Alfred Russel Wallace was a naturalist who independently proposed the theory of evolution by natural selection. Wallace proposed a theory of evolution. Malthus wrote an Essay on the Principle of Population, which explained his predictions and changed the view of many people. Thomas Malthus believed that the human population exhibits exponential growth. Hardy Weinberg principle says that allele frequencies in a population are stable and is constant from generation to generation. The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium. For example, can be named , etc. In a diploid, p and q represent the frequency of allele A and allele a . The frequency of AA individuals in a population is simply . This is simply stated in another way, i.e.,the probability that an allele with a frequency of appears on both the chromosomes of a diploid individual is simply the product of the probabilities, i.e., . Similarly, of aa is , of . Hence, . This is a binomial expansion of . The classroom example of a visible evolutionary response is industrial melanism in the peppered moth (Biston betularia): the replacement, during the Industrial Revolution.