During his travels, Darwin essentially developed the fundamental principles of evolution. Included in these observations were that traits are inherited from parent to offspring, members of a population exhibit genetic diversity (numerous traits), and that organisms are capable of producing more offspring than the environment can support. The fourth observation was that, owing to a lack of food or other resources, many offspring do not survive. Together, these observations helped shape Darwin's theories of natural selection and biological fitness to explain the trends that he had seen.

A key point about natural selection is that it is the population that evolves over time, not individual members of a population lifetime. Individuals that may have inherited advantageous traits are likely to produce more offspring than other members of the population, which drives natural selection within the context of the environment. Environmental pressures can affect the forces of natural selection, but cannot directly result in the production of new traits or the inheritance of those traits.

Darwin described his theory as "descent with modification," meaning that offspring inherit their genetic material from their parents, but with slight modifications. Darwin proposed that those slight modifications which provided a fitness benefit (made it more likely that an organism would reproduce) were more likely to be passed on.

Charles Darwin studied finches in the Galapagos, which prompted his work on natural selection. This was observed as only the finches who were evolutionarily prepared reproduced, thus increasing their traits in the gene pool 

Both "microevolution" and "macroevolution" are driven by the same biological process: natural selection. While the former refers to change within species and the latter refers to change to a new species, the line between what constitutes a species change is difficult to define. Since both terms refer to the same process of natural selection, the term evolution applies to both, whether as a single adaptation or a series of adaptations that culminate into a single species.

Speciation occurs when two populations are separated from each other and accumulate different changes due to different environments. When the birds migrated to the Galapagos and made homes on each island, they became physically different from each other over time. This was one observation to the theory of natural selection.

Influenza (the flu virus) is a real-life example of natural selection that is most evident during an annual time period when the influenza virus infection peaks. The repeated or cyclical nature of flu outbreaks is caused by the natural selection of the virus for traits that allow it to evade the immune system and replicate, a process that actually results in new distinct strains of influenza virus.

In contrast, the other answers are all examples of artificial selection by which the specific variety or breed has been purposely selected for specific traits. Poodles and Angora rabbits have been purposely bred by humans to select for traits that affect their fur. Merlot wine is produced from a certain strain of grapes that have been bred for their taste. Note that artificial selection differs from genetic modification, as no new traits have been introduced to these strains.

Directional selection is natural selection that favors a phenotype at an "extreme" for a particular trait. In this example, we are looking at the height trait. Individuals with taller phenotypes exhibit greater fitness due to their ability to reach the food source more easily. The new environment favors taller giraffes, and thus, after many generations, the phenotype of the population as a whole will be skewed towards the tall end of the height spectrum.

Stabilizing selection favors the average phenotype for a given trait. Disruptive selection favors both extremes of a phenotype, but favors against the average.

Heterozygote advantage occurs when heterozygotes at a particular locus, such as pigmentation, have greater fitness than do both kinds of homozygotes. If the heterozygote favors an intermediate phenotype, such as beige mice, it is also stabilizing selection. Frequency-dependent selection occurs when the fitness of a phenotype depends on how common it is in the population. Together, heterozygote advantage and frequency-dependent selection lead to a balancing system. The founder effect occurs when a few individuals of a population become isolated and form a new population whose gene pool differs from the original population. Directional selection occurs when conditions favor individuals at one extreme of a phenotypic range, thereby shifting a population’s frequency curve for the phenotypic character in one direction or another.

Purifying selection is a type of natural selection impacts genetic diversity. Purifying selection removes variation from a population, meaning that there is a decrease in genetic diversity.

Balancing selection is a type of natural selection that maintains genetic variation in a population, making it the opposite of purifying selection. Examples of balancing selection include heterozygote selection, in which there is adaptive value for heterozygotes of an allele.