Differential Reproductive Success in Evolutionary Science

Flies mating on wood. Getty/Pamela Flora / EyeEm

The term differential reproductive success sounds complicated, but it refers to a rather simple idea common in the study of evolution. The term is used when comparing the successful reproduction rates of two groups of individuals in the same generation of a species population, each exhibiting a different genetically determined characteristic or genotype. It is a term that is central to any discussion of natural selection—the cornerstone principle of evolution. Evolutionary scientists might, for example, want to study whether short height or tall height is more conducive to a species' continued survival. By documenting how many individuals of each group produces offspring and in what numbers, scientists arrive at a differential reproductive success rate. 

Natural Selection

From an evolutionary perspective, the overall goal of any species is to continue to the next generation. The mechanism is ordinarily rather simple: produce as many offspring as possible to ensure that at least some of them survive to reproduce and create the next generation. Individuals within the population of a species often compete for food, shelter, and mating partners to make sure that it is their DNA and their traits that are the ones passed down to the next generation to carry on the species. A cornerstone of the theory of evolution is this principle of natural selection.

Sometimes called "survival of the fittest," natural selection is the process by which those individuals with genetic traits better suited to their environments live long enough to reproduce many offspring, thereby passing the genes for those favorable adaptations to the next generation. Those individuals lacking the favorable traits, or possessing unfavorable traits, are likely to die off before they can reproduce, removing their genetic material from the ongoing gene pool.

Comparing Reproductive Success Rates

The term differential reproductive success refers to a statistical analysis comparing successful reproduction rates between groups in a given generation of a species—in other words, how many offspring each group of individuals is able to leave behind. The analysis is used to compare two groups holding different variations of the same trait, and it provides evidence of which group is "the fittest."

If individuals exhibiting variation A of a trait are demonstrated to reach reproductive age more often and produce more offspring than individuals with variation B of the same trait, the differential reproductive success rate allows you to infer that natural selection is at work and that variation A is advantageous—at least for conditions at the time. Those individuals with variation A will deliver more genetic material for that trait to the next generation, making it more likely to persist and carry on to future generations. Variation B, meanwhile, is likely to gradually vanish. 

Differential reproductive success can manifest in a number of ways. In some instances, a trait variation might cause individuals to live longer, thereby having more birth events that deliver more offspring to the next generation. Or, it may cause more offspring to be produced with each birth, even though lifespan remains unchanged.

Differential reproductive success can be used to study natural selection in any population of any living species, from the largest mammals to the smallest microorganisms. The evolution of certain antibiotic-resistance bacteria is a classic example of natural selection, in which bacteria with a gene mutation making them resistant to drugs gradually replaced bacteria that had no such resistance. For medical scientists, identifying these strains of drug-resistant bacteria (the "fittest") involved documenting the differential reproductive success rates between different strains of the bacteria.