An example of adaptation by natural selection is the retina of the vertebrate eye (in contrast to some other eyes, such as those of some molluscs) is “inside out” relative to an efficient design 10. The vascular tree shadows the photoreceptor cells. Because it is on the “wrong side”, the optic nerve has to go through a hole in the retina, resulting in a blind spot. These features imply that vertebrate retina evolution was processive, not designed in a single creative step.
From its origin in a flat, light-sensitive layer in the dorsal side of the anterior nerve system, development of the vertebrate nerve system (making a tube from a planar surface) resulted in the inverted orientation of the future retina. This retinal “inversion” is not a result of some underappreciated function, because some independently evolved eyes have “noninverted” retinas. Once a pathway of successive adaptations begins, reversals may be competitively disadvantageous (e.g., to completely reorganize the vertebrate retina, a redesign of neural tube development might be required. Again, ancestry apparently constrains the range of variation available to natural selection.
Adaptation is used differently in evolutionary biology than it is in physiological anthropology. Aspects of an organism that suggest its adaptability are the complexity between structure and function and the comparative method of correlating species differences with ecological factors. Direct evidence for adaptation can only come from experimental studies 8.
The problems with adaptive significance and studying diseases such as albinism are many, but not invalidating. The possible problems are an adaptation may not appear to result in better performance with respect to the environment. Adaptation increases relative fitness, not necessarily absolute fitness but better competition with other genotypes 11. A variation of a structure could simply be neutral. These adaptations may not have evolved for purposes for which they now appear to be useful. A trait might not be determined genetically, but be a direct consequence of environment or learning. The same trait might be a simple consequence of chemical or physical “laws”. It is evident that different species may have (neutral) variation for the same adaptive feature merely because of different ancestry; e.g., although a pattern that provides good camouflage is likely to be an adaptive trait, alternative patterns that also provide good camouflage may work just as well 11. Many variations that appear may be “constrained” by the developmental system or the genome (e.g., that tetrapods have 4 limbs may not be an adaptation–why couldn t 6 work just as well?–but this number depends on what is allowed by the developmental mechanisms that pattern the organism and on the ancestry of the genome). Any trait is likely to be anachronistic, since the conditions under which a feature evolved existed in the past.
Adaptation, presents obstacles to validity, however, given the formula for environmental factors, genetic influences, and biological variation, it is evident that albinism falls into this category.
Works Cited
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