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.

Natural selection

Populations show variation, and more offspring are produced than can survive. Individuals whose inherited characteristics suit the environment are more likely to survive, reproduce and pass those alleles on, so over many generations the frequency of those alleles increases. The peppered moth during industrialisation and the spread of antibiotic-resistant bacteria are the standard examples: the selection pressure is soot-blackened bark in one case and the antibiotic itself in the other.

Artificial selection

In artificial selection (selective breeding) humans choose which individuals breed. Parents showing a desired characteristic - high milk yield, disease-resistant wheat, larger fruit, a particular dog shape - are crossed, the best offspring are selected, and the process is repeated for many generations.

Comparing them

Both act on existing variation and both change allele frequency in the population, but artificial selection is far faster and is directed at traits useful to humans rather than traits that aid survival. That is its weakness too: selectively bred populations often have reduced genetic diversity, which leaves them vulnerable to disease and environmental change, and some breeds carry inherited health problems. Modern genetic technologies can produce similar results in a single generation, which raises ethical questions selective breeding never did.

A species is a group of organisms that can interbreed to produce fertile offspring. A horse and a donkey produce a mule, but the mule is sterile, so they remain two species.

How a new species forms

Speciation needs three things: variation in the population, isolation so the groups stop interbreeding, and different selection pressures acting on each group.

Allopatric speciation begins with a geographic barrier such as a river, mountain range or stretch of sea. Gene flow stops. Mutations arise independently in each group, and natural selection favours different alleles in each environment. Over many generations the two gene pools differ so much that, even if the barrier is removed, the groups can no longer interbreed successfully.

Sympatric speciation happens without a physical barrier, for example when a group starts breeding at a different time of year or on a different host plant.

Isolating mechanisms keep the gene pools separate: geographic separation, different breeding seasons, different courtship behaviour, incompatible gametes, or hybrids that are infertile.

Antibiotic resistance: evolution you can watch

1. A bacterial population contains natural variation; a chance mutation makes a few cells resistant to an antibiotic.
2. The antibiotic is a selection pressure and kills the non-resistant cells.
3. The resistant cells survive, reproduce rapidly by binary fission and pass on the resistance allele.
4. Within a short time almost the whole population is resistant, and the antibiotic no longer works.

Resistance genes are often carried on plasmids, which bacteria can pass to each other directly, so resistance can spread between species. Overprescribing, not finishing a course, and routine use in agriculture all speed this up. MRSA is the best-known Irish example.

Common mistake

Bacteria do not "become resistant because they need to". The mutation happens first, by chance; the antibiotic only selects for the cells that already carry it.