Species are characterized by the fact that they are reproductively isolated from other groups, which means that the organisms in one species are incapable of reproducing with organisms in another species.
Chromosomes are thread-like structures present in the nucleus, which carries genetic information from one generation to another. They play a vital role in cell division, heredity, variation, mutation, repair and regeneration.
An independent variable that is manipulated means to achieve a particular effect or tested to determine if it is the cause of the effect.
For example, to find out if your weight depends on the amount of calories you consume, you could lower your caloric intake (calories is the manipulated variable) for a period of time and see how your weight changes in response.
