Final answer:
Natural selection was proposed by Charles Darwin and Alfred Wallace as a mechanism of evolution, leading to variability within and among species. Their theory, combined with Gregor Mendel's genetic discoveries, resulted in the modern synthesis of evolutionary biology.
Step-by-step explanation:
Natural selection, as a mechanism of evolution that acts on variants within populations leading to the evolution of different species, was proposed by both Charles Darwin and Alfred Wallace. They independently observed patterns in organisms and realized that certain traits became more common in a population due to these traits providing an advantage in surviving and reproducing. The concept of natural selection was pivotal in understanding how species adapt and evolve over time, providing a scientific explanation for the diversity seen in the natural world.
In addition to Darwin and Wallace's contributions to the principle of natural selection, the work of Gregor Mendel, published in 1866, laid the foundation for the modern understanding of genetics, which was later integrated with the theory of evolution in the modern synthesis. This synthesis united the principles of genetics with natural selection, explaining how evolution can lead to changes in a population's genetic makeup and the emergence of new species over time. Darwin's and Wallace's theories set the groundwork for this integrated perspective on evolution and genetics.