Microevolution

Microevolution is the process of change in allele frequencies within a population over time. An allele is a specific version of a gene. When the proportion of these versions changes, the population is evolving at a micro level. This process occurs over relatively short periods compared to macroevolution. Macroevolution involves larger changes, such as the sorting of interspecific variation, which is the difference between different species. Microevolution, however, deals with intraspecific variation, or differences within a single species. This process provides the essential raw material that eventually drives macroevolutionary changes. Scientists study these shifts through population genetics, which uses mathematical structures. They also use ecological genetics to observe these changes occurring in the wild.

Four distinct processes drive microevolution: mutation, selection, genetic drift, and gene flow. Mutation is a change in the DNA sequence of a cell's genome. These changes can be caused by radiation, viruses, or mutagenic chemicals. They also occur due to errors during meiosis or DNA replication. DNA replication errors happen frequently during the polymerization of the second strand. To prevent too many errors, cells use DNA polymerases for proofreading. This keeps error rates very low, often only one error in every 10 to 100 million bases. Without this proofreading, error rates could be a thousandfold higher.

Mutations can have various effects on an organism's phenotype, which is its observable physical traits. Some mutations are harmful, some are neutral, and some are weakly beneficial. Studies on the fly Drosophila melanogaster show that about 70 percent of mutations changing a protein are damaging. Mutations can also involve large structural changes like duplications, inversions, or deletions. Gene duplications are particularly important for evolution. They allow one gene copy to keep its original job while the other copy acquires a new function. In animal genomes, tens to hundreds of genes are duplicated every million years. This redundancy helps create novel genes through recombination or mutation.

Selection is the process where heritable traits that aid survival and reproduction become more common. Natural selection is the most dominant form of this process. It acts on the phenotype to favor individuals better suited to their environment. Over time, this can lead to adaptations and even speciation. Charles Darwin famously described this in his 1859 book, On the Origin of Species. He compared natural selection to artificial selection, where humans choose specific traits in plants and animals. The modern evolutionary synthesis was later formed by combining Darwin's ideas with molecular genetics.

Genetic drift is another mechanism that changes allele frequencies through random sampling. In any population, the alleles passed to offspring are a random sample of the parents' alleles. Unlike natural selection, genetic drift is not driven by environmental pressures or adaptive success. Changes from drift can be beneficial, neutral, or even harmful. The impact of genetic drift is much stronger in small populations than in large ones. It can cause certain gene variants to disappear entirely, reducing genetic variability. Scientists have debated its importance for decades. Motoo Kimura proposed the neutral theory of molecular evolution in 1968, suggesting drift causes most genetic changes.

Gene flow is the exchange of genes between different populations of the same species. This often happens through migration or the movement of pollen. Immigration adds new genetic material to a gene pool, while emigration removes it. Gene flow can actually slow down the process of speciation by spreading differences between populations. Physical barriers like oceans, mountains, or deserts can hinder this flow. Even man-made structures, such as the Great Wall of China, can stop plant gene flow. Sometimes, gene flow occurs between different species through hybridization. For example, horses and donkeys can mate to produce a mule.

While many hybrids are infertile because their chromosomes cannot pair up during meiosis, some are viable. These hybrids may show traits that are intermediate between their parents or possess entirely new phenotypes. Microevolutionary processes are visible in many real-world examples. A common instance is the development of antibiotic resistance in bacterial strains. This shows how quickly a population can change in response to its environment. By understanding these four processes, biologists can trace how life adapts and changes across the globe.