How Do Snow Leopards Come to Fit the Mountains?: Going Further 6–8

Made for the mountains

You learned in Grade 6 that living things have adaptations: features that help them live where they live. A fish has gills and fins for water; a camel stores fat in its hump and loses little water, for the desert. A snow leopard has a whole set of adaptations for cold, steep, rocky mountains.

Figure 19.2 A snow leopard and what fits it to the high mountains.

Feature	How it helps in the mountains
thick fur, up to 12 cm long on the belly in winter	keeps it warm on the coldest nights
wide, furry paws	spread its weight on snow, like snowshoes, and grip the rock
a thick tail about 1 m long, nearly as long as its body	balance when it leaps and turns on cliffs; a warm scarf when it sleeps
small, rounded ears	less skin to lose heat from
a large space inside its nose	warms the icy air before it reaches the lungs
long, strong back legs	powerful leaps up rocks after blue sheep
smoky gray coat with dark rosettes	hard to see against gray rock, so it can creep close to its prey

A snow leopard walking towards the camera, its very thick spotted tail curled high above its back — **Figure 19.3** A snow leopard’s tail is thick and furry all the way to its tip. Photo: Eric Kilby from Somerville, MA, USA, CC BY-SA 2.0

The snow leopard’s nearest relative is the tiger. Studies of their DNA show that the two come from one shared ancestor that lived roughly 2 to 5 million years ago. Over that time, the tigers’ family spread through the forests and grasslands of Asia, and the snow leopards’ family came to live high in the mountains of Asia. The two now look very different. The question of this topic is how that happened.

Which of these is an adaptation of the snow leopard to the cold?

Pick an answer to see why.

Two ways to explain it

There are two ways to explain how an animal comes to fit its home, and you will meet both in Grade 9 (Lamarckism and Darwinism).

a.By trying. Each snow leopard grew thicker fur because it was cold, and passed its thicker fur on to its cubs. This was Jean-Baptiste de Lamarck’s idea, in 1809. It is wrong. Changes an animal gets during its life are not passed on: a wrestler’s muscles, a scar or a cut tail don’t appear in the next generation.
b.By selection. In every litter of cubs, some happen to be born with thicker fur than others. In a hard winter, those with thicker fur are a little more likely to live, and to have cubs of their own, which are often born with thick fur like them. Generation after generation, there are more and more thick-furred snow leopards. This is natural selection, the idea of Charles Darwin and Alfred Russel Wallace, in 1858.

Grade 9 tells the idea in words. In this topic you will see how it works with numbers: why “a little more likely” is enough, and how fast a trait can spread or disappear.

A farmer says: “The yaks in our village got thicker coats because their grandparents spent so many winters in the snow.” What is wrong with this?

Pick an answer to see why.

No two snow leopards are the same

Look round your class. Everyone is a human, but no two people are the same height, with the same nose or the same voice. The differences between the members of one kind of living thing are called variation. Snow leopards vary too: in the thickness of their fur, the length of their tails, the size of their paws, the shade of their coat. The whole group of snow leopards that live and breed together in one place, for example in Dolpo, is a population.

Some of this variation is inherited: it comes from the parents’ genes. A cub whose parents both have very thick fur is likely to have thick fur too. Some of it is not inherited: it comes from what happens in the animal’s life. A cub that eats well grows bigger than its brother, and a scar from a fight is not passed on.

Where does inherited variation come from? You learned in Grade 8 Grade 8: Sexual reproduction that young made by sexual reproduction are each a new mix of their two parents, so brothers and sisters differ. New variations also appear from time to time, when a gene is copied with a small change, a mutation (you will meet it in Grade 9: Variation and mutation).

Which difference between two snow leopards could natural selection act on?

Pick an answer to see why.

A better chance, not a sure thing

Here is the heart of natural selection. A useful variation doesn’t make an animal sure to survive. It only makes its chance a little better.

