Evolution, natural selection and adaptation

🦎Adaptations

An adaptation is a feature that helps an organism survive and reproduce in its environment.

• Structural — body features: the thick fur and small ears of an Arctic fox; the long, sticky tongue of a chameleon; the spines of a cactus; the webbed feet of a duck.
• Behavioural — actions: migration, hibernation, hunting in packs, nocturnal activity in hot deserts.
• Physiological — body processes: camels producing very concentrated urine; snake venom; plants in hot, dry places opening stomata only at night.

Mangrove trees along Indonesian coasts are a good example: they have roots that stick up above the mud to take in air, and they can get rid of excess salt.

🔄Natural selection

1. Variation: individuals in a population vary, partly because of different alleles (from mutation and sexual reproduction).
2. Overproduction and competition: more offspring are produced than can survive; they compete for food, space and mates, and face predators and disease.
3. Survival of the better adapted: individuals whose variations suit the environment are more likely to survive (the environment exerts a selection pressure).
4. Reproduction and inheritance: survivors reproduce and pass on their advantageous alleles.
5. Change over time: over many generations these alleles become more common in the population — the population evolves.

Individuals do not evolve during their lifetime, and organisms do not change “because they need to”. The variation already exists; the environment selects among it.

🦴Evidence for evolution

• Fossils — the preserved remains of ancient organisms, in rock layers ordered by age, show how organisms changed over time (for example the evolution of whales from land mammals, or of the horse). The fossil record is incomplete, because fossils form only in special conditions.
• Comparative anatomy — the forelimbs of humans, bats, whales and birds share the same basic bone pattern (the pentadactyl limb), suggesting a common ancestor.
• DNA and proteins — the more similar the DNA of two species, the more recently they shared a common ancestor. Humans share about 97% of their DNA with orangutans.
• Evolution observed today — antibiotic-resistant bacteria, insecticide-resistant mosquitoes, and the peppered moth in industrial Britain.

🏝️New species and extinction

A new species forms (speciation) when populations of one species become isolated, for example by sea, mountains or rivers. Each population meets different conditions, mutations and selection pressures, so they evolve differently until they can no longer interbreed to produce fertile offspring.

Wallace noticed that the animals of Bali and Lombok, only 35 km apart across a deep sea strait, are strikingly different: Asian species to the west, Australasian species to the east. This boundary, now called the Wallace Line, is evidence that populations separated for millions of years evolve independently.

Extinction happens when no individuals of a species remain. Causes include new predators or diseases, competition, climate change, and above all human activity: habitat destruction, hunting and pollution.