Adaptationism
Encyclopedia
Adaptationism is a set of methods in the evolutionary sciences for distinguishing the products of adaptation from traits that arise through other processes. It is employed in fields such as ethology
and evolutionary psychology
that are concerned with identifying adaptations. George Williams
' Adaptation and Natural Selection
was highly influential in its development, defining some of the heuristics, such as complex functional design, used to identify adaptations.
adaptations. Critics (most notably Richard Lewontin
and Stephen Jay Gould
) contend that the adaptationists (John Maynard Smith
, W.D. Hamilton and Richard Dawkins
being frequent examples) have over-emphasized the power of natural selection
to shape individual traits to an evolution
ary optimum, and ignored the role of developmental constraints, and other factors to explain extant morphological and behavioural traits.
Adaptationism could also be characterized as an approach to studying evolution of form and function that attempts to frame the existence and persistence of traits on the scenario that each of them arose independently due to how that trait improved the reproductive success of the organism's ancestors. Adaptationism is also a description of "folk biology" where non-experts see that, in general, organisms have an amazing array of adaptations, then apply this principle too broadly and describe everything as adaptive.
Adaptationists too agree with these rules, but their opponents maintain adaptationists are sometimes too eager to take an imaginative leap where the evidence is spotty or ambiguous.
Evolution cannot form perfect organisms for numerous reasons. Foremost is that some elements of an organism's physiology are subject to constraints that environmental pressure cannot alter.
that are prevented from change by being constrained in some way. When organisms diverge from a common ancestor and inherit certain characteristics which become modified by natural selection of mutant phenotypes, it is as if some are traits are locked in place and are unable to change in certain ways. Some textbook anatomical constraints often include examples of structures that connect parts of the body together though a physical link.
These links are hard if not impossible to break because evolution usually requires that anatomy be formed by small consecutive modifications in populations through generations. Dr. Randolf Nesse, in his book: Why We Get sick explains that the "blind spot"
in the vertebrate eye caused by the nerve fibers running in front of the retina
and blocking vision in parts. He says that natural selection
has come up with an elaborate work-around of the eyes wobbling back-and-forth to correct for this, but vertebrates have not found the solution embodied in cephalopod
eyes, where the optic nerve does not obstruct the view. He goes on to say that a likely reason for this is that in order for the nerve to migrate over evolutionary time it would have to be disconnected or "cut" at some point to complete the trip. This disconnection would not allow for the message to go from the eye to the brain and this would render those mutant individuals blind, so making the transition virtually impossible. See also: Evolution of the eye
.
Other examples often cited for the same reason are the cranial nerves
in tetrapods. Some of the best derived models of early vertebrate evolution have been sharks, skates, and rays (collectively chondrichthyes
). In chondrichthyes the cranial nerves run from the part of the brain that interprets sensory information, and radiate out towards the organs that produce those sensations. In tetrapods however, and mammals in particular, the nerves take an elaborate winding path through the cranium. They go up down and around structures that evolved after the common ancestor with sharks. This is because when their locations shifted over time each stage had to remain connected. See also: Cranial nerves
.
It is not possible to have a number of generations of organisms that are blind (as in the vertebrate eye example) or generations with areas that are not "innervated"
, nor is it possible that in transition the "spermatochord"
becomes severed, then continue to migrate to reach around the pelvis. These would not work because the intermediates with severed connections would be poorly suited for their environments and would fail to reproduce. So these links are maintained throughout millions of years simply because departure from the status quo is inviable.
, epistasis
, and polygenic traits.
With pleiotropy
a hypothetical gene would be selected by natural selection or other related mechanisms. The trait would be selected for its effects on reproductive success. This is well established as how traits evolve. However, some genes control multiple traits. Selection that influences epistasis
is a case where the regulation or expression of one gene, depends on one or several others. This is true for a good number of genes though to differing extents. The reason why this leads to muddied responses is that selection for a trait that is epistatically based can mean that an allele
for a gene that is epistatic when selected would happen to affect others. This leads to the coregulation
of others for a reason other than there is an adaptive quality to each of those traits. Like with pleiotropy, traits could reach fixation in a population as a by-product of selection for another.
