Called bioconversion although it can also be referred to as bio transformation
The answer to this question would be: by producing buffer
The pH of optimal growth of bacteria might be different. Some bacteria can grow in an acid condition called acidophile, other live in base condition called alkaliphiles and the rest live in neutral pH condition called neutrophiles. The bacteria can maintain their internal pH by producing acid, base or buffer. If the surrounding pH is too acid, the bacteria can reduce the H+ concentration by producing base. If the condition is too alkali/base, the bacteria can produce acid by doing fermentation.
This might have been the cause of not being exposed to this specific pollen during his years in Chicago. Allergic reactions can develop when the stimuli presented even when its not contagious causes the immune system to trigger and act in a defensive stance. This delayed effect may have been the cause of the sudden expression of the immune system.
Answer;
He conducted an experiment with breeding pigeons.
Explanation;
The pigeons provided the perfect animal to test his theory of selection for quite a number of reasons including its trait diversity from wing structures to color patterns to size to flight patterns. From these experiments he concluded that by natural selection of randomly occurring traits that make species better suited for survival and reproduction, evolve in the astounding diversity of organisms on Earth today.
