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
ethologyEthology is the scientific study of animal behavior, and a sub-topic of zoology....
and
evolutionary psychologyEvolutionary psychology attempts to explain psychological traits—such as memory, perception, or language—as adaptations, that is, as the functional products of natural selection or sexual selection. Adaptationist thinking about physiological mechanisms, such as the heart, lungs, and immune system,...
that are concerned with identifying adaptations.
George WilliamsProfessor George Christopher Williams is an American evolutionary biologist.Williams is a professor emeritus of biology at the State University of New York at Stony Brook. He is best known for his vigorous critique of group selection. The work of Williams in this area, along with W. D...
'
Adaptation and Natural SelectionAdaptation 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.
Adaptationism is sometimes characterized by critics as an unsubstantiated assumption that all or most traits are
optimalIn mathematics, optimization, or mathematical programming, refers to choosing the best element from some set of available alternatives.In the simplest case, this means solving problems in which one seeks to minimize or maximize a real function by systematically choosing the values of real or...
adaptations.
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
ethologyEthology is the scientific study of animal behavior, and a sub-topic of zoology....
and
evolutionary psychologyEvolutionary psychology attempts to explain psychological traits—such as memory, perception, or language—as adaptations, that is, as the functional products of natural selection or sexual selection. Adaptationist thinking about physiological mechanisms, such as the heart, lungs, and immune system,...
that are concerned with identifying adaptations.
George WilliamsProfessor George Christopher Williams is an American evolutionary biologist.Williams is a professor emeritus of biology at the State University of New York at Stony Brook. He is best known for his vigorous critique of group selection. The work of Williams in this area, along with W. D...
'
Adaptation and Natural SelectionAdaptation 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
optimalIn mathematics, optimization, or mathematical programming, refers to choosing the best element from some set of available alternatives.In the simplest case, this means solving problems in which one seeks to minimize or maximize a real function by systematically choosing the values of real or...
adaptations. Critics (most notably
Richard LewontinRichard 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 GouldStephen 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. Gould spent most of his career teaching at Harvard University and working at the American Museum...
) contend that the adaptationists (
John Maynard SmithJohn Maynard Smith, F.R.S. was a British theoretical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he then took a second degree in genetics under the well-known biologist J.B.S. Haldane...
, W.D. Hamilton and
Richard DawkinsClinton Richard Dawkins, FRS, FRSL is a British ethologist, zoologist, Neo-Darwinian evolutionary biologist and theorist and a popular science author....
being frequent examples) have over-emphasized the power of
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
to shape individual traits to an
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
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
To describe a trait as adaptive means that the trait arose, and existed as variation was passed down through generations and reached a frequency because of its effects on reproductive success. The criteria could also be viewed as a minimum checklist.
Does the feature:
- Exist as a variation on a previous form?
- Is a critical aspect of the trait highly genetically based and heritable?
- Does it enhance reproductive success?
Only when a trait fulfills these criteria is it generally accepted as an adaptation. The people accused or otherwise self described as adaptations agree with these rules they just tend to accept less plausible evidence for these.
The reasons why evolution cannot form perfect organisms is numerous. Foremost would be the fact that organisms are subject to a number of constraints.
Anatomical Constraints
Anatomical constraints are features of organism's
anatomyAnatomy 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"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
retinaThe 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 selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
has come up with an elaborate work-around of the eyes wobbling back-and-forth to correct for this, but it has not transitioned to like that of cephalopods 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, som making the transition virtually impossible. See also:
Evolution of the eyeThe 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 nervesCranial nerves are nerves that emerge directly from the brain stem in contrast to spinal nerves which emerge from segments of the spinal cord.-Origin and function:...
in tetrapods. Some of the best derived models of early vertebrate evolution have been sharks, skates, and rays (collectively
chondrichthyesChondrichthyes 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 nervesCranial nerves are nerves that emerge directly from the brain stem in contrast to spinal nerves which emerge from segments of the spinal cord.-Origin and function:...
.
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"The nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
, nor is it possible that in transition the
"spermatochord"The vas deferens , also called ductus deferens, , is part of the male anatomy of some species; they transport sperm from the epididymis in anticipation of ejaculation.-Structure:...
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 occurs when a single gene influences multiple phenotypic traits. Consequently, a new mutation in the gene may have an effect on some or all traits simultaneously...
,
epistasisEpistasis is the interaction between genes. Epistasis takes place when 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 said to be epistatic, while the phenotype altered or suppressed is said to...
, and polygenic traits.
With
pleiotropyPleiotropy occurs when a single gene influences multiple phenotypic traits. Consequently, a new mutation in the 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
epistasisEpistasis is the interaction between genes. Epistasis takes place when 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 said to be epistatic, while the phenotype altered or suppressed is said to...
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 effect others. This leads to the co-regulation 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 that 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-hocAd hoc is a Latin phrase which means "for this [purpose]". It generally signifies a solution designed for a specific problem or task, non-generalizable, and which cannot be adapted to other purposes....
"just-so stories"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 or 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 menA straw man argument is an informal fallacy based on misrepresentation of an opponent's position. To "attack a straw man" is to create the illusion of having refuted a proposition by substituting a superficially similar proposition , and refuting it, without ever having actually refuted the...
, rather than the actual views of supposed adaptationists.
Adaptiationist rebuttals
Adaptationist researchers respond by asserting that they, too, follow
George WilliamsProfessor George Christopher Williams is an American evolutionary biologist.Williams is a professor emeritus of biology at the State University of New York at Stony Brook. He is 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 BussDavid 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 PinkerSteven Arthur Pinker is a Canadian-American experimental psychologist, cognitive scientist, and author of popular science...
have also suggested that the debate has a strong
ad hominemAn ad hominem argument, also known as argumentum ad hominem is an argument which links the validity of a premise to a characteristic or belief of a person advocating the premise....
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
- Adaptation
Adaptation is the process whereby a population becomes better suited to its habitat. This process takes place over many generations, and is one of the basic phenomena of biology....
- Gene-centered view of evolution
The gene-centered view of evolution, gene selection theory or selfish gene theory holds that natural selection acts through differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation...
- Spandrel
Spandrel is a term used in evolutionary biology to describe a phenotypic characteristic that is a byproduct of the evolution of some other character, rather than a direct product of adaptive selection...
- Beneficial acclimation hypothesis
The Beneficial Acclimation Hypothesis is the physiological hypothesis that acclimating to a particular environment provides an organism with advantages in that environment...
External links