In the context of development the difference between pleiotropy and epistasis is not so clear but at the genetic level the distinction is more clear. With these traits as being by-products of others it can ultimately be said that these traits evolved by not that they necessarily represent adaptations.
Polygenic traits are traits that are controlled by a number of different genes. Counter to what is commonly taught in high school and introductory biology classes with beginner's Mendelian Genetics, traits are rarely controlled by one single discrete, either/or allele. Things like human height vary with a great range because this trait is controlled by several different genes.
To drastically change some quantitative trait controlled by many genes, this could require mutations in more than one gene or changes in regulation of more than one gene. This means that mutations effect these systems more on a one-at-a-time basis. Because of this
Adaptationists are accused by their critics of using ad-hoc
"just-so stories"
to make their theories unfalsifiable. The critics, in turn, have often been accused of attacking straw men
, rather than the actual views of supposed adaptationists.
' depiction of adaptation as an "onerous concept" that should only be applied in light of strong evidence. This evidence can be generally characterized as the successful prediction of novel phenomena based on the hypothesis that design details of adaptations should fit a complex evolved design to respond to a specific set of selection pressures. In evolutionary psychology, researchers such as David Buss
contend that the bulk of research findings that were uniquely predicted through adaptationist hypothesizing comprise evidence of the methods' validity.
Adaptationists such as Steven Pinker
have also suggested that the debate has a strong ad hominem
component. Some suggest that the controversy over the relative importance of various factors would be a quiet debate over subtleties if the critics were less prone to caricaturing their opponents.
Ethology
Ethology is the scientific study of animal behavior, and a sub-topic of zoology....
and evolutionary psychology
Evolutionary psychology
Evolutionary psychology is an approach in the social and natural sciences that examines psychological traits such as memory, perception, and language from a modern evolutionary perspective. It seeks to identify which human psychological traits are evolved adaptations, that is, the functional...
that are concerned with identifying adaptations. George Williams
George C. Williams
Professor George Christopher Williams was an American evolutionary biologist.Williams was a professor emeritus of biology at the State University of New York at Stony Brook. He was best known for his vigorous critique of group selection. The work of Williams in this area, along with W. D...
' Adaptation and Natural Selection
Adaptation and Natural Selection
Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought is a 1966 book by the American evolutionary biologist George C. Williams...
was highly influential in its development, defining some of the heuristics, such as complex functional design, used to identify adaptations.
Debate
Adaptationism is sometimes characterized by critics as an unsubstantiated assumption that all or most traits are optimalOptimization (mathematics)
In mathematics, computational science, or management science, mathematical optimization refers to the selection of a best element from some set of available alternatives....
adaptations. Critics (most notably Richard Lewontin
Richard Lewontin
Richard Charles "Dick" Lewontin is an American evolutionary biologist, geneticist and social commentator. A leader in developing the mathematical basis of population genetics and evolutionary theory, he pioneered the notion of using techniques from molecular biology such as gel electrophoresis to...
and Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
) contend that the adaptationists (John Maynard Smith
John Maynard Smith
John Maynard Smith,His surname was Maynard Smith, not Smith, nor was it hyphenated. F.R.S. was a British theoretical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics under the well-known biologist J.B.S....
, W.D. Hamilton and Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
being frequent examples) have over-emphasized the power of natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
to shape individual traits to an evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary optimum, and ignored the role of developmental constraints, and other factors to explain extant morphological and behavioural traits.
Adaptationism could also be characterized as an approach to studying evolution of form and function that attempts to frame the existence and persistence of traits on the scenario that each of them arose independently due to how that trait improved the reproductive success of the organism's ancestors. Adaptationism is also a description of "folk biology" where non-experts see that, in general, organisms have an amazing array of adaptations, then apply this principle too broadly and describe everything as adaptive.
