Evolution is the process of change in the heritable traits of biological populations over successive generations. This concept explains the diversity of life on Earth, showing how all living things are related by descent from common ancestors. The theory of evolution provides a framework for understanding how species adapt to their environments and how new species arise.
The Raw Material for Change
Evolutionary change depends on genetic variation within a population, and the source of all new genetic traits is mutation. A mutation is a random change in an organism’s DNA sequence. These changes can be compared to a typographical error in an instruction manual; most are neutral or harmful, but occasionally, one might introduce a beneficial alteration.
Mutations introduce new alleles, or alternative forms of a gene, into a population’s gene pool. For these traits to be subject to evolution, they must be heritable, meaning they occur in the egg or sperm cells and can be passed to the next generation. Mutations that happen in other body cells during an organism’s lifetime will not be passed on. These changes do not occur in response to an organism’s needs or environmental challenges.
Genetic diversity is also increased by recombination, which shuffles existing genetic material during sexual reproduction. During meiosis, pairs of chromosomes exchange segments of DNA, creating new combinations of alleles that are passed to offspring. This process is why siblings are not genetically identical. Recombination does not create new alleles, but it generates novel combinations of them, increasing the variety of traits within a population.
The Driving Force of Natural Selection
The most well-known mechanism of evolution is natural selection, a process where organisms with traits better suited to their environment survive and reproduce more successfully. This concept relies on the fact that variation exists within any population. Individuals are not identical and possess a range of different traits.
Organisms also produce more offspring than can survive, leading to a “struggle for existence.” This struggle involves competition for limited resources like food and water, as well as pressures from predators, climate, and diseases. In this competitive landscape, not all individuals will live long enough to reproduce.
This competition leads to differential survival and reproduction. Individuals with inherited traits that give them an advantage are more likely to survive and produce offspring. For example, in a forest with dark soil, deer mice with darker fur are better camouflaged from predators. They are more likely to survive and pass on the genes for dark fur to their young.
Over many generations, this process leads to a gradual shift in the population’s characteristics. Advantageous traits become more common, while less advantageous traits become rarer. This is not a process of organisms “trying” to adapt, but an outcome of the environment “selecting” for individuals who possess suitable traits. The accumulation of these traits over time results in adaptation.
Other Evolutionary Pathways
While natural selection drives adaptation, evolution also occurs through mechanisms based on chance. Genetic drift describes random fluctuations in the frequencies of alleles from one generation to the next. These changes are not driven by environmental advantage but by chance, and their effects are most pronounced in small populations.
The Bottleneck Effect
This effect occurs when a population’s size is drastically reduced by a random event, like a natural disaster. The surviving individuals may, by chance, have a different allele frequency than the original population. As this smaller group reproduces, its genetic makeup will reflect the composition of the survivors, not the original population.
The Founder Effect
This happens when a small number of individuals break off from a larger population to establish a new one. The new population’s gene pool is limited to the alleles carried by those founders. If the founders have a different proportion of alleles than the source population, the new population will also be genetically different.
Gene flow, or migration, is another pathway for evolutionary change involving the transfer of genetic material between populations. When migrating individuals interbreed with a new population, they introduce their alleles into that gene pool. This can alter allele frequencies and increase the genetic similarity between the populations.
The Formation of New Species
The processes of natural selection and genetic drift, acting over vast stretches of time, can lead to the formation of new species. This phenomenon, known as speciation, is the process by which one ancestral species splits into two or more descendant species, creating biodiversity.
A common mode is allopatric speciation, which begins when a population is divided by a geographic barrier like a mountain range or river. Once separated, the two groups are reproductively isolated. This means they can no longer interbreed and exchange genes.
With gene flow cut off, the groups evolve independently. The separate populations are subject to different environmental pressures, leading to different paths of natural selection. Random events like mutations and genetic drift also occur differently in each population, contributing to their divergence.
Over many generations, the accumulated genetic differences can become so substantial that they can no longer interbreed, even if the barrier disappears. At this point, they are considered distinct species. These differences might include incompatible reproductive organs, different mating seasons, or new mating behaviors.
Evolution Observed in Real-Time
Evolution is an ongoing phenomenon that can be observed on short timescales. An example is the evolution of antibiotic resistance in bacteria. When a population of bacteria is exposed to an antibiotic, individuals with a random mutation for resistance survive. These survivors then reproduce, passing on the resistance gene and creating a new population of bacteria immune to the drug.
This process can lead to the emergence of “superbugs” resistant to multiple antibiotics. The overuse of antibiotics in medicine and agriculture has accelerated this evolutionary process, creating a public health challenge.
Another example is the peppered moth in England during the Industrial Revolution. Before pollution, the light-colored moth was well-camouflaged against lichen-covered trees, while the dark form was rare. As industrial soot darkened the tree trunks, the dark moths gained a camouflage advantage against birds, and their numbers increased. This shift was a direct response to a changing environment.
Similarly, many insect species have evolved resistance to pesticides. When a field is sprayed, insects with a genetic predisposition to resist the chemical are more likely to survive and reproduce. Over generations, the pesticide becomes less effective as the insect population becomes dominated by resistant individuals. These examples show evolution is a continuous process shaping life around us.