Variation is essential
Without differences between the individuals in a population, populations cannot change over time—that is, they cannot evolve. Some features must exhibit variability so that change is possible. This variation arises when novel genes are produced as mutations. In other words, mutations—random genetic changes—are the ultimate source of variation in changing populations.
When a novel mutation arises, the population has changed; thus, mutation alone is a mechanism of evolution. However, the occurrence of a single random mutation is unlikely to have a profound effect on a population. Other evolutionary mechanisms—such as gene flow, drift, and selection—play a larger role in shaping the diversity we see on earth today.
However, without this pre-existing and recurring variation, we would not have an evolutionary story to tell.
Gene flow is a profound evolutionary mechanism
Gene flow, or migration, occurs when individuals move between populations. When this happens, some of the novel mutations in one population will migrate to the new population with the migrating individuals. The original population may become less diverse as a result of losing one or more of its individuals. Similarly, the migrants are likely to make their new population more diverse through the introduction of new gene variants. There are many examples of populations evolving through gene flow. An appreciation of gene flow can help us understand the global distribution of the CCR5 mutation in humans (discussed above, in the introduction to the chapter). The CCR5 mutation confers resistance to some forms of HIV, yet is not most common in areas with a high prevalence of HIV and AIDS. The mutation is relatively new: biochemical and biogeographic evidence suggest an origin in Northern Europe approximately 1200 years ago. However, the mutation was distributed long before HIV and AIDS were relevant to human health. In fact, the mutation’s distribution pattern mirrors the Viking migration of the 9th through 11th centuries (Figure 8). Thus, we can hypothesize that Vikings carried the mutation with them as they conquered new territories, and passed the mutation to their descendants. But why was this genetic feature prevalent in Vikings? We’ll develop that story further, below.
Natural selection leads to adaptive evolutionary change
Any feature that benefits an individual in its present environment is considered adaptive, and the feature is referred to as an adaptation. Simply, individuals who possess an adaptation are more likely to survive and reproduce than are individuals without the adaptation. If some or all of the adaptation is inherited, we expect individuals with the adaptation to survive and pass the feature to their offspring in greater numbers than those in the population who do not have the adaptation.
Natural selection refers to adaptive evolutionary change or change that occurs when heritable adaptations confer a survival and reproductive benefit that, in turn, changes the genetic makeup of a population. Natural selection can change populations, species, and whole groups of organisms.
If you consider the speed at which some populations can grow, and the fact that resources are limited, it should be clear that there must be some culling mechanism working to limit population sizes. This mechanism is natural selection: individuals who inherit adaptations simply out-compete (by out-surviving and out-reproducing) individuals that do not possess the adaptations. Evolutionary “winners” are those individuals that survive to reproduce and pass on adaptations to their offspring.
- Give an example of evolution by gene flow.
- What is the relationship between adaptation and natural selection?
- What are some examples of adaptations?
- What is the relationship between natural selection and evolution?
- How might you determine whether a certain individual is an evolutionary “winner”?