Development of Genetics and Evolution (B6.3)
Darwin and Wallace
Charles Darwin (1809–1882) proposed evolution by natural selection in On the Origin of Species (1859). Alfred Russel Wallace independently developed the same theory and their joint papers were presented in 1858.
Darwin's theory was controversial because:
- It contradicted religious accounts of creation
- Darwin had no mechanism for inheritance (genes were not yet known)
- The fossil record had gaps at the time
Today, the theory is supported by extensive fossil evidence, molecular biology (DNA comparisons), and directly observable natural selection (antibiotic resistance, Peppered Moths).
Fossils
A fossil is the preserved remains or trace of an organism that lived millions of years ago. Fossils form by:
- Hard parts replaced by minerals (mineralisation)
- The organism becoming trapped in amber, ice, peat (slows decay)
- Impression/trace fossils (footprints, burrows)
Why the fossil record is incomplete:
- Most organisms decay completely (soft tissues rarely fossilised)
- Conditions for fossilisation are rare
- Many fossils not yet discovered
- Geological activity destroys fossils
Extinction
A species becomes extinct when all members die without reproducing. Causes:
- Climate change (rapid — species cannot adapt)
- New predators or competitors
- Disease (new pathogens)
- Destruction of habitat
- Catastrophic events (asteroid impact)
Antibiotic-resistant bacteria — evolution in action
MRSA (methicillin-resistant Staphylococcus aureus) and other resistant bacteria demonstrate natural selection occurring today:
- Random mutation gives some bacteria resistance.
- Antibiotics kill non-resistant strains.
- Resistant bacteria reproduce → resistant population.
Reducing resistance: complete courses, don't use for viral infections, new antibiotic development, agricultural restrictions.
Lamarck's theory
Jean-Baptiste Lamarck proposed that organisms develop characteristics during their lifetime and pass acquired characteristics to offspring. This is now known to be incorrect — traits acquired during life do not change the genetic code and cannot be inherited. Natural selection (Darwinian mechanism) is the accepted explanation.
Common exam errors
- Saying fossils are bones — many fossils are mineralised replacements of bone, or impressions.
- Confusing Lamarckian (acquired characteristics inherited) with Darwinian (genetic variation + selection) evolution.
- Forgetting Wallace's contribution to the theory.
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