How are evolutionary patterns and processes affected by the dual operation of mutation and natural selection? Give examples.

Understanding Mutation and Natural Selection

Mutation is defined as a change in the nucleotide sequence of an organism’s genome. These changes can be spontaneous or induced by external factors like radiation or chemicals. Mutations are the ultimate source of all genetic variation. They can be beneficial, neutral, or deleterious, depending on their effect on the organism’s phenotype. Mutation rates vary across species and even within genomes.

Natural Selection, as proposed by Charles Darwin and Alfred Russel Wallace, is the process whereby organisms better adapted to their environment tend to survive and reproduce more successfully than those less adapted. This differential survival and reproduction leads to changes in the genetic makeup of a population over time. Natural selection acts on the phenotype, but it is the underlying genotype that is inherited.

The Interplay of Mutation and Natural Selection

Mutation and natural selection are not independent processes; they operate in tandem. Mutation generates genetic variation, and natural selection filters this variation, favoring traits that enhance survival and reproduction in a specific environment. This interaction drives adaptive evolution. Without mutation, there would be no variation for natural selection to act upon, and evolution would grind to a halt. Conversely, without natural selection, mutations would accumulate randomly, potentially leading to maladaptation and reduced fitness.

Evolutionary Patterns and Processes

1. Directional Selection

Directional selection occurs when natural selection favors one extreme phenotype, causing the allele frequency to shift over time in that direction. This often happens when the environment changes in a consistent way.

Example: The evolution of antibiotic resistance in bacteria. Initially, most bacteria are susceptible to antibiotics. However, a mutation conferring resistance arises. In the presence of antibiotics, susceptible bacteria are killed, while resistant bacteria survive and reproduce, leading to a population dominated by resistant strains. (Data from WHO, 2023 indicates a global rise in antibiotic resistance).

2. Stabilizing Selection

Stabilizing selection favors intermediate phenotypes and selects against extreme variations. This maintains the status quo and reduces genetic diversity.

Example: Human birth weight. Babies with very low or very high birth weights have lower survival rates than babies with intermediate birth weights. Natural selection therefore favors intermediate birth weights, maintaining a relatively stable distribution.

3. Disruptive Selection

Disruptive selection favors both extreme phenotypes and selects against intermediate phenotypes. This can lead to the divergence of a population into two distinct groups.

Example: Darwin’s finches on the Galapagos Islands. Different beak shapes evolved to exploit different food sources. Finches with either very large or very small beaks were favored, while those with intermediate beak sizes were less successful. This ultimately contributed to the diversification of finch species.

4. Balancing Selection

Balancing selection maintains multiple alleles at a locus, preventing any single allele from becoming fixed. This can occur through heterozygote advantage or frequency-dependent selection.

Example: Sickle-cell anemia in regions with malaria. Individuals heterozygous for the sickle-cell gene (carrying one normal and one sickle-cell allele) are resistant to malaria, while individuals homozygous for the sickle-cell allele suffer from sickle-cell anemia. This heterozygote advantage maintains both alleles in the population.

Mutation and Speciation

The accumulation of mutations, coupled with the selective pressures of different environments, can lead to reproductive isolation and ultimately, speciation. Allopatric speciation, where populations are geographically separated, is a common example. Different mutations arise in each isolated population, and natural selection favors different traits in each environment. Over time, these populations become so genetically divergent that they can no longer interbreed, resulting in the formation of new species.