Suppose that in a hard winter, a cub with thick fur has a 6 in 10 chance of living to grow up and have cubs of its own, and a cub with thinner fur a 4 in 10 chance. (These are made-up numbers, to show how it works. Real chances are hard to measure in a wild cat, but scientists have measured them in other animals: there are some below.) A chance of 6 in 10 means that out of many such cubs, about 6 in every 10 will live. It doesn’t say which ones. A thick-furred cub can still be caught in an avalanche, and a thin-furred one can be lucky.

Worked example

How many survive?

In one mountain valley, 50 cubs with thick fur and 50 with thinner fur are born in one year. In a hard winter, a thick-furred cub has a 6 in 10 chance of surviving, a thin-furred one 4 in 10.

a.About how many of each survive?
b.At the start, half the cubs had thick fur. What fraction of the survivors have thick fur?

Answer

a.Thick fur: $\tfrac{6}{10} \times 50 = 30$ survive. Thin fur: $\tfrac{4}{10} \times 50 = 20$ survive.
b.There are 30 + 20 = 50 survivors, and 30 of them have thick fur: $\tfrac{30}{50} = \tfrac{6}{10}$ , so 60% of the survivors have thick fur, where 50% of the cubs had. The survivors are the parents of the next generation, so thick fur has become more common in one winter.

With 50 cubs of each kind, chance evens out and the numbers come close to 30 and 20. With only a few cubs, luck can win. Try it.

Ten cubs with thick fur and ten with thinner fur. In each winter, every thick-furred cub has a 6 in 10 chance of surviving, and every thin-furred cub a 4 in 10 chance. Play one winter at a time, and see whether thick fur always wins. Then play a hundred.

Winters played: 0
Thick fur, last winter: –
Thinner fur, last winter: –

The cubs differ only in their fur. Press One winter: the faded cubs are those that didn’t survive.

This winter more thick-furred cubs lived, as the odds say. Play again: does it always happen?

This winter luck won: as many thin-furred cubs lived, or more. A better chance isn’t a promise.

Over 0 winters, about 6 in 10 thick-furred cubs and 4 in 10 thin-furred ones lived: over many cubs, the better chance shows.

In a valley, 6 in 10 thick-furred cubs survive a hard winter and 4 in 10 thin-furred ones. A cub with thick fur is born. Which statement is true?

Pick an answer to see why.

Adding up, generation by generation

A small advantage in one winter makes only a small change. But it happens again in the next generation, and the next. Let’s follow it, with numbers.

Suppose that, among 1,000 cubs born in the mountains, 100 happen to have thick fur and 900 thinner fur: 10% have thick fur. Suppose a thick-furred cub has a 6 in 10 chance of surviving to have cubs, and a thin-furred one 5 in 10: only a small advantage. And suppose every survivor raises the same number of cubs, with fur like its own, so the next generation has the same mix as the survivors.

Figure 19.4 One generation of selection. Out of 1,000 cubs, 10% have thick fur. Thick-furred cubs survive 6 in 10, thin-furred ones 5 in 10, so 11.8% of the survivors have thick fur, and so do 11.8% of their cubs. Bars to scale.

A shortcut: the ratio

Look at the ratio of thick-furred to thin-furred cubs. At the start it was 100 : 900, or 1 : 9. Among the survivors it is 60 : 450. Divide both by 50 and it is 1.2 : 9. The ratio has been multiplied by

$\frac{6 \text{ in } 10}{5 \text{ in } 10} = \frac{6}{5} = 1.2$

the number of times more likely a thick-furred cub is to survive. That happens again every generation: the ratio of thick to thin is multiplied by 1.2 each time. After two generations it is 1.2 × 1.2 = 1.44 : 9; after ten, $1.2^{10}$ ≈ 6.19 : 9.

Carry on the same way and you get this table. Notice the shape: slow at first, while thick fur is rare, then fast, then slow again as the last thin-furred ones become rare.

