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Developmental Constraints on Evolution

A lineageís development may limit the sorts of phenotypes that it can evolve. This limitation is called a developmental constraint.

Pig with wings
The idea of constraint helps us explain why some things didnít happen in evolution that we might think would be advantageous: why didnít any tetrapods evolve more than 5 real fingers and toes, why didnít caterpillars evolve to have the complex eye of adult butterflies, and why didnít pigs evolve wings? Although difficult to figure out, the answers to these questions likely have to do with the developmental processes of tetrapods, insects, and pigs. Perhaps these features would fatally interrupt other aspects of the organismís developmentóor perhaps these features would require so many other drastic changes in development that they are unlikely to arise through mutation.

To look at an example in more detail, horses (and all tetrapodsófrom sloths to salamanders) develop through a stage where the embryo has limbs with five digits, even though some of these will be lost or greatly modified. One might think that it would be advantageous for horses to develop hooves directly, but they donítóthey retain the five-digit developmental stage. The explanation for this may be developmental in natureóskipping the five-digit stage may simply not be an option in tetrapodsí developmental processes.

Horse Sloth Salamander
Horses, sloths, and salamanders all develop through an embryonic stage with five-digit limbs—even though, the digits of the adult forms look quite different.

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Explore further
•  Types of developmental change
•  Ontogeny and phylogeny
•  Understanding complexity
•  Hox genes

Teach this!
Lesson plans for teaching about evolutionary baggage and constraints

• Horse image © 2001 John White.
• Sloth image © California Academy of Sciences; photo by Dr. Lloyd Glenn Ingles
• Salamander image © 2002 William Flaxington

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Genetic Drift

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