Criteria of calling something adaptive
If and only if a trait fulfills the following criteria will evolutionary biologists in general declare the trait an adaptation:- The trait is a variation of an earlier form.
- The trait is heritable through the transmission of genes.
- The trait enhances reproductive success.
Adaptationists too agree with these rules, but their opponents maintain adaptationists are sometimes too eager to take an imaginative leap where the evidence is spotty or ambiguous.
Evolution cannot form perfect organisms for numerous reasons. Foremost is that some elements of an organism's physiology are subject to constraints that environmental pressure cannot alter.
Anatomical Constraints
Anatomical constraints are features of organism's anatomyAnatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
that are prevented from change by being constrained in some way. When organisms diverge from a common ancestor and inherit certain characteristics which become modified by natural selection of mutant phenotypes, it is as if some are traits are locked in place and are unable to change in certain ways. Some textbook anatomical constraints often include examples of structures that connect parts of the body together though a physical link.
These links are hard if not impossible to break because evolution usually requires that anatomy be formed by small consecutive modifications in populations through generations. Dr. Randolf Nesse, in his book: Why We Get sick explains that the "blind spot"
Blind spot (vision)
A blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature, is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
in the vertebrate eye caused by the nerve fibers running in front of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
and blocking vision in parts. He says that natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
has come up with an elaborate work-around of the eyes wobbling back-and-forth to correct for this, but vertebrates have not found the solution embodied in cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
eyes, where the optic nerve does not obstruct the view. He goes on to say that a likely reason for this is that in order for the nerve to migrate over evolutionary time it would have to be disconnected or "cut" at some point to complete the trip. This disconnection would not allow for the message to go from the eye to the brain and this would render those mutant individuals blind, so making the transition virtually impossible. See also: Evolution of the eye
Evolution of the eye
The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of taxa. Certain components of the eye, such as the visual pigments, appear to have a common ancestry – that is, they evolved once, before the animals radiated...
.
Other examples often cited for the same reason are the cranial nerves
Cranial nerves
Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
in tetrapods. Some of the best derived models of early vertebrate evolution have been sharks, skates, and rays (collectively chondrichthyes
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
). In chondrichthyes the cranial nerves run from the part of the brain that interprets sensory information, and radiate out towards the organs that produce those sensations. In tetrapods however, and mammals in particular, the nerves take an elaborate winding path through the cranium. They go up down and around structures that evolved after the common ancestor with sharks. This is because when their locations shifted over time each stage had to remain connected. See also: Cranial nerves
Cranial nerves
Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
.
It is not possible to have a number of generations of organisms that are blind (as in the vertebrate eye example) or generations with areas that are not "innervated"
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, nor is it possible that in transition the "spermatochord"
Vas deferens
The vas deferens , also called ductus deferens, , is part of the male anatomy of many vertebrates; they transport sperm from the epididymis in anticipation of ejaculation....
becomes severed, then continue to migrate to reach around the pelvis. These would not work because the intermediates with severed connections would be poorly suited for their environments and would fail to reproduce. So these links are maintained throughout millions of years simply because departure from the status quo is inviable.
Genetic Constraints
Genetic constraints provide another force that inhibits the effectiveness of natural selection. Examples include pleiotropyPleiotropy
Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
, epistasis
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
, and polygenic traits.
With pleiotropy
Pleiotropy
Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
a hypothetical gene would be selected by natural selection or other related mechanisms. The trait would be selected for its effects on reproductive success. This is well established as how traits evolve. However, some genes control multiple traits. Selection that influences epistasis
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
is a case where the regulation or expression of one gene, depends on one or several others. This is true for a good number of genes though to differing extents. The reason why this leads to muddied responses is that selection for a trait that is epistatically based can mean that an allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
for a gene that is epistatic when selected would happen to affect others. This leads to the coregulation
Coregulation
Coregulation is a term described by psychologist , as a "continuous unfolding of individual action that is susceptible to being continuously modified by the continuously changing actions of the partner." An important aspect of this idea is that communication is a continuous and dynamic process,...
of others for a reason other than there is an adaptive quality to each of those traits. Like with pleiotropy, traits could reach fixation in a population as a by-product of selection for another.
