Population genetics

The genetic makeup of one individual slightly varies from that of the other individual. It becomes essential to determine the occurrence of evolutionary changes in the individuals or a group of individuals (population). Population genetics involves the principles of the transmission genetics applicable to a population. It involves the study of the genes and their way of passing on from one generation to the other. It involves both empirical and theoretical studies. Hence, we measure and quantify the genetic variation and use the mathematical base for understanding the patterns of variation.

Image 1: Population genetics (introductory image depicting variation in butterflies)

The genetic structure of the population:

It mainly depends on the genotypic frequency and the allelic frequency. The gene pool or the total of the alleles helps in determining the genetic structure of a population. The distribution of the alleles into the genotype helps in determining the genetic structure of the sexually interbreeding individuals. Most of the individuals die with the same set of alleles that they had during their birth. Rare mutations occur in exceptional cases. The gene counting method helps in determining the allelic frequency. The number of copies of an allele gets divided by the sum of counts of all the alleles in a given population. It covers a wide variety of genes such as the X-linked genes, multiple alleles, and the mitochondrial genes.

The Hardy-Weinberg Law:

Both the gene and the genotypic frequencies remain constant from the generation to generation in an infinitely large population. Preferably, the population involves interbreeding. It involves random mating and avoids selection, migration or mutation. The above statements define Hardy-Weinberg law. It gets applied in the case of random fusion between the gametes. Under the above conditions, the allelic frequencies remain as they were. They do not change from the generation to generation. The genotypic frequencies stabilize after a generation in the proportions such as p², 2pq, and q² (p and q are the allelic frequencies). The law also gets applied with the loci having more than two alleles. It also covers the SNPs or the single nucleotide polymorphisms, protein variants, or any other factors showing the ability of segregation as per the Mendelian gene segregation pattern.

Image 2: Hardy-Weinberg Principle

Genetic variation in space and time:

The genetic structure varies as per space and the time. The samples collected from the same species belonging to different areas might show the genetic variation. Alternatively, the variations occur in the samples collected from the same area at a different time. The changes in the allelic frequencies across a geographical transect as per space, timings of sample collection, and the type of species are known as clines. They involve changes such as temperature, water, and nutrient availability, and weather. Also, the other environmental and inner cellular changes cause variation.

Genetic variation in natural populations:

The study of genetic variation in a natural population helps in understanding the evolutionary changes and their impact on the genome. We measure the genetic variation either at the protein level or the DNA level. Protein electrophoresis technique helps in revealing the genetic variation at the protein level. It helps in determining the polymorphic locus. The neutral mutation model explains the extensive genetic variation in the proteins. RFLP analysis involving restriction fragment length polymorphisms help in determining the genetic variation at the DNA level.

Forces changing the gene frequencies within a population:

· Mutations: They involve heritable changes in the DNA. The Hardy-Weinberg equilibrium excludes mutation while discussing the genetic variation. However, now we realize that mutation is the best way to cause genetic variation in the population. Novel mutations do arise in a population. The potential for the novel mutation increases with the increase in the population size. However, we must understand that mutation changes a little of the allelic frequencies.

·   Random genetic drift: Assumption of Hardy-Weinberg law states an infinitely large population. In reality, the population never shows infinite size. However, large populations exist. A random change occurs in the allelic frequency by chance. It is known as random genetic drift. It occurs due to the sampling error (related to the size of the population). The genetic drift also leads to the loss of genetic variation. The cases of genetic diversity sometimes involve genetic drift. Apart from the above points, the genetic drift fluctuates the allelic frequencies over a period. The bottleneck effect involves the sampling error. It occurs due to the drastic reduction of the population size.

·    Migration: The Hardy-Weinberg law also excludes the factor of migration and assumes a completely isolated population. However, it is not possible. Many populations migrate and exchange genes with the other populations. Movement of genes occurs due to the contribution of genes of a migrated individual to the other population. It is known as gene flow. One individual from a place mates with another individual after migration and contributes the genes. Hence, migration contributes to the genetic variation. Migration also tends to reduce genetic divergence and increase the effective size of the population.

·      Natural selection: It directly relates to the adaptation. It helps the organism to survive in certain environments and make them suitable. Natural selection leads to the evolution of the traits thereby increasing the adaptability of the organism in a particular environmental condition. It also defines a differential reproduction of the genotypes and measured by Darwinian fitness. It depends on the reproductive contribution of an individual or an organism’s genotype. The disfavoured allele gets eliminated from the population. Balancing selection maintains the genetic variation (for example heterozygote superiority). Otherwise, the directional selection decreases the genetic variation in a population.

·   Assortative mating: Many populations do not mate randomly. It is known as non-random mating. An example includes positive assortative mating. The individuals with similar phenotypes mate with each other. An example includes a mating between a tall man and a tall woman. A mating also involves phenotypically dissimilar individuals (example includes mating between a tall man and a short woman). It is known as negative assortative mating. It affects both the allelic and the genotypic frequencies.

·   Inbreeding: It involves mating between the close relatives. If continued for many generations, it leads to homozygosity within a population. It also reduces fitness.

Rare species and endangered species possess a great risk of losing the genetic variability. Certain attempts help in managing the genetic diversity of such species. They include avoiding inbreeding and keeping the adequate population size.  

References:
[1] Population Genetics, A.N. Shukla
[2] Population Genetics, Matthew Hamilton

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