Evolution

Evolution is the formation of new species from existing organisms through a series of inherited changes over very long time periods. All evidence for evolution is circumstantial - it fits the observations but the process itself cannot be directly proven.

Origins of the Theory

Charles Darwin and Alfred Russel Wallace jointly proposed the theory of natural selection in 1858. Darwin gathered much of his evidence during a five-year mapping voyage on HMS Beagle beginning in 1832, and published On the Origin of Species in 1859.

Natural Selection - Observations

• Overbreeding: organisms produce far more offspring than the environment can support (e.g. a single tree produces thousands of seeds).
• Despite this, population sizes stay roughly constant.
• Inherited variation exists within every population - no two individuals of the same species (except identical twins) are exactly alike.

Natural Selection - Conclusions

1. Struggle for existence: overbreeding forces individuals to compete for limited resources.
2. Survival of the fittest: individuals with favourable inherited variations are better adapted, more likely to survive and more likely to reproduce. E.g. deer that run faster escape predators; rabbits with sharper hearing detect danger sooner.
3. Origin of species: as advantageous variations accumulate across generations, offspring drift further from the original type. Eventually they can no longer interbreed with the original population - a new species has formed.

Modern Examples of Natural Selection in Action

• Peppered moth: the pale, speckled form was well camouflaged on lichen-covered trees. A dark mutant appeared in Manchester in 1848 and became the dominant form on soot-covered trees during the Industrial Revolution because birds could no longer see it. After the Clean Air Act, the pale form recovered in cleaner areas.
• Antibiotic-resistant bacteria: bacteria continuously evolve resistance to new antibiotics, a modern and ongoing example of selection pressure.
• Warfarin-resistant rats: warfarin causes lethal bleeding, but a dominant allele in some rats confers resistance. As warfarin use spread, resistant rats became more common (though they require more vitamin K).
• Galapagos finches: finches on different islands evolved distinct beak shapes suited to different food sources, reducing competition and giving rise to new species from a common ancestor.

1. Fossil Evidence (Palaeontology)

Fossils are the preserved remains or traces of organisms that lived long ago - bones, teeth, shells, seeds, pollen, leaf prints, footprints and even faeces. They are found in sedimentary rock, ice, amber and peat bogs. They are dated using radiocarbon dating (accurate up to ~30,000 years) and potassium-argon dating (up to ~1,200 million years).

What the fossil record shows:

• Younger fossils are more complex and more similar to living organisms.
• Older rock layers contain fewer species - biodiversity has increased over time.
• Many modern species (e.g. the modern horse) have no fossil counterparts.
• Many fossil species (e.g. dinosaurs, the dodo) are extinct today.

Evolution of the horse - a classic fossil sequence tracking one trait, height:

• 60 million years ago: Eohippus, ~0.4 m tall (fox-sized)
• 30 million years ago: ~0.6 m tall
• 10 million years ago: ~1 m tall
• 1 million years ago: Equus, the modern horse, ~1.6 m tall

Over 60 million years horses grew larger, reduced from four toes to a single hoof (better for running on hard ground) and developed ridged molars for grinding tough grass rather than soft foliage. These changes match the shift from marshy woodland to open grassland.

Limits of the fossil record: soft tissues rarely fossilise, older fossils are hard to date, most organisms never fossilised at all, and many fossils are destroyed by erosion.

2. Comparative Anatomy

The pentadactyl (five-fingered) limb appears in the arm of a human, the wing of a bat, the flipper of a whale, the foreleg of a mole and the leg of a horse. All share the same underlying bone pattern but are adapted to very different functions - grasping, flying, swimming, digging or running. This is called adaptive radiation from a common ancestor.

Homologous structures - organs with the same basic structure but different functions - point to shared ancestry. Vestigial structures are reduced remnants of features that once had a purpose (e.g. the second and fourth toes of the horse survive as tiny splint bones).

3. Comparative Embryology

Early embryos of vertebrates as different as humans, fish, birds and tortoises look strikingly similar - all have a tail, gill slits (which become the Eustachian tube in humans), a notochord and similar brain and eye position. This suggests they inherited a shared embryological plan from a common ancestor.

4. Comparative Biochemistry

All living cells share the same core biochemical pathways (e.g. respiration) and use very similar molecules (e.g. haemoglobin in vertebrate blood). Comparing DNA and protein sequences between species is the most powerful modern evidence - the closer the sequences match, the more closely the species are related.