Darwin's theory of natural selection

Charles Darwin proposed that species evolve by natural selection. The steps are:

• Individuals in a species show variation caused by differences in their genes.
• There is competition to survive (for food, mates, space) — more offspring are produced than can survive.
• Individuals with the most useful (advantageous) characteristics are more likely to survive and reproduce — “survival of the fittest”.
• They pass on the alleles for these useful characteristics to their offspring.
• Over many generations the useful alleles become more common, and the species gradually changes (evolves).

Key idea: the environment “selects” which variations survive. New variation ultimately comes from random mutations in the DNA.

Survival of the fittest

Evidence for evolution

• Fossils — the fossil record shows how organisms have gradually changed over millions of years. Gaps exist because many organisms were soft-bodied or fossils have not been found or were destroyed.
• Antibiotic-resistant bacteria — a clear, modern example of evolution. A random mutation makes some bacteria resistant; when antibiotics are used, the non-resistant bacteria die but resistant ones survive and reproduce, so resistance spreads (e.g. MRSA).

Slowing resistance: doctors should not over-prescribe antibiotics, patients should finish the full course, and antibiotics should not be used for viral infections. This is why antibiotic-resistant bacteria are strong evidence for natural selection in action.

Your turn — how fast bacteria multiply

Darwin & Wallace

Alfred Russel Wallace independently came up with the idea of evolution by natural selection at the same time as Darwin. In 1858 they published their ideas jointly, and Darwin published On the Origin of Species in 1859.

• Wallace's fieldwork provided extra evidence, for example on warning colouration in animals.
• Darwin's ideas were controversial at first because they challenged religious beliefs, there was little other evidence, and the mechanism of inheritance (genes) was not yet understood.

Why it was accepted slowly: a scientific theory is only accepted once there is enough evidence. Later discoveries in genetics explained how characteristics are inherited, strengthening the theory.

Classification & the three domains

Living things are classified into groups. The Linnaean system uses a hierarchy of groups from largest to smallest:

Each species gets a two-part binomial name (genus + species), e.g. Homo sapiens.

As microscopes and DNA analysis improved, Carl Woese proposed the three-domain system, splitting life into:

• Archaea — primitive bacteria, often from extreme environments.
• Bacteria — true bacteria.
• Eukaryota — organisms with a nucleus (animals, plants, fungi, protists).

Why it changed: evidence from RNA/DNA sequencing showed that archaea and bacteria are very different, so the older systems were revised — a good example of how models change with new evidence.

Your turn — how many domains?

Selective breeding or genetic engineering?

Tap a statement, then tap the technique it describes.

Selective breeding

Selective breeding (artificial selection) is when humans choose which organisms breed, to produce offspring with desired characteristics. Over many generations the desired feature is enhanced. Steps:

• Choose parents that show the desired characteristic (e.g. high milk yield, disease resistance, more meat, attractive flowers).
• Breed them together.
• From the offspring, choose those with the best feature and breed again.
• Repeat over many generations.

Risk: selective breeding reduces the gene pool (less variation / more inbreeding). This can lead to inherited health problems and leaves the population vulnerable to new diseases, because there are few different alleles.

Genetic engineering (GM)

Genetic engineering means moving a gene from one organism into another so the second makes a useful protein or gains a new characteristic. Outline of the process:

• Cut out the desired gene using an enzyme.
• Insert the gene into a vector (often a plasmid or a virus).
• Use the vector to transfer the gene into the cells of the target organism, which then makes the protein.

Examples: bacteria engineered to make human insulin; Bt crops that make a protein toxic to insect pests; and golden rice, engineered to make beta-carotene (vitamin A) to reduce deficiency.

Benefits vs risks: GM can raise yields, reduce pesticide use and produce medicines. Concerns include effects on wild populations/biodiversity, the spread of inserted genes, and ethical worries — so GM is carefully regulated.

Making human insulin

Tissue culture

Tissue culture grows new organisms or tissues from small groups of cells on a nutrient medium under sterile conditions.

• Plant tissue culture uses meristem cells to produce many identical (cloned) plants quickly — useful for rare or disease-resistant plants.
• Animal tissue culture grows cells for medical research and to test new drugs, reducing (but not removing) the use of whole animals.

Why sterile? to prevent contamination by microbes, which would compete with or kill the growing cells.

Your turn — Linnaean levels

Match the example to the technique

Tap an example on the left, then its matching technique on the right.

The big ideas to know

You've covered the core of Edexcel Topic 4 — Natural selection and genetic modification. Press Finish to see your score.