Every species exhibits variations based on various genes being activated at various times. An example with humans can be eye color or hair color. What species means is that all members of a species can procreate with healthy, capable of procreating progeny, but anything else is just a variation.
Living organisms such as bacteria, fungi, algae are very important in the soil, they perform several functions, some of which include the following;
- Living organisms help break down dead plant parts, this helps to add more nutrients to the soil
- Living organisms dig up holes in the soil, this helps to maintain aeration in the soil.
- Living organisms play a major role on the production of humus
- Living organisms help to break large particles of rock
- Living organisms such as Rhizobium bacteria help in the fixing of nitrogen
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the answer is the first one: by producing random changes in an organism's genetic code
