Variation and evolution: WJEC A level Biology revision notes

Lesson 1 of 19

What causes variation between individuals?

These notes cover topic 4.4, and , in WJEC A level Biology Unit 4: , Inheritance and Options. Each lesson follows the WJEC specification, and the exam tips come from BioWelsh analysis of WJEC past papers.

is the differences between individuals of the same . No two individuals are exactly alike, even in a large . Differences in come from two sources: differences in , and differences in the environment each individual lives in. Most characteristics are affected by both.

Where genetic variation comes from
Source	What happens
	A change in the base sequence of DNA creates a new . This is the only source of completely new .
	Chromatids of swap lengths of DNA in prophase I of meiosis, making new combinations of on each chromosome.
	line up at random at metaphase I, and chromatids at metaphase II, so each gets a different mix of chromosomes.
Random fertilisation	Any male can fuse with any female , combining two different sets of .
Random mating	In a , which individuals mate with each other is largely a matter of chance.

The environment affects how are expressed. Diet, light, temperature, water, soil minerals, disease and exercise can all change a . A plant with for tall growth will stay short in poor soil, and two plants grown from cuttings of the same parent, with identical , can grow to very different sizes in sun and in shade.

caused by differences in is : it can be passed to offspring in gametes. caused only by the environment is , because it does not change the in the gametes. A scar or a muscle built by training is not passed on. Some changes, which alter expression without changing the base sequence, can be passed on for a generation or two.

Lesson 2 of 19

How do continuous and discontinuous variation differ?

comes in two kinds. In , individuals fall into a few distinct categories with no intermediates. In , individuals show a complete of values between two extremes.

The photographs below show the two common forms of the peppered moth side by side. Wing colour is a classic example of .

Two photographs side by side of a moth resting with its wings spread flat. On the left, on sandy stone, the wings and body are white, densely covered in small black speckles and short dark lines. On the right, on pale grey bark with orange patches, the wings and body are almost completely black, with faint darker veins on the wings. — The speckled form (left) and the melanic form (right) of the peppered moth. Photo: Ben Sale (both photographs), CC BY 2.0. Resized.

The moth on the left is the speckled (pale) form, with white wings covered in small black flecks.
The moth on the right is the melanic (dark) form, with almost black wings.
Each moth clearly belongs to one form or the other. The difference is controlled by one , and the for the melanic form is .

data are shown as a bar chart with gaps between the bars, because each bar is a separate category. The chart below shows the ABO blood groups in a .

Bar chart with four separate gold bars. The horizontal axis shows blood groups A, B, AB and O. The vertical axis is percentage of the population from 0 to 50. The bars read about 42 for A, 10 for B, 4 for AB and 44 for O, with gaps between them. — Percentage of people with each ABO blood group in a population, using rounded values of the kind quoted for the UK. Original BioWelsh graph.

There are only four categories, A, B, AB and O, and every person is in exactly one of them.
There are no intermediates, so the bars do not touch.
Blood group is controlled by a single with three , and the environment has no effect on it.

data are grouped into classes and shown as a , with no gaps between the bars. The below shows the height of a sample of adults.

Histogram with nine touching blue bars. The horizontal axis is height in centimetres in classes from 150 to 154 up to 190 to 194. The vertical axis is number of people from 0 to 50. The bars read 3, 9, 21, 36, 44, 38, 22, 10 and 3, rising to a single peak at 170 to 174 and falling away evenly on both sides. — Height of 186 adults in a sample, grouped into 5 cm classes, using illustrative values. Original BioWelsh graph.

Height can take any value within the , so the classes run on from each other.
Most people are near the middle, with fewer and fewer towards each extreme.
The shape is close to a symmetrical bell curve, called a .
Height is controlled by many , each with a small effect, and is also affected by diet and health.

Discontinuous and continuous variation compared
Feature	Discontinuous	Continuous
Values	A few distinct categories, no intermediates	A of values with every intermediate
involved	One , or a few ()	Many at different ()
Effect of the environment	Little or none	Large
Graph	Bar chart with gaps	or curve
Examples	ABO blood group, peppered moth form, pea seed shape	Height, body mass, leaf length, wingspan

Lesson 3 of 19

How do you describe a set of measurements?

is described with averages and with measures of spread. You need to calculate them and to read them from graphs.

