What problem is faced by organisms that live in fresh water?
their bodies tend to take in too much water.
they have no way of controlling their tonicity.
only salt water poses problems for animals that live in it.
their bodies tend to lose too much water to their environment.

A. Their bodies tend to take in too much water.

Explanation:

Osmoregulation is the maintenance of osmotic concentration inside the body cells and in the extracellular fluid by controlling the amount of water and salts.  Organisms living in water can be divided into two groups: Some are osmoconformers which change the osmolarity of body fluids with respect to the surrounding medium. Some are osmoregulators which do not allow change in internal osmolarity and try to maintain it by various means.

If as osmoregulator is placed in fresh water environment then their body is hypertonic to their environment. Osmosis will occur which is the movement of water from low solute to high solute concentration. Thus the problems faced by such organisms would be:

- Entry of excess water

- Loss of body salts to outside.

Adaptations in such organisms would be:

- Body cover such as scales or adipose covers

- They do not drink more water

- Excess water is eliminated as dilute urine

- Special cells called ionocytes or chloride cells help in active uptake of sodium ions and chloride ions.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

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