Biological constraints are factors which make populations resistant to evolutionary change. Constraint has played an important role in the development of such ideas as homology

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 and body plan

Body plan

A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...

s.

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. In order to make the concept more useful, then, it is necessary to divide it into smaller units. First, one can consider the pattern of constraint as evidenced by phylogenetic analysis. 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. 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 stabilising selection acts on an organism to prevent it changing, for example, so that it can continue to function in a tightly-defined niche

Ecological niche

In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

. 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 stabilising selection is that of the requirement that organisms function adequately in their environment. Thus, where stabilising 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 due to gravity places constraints on bone density and strength for a particular sized animal. Similarly, the properties of water mean that tissues must have certain osmotic 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

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

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. As Rupert Riedl

Rupert Riedl

Rupert Riedl was an Austrian zoologist who made contributions in the fields of:* Marine biology* Morphology* Theory of evolution * Evolutionary Epistemology* Environment and society...

 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 stabilising 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 descendents would be lower than those of a heterozygous one. Similarly, developmental systems may be highly canalised

Canalization

Canalization may refer to:* Canalization, the process of introducing weirs and locks to a river so as to secure a defined depth suitable for navigation*Channelization, the process of modifying the course of a stream so that it follows a restricted path...

, 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, stabilising 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 phylogeny or the fossil record remains problematic.

Further reading

• Riedl, R. (1978). Order in Living organisms: a systems analysis of evolution. John Wiley & Sons.
• Schwenk, K. (1995). A utilitarian approach to evolutionary constraint. Zoology 98, 251-262.