Averages and spread
Measure	What it is	When it is useful
	The sum of the values divided by the number of values	The usual average for data
	The middle value when the values are put in order	Small samples, or data with a few extreme values
	The value, or class, that occurs most often	Reading the most common class from a
	The largest value minus the smallest	A quick idea of spread, but one extreme value changes it
	A measure of how spread out the values are around the	Comparing the spread of two samples

The graph below shows two samples with the same but different standard deviations.

Line graph of number of shells from 0 to 40 against shell length from 20 to 60 millimetres. A solid blue bell-shaped curve labelled sample A peaks at 36 at 40 millimetres and falls to almost zero by 32 and 48. A dashed orange curve labelled sample B peaks at only 16 at 40 millimetres and spreads from about 22 to 58. A vertical dashed grey line at 40 is labelled mean of both equals 40 millimetres. — Two samples of shell lengths with the same mean but different standard deviations, using illustrative values. Original BioWelsh graph.

Both curves are centred on 40 mm, so the two samples have the same .
Sample A is tall and narrow: most values are close to the , so its is small.
Sample B is low and wide: values are spread far from the , so its is large.
In a the , and are all the same value, at the peak.

Not every is symmetrical. If a tail stretches further to one side, the data are . When a changes, a may shift: if the peak moves to larger values in a later year, the has increased.

Lesson 4 of 19

How does competition affect survival and breeding success?

Organisms produce far more offspring than can survive. A pair of rabbits can have dozens of young a year, and an oak tree makes thousands of acorns. Yet usually stay about the same size, because resources are limited and most offspring die before they can breed. Individuals must compete for what they need.

Two kinds of competition
Type	Between	Example
	Individuals of the same	Seabirds on a crowded cliff competing for nesting ledges; males competing for mates
	Individuals of different	Two of grass competing for light, water, mineral ions and space

is usually the most intense, because members of the same need exactly the same resources. It affects survival, because the weakest individuals get too little food, water or shelter. It also affects breeding success: an individual that survives but cannot win a , a nest site or a mate leaves no offspring.

This is why competition matters for . Individuals whose help them compete successfully are more likely to survive and breed, so those are passed on more often. Competition is one of the ways the environment selects between individuals.

Plants compete for light, water, mineral ions and space.
Animals compete for food, water, shelter, , nesting or breeding sites and mates.

Lesson 5 of 19

What are selective agencies, and how do they affect survival?

A , or , is any factor in the environment that makes some individuals more likely than others to survive and reproduce. Because it favours some over others, it changes which are passed on.

Selective agencies named in the specification, with examples
Selective agency	How it selects
Supply of food	When food is scarce, individuals that find, catch or digest it better survive, for example seabirds with longer wings reaching distant fishing grounds.
Breeding sites	Only individuals that win a nest site, or den can raise young.
Climate	Temperature, rainfall and snow cover decide which coat colours, body sizes or flowering times succeed.
Predation	Predators catch the individuals that are easiest to see or slowest to escape.
Disease	Individuals with giving survive .
Human impact	Pollution, pesticides, , hunting, poaching and all select, often very strongly.

The value of an depends on the environment. The photographs below show snowshoe hares, which live in the forests of North America.

Two photographs side by side. On the left, a hare with a greyish-brown coat and long dark-edged ears crouches among yellow and brown fallen leaves and green plants, closely matching the colours around it. On the right, a hare with an entirely white coat and a dark eye sits on snow beneath dark conifer twigs. — Snowshoe hares in a brown autumn coat (left) and a white winter coat (right). Photo: U.S. Fish and Wildlife Service (left); NPS/Jim Peaco (right), Public domain.

On the left, a hare in autumn has a brown coat, which blends with dead leaves and soil.
On the right, a hare has moulted into its white winter coat, which blends with snow.
In a few areas with little snow, some hares stay brown all winter, and this difference is inherited.
A white hare on bare brown ground is easy for a predator to see. Where winters have become shorter, the for staying brown gives an advantage.

The bar chart below shows the two forms of peppered moth caught in two kinds of woodland.

