Spandrel (biology) - AbsoluteAstronomy.com

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...

, a Spandrel is a phenotypic

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

characteristic that is a byproduct of the evolution of some other characteristic, rather than a direct product of adaptive 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....

Origin of Term

The term was coined by the Harvard

Harvard University

Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...

paleontologist

Paleontology

Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

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....

and population geneticist

Population genetics

Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...

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...

in their influential paper "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" (1979).

In this paper Gould

Stephen Jay Gould

and Lewontin

Richard Lewontin

employed the analogy of spandrel

Spandrel

A spandrel, less often spandril or splaundrel, is the space between two arches or between an arch and a rectangular enclosure....

s in Renaissance architecture

Renaissance architecture

Renaissance architecture is the architecture of the period between the early 15th and early 17th centuries in different regions of Europe, demonstrating a conscious revival and development of certain elements of ancient Greek and Roman thought and material culture. Stylistically, Renaissance...

: curved areas of masonry between arches supporting a dome that arise as a consequence of decisions about the shape of the arches and the base of the dome, rather than being designed for the artistic purposes for which they were often employed. Properties that they singled out were the necessary number of four and their specific three-dimensional shape. In the biological sense, a 'spandrel' or 'exaptation' (as Gould and Lewontin referred to them) might be the result of an architectural requirement inherent in the Bauplan of an organism, or to some other constraint on adaptive evolution

Natural selection

. In Gould’s and Vbra’s (1982) theory of exaptation

Exaptation

Exaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour...

, exaptations were characteristics that enhance fitness in their present role but were not built for this role by natural selection and may be divided into two subcategories; preadaptation and spandrels. Spandrels are characteristics that did not originate by the direct action of natural selection and that were later co-opted for a current use. Gould

Stephen Jay Gould

saw the term to be optimally suited for evolutionary biology for “the concept of a nonadaptive architectural by-product of definite and necessary form – a structure of particular size and shape that then becomes available for later and secondary utility” (Gould 1997)

Criticism of the Term

Their suggestive proposal generated a large literature of critique, which Gould characterised (Gould 1997) as being grounded in two ways. First, a terminological claim was offered that the "spandrels" of Basilica di San Marco

St Mark's Basilica

The Patriarchal Cathedral Basilica of Saint Mark is the cathedral church of the Roman Catholic Archdiocese of Venice, northern Italy. It is the most famous of the city's churches and one of the best known examples of Byzantine architecture...

were not spandrels at all, but rather were pendentives. Gould (1997) responded, "The term spandrel may be extended from its particular architectural use for two-dimensional byproducts to the generality of 'spaces left over', a definition that properly includes the San Marco pendentives."

Other critics, such as Daniel Dennett

Daniel Dennett

Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

, further claimed that these pendentives are not merely architectural by-products as Gould and Lewontin supposed. Dennett argues that alternatives to pendentives, such as corbels or squinches would have served equally well from an architectural standpoint, but pendentives were deliberately selected due to their aesthetic

Aesthetics

Aesthetics is a branch of philosophy dealing with the nature of beauty, art, and taste, and with the creation and appreciation of beauty. It is more scientifically defined as the study of sensory or sensori-emotional values, sometimes called judgments of sentiment and taste...

value. Critics argue that Lewontin and Gould's oversight in this regard illustrates their underestimation of the pervasiveness of adaptations found in nature.

Response to Criticism

Gould responded that critics ignore that later selective value is a separate issue from origination as necessary consequence

Consequence

Consequence may refer to:* In logic, consequence relation, also known as logical consequence, or entailment* In operant conditioning, a result of some behavior...

s of structure; he summarised his use of the term 'spandrel' in 1997: "Evolutionary biology needs such an explicit term for features arising as byproducts, rather than adaptations, whatever their subsequent exaptive utility

Exaptation

.... Causes of historical origin must always be separated from current utilities; their conflation has seriously hampered the evolutionary analysis of form in the history of life." Gould cites the masculinized genitalia of female hyenas and the brooding chamber of some snails as examples of evolutionary spandrels. (Gould 1997:Abstract)

Gould

Gould

-Places:* Gould , a lunar crater formation* The Gould Coast, AntarcticaUnited States* Gould, Arkansas* Gould, Colorado* Goulds, Florida* Gould, Oklahoma* Gould Township, Minnesota* Gould Academy in Bethel, Maine-Other uses:...

