Theories of evolution and evidence for them
Darwin's theory of evolution by natural selection is one of the foundations of modern biology. To appreciate why it took hold (and why an earlier theory by Lamarck didn't), you need to know the historical context and the evidence that supports it today.
Lamarck (early 1800s) — the inheritance of acquired characteristics
Jean-Baptiste Lamarck proposed that:
- Organisms change during their lifetime in response to use of body parts.
- These acquired characteristics are passed on to offspring.
Famous example: a giraffe stretches its neck to reach high leaves; its neck gets a bit longer; that longer neck is inherited by its offspring.
This theory is now rejected because:
- Acquired characteristics (e.g. injuries, muscles built up by exercise) are not inherited.
- Genetic information passed to offspring comes from gametes, which are not changed by use of body parts.
Lamarck's theory was wrong, but he was the first to suggest that species change — an important step away from "fixed species".
Darwin (1859) — natural selection
Charles Darwin spent five years on HMS Beagle making observations, especially in the Galápagos Islands where he studied finches and tortoises. He concluded that:
- Variation exists in every species.
- Many more offspring are produced than survive.
- Those best suited to their environment survive and reproduce.
- They pass on their characteristics, leading to gradual change over many generations and eventually new species.
He published On the Origin of Species in 1859.
Wallace — independent discovery
Alfred Russel Wallace developed the same theory independently, working on islands in South-East Asia. He sent his ideas to Darwin, who then rushed to publish. The 1858 Linnean Society paper was credited to both Darwin and Wallace.
Why Darwin's theory was slow to be accepted
- It contradicted the Christian belief that God created life as it is.
- The mechanism of inheritance (genes, DNA) was not yet known.
- Insufficient evidence at the time — the fossil record was patchy.
By the early 20th century the rediscovery of Mendel's work and later the discovery of DNA (Watson & Crick, 1953) provided the genetic mechanism Darwin lacked. Today the modern synthesis of evolution combines Darwin's natural selection with genetics.
Evidence for evolution today
- Fossils. Layers of sedimentary rock preserve organisms from different ages. Fossils show clear gradual change (e.g. horse evolution, transitional fossils like Tiktaalik and Archaeopteryx).
- Antibiotic resistance. Bacteria evolving resistance in real time (B6.6).
- Comparative anatomy. Similar bone structures in mammal limbs (homologous features) suggest common ancestry.
- DNA evidence. The more closely two species are related, the more similar their DNA sequences. Used to build evolutionary trees (B6.10).
Limitations of fossil evidence
- Many early life forms were soft-bodied and did not fossilise.
- The fossil record is incomplete — organisms only fossilise under specific conditions.
- Fossils have been destroyed by geological activity.
Extinction
A species becomes extinct when no individuals remain. Causes:
- New disease (e.g. chytrid fungus in amphibians).
- Environmental change beyond tolerance.
- New predator or competitor introduced.
- Catastrophic events (asteroid impact at end of Cretaceous → dinosaurs).
⚠Common mistakes
- Saying Lamarck "was right". He was wrong about how change occurred (acquired characteristics aren't inherited), even though right that species change.
- Saying evolution is "just a theory" so unproven. In science a "theory" is a well-evidenced explanation, not a guess.
- Saying we evolved from monkeys. We share a common ancestor with modern apes; we did not descend from current monkeys.
- Saying fossils prove every step of evolution. Fossil record is incomplete; DNA and comparative anatomy fill in many gaps.
Links
Builds on B6.5 (variation and mutation) and B6.6 (natural selection). Leads into B6.10 (classification and evolutionary trees).
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