There are at least 31 climatic zones around the world ranging from year-round freezing conditions to daily hot temperatures of around 45 degrees C. Each zone is inhabited by people who have adapted their lifestyles to accommodate the environmental conditions. There are many examples showing physiological and morphological differences between groups living in different environmental conditions (i.e. climate has been shown to influence characteristics including birth weight, body shape and composition, cranial morphology and skin color and sensitivity). Whilst the phenotypic differences are very clear, the genotypic differences are less easy to discern. This can be attributed to the logistical difficulties in executing the definitive study which controls for the environmental and lifestyle factors which themselves induce physiological and morphological changes. However, considering that at least 50 genes have been identified which have altered expression after exposure to heat and at least 20 genes are altered by cold exposure, it is reasonable to assume that more physiological and morphological differences will be attributed to genetic origins as the data becomes available.