Bar chart with two pairs of bars. The vertical axis is percentage of moths caught from 0 to 100. In woodland with soot-darkened bark, a dark grey bar for the melanic form reads 88 and a pale bar for the speckled form reads 12. In woodland with pale, lichen-covered bark, the melanic bar reads 9 and the speckled bar 91. — Percentage of each form of peppered moth caught in two kinds of woodland, using illustrative values. Original BioWelsh graph.

Where soot from factories had darkened the bark, most moths caught were melanic.
Where the bark was pale and covered in lichen, most were speckled.
Birds eat the moths that stand out against the bark they rest on, so predation is the , and pollution by humans changed which form was favoured.
Since clean air laws soot, the speckled form has become common again across Britain.

The photograph below shows a banded snail, whose shells vary in colour and in the number of dark bands.

Close-up of a snail shell hanging from a green plant stem against a blurred green background. The rounded shell is pale yellow with several broad dark brown bands spiralling round it. — A banded snail with a yellow shell and dark bands, on a plant stem. Photo: Anders Sandberg, CC BY 2.0. Resized.

This shell is yellow with dark bands running round it.
Song thrushes eat these snails and find the ones that stand out from their background.
In beech woodland with brown leaf litter, brown unbanded shells are harder to see; in and long grass, banded shells are.
So the most common shell type differs from one to the next. Climate also selects, because dark shells warm up faster and pale shells overheat less.

Lesson 6 of 19

How does natural selection change allele frequencies?

is the process by which individuals with better suited to their environment are more likely to survive and reproduce, so the that produced those are passed on more often. Over many generations the of those in the rises.

, step by step

1) creates new , so individuals in a vary.
2) More offspring are produced than can survive, and a acts.
3) Individuals with an advantageous have a : they are more likely to survive.
4) They reproduce and pass on the advantageous to their offspring.
5) Over many generations, the of that in the increases.

The diagram below follows the same five steps for the peppered moth on soot-darkened bark.

Four dark grey panels of tree bark joined by arrows. The first holds six moths: five pale grey with black speckles and one almost black. In the second, four of the pale moths are crossed out in red. The third holds one pale moth and three dark moths. The fourth holds five dark moths and one pale moth. Headings read variation, selection pressure, survival and reproduction, and change over generations, each with a short sentence explaining the step. — Natural selection in four steps, using the peppered moth on soot-darkened bark. Original BioWelsh diagram.

The for dark wings arose by before the bark became dark. It was not caused by the soot.
Birds eat the pale moths that stand out on dark bark, so the dark moths are more likely to survive.
The survivors breed, so more of the next generation carry the for dark wings.
Repeated each generation, the of the dark rises. The has evolved.

can be fast when the is strong. Waste from old metal mines in north Wales contains so much copper that most plants die, yet some grasses now grow there. The graph below shows how the of an for copper tolerance might change after a site is polluted.

Line graph of frequency of the tolerance allele from 0 to 1 against time since the site was polluted from 0 to 100 years. A green S-shaped curve starts near 0.03, rises slowly for about 25 years, climbs steeply between about 30 and 60 years, and levels off near 0.95 after about 80 years. — Frequency of a copper-tolerance allele in a grass population colonising polluted mine waste, using illustrative values. Original BioWelsh graph.

At first the tolerance is rare, present only because of earlier .
Tolerant plants survive and set seed on the polluted soil, while others die, so the increases.
The rise is fastest in the middle years, when many plants carry the and it still gives a large advantage.
The curve levels off near the top, when almost every plant carries it.

Lesson 7 of 19

What are stabilising, directional and disruptive selection?

For a characteristic, selection can act on the middle of the or on one or both extremes. The three graphs below each show a before selection and many generations later.

The first graph shows .

Line graph of number of individuals against value of the characteristic from 0 to 100. A solid blue bell curve peaks at 50 individuals at a value of 50. A dashed red curve, labelled many generations later, is centred on the same value but is narrower and peaks at about 92. Labels near each tail read extremes selected against, and a label above the peak reads mean unchanged. — Stabilising selection: a population before selection and many generations later. Original BioWelsh graph.

Individuals at both extremes are selected against.
The stays the same, but the curve becomes narrower and taller, so the falls.
happens in an environment that stays the same. Human birth mass is an example: very small babies lose heat and very large babies have difficult births.

The second graph shows .