In the context of development the difference between pleiotropy and epistasis is not so clear but at the genetic level the distinction is more clear. With these traits as being by-products of others it can ultimately be said that these traits evolved by not that they necessarily represent adaptations.
Polygenic traits are traits that are controlled by a number of different genes. Counter to what is commonly taught in high school and introductory biology classes with beginner's Mendelian Genetics, traits are rarely controlled by one single discrete, either/or allele. Things like human height vary with a great range because this trait is controlled by several different genes.
To drastically change some quantitative trait controlled by many genes, this could require mutations in more than one gene or changes in regulation of more than one gene. This means that mutations effect these systems more on a one-at-a-time basis. Because of this
Adaptationists are accused by their critics of using ad-hoc
Ad hoc
Ad hoc is a Latin phrase meaning "for this". It generally signifies a solution designed for a specific problem or task, non-generalizable, and not intended to be able to be adapted to other purposes. Compare A priori....
"just-so stories"
Just-so story
A just-so story, also called the ad hoc fallacy, is a term used in academic anthropology, biological sciences, social sciences, and philosophy. It describes an unverifiable and unfalsifiable narrative explanation for a cultural practice, a biological trait, or behavior of humans or other animals...
to make their theories unfalsifiable. The critics, in turn, have often been accused of attacking straw men
Straw man
A straw man is a component of an argument and is an informal fallacy based on misrepresentation of an opponent's position, twisting his words or by means of [false] assumptions...
, rather than the actual views of supposed adaptationists.
Adaptationist rebuttals
Adaptationist researchers respond by asserting that they, too, follow George WilliamsGeorge C. Williams
Professor George Christopher Williams was an American evolutionary biologist.Williams was a professor emeritus of biology at the State University of New York at Stony Brook. He was best known for his vigorous critique of group selection. The work of Williams in this area, along with W. D...
' depiction of adaptation as an "onerous concept" that should only be applied in light of strong evidence. This evidence can be generally characterized as the successful prediction of novel phenomena based on the hypothesis that design details of adaptations should fit a complex evolved design to respond to a specific set of selection pressures. In evolutionary psychology, researchers such as David Buss
David Buss
David M. Buss is a professor of psychology at The University of Texas at Austin, known for his evolutionary psychology research on human sex differences in mate selection.-Biography:...
contend that the bulk of research findings that were uniquely predicted through adaptationist hypothesizing comprise evidence of the methods' validity.
Adaptationists such as Steven Pinker
Steven Pinker
Steven Arthur Pinker is a Canadian-American experimental psychologist, cognitive scientist, linguist and popular science author...
have also suggested that the debate has a strong ad hominem
Ad hominem
An ad hominem , short for argumentum ad hominem, is an attempt to negate the truth of a claim by pointing out a negative characteristic or belief of the person supporting it...
component. Some suggest that the controversy over the relative importance of various factors would be a quiet debate over subtleties if the critics were less prone to caricaturing their opponents.
See also
- AdaptationAdaptationAn adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
- Gene-centered view of evolutionGene-centered view of evolutionThe gene-centered view of evolution, gene selection theory or selfish gene theory holds that evolution occurs through the differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation, with gene defined as...
- SpandrelSpandrel (biology)In evolutionary biology, a Spandrel is a phenotypic characteristic that is a byproduct of the evolution of some other characteristic, rather than a direct product of adaptive selection.-Origin of Term:...
- Beneficial acclimation hypothesisBeneficial acclimation hypothesisThe Beneficial Acclimation Hypothesis is the physiological hypothesis that acclimating to a particular environment provides an organism with advantages in that environment...
External links
- Information from "Deep Ethology" course website, by Neil Greenberg
- Tooby & Cosmides comments on Maynard Smith's New York Review of Books piece on Gould et al.