(1991) outlines some considerations for grounds for assigning or denying a structure the status of spandrel pointing first to the fact that a structure which originated as a spandrel through primary exaptation may have been further crafted for its current utility by a suite of secondary adaptations, thus the grounds of how well crafted a structure is for a function cannot be used as a ground for assigning or denying spandrel status. The nature of the current utility of a structure also does not provide a basis for assigning or denying spandrel status nor does he see the origin of a structure as having any relationship to the extent or vitality of a later co-opted role but places importance on the later evolutionary meaning of a structure. This seems to imply that the design and secondary utilization of spandrels may feed back into the evolutionary process and thus determine major features of the entire structure. The grounds Gould

Gould

(1997, pp. 10752-10753) does accept to have validity in assigning or denying a structure the status of spandrel are historical order and comparative anatomy

Comparative anatomy

Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is closely related to evolutionary biology and phylogeny .-Description:...

. Historical order involves the use of historical evidence to determine which feature arose as a primary adaptation and which one appeared subsequently as a co-opted by-product. In the absence of historical evidence, inferences are drawn about the evolution of a structure through comparative anatomy. Evidence is obtained by comparing current examples of the structure in a cladistic context and by subsequently trying to determine a historical order from the distribution yielded by tabulation.

Language as a Spandrel

The linguist Noam Chomsky

Noam Chomsky

Avram Noam Chomsky is an American linguist, philosopher, cognitive scientist, and activist. He is an Institute Professor and Professor in the Department of Linguistics & Philosophy at MIT, where he has worked for over 50 years. Chomsky has been described as the "father of modern linguistics" and...

has argued that the 'language faculty', specifically the property of discrete infinity or recursion

Recursion

Recursion is the process of repeating items in a self-similar way. For instance, when the surfaces of two mirrors are exactly parallel with each other the nested images that occur are a form of infinite recursion. The term has a variety of meanings specific to a variety of disciplines ranging from...

that plays a central role in his theory of Universal Grammar

Universal grammar

Universal grammar is a theory in linguistics that suggests that there are properties that all possible natural human languages have.Usually credited to Noam Chomsky, the theory suggests that some rules of grammar are hard-wired into the brain, and manifest themselves without being taught...

, may have evolved as a spandrel: in this view, Chomsky initially pointed to language being a result of increased brain size and increasing complexity, though he provides no definitive answers as to what factors led to the brain attaining the size and complexity of which discrete infinity is a consequence. Steven Pinker

Steven Pinker

Steven Arthur Pinker is a Canadian-American experimental psychologist, cognitive scientist, linguist and popular science author...

and Ray Jackendoff

Ray Jackendoff

Ray Jackendoff is an American linguist. He is professor of philosophy, Seth Merrin Chair in the Humanities and, with Daniel Dennett, Co-director of the Center for Cognitive Studies at Tufts University...

say Chomsky's case is 'unconvincing' and that 'language maps among recursive systems rather than being a straightforward externalization of a single recursive system', and as an example, numerical recursion ‘is parasitic on language (rather than vice-versa)’ among other arguments (Pinker and Jackendoff, 2005). Pinker contends that the language faculty is not a spandrel, but rather a result of 'boring' natural selection (Pinker and Bloom, 1990). Newmeyer (1998) instead views the lack of symmetry, irregularity and idiosyncrasy that Universal Grammar tolerates and the widely different principles of organization of its various subcomponents and consequent wide variety of linking rules relating them as evidence that such design features do not qualify as an exaptation. He also points to the fact that Universal Grammar cannot be derivative and autonomous at the same time and that Chomsky seems to want language to be an epiphenomenon and an ‘organ’ at the same time, where an organ is defined as a product of a dedicated genetic blueprint. These criticisms, though valid in some regards, must take into account the fact that Chomsky has offered a conception of a specific feature of the language faculty, namely discrete infinity or recursion; he has not offered argument as a conception or theory of the evolution of the language faculty or universal grammar as a whole. Thus any theory of language origin cannot be viewed as if it were a theory of the evolution of the complexified modern language faculty as a whole. (Botha, 2001)

The spandrel theory of language origin thus cannot be adequate unless it is embedded in a principled theory of evolutionary spandrel-hood or by-producthood. The theory of exaptation and the principled grounds for assigning or denying a structure spandrel status as outlined by Gould

Gould

leave the attribution of spandrel status to human language arbitrary, for there is no direct historical evidence of the events through which language originated, and comparative anatomy does not apply, since language is believed to be species-specific and comparative analysis must involve homologous structures or features. A conception that fails to meet this condition cannot assign or deny spandrel status to a linguistic entity in a non-ad hoc and non-arbitrary way. Thus we do not currently know how much of language arose as a spandrel.

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