VARIATION IN POPULATION CHARACTERISTICS

DIFFERENCE IN STRUCTURE, FUNCTION AND BEHAVIOUR BETWEEN INDIVIDUALS BELONGING TO THE SAME SPECIES ARE COMMON. IN FACT, NO TWO INDIVIDUALS BELONGING TO THE SAME SPECIES ARE COMMON. IN FACT, NO TWO INDIVIDUALS, EXCEPT PERHAPS IDENTICAL TWINS, HAVE EXACTLY SIMILAR CHARACTERISTICS. IT IS THE GRADUAL ACCUMULATION OF MANY VARIATIONS OVER MANY GENERATIONS WHICH EVENTUALLY GIVES A POPULATION REPRODUCTIVE ISOLATION AND, CONSEQUENTLY SPECIES IDENTITY.

POPULATION MAY VARY IN CHARACTERISTICS  AS ADAPTATIONS TO LOCAL HABITAT CONDITIONS. AN INCREASE OR DECREASE IN THE FREQUENCY OF A GIVEN CHARACTERISTIC  APPEARS TO BE A RESULT OF VARIATIONS  IN THE SELECTIVE PRESSURE OF THE ENVIRONMENT, PERMITTING INDIVIDUALS WITH CERTAIN CHARACTERISTICS OR GENE COMBINATIONS TO SURVIVE AT ONE TIME OR PLACE; OTHER INDIVIDUALS, AT OTHER TIMES AND OTHER PLACES.

MOST SPECIES DIFFER NOT BY SINGLE GENES BUT BY HUNDREDS AND THOUSANDS OF GENES. WHEN FREE BREEDING, PREVAILS  IN A SPECIES, THESE GENES MAY BE ARRANGED IN ALL SORTS OF COMBINATION TO FORM AN ALMOST INFINITE MIXTURE OF CHARACTER MODIFICATION. HETEROZYGOTE ARE, THEREFORE, MUCH MORE FLEXIBLE IN ADAPTIVELY RESPONDING TO THE ENVIRONMENT  THAN ARE HOMOZYGOTE. THE MORE CHARACTERS FOR WHICH AN INDIVIDUAL IS HETEROZYGOUS, THE MORE ADAPTABLE ITS OFFSPRINGS ARE LIKELY TO BE. THESE POPULATIONS ARE MORE EFFICIENT IN EXPLOITING THE ENVIRONMENT THAN ARE GENETICALLY UNIFORM ONES.