Explain the term selective pressure as it relates to changes in populations

Selection pressures are external agents which affect an organism's ability to survive in a given environment. Selection pressures can be negative (decreases the occurrence of a trait) or positive (increases the proportion of a trait)

Selection pressure are external agents which affect an organism ability to survive in a given environment, selection pressures can be negative ( decreases to occurrence of a trait) or positive ( increases the proportion of a trait)

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i guess you forgot putting it.

what's the question ?