"Waste" -- in the form of urine and feces -- how the body removes the parts of food we ingest that is not used for nutrition and also is a way to rid the body of toxins. The kidneys filter the blood, removing "waste" products such as excess vitamins or drugs (this is why your urine can have a bright color if you take high doses of vitamin c) and liquid waste is held in the bladder before being released. Food travels through the gut to be digested -- broken down into usable bits and waste. After breaking down in the stomach, the material travels through the small and large intestines. The small intestine is lined with villi -- tiny protrusions that add surface area so nutrients can be absorbed into the bloodstream. In the large intestine and colon, water is pulled from the mass so it becomes more solid. Eventually the solidified waste passed through the rectum and out the anus as feces. The build-up of waste in the body can itself be toxic -- if the kidneys do not function properly to clean the waste out, the buildup can be fatal. When the body goes into emergency mode to eliminate a toxic substance -- such as e. Coli in the case of food poisoning -- the intestines don't both absorbing water and the result is the liquid fecal matter being quickly passed through and ejected as diarrhea.
In order for natural selection to take place we must have "variety in a population". This variety of traits is what allows natural selection to "pick" the most favorable traits.
<span>Sterilization of animals reduces the level of circulating reproductive hormones leading to increased longevity, typically due to reduce cancer mortality. The primary aim of animal sterilization is avoiding overpopulation, but this increased longevity could be an added benefit for an individual animal.</span>
For plants, it's photosynthesis.
