Selection Coefficient and Relative Fitness in Population Genetics

There are four main modes of natural selection that affect phenotypic variation in populations. Stabilizing selection favors intermediate phenotypes and maintains the average phenotype, reducing genetic variation (e.g., human babies of average size have higher survival rates). Directional selection favors one extreme phenotype, shifting the population mean in one direction and reducing genetic variation. Disruptive selection favors extreme phenotypes at both ends of the distribution, increasing genetic variation by eliminating intermediates. Balancing selection maintains multiple phenotypes simultaneously without favoring any single type, preserving genetic variation across generations.

There are three main modes of natural selection: (1) Directional selection shifts populations toward one extreme, such as when only very tall individuals survive; (2) Stabilizing selection favors intermediate traits and selects against extremes, such as average birth weight in humans; (3) Disruptive selection favors both extremes while selecting against intermediate forms, such as light and dark butterfly coloration. These modes explain how populations adapt to different environmental pressures and can lead to speciation.

Four main types of natural selection patterns shape evolutionary trajectories: (1) Directional selection shifts the average trait value toward a new, more adaptive value, reducing genetic diversity—demonstrated by Galapagos finches whose beak sizes increased during droughts when larger-beaked individuals could crack tougher seeds; (2) Stabilizing selection maintains the current average as most adaptive, reducing trait variability—illustrated by human birth weight maintained around 7 pounds for millennia because babies born significantly smaller or larger face survival disadvantages; (3) Disruptive selection favors extreme trait values over intermediate ones, increasing genetic diversity and potentially leading to speciation—shown by Whitefish populations where fish with many closely-spaced gill rakers efficiently filter tiny plankton while those with few widely-spaced rakers efficiently filter larger prey, with medium-numbered rakers performing poorly at both tasks; (4) Balancing selection maintains multiple trait values simultaneously as equally adaptive, preserving or increasing genetic diversity—exemplified by heterozygote advantage in immune system genes where heterozygotes have stronger immune responses than either homozygous type.

Natural selection is Darwin's mechanism explaining evolution through differential survival and reproduction based on heritable traits. There are three main types: stabilizing selection eliminates extreme traits, favoring the average phenotype and reducing genetic variation (e.g., average-weight babies have higher survival); directional selection favors one extreme trait, shifting the population mean and potentially fixing favorable alleles (e.g., larger birds surviving snowstorms due to greater fat reserves); and disruptive selection favors both extremes over the average, potentially leading to speciation (e.g., birds with very short or very long beaks outcompeting those with intermediate beaks when food sources are either very large or very small).

This video explains four key evolutionary concepts: (1) Balanced polymorphism maintains multiple alleles in a population when heterozygotes have higher fitness than either homozygote, as seen in sickle cell anemia where carriers resist malaria; (2) Disruptive selection favors extreme phenotypes over intermediates, potentially leading to speciation when populations specialize on different resources; (3) Heterozygous advantage occurs when hybrid offspring from purebred lines show superior fitness compared to either parent; (4) Directional selection shifts population averages toward one extreme when individuals with that trait have higher survival and reproduction rates, as demonstrated by gazelles evolving faster running speeds to escape predators.

Mutation-selection balance describes the equilibrium state where the rate of new deleterious mutations entering a population equals the rate at which natural selection removes them; for a single locus with partial dominance, the equilibrium frequency of deleterious alleles is q̂ = μ/(hs), and the mutation load (reduction in mean population fitness) is approximately 2μ per locus, meaning the total genome-wide load is 1 - e^(-U) where U is the genome-wide mutation rate, demonstrating that even though each individual carries many deleterious mutations, the overall impact on population fitness depends on the balance between mutation rates, selection pressures, and dominance relationships.

Mutation-selection balance explains why slightly deleterious traits persist in populations. Even weakly harmful mutations cannot be completely purged by natural selection due to random genetic drift, especially in finite populations. The speaker explains that this creates a baseline level of genetic load that must be overcome for advantageous traits to spread.

