The role of environments with extreme ecological conditions in the reductive evolutionary development processes of animal

Different groups of animals show phenotypic characters, which have been resulted by the reductive phenomena. The examples are the absence of pigmentation; dwindle of eyes in some cave-living animals, and also the absence of scale in some fishes. These characters are often leaded to evolution of new species with special adaptation that is so called "Regressive evolution". The reductive phenomena occur as a result of a long time adaptation of populations to the environments with extreme ecological conditions such as high salinity, high solphure and low oxygen concentration. Many fish species such as the only Iranian scaleless tooth-carp (Aphanius furcatus</em>) have been evolved by this event. The principle mechanisms of this event have not been understood well, but it is suggested that some specific genes such as Ectodysplasin probably play role in the reductive event because Ectodysplasin is responsible to produce the ectodermal structures such as teeth and scale. Therefore, mutation in this gene pathway causes reduction in development and forming of the ectodermal structures. As results, the reductive events in most of the animals caused speciation, while the result of this event in human seems to be different than other animals.