhaps the most controversial aspect of evolutionary thought is its applicability to human beings. The idea that all diversity in life, including human beings, arose through natural processes without a need for supernatural intervention poses difficulties for the belief in purpose inherent in most religious faiths — and especially for the Abrahamic religions. Many religious people are able to reconcile the science of evolution with their faith, or see no real conflict; Judaism, Catholicism and Anglicanism are notable as major faith traditions whose adherents generally see no conflict between evolutionary theory and religious belief. The idea that faith and evolution are compatible has been called theistic evolution. Another group of religious people, generally referred to as creationists, consider evolutionary origin beliefs to be incompatible with their faith, their religious texts and their perception of design in nature, and so cannot accept what they call "unguided evolution".

One particularly contentious topic evoked by evolution is the biological status of humanity. Whereas the classical religious view can broadly be characterized as a belief in the great chain of being (in which people are "above" the animals but slightly "below" the angels), the science of evolution shows that humans, as animals, share common ancestry with all other animals and are particularly close to primates such as chimpanzees, gibbons, gorillas, and orangutans. Some people find this offensive because it "degrades" humankind. A related conflict arises when critics combine the religious view of people's superior status with the mistaken notion that evolution is necessarily "progressive". If human beings are superior to animals yet evolved from them, these critics claim, inferior animals would not still exist. In fact, humans and other species share common ancestors, and it is these common ancestors that are extinct.

In some countries — notably the United States — these and other tensions between religion and science have fueled what has been called the creation-evolution controversy, which, among other things, has generated struggles over teaching curricula. While many other fields of science, such as cosmology and earth science also conflict with a literal interpretation of many religious texts, evolutionary studies in biology have borne the brunt of these debates. It also affects the public school system, where religion is assumed to have no effect on the teachings.

Evolution has been used to support philosophical and ethical choices which most contemporary scientists consider were neither mandated by evolution nor supported by science. For example, the eugenic ideas of Francis Galton were developed into arguments that the human gene pool should be improved by selective breeding policies, including incentives for reproduction for those of "good stock" and disincentives, such as compulsory sterilization, "euthanasia", and later, prenatal testing, birth control, and genetic engineering, for those of "bad stock". Another example of an extension of evolutionary theory that is now widely regarded as unwarranted is "Social Darwinism"; a term given to the 19th century Whig Malthusian theory developed by Herbert Spencer into ideas about "survival of the fittest" in commerce and human societies as a whole, and by others into claims that social inequality, racism, and imperialism were justified.