Final answer:
According to the theory of evolution, changes in horse populations over generations occur due to natural selection. This process, proposed by Darwin and Wallace, states that individuals who are most fit to survive and reproduce pass on their traits, leading to a change in population characteristics.
Step-by-step explanation:
According to the theory of evolution, the changes in horse populations over many generations occur due to natural selection. This process, proposed by Charles Darwin and Alfred Russel Wallace, states that individuals who are most fit to survive and reproduce will pass on their traits to the next generation. Over time, this leads to a change in the characteristics present in the population. In the case of horse populations, the transformation of leg bones from having four distinct digits to the structure of today's horse is an example of the changes brought about by natural selection.