Within a given taxon, development generally results in the production of individuals that are recognizable as members of that specific group. Vertebrate embryos can be recognized as such early in development, regardless of whether they will later become fish, birds, or mammals. Humans nearly always have the same number of fingers and toes at birth (rare exceptions do exist, however), and, although there are slight variations in the size and shape of digits and limbs from one individual to the next, they are recognizable as human features (for example, wings, hooves, and fins never appear in humans). Normal development seems to follow the same pathway, and resulting variation is limited. This is because developmental constraints favor certain outcomes and prevent others. Developmental constraints are any aspects of a developmental system that increase the probability of a particular outcome and limit the production of variable phenotypes. Just as natural selection favors change to suit environment (adaptation and convergent evolution), developmental constraints limit adaptation and favor conservation of the morphology, or form, of animals. The internal organization of living organisms limits the range of possible phenotypes on which natural selection can operate. Developmental mechanisms are fundamental in generating diversity. At the same time, they impose constraints on the direction of evolutionary change.
Developmental Mechanisms
Because of the particular pattern of embryonic development characteristic of a given species, some structural patterns are more likely to form than others. The probability that a mutation will result in a potentially functionalbody form depends on when the mutation is expressed. Mutations that act on early development are likely to have drastic effects on phenotype because normal development of later structures depends on that of earlier structures, a phenomenon known as epistasis. Drastic changes early in ontogeny are unlikely to result in benefits to the organism and in fact are usually lethal. In contrast, mutations that are expressed late in development are less likely to disrupt the developmental process and more likely to result in functional phenotypes that would benefit the organism.
Mutations, such as the blue pigment in this green frog's skin, may take place late in the developmental cycle. Late-developing mutations are more likely to benefit an organism's ability to function in its environment.
Morphogenesis includes those processes of development that produce the final form of the organism. Anything that alters the final form through evolutionary time must do so through alterations in development. Yet development is a very tightly integrated process in which it is difficult to change one thing without adversely affecting many other things.
Constraints on Development
Regardless of the direction and magnitude of external selective pressure, it may be impossible for the organism to change because of internal constraints. Natural selection might not favor even seemingly adaptive changes because of trade-offs among developmental costs because of pleiotropy, the action of genes in multiple tissues that may be otherwise unrelated. Pleiotropy can result in constraints in which no possible genetic change can produce beneficial morphological change without causing other undesirable changes. Groups of characters may also be associated because of pleiotropy, resulting in suites of characters that are inherited together. In this case, change in one character is impossible without change in the others.
Constraints on development can be generally classified as structural or phylogenetic. Structural constraints can be physiological, cellular, genetic, metabolic, or mechanical. For example, the respiration rate across cell membranes presents physiological limits to cellular surface-to-volume ratios, andmechanical constraints limit how long or thin a limb can be and still support the weight of the organism.
Cellular constraints are limits to rates of cell division, secretion of cell products, and cell migration and/or metabolic efficiency. Metabolic constraints such as the maximum rate of respiration limit the abundance of tissues that have high rates of oxygen consumption. Functional constraints arise in embryos as the organ systems responsible for functions such as feeding and respiration become functionally connected.
Limits to the maximum rate of mutation and recombination that reduce the potential rate of evolutionary change are one form of genetic constraint. The other is a form of historical constraint. Some genes are highly conserved, occurring in many species and higher taxonomic units, because they are involved with fundamental aspects of development.
Phylogenetic constraints (also called historical constraints) are reflected in differences among species that result from having different patterns of descent. Phylogenetic constraints are one reason why variation associated with the production of a given baüplan (body plan) is minimal. Therefore, development of the baüplan may be canalized (guided or controlled) by both structural and historical aspects of genetic constraints.
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