In a population at mutation-selection balance with segregating non-recessive deleterious alleles, the genetic load (reduction in mean fitness) equals twice the mutation rate (L = 2μ), independent of the mutation's severity (selection coefficient S); this counterintuitive result, known as the Haldane-Muller principle, occurs because stronger mutations are purged more rapidly by selection while weaker mutations persist longer, causing their effects to cancel out and leave only the mutation rate as the determining factor.

All biological objects have undergone billions of years of evolutionary change. Heritability measures the proportion of phenotypic variation determined by genetic factors, with human height having approximately 80% heritability. Genetic variation exists despite evolutionary forces that reduce it. Evolution can be conceptualized as movement through genotype space, with each genotype representing a point. The genotype space is enormous (3.5 billion base pairs in humans). Mutation is the ultimate source of genetic variation, with rates approximately 10^-8 per generation for humans. A simple model of mutation-selection balance considers two alleles with mutation rates between them. The equilibrium frequency of a deleterious allele depends on the mutation rate and the selection coefficient. The mutation-selection equilibrium is stable because selection acts as a restoring force. The prevalence of genetic diseases in populations is determined by mutation-selection balance. Genetic load refers to the reduction in population fitness due to deleterious mutations. Mutation-selection balance is a fundamental evolutionary process that maintains genetic variation in populations.

This segment introduces the concept of genetic load, which refers to the reduction in a population's average fitness due to the presence of deleterious alleles. The instructor explains that genetic load can accumulate through mutation-selection balance, where new harmful mutations arise faster than selection can eliminate them. The segment discusses how genetic load affects population viability and why conservation biologists monitor genetic load in endangered species. The instructor uses the example of sickle cell anemia to illustrate how genetic load can be balanced against heterozygote advantage in certain environments.

Population genetics has diverse applications across fields: (1) Integrated pest management - studying pest population genetics helps develop control strategies, as demonstrated by research on the alfalfa weevil in Argentina; (2) Conservation genetics - analyzing genetic diversity in species like llamas helps ensure sustainable resource management; (3) Pathogen evolution - studying pathogen population genetics (e.g., Sclerotinia in soybeans, E. coli in humans) helps predict and prevent disease outbreaks by understanding evolutionary potential.

Pest populations contain three genotype categories: RR (homozygous resistant), SS (homozygous susceptible), and RS (heterozygous). Initially, resistant alleles exist at very low frequencies. Each pesticide application kills susceptible individuals while allowing resistant ones to survive and reproduce. Over successive generations (typically 8-13), the proportion of resistant individuals increases dramatically while susceptible individuals decline. This demonstrates how repeated pesticide use accelerates resistance development through natural selection.

Pesticide resistance evolution follows theoretical predictions based on selection strength. Low-dose pesticide exposure (where some individuals survive) leads to polygenic resistance architecture involving many mutations of small effect size across multiple loci. High-dose pesticide exposure (where most individuals die) typically requires monogenic resistance, usually a loss-of-function mutation in a single gene rendering pesticide receptors unable to bind toxins. Empirical evidence for polygenic resistance in wild arthropod populations has been limited due to historical sequencing technology limitations, giving the impression that most field-evolved resistance has monogenic architecture.

Drosophila melanogaster evolved resistance to organophosphate pesticides through amino acid changes in the acetylcholinesterase enzyme. Pesticides were introduced in the 1950s, and resistance arose within approximately 50 years (500 generations). Researchers found that resistance mutations arose at three different sites in the gene, and populations contained flies with all possible combinations of these mutations. This pattern was inconsistent with the hard sweep model, which would predict only one mutation arising and fixing before others could appear. An alternative explanation was that resistant alleles were already present in the population as standing variation from natural selection by other mimics of acetylcholine.

Drug resistance is a widespread phenomenon across different types of pathogens. HIV can develop resistance to antiretroviral drugs, with resistance increasing when patients do not take prescribed medications as recommended. Similarly, DDT, an insecticide used to kill mosquitoes that spread malaria, initially eliminated most mosquito populations but over time, some mosquitoes developed resistance. These resistant mosquitoes survived, bred, and increased their population, eventually requiring countries to ban DDT due to its harmful effects on the ecosystem. Mutations that provide resistance to drugs or insecticides can become established in populations over time, resulting in new populations of organisms that no longer die from specific applications or treatments.