Gene pools and allele frequencies Knowledge-Based Study

Study notes

1. What Is a Gene Pool?

   In population genetics, the fundamental unit of evolution is not the individual organism, but the population. A population is defined as a localized group of individuals belonging to the same species that inhabit the same geographical area and have the potential to interbreed, producing fertile offspring.

   The total collection of genes and their alternate forms (alleles) present in a population at any given time is termed the gene pool. The gene pool contains all the genetic variation available to the population, encompassing every gene locus across every individual member.

   • In a diploid organism, each individual possesses two alleles for each autosomal locus.
   • If all individuals in a population are homozygous for the exact same allele at a given locus, that allele is said to be fixed in the gene pool.
   • When two or more alleles exist for a given gene locus in the population, individuals may be either homozygous or heterozygous, creating genetic diversity.

   Measuring Genetic Variation

   To characterize the genetic makeup of a population, geneticists track two primary parameters: genotype frequencies and allele frequencies.

   Genotype frequency is the proportion of a specific genotype among all individuals in the population. In contrast, allele frequency (often called gene frequency) refers to the relative proportion of a specific allele among all the copies of that gene present in the gene pool.

   Mathematical Conventions

   Consider a gene locus with two alternative alleles, typically designated as $A$ and $a$:

   • The letter $p$ usually denotes the frequency of the dominant allele ($A$).
   • The letter $q$ usually denotes the frequency of the recessive allele ($a$).

   Because these two alleles represent all possible variants at this locus in a biallelic system, the sum of their frequencies must always equal 1 (or 100%):

   $$p + q = 1$$

   Step-by-Step Calculation

   Allele frequencies can be calculated directly by counting alleles in a sample where the genotypes of individuals are known. Because diploid organisms carry two alleles per locus, a population of $N$ individuals contains a total of $2N$ alleles for that gene.

   Worked Example

   Imagine a population of 500 diploid flowering plants where flower color is governed by an incomplete dominance locus with two alleles: $C^R$ (red) and $C^W$ (white).

   • 320 red-flowered individuals ($C^R C^R$)
   • 160 pink-flowered individuals ($C^R C^W$)
   • 20 white-flowered individuals ($C^W C^W$)

   Step 1: Calculate the total number of alleles in the gene pool.
   Total alleles = $500 \times 2 = 1000$.

   Step 2: Count the total number of $C^R$ alleles.
   Each $C^R C^R$ individual carries two $C^R$ alleles, and each $C^R C^W$ individual carries one $C^R$ allele:
   $\text{Total } C^R = (320 \times 2) + 160 = 640 + 160 = 800$.

   Step 3: Calculate the frequency of $C^R$ ($p$).
   $p = \frac{800}{1000} = 0.8$.

   Step 4: Calculate the frequency of $C^W$ ($q$).
   $\text{Total } C^W = (20 \times 2) + 160 = 40 + 160 = 200$.
   $q = \frac{200}{1000} = 0.2$.

   Notice that $p + q = 0.8 + 0.2 = 1.0$, confirming the calculations are consistent.

   Evolution at the Population Level

   In modern evolutionary biology, microevolution is defined precisely as a generation-to-generation change in a population's allele or genotype frequencies. If allele frequencies remain constant across generations, the population is in a non-evolving state known as genetic equilibrium.

   The Hardy-Weinberg Baseline

   The Hardy-Weinberg principle states that allele frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. It serves as an essential null model. When observed allele frequencies deviate from expected baseline values over time, scientists conclude that evolutionary forces are acting on the gene pool.

   Mechanisms That Alter Allele Frequencies

   Five main processes can alter allele frequencies in a population's gene pool:

   1. Mutation: Generates novel alleles, introducing new variations directly into the gene pool.
   2. Gene Flow: The movement of fertile individuals or gametes between populations, transferring alleles into or out of the gene pool.
   3. Genetic Drift: Unpredictable, random fluctuations in allele frequencies from one generation to the next, which have particularly strong effects in small populations (e.g., population bottlenecks and founder effects).
   4. Natural Selection: Differential reproductive success of individuals carrying distinct alleles, consistently increasing the frequencies of adaptive traits.
   5. Non-Random Mating: Assortative mating or inbreeding that alters genotype frequencies, though it does not necessarily change allele frequencies directly.