The development of understanding of genetics and evolution

Charles Darwin proposed the theory of evolution by natural selection after extensive observations on his voyage.

• •Individual organisms within a species show a wide range of phenotypic variation caused by differences in their genes.
• •Organisms with characteristics most suited to the environment are more likely to survive and reproduce.
• •The beneficial alleles responsible for the advantageous phenotype are passed on to the next generation.
• •Over many generations, the frequency of the advantageous allele increases within the population — this gradual change is evolution by natural selection.
• •Jean-Baptiste Lamarck incorrectly suggested that changes acquired during an organism's lifetime could be inherited.

Darwin's theory was only gradually accepted by other scientists, for several reasons.

• •It contradicted the widely-held religious belief that God created every species on Earth.
• •There was insufficient evidence at the time to convince many scientists, as few studies had examined how organisms change over time.
• •The mechanism of inheritance and variation (genes and DNA) was not understood until decades after the theory was published.

Alfred Russel Wallace independently worked on evolution and proposed the mechanism of speciation.

• •Geographical isolation occurs when a physical barrier, such as a river or mountain range, separates populations of a species.
• •Different environmental conditions exist in each isolated location, leading to different natural selection pressures.
• •Over time, genetic variation and natural selection lead to differences in the genotypes and phenotypes of each population.
• •A new species forms when the two populations can no longer interbreed to produce fertile offspring.

Gregor Mendel carried out breeding experiments on pea plants to discover the basic principles of inheritance.

• •Gregor Mendel observed that characteristics were inherited in predictable units, now known as genes — these units could be dominant or recessive, and were never blended together.
• •Mendel's work was not accepted at first because scientists did not know about chromosomes or DNA.
• •In the late 19th century, the behaviour of chromosomes during cell division was observed under microscopes.
• •In the mid-20th century, the double helix structure of DNA was determined by Watson, Crick, Franklin, and Wilkins.

The theory of evolution is now widely accepted due to extensive fossil records and genetic data.

• •Fossil records show how organisms have changed slowly over long periods of geological time.
• •The rapid evolution of resistant bacteria provides direct modern observable evidence of natural selection.
• •Comparing DNA sequence data reveals evolutionary relationships and common ancestors between different species.
• •Chemical evidence, such as protein structure similarities, further supports shared evolutionary history.

Fossils provide evidence about early forms of life and how modern organisms evolved.

• •Fossils can form from hard parts of organisms, such as bones or teeth, that do not decay easily.
• •Fossils form when soft tissues decay and hard parts are replaced by minerals during mineralisation.
• •Preservation can occur in conditions where decay is prevented, such as in amber, peat bogs, or ice.
• •Traces of organisms, such as footprints, burrows, and preserved rootlet tracks, can also fossilise.

Extinction can be caused by changes to the environment or biological factors.

• •Extinction can be caused by new predators hunting a species to extinction.
• •Outbreaks of new diseases can quickly wipe out populations that lack immunity.
• •Increased competition for food or resources from a better-adapted species can cause extinction.
• •Catastrophic single events, such as asteroid impacts or major volcanic eruptions, cause mass extinctions.

Bacteria evolve rapidly because they reproduce at a very fast rate.

• •Random genetic mutations can produce new strains of bacteria that are resistant to antibiotics.
• •When an antibiotic is used, the non-resistant bacteria are killed, but the resistant strain survives and reproduces.
• •MRSA is a well-known example of a bacterial strain resistant to multiple antibiotics.
• •To prevent resistance, doctors should not prescribe antibiotics for viral infections and patients must complete the full course.