Its is by natural selection; that is, species that are better adapted to their environment are more likely to survive and reproduce. It was assumed that the process of evolution halted a long time ago. But we have been evolving for the past 50,000 years. It all begins with your genes. In your genes you have these small variations, random genetic variations called mutations. They can lead to differences among individuals, differences which affect the ability to adapt to change. Thats what natural selection is...to survive. And survival permits then to reach sexual maturity, to reproduce, and to transmit their features to the next generation. We can explain heredity by looking at Gregory Mendel's experiment on garden peas. His study concluded that characteristics transfered from the parent to the offspring. By using seeds considered to be of the same breed and type he was able to duplicate the same flower. Also by intermingling them he found out that it was posible that humans can be breed much like other species and made to be of certain characteristics if common traits were used. But there are many factors in the case of evolution. Sciences such as Palaeontology, the study of fossils, and geological distribution, the study of different species scattered around the Earth can show how it use to be thousands of years ago all help the theory of Evolution. Still the actual mechanism of evolution, however, is not yet conclusively proven.
TutorVista- Heredity and Evolution (No date)
Rathus, S. (2011,2007)- Psychology Concepts and Connections