Generation	About how many years	Ratio thick : thin	Thick fur
0	0	1 : 9	10%
1	8	1.2 : 9	12%
5	40	2.5 : 9	22%
10	75	6.2 : 9	41%
15	110	15 : 9	63%
20	150	38 : 9	81%
30	225	237 : 9	96%

So an advantage of 6 in 10 against 5 in 10 takes thick fur from one cub in ten to nearly all of them in about 30 generations, a couple of hundred years. Even a tiny advantage, 1% more likely to survive (a ratio of 1.01), would take it from 10% to 90% in about 440 generations: some 3,300 years. The snow leopards’ family has had millions of years.

The graph follows the share of snow leopards with thick fur, generation by generation, using the ratio shortcut. Choose how much better the better-suited cubs survive, then play. Then try what happens if the winters became milder and thick fur turned into a burden: switch the climate partway through.

Better-suited cubs survive: 6 in 10 (others 5 in 10)

Winters:

Generation: 0 (about 0 years)
Thick fur: 10%
Ratio thick : thin: 1 : 9.0

One cub in ten has thick fur. Each generation, the ratio of thick to thin is multiplied by how many times more likely thick-furred cubs are to survive. Press Play.

Thick fur is spreading: the ratio is multiplied by 1.2 each generation. Slow while it is rare, fast in the middle.

Nearly every snow leopard now has thick fur. It slows down at the end: few thin-furred ones are left to replace.

In milder winters thick fur is a burden: the ratio is divided by 1.2 each generation, and thick fur becomes rarer.

Thick fur has almost gone. The same selection that spread it has made it rare, because the environment changed.

Up and down: selection people have measured

Natural selection is usually far too slow to watch in a snow leopard. In animals that breed every year, or when the environment changes suddenly, scientists have seen it happen, and counted it.

The peppered moth

Two moths resting on a pale birch trunk: one black, which stands out clearly, and one speckled white, which is hard to see against the bark — **Figure 19.5** The two forms of the peppered moth, dark and pale, on a birch tree with pale bark. Which would a hungry bird find first? Photo: Martinowksy at Dutch Wikipedia ( Original text: Maarten Sanne ), CC BY-SA 3.0

The peppered moth of Britain rests on tree trunks by day, and birds hunt it. Most peppered moths are pale, speckled with black, and almost invisible on trees covered with pale lichens. A rare inherited variation makes them black all over.

A story from science

The moths of Manchester

In 1848 a collector in Manchester caught a black peppered moth, a rare curiosity. Manchester was then full of coal-burning factories, and the soot killed the lichens and blackened the trees. Pale moths now stood out on the dark bark, and the black ones were hidden. By 1895, about 98 in every 100 peppered moths caught around Manchester were black. A peppered moth has one generation a year, so this took fewer than 50 generations. In 1924 the British scientist J. B. S. Haldane worked out that to spread that fast, black moths must have been about one and a half times as likely to survive and leave young as pale ones.

In the 1950s Bernard Kettlewell tested why. He marked hundreds of moths of both kinds, let them go, and caught as many as he could again a few days later. In a sooty wood near Birmingham, he got back 27.5% of the black moths he had let go but only 13% of the pale ones: birds had eaten more of the pale. In a clean wood in Dorset it was the other way round: he got back about three times as large a share of the pale moths as of the black.

Then Britain passed clean-air laws, starting in 1956. The soot went, the lichens came back, and the trees grew pale again. Now the black moths stood out, and they became rare again: in parts of Manchester, from about 95% to under 15% in a study published in 2011. Between 2001 and 2007 the biologist Michael Majerus let 4,864 moths go, a few each night, in his garden in Cambridge, and watched which ones birds ate the next day: they ate more of the black ones. A trait had risen and then fallen, both times by selection.

Why did black peppered moths become rare again after the 1950s?

Pick an answer to see why.

Elephants without tusks

Some African elephants are born without tusks. In most populations, only about 2 to 4 in every 100 females are tuskless. During the civil war in Mozambique, from 1977 to 1992, poachers killed more than 90% of the elephants in Gorongosa National Park, to sell the ivory. They shot mostly elephants with tusks, and mostly left the tuskless ones.

