NCEA Level 3 · Year 13 · Biology · 2 minutes
Natural selection: how Darwin worked it out
Variation, a selection pressure, and survivors who pass on their alleles.
Key terms
- Natural selection
- The process where individuals with favourable alleles are more likely to survive, reproduce and pass those alleles on.
- Variation
- Differences between individuals, with mutation as the original source of new alleles.
- Selection pressure
- Something like predators, disease or climate that affects which individuals survive.
- Allele
- A version of a gene that can be passed on from parents to offspring.
- Allele frequency
- How common an allele is in a gene pool, which shifts over generations during evolution.
- Mutation
- A change that is the original source of new alleles.
Transcript
In the summer of eighteen fifty eight, Charles Darwin opened a letter from the other side of the world. Inside was his own big idea, the one he'd been working on for twenty years, written by someone else. Awkward.
It came from Alfred Russel Wallace, a collector in the islands of what's now Indonesia. Darwin wrote: I never saw a more striking coincidence. Extracts from both men were read together in London that July. A year later, Darwin published On the Origin of Species.
So what was the idea? Natural selection, in four steps. One: variation. Individuals differ, and mutation is the original source of new alleles. Two: inheritance. Some of those differences are passed on in genes. Three: a selection pressure, like predators, disease or climate, affects who survives.
Four: differential survival and reproduction. Individuals with favourable alleles are more likely to survive, reproduce, and pass those alleles on. Over generations, allele frequencies in the gene pool shift. That's evolution.
The classic example: the peppered moth. Most were pale and speckled, hidden on lichen covered trees. But coal soot darkened the bark and killed the lichens, so pale moths stood out to hungry birds. A dark form, first recorded in Manchester in eighteen forty eight, made up about ninety eight percent of moths there by eighteen ninety five.
In the nineteen fifties, Bernard Kettlewell showed birds took more of whichever form stood out. And when the air was cleaned up, the pale moths came back.
It's happening in Aotearoa too. In twenty twelve, ryegrass in a Marlborough vineyard was confirmed resistant to glyphosate, a common weedkiller. Years of spraying were the selection pressure. Resistant plants survived, reproduced, and passed their resistance on.
So: mutation makes variation. A selection pressure acts on it. Survivors reproduce and pass on their alleles, so allele frequencies shift. And remember: individuals don't evolve. Populations do.
Sources
Every fact in the video, and where it comes from.
- 1darwinproject.ac.uk/entry-2285
- Darwin wrote to Charles Lyell on 18 June 1858 that Wallace had sent him an essay, saying 'I never saw a more striking coincidence'.
- 2amnh.org/exhibitions/darwin/the-idea-takes-shape/a-theo…
- Darwin had worked out natural selection by late 1838 (after reading Malthus) but did not publish for nearly twenty years, until Wallace's essay arrived.
- 3wallaceletters.myspecies.info/node/151
- Alfred Russel Wallace wrote his essay at Ternate (in today's Indonesia) in February 1858 while collecting in the Malay Archipelago, and sent it to Darwin.
- 4britannica.com/biography/Alfred-Russel-Wallace
- Alfred Russel Wallace wrote his essay at Ternate (in today's Indonesia) in February 1858 while collecting in the Malay Archipelago, and sent it to Darwin.
- 5linnean.org/the-society/history
- Extracts from Darwin's manuscripts and Wallace's essay were read together at the Linnean Society of London on 1 July 1858.
- 6artsandculture.google.com/asset/LQGdurk5NKd6sA
- Extracts from Darwin's manuscripts and Wallace's essay were read together at the Linnean Society of London on 1 July 1858.
- 7nls.uk/collections/john-murray/authors/charles-darwin
- On the Origin of Species was published by John Murray on 24 November 1859.
- 8evolution.berkeley.edu/evolution-101/mechanisms-the-processes-of-evol…
- Natural selection: individuals vary; variation is heritable; more offspring are produced than survive; those with favourable heritable traits survive and reproduce more, so favourable alleles increase in frequency in the population's gene pool over generations. Mutation is the original source of new alleles.
- 9britannica.com/science/natural-selection
- Natural selection: individuals vary; variation is heritable; more offspring are produced than survive; those with favourable heritable traits survive and reproduce more, so favourable alleles increase in frequency in the population's gene pool over generations. Mutation is the original source of new alleles.
- 10en.wikipedia.org/wiki/Peppered_moth_evolution
- Typical peppered moths are pale and speckled, camouflaged on lichen-covered bark; industrial soot darkened trees and pollution killed lichens, so pale moths became easier for birds to see.
- The first live dark (carbonaria) peppered moth was caught in Manchester in 1848 (R. S. Edleston); by 1895 about 98% of Manchester's peppered moths were dark.
- In the 1950s Bernard Kettlewell's release-recapture experiments found birds took more pale moths in polluted Birmingham and more dark moths in unpolluted Dorset.
- As air pollution fell (since about the 1960s), the dark form declined and the pale form became common again.
- 11link.springer.com/article/10.1007/s12052-008-0107-y
- Typical peppered moths are pale and speckled, camouflaged on lichen-covered bark; industrial soot darkened trees and pollution killed lichens, so pale moths became easier for birds to see.
- The first live dark (carbonaria) peppered moth was caught in Manchester in 1848 (R. S. Edleston); by 1895 about 98% of Manchester's peppered moths were dark.
- As air pollution fell (since about the 1960s), the dark form declined and the pale form became common again.
- 12rnz.co.nz/news/national/123411/superweeds-found-in-nz-fo…
- In December 2012 glyphosate (Roundup) resistance was confirmed in annual ryegrass from a Marlborough vineyard, the first documented case in New Zealand.
- 13journal.nzpps.org/index.php/nzpp/article/view/5713
- Ryegrass populations from Marlborough and Nelson vineyards with many consecutive years of glyphosate use were about 10 to 30 times more resistant than unsprayed populations: the first confirmed glyphosate resistance in New Zealand.
Image and sound credits
- Charles Darwin, 1855: Maull and Polyblank, public domain, via Wikimedia Commons
- Alfred Russel Wallace: Maull & Fox, London (Bibliothèque nationale de France, Gallica), public domain, via Wikimedia Commons
- Darwin's tree sketch, Notebook B (1837): Charles Darwin, public domain, via Wikimedia Commons
- On the Origin of Species title page (1859): John Murray, public domain, via Wikimedia Commons
- Darwin's finches (1845): John Gould, public domain, via Wikimedia Commons
- Peppered moth, typical form: janet graham, CC BY 2.0, via Wikimedia Commons/Openverse
- Peppered moth, typical form: Kristof Zyskowski & Yulia Bereshpolova, CC BY 2.0, via Openverse
- Peppered moth, carbonaria form: Jerzy Strzelecki, CC BY 3.0, via Wikimedia Commons
- Manchester from Kersal Moor: William Wyld, public domain, via Wikimedia Commons (Google Art Project)
- Wairau Valley vineyards in Marlborough, New Zealand: Jonathan Harker, CC BY 4.0, via Wikimedia Commons
- Ryegrass: Shaun (iNaturalist NZ, observation 47372033), CC BY 4.0, via DigitalNZ
- Map data: Natural Earth (public domain)
- Moths, bark, charts and grass diagrams: drawn in code (schematic)
- Paper textures: ambientCG (CC0) · Torn paper edges generated with AI
- Sound effects: Freesound contributors (CC0)
- Music: generated with ACE-Step · Voice: Gemini TTS
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