Microevolution refers to the changes in the frequency of specific gene versions, known as alleles, within a single population over generations. These shifts represent evolution occurring on a small scale, often over relatively short periods. For example, a population of rabbits might show a subtle shift in average fur color from lighter brown to a slightly darker shade across several generations. This gradual alteration in the genetic makeup of the population exemplifies microevolution.
The Driving Forces of Microevolution
Genetic changes within a population are influenced by several fundamental processes that alter allele frequencies. These processes shape the characteristics observed in subsequent generations.
Natural Selection
Natural selection is a process where certain traits become more prevalent in a population because they offer advantages in survival and reproduction. Individuals possessing traits that better suit their environment are more likely to survive, find mates, and produce offspring. These advantageous traits are then passed on, leading to their increased representation in the gene pool of future generations. This mechanism is the only one among the evolutionary forces that consistently leads to adaptation, making organisms better suited to their surroundings.
Genetic Drift
Genetic drift describes random fluctuations in allele frequencies that occur due to chance events, independent of any selective pressure. This process is particularly pronounced in smaller populations, where random occurrences can have a significant impact on the overall gene pool. For instance, if a small group of individuals is isolated, their allele frequencies might differ randomly from the original larger population.
Two specific scenarios illustrate genetic drift. The bottleneck effect happens when a population undergoes a sudden, drastic reduction in size, such as from a natural disaster, leaving a smaller, non-representative sample of the original genetic diversity. The founder effect occurs when a new population is established by a small number of individuals who separate from a larger group, carrying only a limited subset of the original population’s alleles.
Gene Flow
Gene flow involves the transfer of genetic material between different populations through the movement of individuals or their gametes. When individuals migrate from one population to another and interbreed, they introduce new alleles or alter the frequencies of existing ones in the recipient population. This exchange of genetic information can reduce genetic differences between populations, making them more similar over time. An example would be pollen from one plant population being carried by wind to fertilize plants in a distant population.
Mutation
Mutation refers to random, spontaneous changes in the DNA sequence of an organism. These alterations are the ultimate source of all new genetic variation within a population. While many mutations are neutral or harmful, some can introduce novel traits that may be beneficial, harmful, or have no immediate effect. Without mutations, the raw material for evolutionary change would not exist, as there would be no new alleles for natural selection or other forces to act upon.
Observing Microevolution in Action
Microevolution is a demonstrable phenomenon observed in various species across diverse environments. These real-world examples highlight how populations respond to environmental pressures and how allele frequencies shift over relatively short periods.
Antibiotic Resistance in Bacteria
The development of antibiotic resistance in bacteria provides a clear example of microevolution. When a population of bacteria is exposed to an antibiotic, most susceptible bacteria are killed. However, a few individual bacteria might possess random genetic mutations that confer resistance. These resistant bacteria survive, multiply rapidly, and pass on their resistance genes to their offspring. This process leads to a new population dominated by resistant strains, making the antibiotic less effective over time.
The Peppered Moth
The peppered moth (Biston betularia) offers a well-known example of microevolution driven by environmental change. Before the Industrial Revolution in England, most peppered moths had light-colored wings, camouflaging them against lichen-covered trees. As industrial pollution darkened tree trunks with soot, light-colored moths became more visible to predators. Darker, melanic forms, previously rare due to mutation, gained a survival advantage, increasing their frequency in polluted areas. When pollution controls were implemented and tree trunks lightened, the selective pressure reversed, and lighter moth forms became more prevalent again.
Pesticide Resistance in Insects
Similar to antibiotic resistance in bacteria, pesticide resistance in insect populations demonstrates how human-introduced pressures can drive rapid microevolution. When an insecticide is applied, it eliminates most susceptible insects. However, some insects within the population may possess pre-existing genetic variations that make them less susceptible. These resistant individuals survive the application, reproduce, and pass their resistance genes to their offspring. Over successive generations, the insect population becomes increasingly resistant to the pesticide.
The Link to Macroevolution
Microevolution and macroevolution are not distinct processes but rather represent different scales of the same underlying evolutionary mechanisms. While microevolution describes genetic changes within a population or species over shorter timeframes, macroevolution encompasses evolutionary changes at or above the species level, occurring over vast geological periods. The emergence of new species or larger taxonomic groups, for instance, falls under macroevolution.
Macroevolution is the cumulative outcome of numerous microevolutionary changes accumulating over extended periods. Consider microevolution as individual footsteps; macroevolution is the entire journey resulting from those steps. When enough microevolutionary changes accumulate within isolated populations, they can become so genetically different that they are no longer able to interbreed successfully. This reproductive isolation marks the formation of a new species, a process known as speciation.