Line graph of number of individuals against value of the characteristic from 0 to 100. A solid blue bell curve peaks at 60 individuals at a value of about 38. A dashed red curve of the same shape peaks at 60 at a value of about 62. Labels read selected against on the left, favoured on the right, and mean shifts to the right above. — Directional selection: a population before selection and many generations later. Original BioWelsh graph.

Individuals at one extreme are favoured and those at the other are selected against.
The whole curve, and its , shifts in one direction.
happens when the environment changes: for example , copper tolerance, or longer wings in seabirds when food moves further away.

The third graph shows .

Line graph of number of individuals against value of the characteristic from 0 to 100. A solid blue bell curve peaks at 58 individuals at a value of 50. A dashed red curve has two peaks of about 52, at values of about 28 and 72, and dips almost to zero at 50. Labels read favoured above each red peak and middle values selected against above the centre. — Disruptive selection: a population before selection and many generations later. Original BioWelsh graph.

Individuals at both extremes are favoured and those near the are selected against.
The single peak splits into two, so the becomes .
In coho salmon, large males win females by fighting and small males sneak in to fertilise eggs, but males do neither well. can be a first step towards two .

Lesson 8 of 19

Why is the sickle cell allele common where malaria is found?

is caused by the Hbˢ of the beta globin . People who are HbˢHbˢ have , and without treatment many die young. You might expect to remove the , yet in parts of Africa about one person in five carries it. (If the genetics of is unfamiliar, read the inheritance lesson on gene mutations.)

How each genotype fares where malaria is common
Genotype	Phenotype	Where malaria is common
HbᴬHbᴬ	Normal haemoglobin	Very likely to suffer severe : selected against
HbᴬHbˢ	Sickle cell trait, usually healthy	Protected from severe : the parasite cannot multiply well in their red blood cells. Selected for
HbˢHbˢ		Often die young: selected against

have the highest chance of surviving and reproducing, so both stay in the . This is . Where there is no , for example among descendants of Africans in North America, have no advantage while HbˢHbˢ individuals are still selected against, so the of Hbˢ falls over the generations.

Lesson 9 of 19

What are the gene pool and genetic drift?

genetics looks at a whole rather than individual crosses. The is all the of all the in a at a given time. The is the proportion of all the copies of a in the that are one particular . is a change in in a .

can change by chance as well as by selection. In each generation only some individuals happen to breed, and each passes on only some of its . This random change in is . In a large the chance effects mostly cancel out, but in a small an can to a of 1, called , or be lost altogether.

The graph below shows a computer simulation of . Every starts with the at a of 0.5, and there is no selection.

Line graph of frequency of the allele from 0 to 1 against generation from 0 to 40. All four lines start at 0.5. The blue line for a population of 1000 stays between about 0.47 and 0.54 throughout. Three orange lines for populations of 10 zigzag widely: one rises to 1 by generation 14 and stays there, one falls to 0 by generation 24 and stays there, and one wanders between about 0.25 and 0.9, ending at 0.65. — Computer simulation of genetic drift: the frequency of one allele in three populations of 10 and one of 1000, with no selection. Original BioWelsh graph.

In the of 1000, the stays close to 0.5 for all 40 generations.
In the three of 10, it swings up and down from one generation to the next.
One small loses the completely, and another has it at a of 1.
acts by chance alone, which is why the small end up different from each other.

The is a special case of . When a few individuals leave to start a new , for example on an island, they carry only a small, chance sample of the in the original . The diagram below shows how this works.

Three ovals joined by arrows. The left oval, labelled mainland gene pool, holds 30 small circles: 16 blue, 10 gold and 4 red, with the percentages 53, 33 and 13. An arrow labelled a few are carried to an island leads to a small middle oval, labelled alleles carried by the founders, holding 3 red circles and 1 gold. An arrow labelled population grows leads to a right oval, labelled island gene pool, many generations later, holding 22 red and 8 gold circles and no blue, with the percentages 0, 27 and 73. A note says each circle stands for one copy of an allele. — The founder effect: a few individuals start a new population. Original BioWelsh diagram.

The mainland contains three .
By chance, the four founders carry no copies of the blue and a lot of the red one.
The island grows from the founders, so it inherits their unusual .
Without from the mainland, the two stay different, and in the small island can make them differ even more.