In a study published in 2021, scientists found that among the female elephants of Gorongosa, about 18% had been tuskless before the war, and about 51% of those that survived it.

This is selection by people, not by nature, and it shows that a useful feature can become less common too: tusks became rarer within a few decades. Among the females born since the war, about a third are tuskless: fewer than among their mothers, partly because a tuskless mother passes the trait on to only about half of her daughters, but still far more than before the war. Tusks are useful to an elephant, for digging and stripping bark, so now that the poaching has stopped, tuskless elephants may slowly become rarer again.

Try it at home: hungry birds

Activity

Paper moths on newspaper

You need: two sheets of newspaper with lots of small print, one sheet of plain colored paper (red, blue or green), scissors, a watch or phone to time 10 seconds, a friend, and a notebook.

Cut 20 small squares, about 1 cm across, from one sheet of newspaper, and 20 the same size from the colored paper. These are your “moths”: two forms of one species.
Spread the other sheet of newspaper flat on a table or the floor: this is the tree bark. While your friend looks away, scatter all 40 moths over it.
Your friend is a bird. They have 10 seconds to pick up moths one at a time, as fast as they can.
Count the moths left on the paper, newspaper ones and colored ones. These are the survivors.
The survivors breed: for every survivor, add one more moth of the same kind (cut more if you need to). Write the numbers of each kind, and the percentage of newspaper moths.
Scatter them again and repeat for three or four generations.

What you should see

The bird finds the colored moths much more easily, so most of the survivors in each round are the newspaper moths, which blend in. Their share rises every generation: you might start at 50%, then reach 60–70%, then 80% or more. If you then swap the background for the colored paper, the advantage turns round. Compare your percentages with the ratio shortcut: in each round, how many times more likely was a newspaper moth to survive?

In short

An adaptation is a feature that helps a living thing survive and reproduce where it lives. A snow leopard’s thick fur, wide paws and long tail fit it to cold, steep mountains.
Members of a population differ: variation. Only inherited variation, from the genes, can be passed on.
A useful inherited variation doesn’t make an animal sure to survive; it gives it a better chance of surviving and having young. In a large group, the share that survives comes close to that chance.
Each generation, the survivors are the parents of the next, so a useful variation becomes more common. The ratio of animals with the trait to those without it is multiplied each generation by how many times more likely they are to survive and breed.
Small advantages add up over many generations: slowly while a trait is rare, fast in the middle, slowly at the end. When the environment changes, selection can turn round and make a trait rarer, as with the black peppered moths after the air was cleaned.

Questions

In a population of hares, 30% are born with white winter coats. In a snowy winter, white hares are twice as likely as brown hares to survive. What will the share of white hares among the survivors be?

Pick an answer to see why.

Which of these would make natural selection impossible?

Pick an answer to see why.

a.

Explain how snow leopards came to have long, thick tails. Use the words variation, inherited, survive and generation.

Answer

Long ago, the snow leopards’ ancestors showed variation in their tails: some were born with longer, thicker tails than others, and this difference was inherited, so their cubs tended to have tails like them. On steep cliffs and in cold winters, cats with longer, thicker tails kept their balance better while hunting and kept warmer while sleeping, so they were a little more likely to survive and raise cubs. In each generation a slightly larger share of the parents had long, thick tails, so a larger share of the cubs did too. Over many thousands of generations, long, thick tails became normal for the whole population. No cat grew a longer tail because it tried or needed to.

b.

In a valley there are 40 snow leopard cubs: 10 with thick fur and 30 with thinner fur. In a hard winter, 8 of the thick-furred cubs and 12 of the thin-furred cubs survive.

a.Write each kind’s chance of surviving as “… in 10”.
b.What percentage of the cubs had thick fur, and what percentage of the survivors?
c.How many times more likely was a thick-furred cub to survive?

