In general what is true about places at higher altitudes ?

Higher altitudes have cooler temperatures than places at lower altitudes

In the high areas the air is less dense, for this reason the heat cannot be absorbed and maintained as in areas of denser air. These areas are cooler.  

In these areas the rains are abundant. The temperature decreases approximately 1 ° C every 100 meters. These areas receive greater heat stroke and a specific wind regime.

In general the diversity of species is low in high altitude areas due to the extreme conditions of temperatures and atmospheric pressure

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.