What must be true for natural selection to occur?

In order for natural selection to occur, reproduction, heredity, variation in fitness or organisms, variation in individual characters among members of the population must occur. If they happen natural selection automatically results.

Explanation:

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d. making up measurements to record.

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.