Module 101. Genetics and Evolution

Genetics and evolution are two interrelated scientific disciplines that explain the mechanisms of heredity and the diversity of life on Earth. Genetics focuses on how traits are transmitted from parents to offspring through genes, while evolution describes how populations of organisms change over time due to genetic variation and natural selection. Together, they form the foundation of modern biology, providing insights into the origin, adaptation, and interconnection of all living organisms.

Historical Development of Genetics

The study of genetics began in the nineteenth century with the pioneering work of Gregor Mendel, an Austrian monk who conducted experiments on pea plants. His observations on inheritance patterns led to the formulation of Mendel’s Laws of Inheritance — the Law of Segregation and the Law of Independent Assortment. Mendel’s work, published in 1866, went largely unnoticed until its rediscovery in 1900 by Hugo de Vries, Carl Correns, and Erich von Tschermak.
Subsequent discoveries, including Thomas Hunt Morgan’s research on fruit flies (Drosophila melanogaster), confirmed that genes are located on chromosomes, leading to the chromosomal theory of inheritance. In 1953, James Watson and Francis Crick, with the aid of Rosalind Franklin’s X-ray crystallography, described the double-helix structure of DNA, marking a monumental advancement in molecular biology.
The latter half of the twentieth century witnessed rapid progress in genetic understanding, including the discovery of the genetic code, the development of recombinant DNA technology, and the sequencing of the Human Genome Project completed in 2003.

Structure and Function of Genetic Material

Genetic information is stored in molecules of deoxyribonucleic acid (DNA) in most organisms, while some viruses contain ribonucleic acid (RNA). DNA consists of two complementary strands of nucleotides, each containing a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases — adenine (A), thymine (T), guanine (G), and cytosine (C). The sequence of these bases encodes genetic instructions.
Genes, specific segments of DNA, direct the synthesis of proteins through two primary processes:

• Transcription: The DNA sequence of a gene is transcribed into messenger RNA (mRNA).
• Translation: The mRNA is decoded by ribosomes to assemble amino acids into a specific protein.

This relationship between genes and proteins is described by the central dogma of molecular biology — DNA → RNA → Protein. The complete set of genetic material within an organism is known as its genome.

Patterns of Inheritance

Inheritance refers to the transmission of genetic traits from one generation to the next. Mendelian inheritance explains simple traits controlled by single genes with dominant and recessive alleles. Examples include the inheritance of seed shape in peas or blood group in humans.
However, most traits are influenced by multiple genes (polygenic inheritance) and environmental factors. Non-Mendelian inheritance patterns include:

• Incomplete Dominance: The heterozygote shows an intermediate phenotype (e.g., pink flowers in snapdragons).
• Codominance: Both alleles are equally expressed (e.g., AB blood group in humans).
• Multiple Alleles: More than two forms of a gene exist in a population (e.g., ABO blood group system).
• Sex-Linked Inheritance: Traits associated with genes on sex chromosomes (e.g., colour blindness, haemophilia).

Mutations — changes in DNA sequences — introduce genetic variation, which is fundamental to evolution.

Molecular Basis of Evolution

Evolution operates through genetic variation and the mechanisms that influence it. Charles Darwin’s theory of natural selection, presented in On the Origin of Species (1859), proposed that organisms with favourable traits survive and reproduce more successfully, leading to gradual changes in populations.
Modern genetics later provided the molecular explanation for Darwin’s ideas, resulting in the Modern Evolutionary Synthesis during the mid-twentieth century. This synthesis integrated Mendelian genetics with Darwinian natural selection, recognising that mutation, genetic recombination, and gene flow generate the variability upon which selection acts.
The sources of genetic variation include:

• Mutation: Spontaneous or induced changes in DNA.
• Genetic Recombination: Exchange of genetic material during meiosis.
• Gene Flow: Movement of genes between populations through migration.
• Genetic Drift: Random changes in allele frequencies, particularly in small populations.

These mechanisms collectively shape genetic diversity and evolutionary adaptation.

Speciation and Evolutionary Processes

Speciation is the evolutionary process through which new species arise. It occurs when populations of the same species become reproductively isolated due to geographic, behavioural, or ecological barriers. The main types of speciation include:

• Allopatric Speciation: Occurs when populations are geographically separated (e.g., Darwin’s finches in the Galápagos Islands).
• Sympatric Speciation: Occurs without physical separation, often due to genetic or behavioural differences.
• Parapatric Speciation: Occurs in neighbouring populations experiencing distinct selective pressures.

Evolutionary change can be gradual (phyletic gradualism) or occur in bursts (punctuated equilibrium). Adaptive radiation, as seen in the diversification of mammals after the extinction of dinosaurs, exemplifies how species evolve to occupy varied ecological niches.

Evidence Supporting Evolution

Multiple lines of scientific evidence support evolutionary theory:

• Fossil Record: Reveals transitional forms and chronological sequences of life.
• Comparative Anatomy: Shows homologous structures (similar origin, different function) such as the forelimbs of vertebrates.
• Embryology: Early developmental stages of vertebrates display common features, indicating shared ancestry.
• Molecular Biology: DNA and protein sequence similarities reveal evolutionary relationships.
• Biogeography: Geographic distribution of species aligns with evolutionary history.

These evidences collectively affirm that all living organisms share a common origin and have diversified over billions of years through descent with modification.

Human Genetics and Evolution

Human genetics explores the inheritance of traits, genetic disorders, and molecular evolution of humans. Research in population genetics has traced human origins to Africa, with mitochondrial DNA studies suggesting a common maternal ancestor, often referred to as “Mitochondrial Eve.”
Human evolution illustrates the gradual transformation of hominins over approximately six million years. Fossil evidence, such as Australopithecus afarensis (“Lucy”) and Homo erectus, along with genetic data, traces the lineage leading to modern humans (Homo sapiens). The development of bipedalism, increased brain size, and tool use are key milestones.
Advances in genomics have deepened understanding of genetic diseases, human diversity, and adaptation. For example, variations in haemoglobin genes provide resistance to malaria, demonstrating the interaction between genetics and environment.

Applications and Modern Developments

Genetics and evolutionary principles underpin numerous scientific and technological applications:

• Genetic Engineering: Enables modification of DNA for improved crops, pharmaceuticals, and gene therapy.
• Genomics and Biotechnology: Facilitate sequencing and manipulation of genetic material.
• Forensic Science: Utilises DNA fingerprinting for identification and criminal investigation.
• Evolutionary Medicine: Studies how evolutionary pressures influence disease susceptibility.
• Conservation Biology: Applies population genetics to preserve biodiversity and manage endangered species.

Modern research also explores epigenetics, which studies heritable changes in gene expression not caused by alterations in DNA sequence, and CRISPR-Cas9 technology, which allows precise genome editing.

Significance of Genetics and Evolution

Genetics and evolution together form the cornerstone of biological sciences, explaining how life is inherited, diversified, and sustained. They provide the conceptual framework for understanding biodiversity, health, and ecological adaptation.