Inheritance, Evolution and Natural Selection

Darwin's Theory of Natural Selection

Charles Darwin proposed evolution by natural selection in On the Origin of Species (1859). The mechanism:

1. Variation: individuals in a population show variation in their characteristics (due to mutations and sexual reproduction).
2. Overproduction: more offspring are produced than can survive (competition for resources).
3. Survival of the fittest: individuals with characteristics best suited to the environment survive and reproduce (natural selection); those less well adapted die.
4. Inheritance: surviving individuals pass their advantageous alleles to offspring.
5. Gradual change: over many generations, the frequency of advantageous alleles increases → population changes → evolution.

Evolution is the change in allele frequency in a population over time, leading to new species.

Alfred Russel Wallace independently proposed natural selection at the same time — Darwin and Wallace presented jointly to the Linnean Society in 1858.

Evidence for Evolution

• Fossil record: shows progression of life forms over geological time; older fossils in deeper strata.
• Comparative anatomy: homologous structures (e.g. pentadactyl limb in mammals — same bones, different functions) suggest common ancestry.
• Antibiotic resistance: bacteria evolving resistance in real time — directly observed natural selection.
• DNA/molecular evidence: similar DNA sequences in closely related species.
• Selective breeding (artificial selection): humans select individuals with desirable traits → rapid change (e.g. dogs from wolves; high-yield crops).

Speciation

Speciation is the formation of a new species.

Allopatric speciation (geographic isolation):

1. Population splits (e.g. by mountain range, ocean, river).
2. Isolated populations face different selection pressures.
3. Different mutations accumulate; allele frequencies diverge.
4. Eventually, the two populations can no longer interbreed → two separate species.

A species = a group of organisms that can interbreed to produce fertile offspring.

Biotechnology and Genetic Engineering

Selective breeding: mating individuals with desirable traits over many generations. Example: dairy cows selected for high milk yield.

Genetic engineering: inserting a specific gene from one organism into another (transgenic organism).

Steps (simplified):

1. Identify and extract the desired gene (e.g. human insulin gene).
2. Cut with restriction enzymes (creates sticky ends).
3. Insert into a vector (usually a bacterial plasmid) using ligase (joins sticky ends).
4. Transform bacteria with the recombinant plasmid.
5. Bacteria multiply → produce the desired protein at scale.

Example: human insulin produced by GM bacteria (E. coli) — used to treat Type 1 diabetes.

GM crops: crops modified for pest resistance (Bt crops), herbicide tolerance, drought resistance, improved nutrition (Golden Rice — vitamin A).

Ethical debates: safety, biodiversity, corporate control of food supply, "playing God," potential ecological impacts.

Cloning

• Natural cloning: identical twins, asexual reproduction in bacteria and plants.
• Artificial plant cloning: cuttings, tissue culture (micropropagation) — produces many identical plants rapidly.
• Animal cloning: somatic cell nuclear transfer (SCNT) — nucleus of body cell inserted into enucleated egg cell → surrogate mother. Example: Dolly the sheep (1996).