Answer

a.Thick: 8 of 10, so 8 in 10. Thin: 12 of 30 = 4 of 10, so 4 in 10.
b.Cubs: $\tfrac{10}{40}$ = 25%. Survivors: $\tfrac{8}{8 + 12} = \tfrac{8}{20}$ = 40%.
c.8 in 10 ÷ 4 in 10 = 2 times as likely. (Check with the ratio: 10 : 30 became 8 : 12, and $\tfrac{8}{12} \div \tfrac{10}{30} = 2$ .)

c.

A beetle comes in green and brown forms. On green leaves, birds eat more brown beetles. A scientist finds that the ratio of green to brown beetles is multiplied by 1.5 each generation.

a.If the ratio starts at 1 : 1, what is it after 1, 2 and 3 generations?
b.What percentage are green after 3 generations?
c.Sketch a graph of the percentage of green beetles over 10 generations. What shape is it, and why does it flatten near 100%?

Answer

a.1.5 : 1, then 2.25 : 1, then 3.375 : 1.
b. $\tfrac{3.375}{4.375} = 0.771$ , so about 77%.
c.After 4 generations about 84%, after 5 about 88%, after 10 almost 98%. The graph rises fast, then flattens. It flattens because the ratio keeps growing but each step adds less to the percentage: once nearly all beetles are green, there are few brown ones left to be replaced. It never quite reaches 100% while some brown beetles are still born.

d.

A student says: “A snow leopard with thick fur will survive the winter, and one with thin fur will die. That is natural selection.” Explain what is wrong, and correct it.

Answer

Natural selection is about chances, not certainties. A thick-furred snow leopard can still die (of hunger, a fall or an avalanche), and a thin-furred one can live. What is true is that thick-furred cats are more likely to survive and raise cubs in cold places. Over a whole population and many generations, that better chance means a larger share of each generation’s parents have thick fur, so thick fur becomes more common.

e.

Black peppered moths rose from rare to 98% around Manchester between 1848 and 1895, then fell to under 15% in parts of Manchester (a study published in 2011). Explain both changes using natural selection, and say what evidence shows that birds were the cause.

Answer

Rise: soot from coal-burning factories killed the pale lichens and blackened the tree trunks. On dark bark, pale moths were easy for birds to see and black moths were hidden, so black moths were more likely to survive and breed. The black color is inherited, so each generation had a larger share of black moths. Fall: after the clean-air laws of the 1950s, the lichens returned and the trunks grew pale again. Now black moths stood out and were eaten more, so their share fell each generation. Evidence: in Kettlewell’s experiments, a larger share of black moths was recaptured in a sooty wood (27.5% against 13% of pale ones), and a larger share of pale moths in a clean wood; and Majerus, releasing thousands of moths between 2001 and 2007, saw birds eat more of the black ones on pale trees.

Think about it

Would snow leopards fit Chitwan?

Suppose snow leopards were moved to the warm forests of Chitwan. Which of their adaptations might become a burden there? If they could survive and breed for many generations, what might natural selection do to them?

Answer

Their thick fur and heavy tail would trap heat in a climate of 35 °C summers, and their gray coat would stand out among green leaves and brown grass. In a warm forest, cats born with thinner fur and a darker, warmer-colored coat would be more likely to survive and raise young, so, over many generations, those variations would become more common, as long as they exist in the population. The population would slowly come to fit the forest, much as the snow leopards’ ancestors came to fit the mountains. (In real life, a few snow leopards moved to Chitwan would probably die before any of this could happen; selection needs time, and enough animals.)

Where to go next

Why Do You Look Like Your Parents, but Not Exactly? Where inherited variation comes from: how genes pass from parents to young, and why brothers and sisters differ.
How Did Farmers Make Our Crops? The same selection, done on purpose: how farmers turned wild plants into maize, rice and our other crops by choosing which ones to sow.
Why Do Animal Numbers Rise and Fall? Why most young animals never grow up: food, hunters and disease, the struggle that does the selecting.