Question: At the beginning of the spring, Dr. Alice Whittlesby notices that there is an equal distribution of long and short grasses in the vacant lot across the street from her house as well as in her own backward which has not been mowed for years. Several years later she collects seeds and grows grass from both locations. She notes that there are mostly long grass plants from the vacant lot seeds where the grass had been left to grow with no human intervention, but mostly short grass plants in her yard which had been grazed regularly by several goats.

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Question: These observations are examples of what biological principle? Develop a hypothesis to explain Dr. Whittlesby's observations. Explain your answer and provide examples.