Different genes make for greater or lesser chances of survival and reproduction, natural selection results in more favorable genes becoming more frequent in a population over time.

Genetic changes that give their carriers a better chance to survive and reproduce than individuals without the genetic change who live in the same environment.

Climate may influence the way the human body grows and develops, and therefore some kinds of human variation may be explainable largely as a function of environmental variation.

Acclimatization involves physiological adjustments in individuals to environmental conditions. Acclimatizations may have underlying genetic factors, but they are not themselves genetic. Individuals develop them during their lifetimes, rather than being born with them.

Describes a general relationship between body size and temperature: The slenderer populations of a species inhabit the warmer parts of its geographic range, and the more robust populations inhabit the cooler areas.

Bergmann and Allen, two 19th century naturalists, suggested some general rules for animals, but it was not until the 1950's that researchers began to examine whether these rules applied to human populations.

Protruding body parts (e.g., limbs) are relatively shorter in the cooler areas of a species' range than in the warmer areas.

Research comparing human populations tends to support Allen's rule.

Rationale behind these theories is that the long-limbed, lean body type often found in equatorial regions provides more surface area in relation to body mass and thus facilitates the dissipation of body heat. In contrast, the chunkier, shorter-limbed body type found among residents of cold regions promotes retention of body heat because the amount of surface area relative to body mass is lessened.

The amount of melanin in the skin seems to be related to the climate in which a person lives.

Populations of birds and mammals living in warmer climates have more melanin and, therefore, darker skin, fur, or feathers, than do populations of the same species living in cooler areas.

Oxygen deficiency

Most often when referring to the conditions in locations of high altitude where barometric pressure is lower, allowing the inatake of lesser oxygen with each breath.

Sickle-cell anemia

or 

 Sicklemia

Abnormality of the red blood cells.

Condition in which normal, disk-shaped red blood cells assume a crescent (sickle) shape when deprived of oxygen.

the sickle-shaped red blood cells do not move through the body as readily as normal cells, and thus cause more oxygen deficiency and damage to the heart, lungs, brain, and other vital organs. 

Red blood cells tend to "die" more rapidly, and the anemia worsens still more.

Through the process of natural selection and genetic drift, populations inhabiting different geographic regions will come to exhibit some differences in biological traits. When differences within a species become sufficiently noticeable, biologists may classify different populations into different varieties, or races.

The way biologists describe slight population variants within a species.

The belief that some "races" are innately inferior to others.

The misuse and misunderstanding of the term race and its associatoin with racist thinking is one reason why many biological anthropologists and others hae suggested that the term should not be applied to human biological differences.