Evolution, extinction and biodiversity · System
Variation and competitive success
No two individuals in a population are identical. Most of the time that is a curiosity. When something goes wrong — a drought, a cold winter, a new predator — it becomes the most important fact about them.
Start here
A thick coat is an advantage. Until it is not.
Two mice in the same litter, one with a noticeably thicker coat. Which of them does better depends entirely on what happens next — and the same is true of every difference between them: size, speed, appetite, colour, boldness.
Which variation in a population is the best one to have?
There is no best one. An advantage is only ever an advantage in a particular set of conditions, and the conditions change. Being large helps you win fights and costs you more food; a thick coat saves you in a cold winter and overheats you in a warm one. The bench below runs the same population through five different environments, and no one variation wins in all of them — in the last, nothing wins at all.
At the bench · one population, five conditions
Change the world, not the mice
1 of 5 conditions tried
The same five mice every time. Only the conditions change. Watch one mouse across all five and see how often the same animal is at the top and at the bottom.
The conditions
A hard winter, snow for eight weeks
Food is scarce and buried, and the cold is relentless. Losing heat is now the main way to die.
Large, heavy build
70% survive
A large body loses heat more slowly than a small one, and fat reserves last. It also needs more food, which is scarce.
Thick coat
90% survive · best here
Insulation is exactly what this winter demands, and it costs almost nothing to carry.
Small and quick
45% survive · worst here
A small body loses heat fast relative to its mass, and speed is no defence against cold.
Bold and exploratory
55% survive
Willing to search further for buried food, which helps — and is exposed to the cold while doing it.
Pale sandy fur
75% survive
Pale fur against snow is difficult for an owl to see, which is worth a great deal in a winter with no cover.
The thick coat wins, and it is the mouse that will suffer most in the next environment on the list.
A long drought, no rain for months
Water is the limit, and heat is the danger. Every gram of body has to be kept cool and supplied.
Large, heavy build
40% survive
A big body needs more water and more food, and there is neither. Size is now a bill rather than an asset.
Thick coat
25% survive · worst here
The insulation that saved this mouse in winter now traps heat it cannot lose. The worst place on the bench to be.
Small and quick
85% survive · best here
Small, light, needs little water and cools easily. The animal that struggled in January is thriving.
Bold and exploratory
60% survive
Finds the few remaining water sources, and takes risks doing it.
Pale sandy fur
70% survive
Pale fur reflects sunlight and matches dry ground, so it stays cooler and stays hidden.
The thick coat has gone from best to worst, and the small quick mouse from worst to best. Nothing about either animal changed.
A barn owl takes up residence
A new predator hunting at night by sight and sound. The ground is dark peaty soil.
Large, heavy build
35% survive
Slower to reach cover and a larger target. Being conspicuous has suddenly become expensive.
Thick coat
55% survive
Neither helped nor harmed by the coat. This mouse survives on average.
Small and quick
80% survive · best here
Quick to cover and a small target. Speed is worth more tonight than anything else.
Bold and exploratory
20% survive · worst here
Bold means out in the open more often, which is precisely the wrong habit to have when something is hunting.
Pale sandy fur
25% survive
Pale fur on dark peat is the most visible thing in the field. The camouflage that saved it in snow now advertises it.
Boldness and pale fur are the liabilities here — and pale fur was an advantage in the snow. The variation has not changed; the background has.
A mild year, and the population doubles
No shortage of weather to worry about — but too many mice, too little food, and constant competition for burrows.
Large, heavy build
80% survive · best here
Wins direct confrontations over food and burrows. In a crowd, size finally pays for itself.
Thick coat
50% survive
No advantage and no cost in a mild year. Average.
Small and quick
45% survive · worst here
Quick, but repeatedly displaced from food by larger mice. Speed does not win an argument.
Bold and exploratory
75% survive
Explores beyond the crowded area and finds unoccupied ground and untouched food.
Pale sandy fur
50% survive
Colour is irrelevant to this problem, so this mouse sits in the middle.
Size wins where the competition is with your own kind rather than with the weather. Note that the mouse that did badly in the drought is this environment’s winner.
A virus sweeps through the population
Nothing to outrun and nothing to hide from. Resistance is decided by which versions of certain genes an animal happens to carry.
Large, heavy build
45% survive
No protection. Size is irrelevant to a virus.
Thick coat
45% survive
No protection. The coat does nothing here.
