1. Variations exist in the individuals within a population.
2. Those variations are passed down from one generation to the next.

    But Darwin had no idea how those variations came to be or how they were passed down from one generation to the next. Mendel’s experiments and the development of the science of genetics provided answers. Genetics explains that the phenotype—the physical attributes of an organism—is produced by an organism’s genotype. Through the mechanism of mutations, genetics explains how variations arose among individuals in the form of different alleles of genes. Meiosis, sexual reproduction, and the inheritance of alleles explain how the variations between organisms are passed down from parent to offspring.

 

    With the modern understanding of genes and inheritance, it is possible to redefine natural selection and evolution in genetic terms. The particular alleles that an organism inherits from its parents determine that organism’s physical attributes and therefore its fitness for survival. When the forces of natural selection result in the survival of the fittest, what those forces are really doing is selecting which alleles will be passed on from one generation to the next.

Once you see that natural selection is actually a selection of the passage of alleles from generation to generation, you can further see that the forces of natural selection can change the frequency of each particular allele within a population’s gene pool, which is the sum total of all the alleles within a particular population. Using genetics, one can create a new definition of evolution as the change in the allele frequencies in the gene pool of a population over time. For example, in the population of moths we discussed earlier, after the trees darkened, the frequency of the alleles for black coloration increased in the gene pool, while the frequency of alleles for light coloration decreased.

 

 

    The Hardy-Weinberg principle states that a sexually reproducing population will have stable allelic frequencies and therefore will not undergo evolution, given the following five conditions:

• large population size
• no immigration or emigration
• random mating
• random reproductive success
• no mutation

    The Hardy-Weinberg principle proves that variability and inheritance alone are not enough to cause evolution; natural selection must drive evolution. A population that meets all of these conditions is said to be in Hardy-Weinberg equilibrium. Few natural populations ever experience Hardy-Weinberg equilibrium, though, since large populations are rarely found in isolation, all populations experience some level of mutation, and natural selection simply cannot be avoided.