Evolution, extinction and biodiversity · System
Biodiversity and gene banks
Variation is what a population has instead of a plan. Keeping it — in the wild where possible, in a freezer where necessary — is the closest thing biology has to an insurance policy.
Start here
Every Cavendish banana in every shop is a clone of every other one.
Not a variety in the ordinary sense — a single genetic individual, propagated by cuttings, grown by the billion. The Cavendish is only the current banana because the previous one, the Gros Michel, was wiped out commercially by a fungus in the 1950s. The same kind of fungus is now moving through Cavendish plantations.
Why is a crop of genetically identical plants so vulnerable?
Because if one plant can be killed by a disease, every plant can — they are the same plant. There is no resistant minority for selection to work with, which is the whole mechanism from Natural selection removed at a stroke. Genetic variation is not a nice-to-have; it is what stops a single event taking everything.
At the bench · four fields, one disease
Plant it, then release the blight
no blight released yet
A thousand potato plants, and a blight that a few varieties happen to resist. Watch what is left to plant next year.
What you planted
One variety, a thousand identical plants
Every plant grown from cuttings of the same original, easiest to harvest by machine, and every plant has exactly the same weaknesses.
1000 of 1000
0 of 1000
none
highest
Nothing survives. Not one plant in a thousand, because there was only ever one plant repeated a thousand times — and this blight kills it. This is what happened to Ireland's potato crop in the 1840s, in miniature, and it is the shape of the Gros Michel banana as well.
Four varieties, 250 plants of each
A little less convenient — four harvest dates, four sets of instructions — and one of the four happens to carry resistance to this particular blight.
1000 of 1000
250 of 1000
4 varieties
good
About 25% survives — not a good year, and not a catastrophe. There is a crop to eat and, more importantly, seed to plant that is known to resist. Variation is what turns a total loss into a bad harvest.
Ten varieties, 100 plants of each
The way small farms grew potatoes for centuries, and still do in the Andes where the crop originated. Awkward to manage, and four of the ten resist this blight.
1000 of 1000
400 of 1000
10 varieties
good
About 40% survives — not a good year, and not a catastrophe. There is a crop to eat and, more importantly, seed to plant that is known to resist. Variation is what turns a total loss into a bad harvest.
A mixed landrace, every plant slightly different
Not varieties at all — a population grown from seed, so no two plants are genetically identical. Yields less per plant and is nearly impossible to harvest mechanically.
1000 of 1000
620 of 1000
very high
lowest
Around 62% of the crop comes through, and next year every one of those survivors is a plant that resisted. The yield per plant was never the highest here; the yield in the year of a blight is the only one that decides whether anyone eats.
Keeping the variation
Two places to store it, and only one of them is alive.
In the wild
Protected habitat
Populations left where they are, still breeding, still varying, still evolving in response to whatever happens. The only method that preserves the ecosystem along with the species.
Needs land, and land has other uses. Vulnerable to exactly the pressures that made protection necessary.
In storage
Seed banks
Dried seeds at around minus eighteen degrees, catalogued and duplicated between countries. Cheap per sample and enormous numbers can be held in one building.
Only works for seeds that survive drying and freezing, and they must be tested and regrown on a cycle or they die quietly in the dark.
In storage
Frozen sperm, eggs and tissue
For animals, where you cannot store a seed. Frozen zoos hold material from thousands of species, including some already extinct in the wild.
Bringing an animal back from frozen cells needs a surrogate mother of a close species, and a stored population cannot be a functioning one.
Living collections
Botanic gardens and zoos
Living organisms maintained and bred deliberately, with studbooks tracking relatedness to keep as much variation as possible.
Expensive per individual, holds small numbers, and captive populations lose variation and adapt to captivity over generations.
Key fact
Biodiversity is the variety of living things — between species, and between individuals within a species. It matters because variation is what allows populations to survive change. Gene banks store seeds, sperm, eggs and tissue so that variation is not lost for ever if a population is.
Think again
“Biodiversity means how many different species there are.”
That is one of the two things it means, and the other one is doing more work in this unit. Biodiversity includes the variation within each species — how genetically different the individuals of one population are from each other. A wood with forty species in it, each reduced to a handful of closely related survivors, is in far more trouble than the species count suggests, and this is exactly the vulnerability that appeared as risk factor four in When the environment changes: extinction. It is also the sense in which a field of one thousand identical potato plants has a biodiversity of almost nothing, despite being a thousand living organisms. Count species when you want to describe a habitat; ask about variation within a species when you want to know whether it can survive what happens next.
“We have gene banks, so it does not matter if species are lost in the wild.”
A seed bank stores a species; it does not store the thing the species was part of. Put a seed back into a landscape and it needs the soil fungi it partners with, the insect that pollinates it, the animal that disperses it, and a climate resembling the one it left — none of which is in the freezer. Seeds also do not last for ever: they need testing, regrowing and replacing on a cycle, which is expensive and only as reliable as the funding. And a stored population cannot go on evolving; it is frozen in the state it was collected in, while the world outside carries on changing. Gene banks are worth every penny as an insurance policy, and the honest description of them is exactly that — insurance, taken out against a loss you are still trying to prevent, not a substitute for preventing it.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · What biodiversity includes
Which field has the lowest biodiversity?
Rung 2 · The one that catches people
A conservation charity says a seed bank means a plant species can never truly be lost. What is the strongest objection?
Rung 3 · Explain the monoculture risk
Explain why a field of genetically identical crop plants can be completely destroyed by a disease, while a field of many varieties usually is not — and link your answer to natural selection.
Rung 4 · Take it somewhere new
A government has a fixed conservation budget and must divide it between protecting habitat and expanding a national seed bank. Set out the argument for each, say which you would fund more heavily and why, and identify one thing your choice would fail to protect.
Key note
Biodiversity is the variety of life: the number of species, and the genetic variation within each one. Variation is what allows a population to survive a change, so losing it raises the risk of extinction and, in crops, of total crop failure. Gene banks preserve hereditary material — seeds, sperm, eggs, tissue — as insurance, but they cannot store an ecosystem and a stored population stops evolving.
Going further
The Svalbard Global Seed Vault sits inside a mountain on an Arctic island, about 1,300 km from the North Pole. It holds well over a million seed samples sent by seed banks all over the world, in sealed foil packets at minus eighteen degrees, and it is designed to keep them cold even if the power fails, because the surrounding rock is permanently frozen. It is a backup of backups: countries deposit duplicates of what their own national banks already hold. It has been used for real. Syria's seed bank at Aleppo, which held varieties of wheat and barley from across the Middle East, became inaccessible during the civil war, and researchers withdrew their duplicate samples from Svalbard, grew them on in Lebanon and Morocco, and sent fresh seed back to the vault. That is the whole argument for gene banks in one story — and also for why the seeds have to be regrown periodically rather than simply left in a freezer.
Before this lesson
Connects to
At GCSE this becomes
- Maintaining biodiversity, the trade-offs in conservation programmes, and the biological arguments behind agricultural policy.
Where to next
Ask Mr Badmus AI
Want to argue the case for or against a conservation decision?
The blight bench is a teaching model: the disease affects whole varieties rather than individual plants, resistance is all-or-nothing, and yield is treated as directly proportional to surviving plants. Real blight resistance is partial and varies with weather, and real fields lose some of every variety. The Gros Michel, Cavendish and Svalbard accounts are real.
Lesson content © MrBadmusAI.