What happens to unnecessary traits during the process of natural selection?

Explanation:

Under an evolutionary phenomenon called relaxed selection, traits that were advantageous in one time and place become obsolete in another. Traits that aren't actively maintained by natural selection tend to become smaller or less functional over time, studies suggest.

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acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.