Natural Selection And Phenotypic Variation Essay

2025 words - 8 pages

Long standing arguments against the theory of natural selection stem from the occurrence of incipient structures and complex traits in organisms despite the seemingly stochastic nature of mutations. Many complex adaptations observed in nature today are thought to have arisen from less complex ones with simpler functions, therefore these characters are thought to have been “pre-adapted.” In order to go from a simple to a complex structures there must have been a transitional phase, where the two structures function simultaneously or where the new function is assumed without interfering with the old function. These structures are termed incipient or incomplete, and given what we know to be true of natural selection and the theory of evolution it becomes hard to reconcile the idea that natural selection continued to favor these structures despite the lack of selective value. Incipient structures are thought to be neither sufficiently large enough not elaborate enough to perform an adaptive function and thus it also becomes difficult to understand how larger complex characters arise. A discussion of morphological and developmental genetics explains that these structures have been performing useful functions since their simple origins, therefore being selectively favored while at the same time evolving to become large enough to accumulate new more complex functions. Modification of pre-existing genes and regulatory circuits in early development has been extensively studied in metazoans, Hox genes and the development of complex structures such as eyes, limbs and appendages. Phenotypic variation is therefore generated via the modification of existing genes, regulatory processes and developmental processes and this variation is acted on to produce significant phenotypic effects. It is the generation of novel characters which have brought on large-scale evolutionary trends.
Evolutionary novelty is a term that is often tossed around because even among the scientific community, there is a lot of debate as to what can be called an adaptation and what should be called an exaptation. Gould and Vrba (1981) argued that not all characters are adaptive, but rather that some structures were not adapted for anything in particular but due to selective pressures of the environment, they now have important functions. They posit that “Adaptations have functions; exaptations have effects” (Gould & Vrba 1981) and this is significant because it implies that evolution is flexible. In other words, structures “though undoubtedly built by selection for some complex set of functions, can, as a result of its intricate structure, work in an unlimited number of ways unrelated to the selective pressure that constructed it” (Gould & Vrba 1981). It is this flexibility which allows species to better adapt to new environments, thus the effect of having a certain exaptation can mark the difference between future survival and extinction. To further drive the point home, we can...

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