The Cave Environment

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The cave environment
In the article Evolution of eye development in the darkness of caves: adaptation, drift, or both, it mentioned that environmental factors in caves greatly differs from the environmental factors outside of caves. According to Sylvie Retaux and Didier Casane, a factor that impacts the development of cave inhabitants is the availability of sunlight. Without sunlight, food is limited and organisms that live in this environment have to rely on food from elsewhere, such as from the surface or during seasonal flooding.

The diversity of cave animals
Troglobionts are cave dwelling animals that are adapted to the cave lifestyle. Retaux and Didier noted that cave environments have contributed to the evolution of regressive and constructive
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As stated by Retaux and Casane, Darwin described these two species as having lost their visual structures and referred to them as eyeless. He did not see these species striving for survival, nor did he claim that adaption was involved. He contributed that the loss of visual structures was because of not using them in the dark. After Darwin’s explanation of regression, two other viewpoints were proposed. One viewpoint suggested that eye loss and loss of pigment in the eyes are the basis of selective neutral mutation that influences the appearance of such species, favoring genetic drift. The other viewpoint suggested that mutations in pleiotropic genes unintentionally produce regressive traits and the selection of constructive traits is important, favoring natural selection.
As noted by Retaux and Casane, an expectation of the neutral theory of evolution involves different degrees of mutations. It is expected that mutations occur at a higher rate in cave animals because the slightly deleterious mutations are fixed within the population due to their genome size, the smaller population size and the difference in chemical mutagens found in
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Even though the over expression of Hedgehog signals is disadvantageous to the eye lens, pleiotropy factors makes up for it.

Small retina Studies have shown that missing the ventral quadrant of the retina would not be significantly disadvantageous to the cavefish, but since the cavefish eye lens do degenerate and undergo eye apoptosis it is a disadvantage to the vertebrates. This was evidenced by the transplantation of the surface fish lens into a cavefish retina that produced a developed eye, but it is not known if the eye was actually functional. It is also noted that the increase of Hedgehog signaling benefits cavefish by increasing foraging behavior.

Conclusions: selection and constraints
Retaux and Casane concluded that eye degeneration is contributed by pleiotropic factors. They believe adaptive mechanisms replace regressive traits and indirect selection guides this evolutionary process. In terms of genetics, the “Hedgehog gene hypothesis” is not relevant because it does not contain the quantitative trait loci, which are responsible for eye development. They also mentioned that cavefish undergo the development of eyes before undergoing the degeneration process because if they didn’t then they will not have viable

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