Biological constraints are factors which make

population Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...

s resistant to

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

ary change. One proposed definition of constraint is "A property of a trait that, although possibly adaptive in the environment in which it originally evolved, acts to place limits on the production of new phenotypic variants." Constraint has played an important role in the development of such ideas as homology and

body plan A body plan, (), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually app ...

Types of constraint

Any aspect of an organism that has not changed over a certain period of time could be considered to provide evidence for "constraint" of some sort. To make the concept more useful, it is therefore necessary to divide it into smaller units. First, one can consider the pattern of constraint as evidenced by

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analysis and the use of

phylogenetic comparative methods Phylogenetic comparative methods (PCMs) use information on the historical relationships of lineages (phylogenies) to test evolutionary hypotheses. The comparative method has a long history in evolutionary biology; indeed, Charles Darwin used diffe ...

; this is often termed phylogenetic inertia, or phylogenetic constraint. It refers to the tendency of related taxa sharing traits based on phylogeny. Charles Darwin spoke of this concept in his 1859 book "

On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life'')The book's full original title was ''On the Origin of Species by M ...

", as being "Unity of Type" and went on to explain the phenomenon as existing because organisms do not start over from scratch, but have characteristics that are built upon already existing ones that were inherited from their ancestors; and these characteristics likely limit the amount of evolution seen in that new taxa due to these constraints. If one sees particular features of organisms that have not changed over rather long periods of time (many generations), then this could suggest some constraint on their ability to change (evolve). However, it is not clear that mere documentation of lack of change in a particular character is good evidence for constraint in the sense of the character being unable to change. For example, long-term

stabilizing selection Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of ...

related to stable environments might cause stasis. It has often been considered more fruitful, to consider constraint in its causal sense: what are the causes of lack of change?

Stabilizing selection

The most common explanation for biological constraint is that

acts on an organism to prevent it changing, for example, so that it can continue to function in a tightly-defined niche. This may be considered to be a form of external constraint, in the sense that the organism is constrained not by its makeup or genetics, but by its environment. The implication would be that if the population was in a new environment, its previously constrained features would potentially begin to evolve.

Functional coupling and physico-chemical constraint

Related to the idea of

is that of the requirement that organisms function adequately in their environment. Thus, where stabilizing selection acts because of the particular niche that is occupied, mechanical and physico-chemical constraints act in a more general manner. For example, the acceleration caused by

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

places constraints on the minimum

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

and

strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...

for an animal of a particular size. Similarly, the properties of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

mean that tissues must have certain

osmotic Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region o ...

properties in order to function properly. Functional coupling takes the idea that organisms are integrated networks of functional interactions (for example, the vertebral column of

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s is involved in the muscle, nerve, and vascular systems as well as providing support and flexibility) and therefore cannot be radically altered without causing severe functional disruption. This may be viewed as one type of

trade-off A trade-off (or tradeoff) is a situational decision that involves diminishing or losing on quality, quantity, or property of a set or design in return for gains in other aspects. In simple terms, a tradeoff is where one thing increases, and anoth ...

. As Rupert Riedl pointed out, this degree of functional constraint — or burden — generally varies according to position in the organism. Structures literally in the centre of the organism — such as the vertebral column — are often more burdened than those at the periphery, such as hair or toes.

Lack of genetic variation and developmental integration

This class of constraint depends on certain types of phenotype not being produced by the genotype (compare stabilizing selection, where there is no constraint on what is produced, but rather on what is naturally selected). For example, for a highly homozygous organism, the degree of observed phenotypic variability in its descendants would be lower than those of a heterozygous one. Similarly, developmental systems may be highly canalised, to prevent the generation of certain types of variation.

Relationships of constraint classes

Although they are separate, the types of constraints discussed are nevertheless relatable to each other. In particular, stabilizing selection, mechanical, and physical constraints might lead through time to developmental integration and canalisation. However, without any clear idea of any of these mechanisms, deducing them from mere patterns of stasis as deduced from phylogenetic patterns or the

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

remains problematic. In addition, the terminology used to describe constraints has led to confusion.

Examples

“''Variational inaccessibility''. Despite mutations, certain character variants are never produced. These variants are therefore developmentally impossible to achieve and are never introduced into a population. This is implied by canalization and has been called both genetic and developmental constraint.”Schwenk K, Wagner GP (2003) "Constraint". In: Hall BK, Olson WM (eds) ''Keywords and Concepts in Evolutionary Developmental Biology''. Harvard University Press, Cambridge (MA), pp. 52–61.

References

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