Final answer:
The statement is false because natural selection can also have observable effects within a single generation, and it does not always take thousands to millions of years for significant changes to occur.
Step-by-step explanation:
The statement that the effects of natural selection are only evident over extremely long periods of time, such as thousands to millions of years, is false. While it is true that major evolutionary changes in the genotype of a species due to natural selection can take thousands or even millions of years, natural selection can also work in a single generation on an individual organism. This can result in observable changes within a relatively short period, especially if the environmental pressures are strong and the biological traits are highly heritable. An example of this would be the development of drug-resistant viruses, which can occur rapidly when there is intense selection pressure from the use of antibiotics.
Natural selection acts on populations by increasing the frequency of beneficial alleles while decreasing the frequency of those that are deleterious. This process, known as adaptive evolution, shapes species over time. Additionally, models like gradual speciation and punctuated equilibrium illustrate how species can diverge slowly or rapidly, followed by periods of little or no change. These mechanisms reflect the dynamic nature of evolutionary processes.