Small and quick
45% survive
No protection. You cannot outrun an infection.
Bold and exploratory
30% survive
Actively worse — more contact with more mice means more chance of catching it.
Pale sandy fur
45% survive
No protection, and no penalty.
None of the visible variations helps. Whatever decides who survives this is something none of these mice can be seen to have — which is why a population needs variation it is not currently using.
What they are competing for
More are born than can possibly survive.
Between members of one species
Food and water
The most common competition, and the one that bites first. Every organism eating the same thing in the same place is a rival.
Between members of one species
Space and shelter
Territory, a burrow, a nest site, a place on the rock. Often more limiting than food, and usually settled by size or by who arrived first.
Between members of one species
Mates
Only some individuals breed. This is where fitness is finally counted, because surviving without reproducing contributes nothing to the next generation.
Against everything else
Predators and disease
Not competition exactly, but the same filter: whichever variations happen to help you avoid being eaten or infected are the ones that get passed on.
Key fact
Individuals of a species vary, and resources are limited, so some compete more successfully than others. Which variation gives an advantage depends entirely on the conditions, and the conditions change — so there is no such thing as a generally superior individual.
Think again
“Survival of the fittest means the strongest survive.”
Fittest here means best fitted to these conditions, in the sense that a key fits a lock — not fittest in the sense of a gym. It is regularly the small, the drab and the unimpressive that come through. In a drought, the animal that needs least water wins; in a famine, the one that survives on least food; against a new disease, the one that happens to be resistant, which may be the sickliest animal in the group in every other respect. A stag with enormous antlers wins fights and starves in a hard winter carrying them. And the measure is not survival at all in the end — it is how many surviving offspring an individual leaves. An organism that lives fifty years and never breeds has a fitness of zero; a mayfly that lives one day and lays five hundred eggs has done everything that counts.
“Some individuals are just better than others.”
Better at what, and in what conditions? Change the environment on the bench above and the ranking reshuffles, using the same five animals with the same five sets of characteristics. A large mouse dominates a food pile and needs more of it; a bold mouse finds new food first and meets the owl first; a thick coat is life in January and a burden in July. This is why biologists are careful to say well adapted to a particular environment rather than advanced or superior, and it is not a politeness. A species that is superbly suited to one place is, by that same specialisation, badly placed if the place changes — which is precisely how extinctions happen, and it is the subject of When the environment changes: extinction.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Advantage in context
Pale sandy fur is an advantage in snow and a disadvantage on dark soil. What does that show?
Rung 2 · The one that catches people
What does "fittest" mean in "survival of the fittest"?
Rung 3 · Explain a reversal
On the bench, the thick-coated mouse is the most likely to survive the winter and the least likely to survive the drought. Explain both results, and say what this means for the idea of a "better" animal.
Rung 4 · Take it somewhere new
On the disease bench, none of the five visible variations protects the mice. Explain why a population still benefits from having lots of variation it is not currently using, and what that implies for a population reduced to very few individuals.
Key note
Individuals within a species vary, and more offspring are produced than the resources can support, so they compete — for food, water, space, mates and escape from predators. Variations that suit the current conditions make an individual more likely to survive and reproduce. Which variations those are depends on the conditions, so no variation is an advantage in general.
Going further
Peter and Rosemary Grant spent forty years measuring finches on Daphne Major, a small island in the Galápagos, catching and weighing almost every bird on it year after year. In the drought of 1977 the small soft seeds ran out and only the large tough ones were left, and the finches with slightly deeper, stronger beaks survived at a much higher rate. The average beak depth in the population measurably increased in a single generation. Then in 1983 came exceptional rains, the small soft seeds returned in abundance, and the average beak size went back down. That is the whole of this lesson in one data set: the same variation, an advantage in one year and a disadvantage six years later, with no bird ever changing its own beak. It is also one of the few places where evolution has been watched happening rather than reconstructed afterwards.
Before this lesson
Connects to
At GCSE this becomes
- Selection pressures, adaptation, and the evidence for evolution from the fossil record and from living populations.
Where to next
- Next: Natural selection
- Previous: What makes a species
Inheritance and DNA
Ask Mr Badmus AI
Want to test a variation against conditions of your own?
The survival percentages are teaching values chosen to show how the ranking changes, not measurements. Real fitness depends on many characteristics at once, on chance, and on what the rest of the population is doing; the bench isolates one variation per mouse to make the point readable.
Lesson content © MrBadmusAI.