Genetic variation refers to the differences in DNA sequences among individuals within a population or between different populations. This variability forms the basis for life’s diversity and is the raw material for evolution. It allows populations to adapt to changing environmental conditions. Without it, populations cannot evolve, potentially leading to their decline or extinction.
The Role of Mutation
Mutation is the fundamental source of all new genetic variation, introducing novel alleles into a gene pool. It is a random change in the DNA sequence. These changes can arise from errors during DNA replication or from environmental factors called mutagens.
Physical mutagens like ultraviolet (UV) radiation or X-rays can damage DNA. Chemical mutagens, such as industrial chemicals or tobacco compounds, can alter its base sequence. Mutations range from point mutations (single base change) to larger insertions and deletions. While many are neutral or harmful, some mutations confer a survival or reproductive advantage, providing raw material for natural selection.
Genetic Recombination through Sexual Reproduction
Sexual reproduction generates new genetic combinations by shuffling existing genetic material. This reshuffling occurs primarily during meiosis, the cell division that produces gametes. Crossing over is one mechanism, where homologous chromosomes exchange DNA segments during prophase I. This creates chromosomes with new gene combinations.
Independent assortment is another mechanism, where homologous chromosome pairs randomly align and separate during metaphase I. This random orientation ensures each gamete receives a unique mix of maternal and paternal chromosomes. Random fertilization further amplifies variation, as any sperm can fertilize any egg, each already genetically unique. These processes ensure offspring are genetically distinct from parents and siblings, leading to extensive diversity.
Gene Flow Between Populations
Gene flow, or gene migration, is the transfer of genetic material (alleles) between populations. This occurs when individuals migrate and reproduce in a new population, introducing their genetic variants. It can also happen through gamete dispersal, like pollen carried by wind.
Gene flow can introduce new alleles, increasing diversity, or remove them if individuals leave. Extensive gene flow reduces genetic differences between populations, making them more similar. For instance, interbreeding between ancient humans and Neanderthals introduced new genetic variations into the human population.
Random Changes: Genetic Drift
Genetic drift refers to random fluctuations in allele frequencies from one generation to the next. This is particularly impactful in small populations, where random events cause disproportionate changes. Over time, genetic drift can lead to the loss or fixation of alleles, reducing genetic variation.
Two instances are the bottleneck effect and the founder effect. The bottleneck effect describes a sharp reduction in population size, often due to environmental disasters. The surviving population has reduced and potentially unrepresentative genetic diversity. For example, the northern elephant seal population experienced a bottleneck in the 1890s.
The founder effect occurs when a new population is established by a small number of individuals who separate from a larger group. This smaller founding group carries only a subset of the original population’s genetic diversity, which can lead to a higher frequency of certain rare traits in the new population, such as specific genetic conditions observed in the Amish communities due to their founding by a small group of German settlers.
Selection by the Environment
Natural selection is a process where individuals with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits. It does not create new genetic variation but acts on existing variation. Natural selection shapes allele frequency and distribution, influencing the genetic landscape of a population.
Different types of natural selection impact genetic diversity. Directional selection favors one extreme phenotype, shifting trait distribution and potentially reducing variation. Stabilizing selection favors intermediate phenotypes, decreasing diversity by maintaining the status quo.
Disruptive selection favors individuals at both extremes, potentially increasing diversity and leading to distinct forms. Balancing selection, like heterozygote advantage, maintains multiple alleles, preventing genetic diversity loss, as seen with the sickle cell allele in malaria-prone regions.