You cannot determine the fittest of the species unless you know the rate of reproduction, and without a rate of reproduction, natural selection cannot determine which of the species is the fittest. Therefore, "Fitness" results from reproduction. You must survive to reproduce, but if you don't reproduce, then no fitness. Rate of reproduction can be determined by, but not limited to, several contributing factors to natural selection including mutations, genetic drift, gene flow, and assortive mating.

Mutations lead to a new genetic variation in population. Mutations can cause varying results, good or bad, and partly neutral. If neutral, it will lead to advancement of the species and evolution. Therefore, if a species has a physical characteristic from natural selection resulting from a particular persistent mutation, they will exhibit increased reproductive success. Thus a higher frequency of the physical characteristic will be observable in the population.

Genetic drift occurs in a smaller population who’s isolated from larger population. In some cases, too many genes can be contributed; therefore physical characteristics will ultimately lead to a lack of variation and a smaller population. A prime example of this would be the Amish, who interbreed within their own population leading to little variation, and in many cases, mutations that do not contribute to the growth of the population size.

Genetic flow occurs during a species migration and the exchange of genes between populations. Therefore, this exchange will lead to new variation in physical characteristics and diversity of the species. If a section of the migrants leave, once again a new variation or gene frequency will occur within the population. Reproduction will take place, but variations will occur over time, as evolution is not an instant idea.

Assortive Mating occurs when species having like with like variations mate to produce a population with little or no variation (many mutations seen). Consequently, unlike with unlike mating leads to a diverse population, and many case, less mutations. This can be seen in breeds of animals, one example being Siberian Huskies.

(Narrated by John Lithgow, the program ranges from a wolf research facility in rural Indiana to the Westminster Dog Show in New York's Madison Square Garden. NOVA makes a fascinating detour to the city dump in Tijuana, Mexico, where viewers get surprising insight into the origin and evolutionary strategy of our canine companions.)

Example of Like with Like: (Pure Breed & Pure Breed) I have a pure bred Sable Siberian Husky. He’s six years old and comes from a line of championship European show dogs. At the age of five, his hips started to weaken and he took on an offensive smell. His knees are weak and he exhibits an activity level of a 70 year old. This would be a like with like breeding result.

Example of Unlike with Unlike: (German Shepherd & Siberian Husky) My neighbor has a mixed breed husky. He’s 14 years old, has no smell, no problems with his knees or hips, and he has high activity-leading me to believe unlike and unlike variation results in an increase in reproductive success in the population.

It’s important to note that although gene frequencies may change in a population, leading to variation, it doesn’t mean that a population will have more reproductive success. In fact, the size of the population may not change at all, just the physical characteristics that can be observed. Therefore, natural selection looks for traits- not who’s the fittest- that are adaptive in a given environment. Natural selection cannot determine fitness from observing a population. Reproductive success must be known to make that determination.

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Sources

Picture via Wikipedia

Video via Archive.org