The explains why some small, isolated human communities have high frequencies of rare . The Old Order Amish of Pennsylvania descend from a few hundred founders and marry within their , and an causing Ellis-van Creveld syndrome, a form of dwarfism with extra fingers, is far more common among them than in other . A has a similar effect: northern elephant seals were hunted until perhaps only a few dozen survived, and today's large has very little genetic .

Lesson 10 of 19

How do you use the Hardy-Weinberg equation?

The states that, under certain ideal conditions, and frequencies in a stay the same from one generation to the next. It lets you work out from frequencies, and the reverse.

For a with two , A and a, let p be the of A and q the of a.
Every is either A or a, so p + q = 1.
If gametes meet at random, the frequencies are AA = p², Aa = 2pq and aa = q², so p² + 2pq + q² = 1.

The diagram below shows where the equation comes from, for a in which p = 0.7 and q = 0.3.

A large square divided into four rectangles. Along the top the allele in the sperm is labelled A, p equals 0.7, over the wider left part and a, q equals 0.3, over the narrower right part; the same labels run down the left side for the allele in the egg. The large top left green square reads AA, p squared equals 0.49. The two gold rectangles read Aa, pq equals 0.21. The small bottom right pink square reads aa, q squared equals 0.09. A list on the right gives AA 0.49, Aa 2pq equals 0.42, aa 0.09, total 1.00. — Where p² + 2pq + q² comes from, for p = 0.7 and q = 0.3. Original BioWelsh diagram.

Seventy per cent of eggs and 70% of carry A, so the chance of an AA is 0.7 × 0.7 = 0.49.
The chance of aa is 0.3 × 0.3 = 0.09.
Aa can form in two ways, A egg with a or a egg with A , so its is 2 × 0.7 × 0.3 = 0.42.
The three frequencies add up to 1.

When one is , you cannot count the AA and Aa individuals separately, because they look the same. Always start from the , which gives q² directly.

If the are , every can be recognised, so can be counted directly. In a sample of 1000 people with 300 MM, 500 MN and 200 NN, there are 2000 : M = (2 × 300 + 500) ÷ 2000 = 0.55 and N = 0.45.

Lesson 11 of 19

When does the Hardy-Weinberg principle apply?

The only holds if nothing is changing . The conditions are the opposite of the things that cause .

The conditions for the Hardy-Weinberg principle
Condition	Why it is needed
The is very large	changes in small
Mating is random	If individuals choose mates by , frequencies change
There is no	creates new and changes frequencies
There is no migration: no or	adds or removes
There is no selection	All must be equally likely to survive and reproduce

The principle also assumes the organisms are and reproduce sexually, that generations do not overlap, and that are the same in males and females.

No real meets every condition, but many large, randomly mating come close. The principle is most useful as a baseline. If the frequencies in a are very different from those the equation predicts, something is changing them: selection, , or migration. For example, a herd of horses living in desert conditions might not be in equilibrium if pale coats are better camouflaged or less likely to overheat.

Lesson 12 of 19

What is a species, and how are populations reproductively isolated?

A is a group of organisms with similar features that can interbreed to produce offspring. is the formation of a new from an existing one. For this to happen, part of a must stop exchanging with the rest.

Within a , individuals usually breed with those nearby, forming local interbreeding groups called . move between when individuals or gametes move, which is called . keeps the of the similar. If something prevents it, the are , and their can apart.

Mechanisms of reproductive isolation

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- : gametes never fuse, so no forms
  - Geographical: a physical barrier separates the
  - Seasonal (temporal): they breed or flower at different times
  - Behavioural: songs or displays are not recognised
  - Morphological (mechanical): body or flower shapes prevent mating
  - : or cannot fertilise the other's gametes
- : a forms, but the do not mix further
  - : the dies before it can breed
  - : the lives but cannot make gametes
  - : first are , but later generations are weak or sterile

is when the are in different places, separated by geography. It is when the share the same area and are isolated by something else, such as behaviour or breeding season.

Lesson 13 of 19

How does geographical isolation lead to new species?

In a physical barrier, such as a mountain , a river, a stretch of sea or a sheet of ice, splits a into two. The diagram below shows the stages.

Four green panels joined by arrows. In the first, blue circles are spread across the panel. In the second, a thick wavy blue barrier runs down the middle with blue circles on both sides. In the third, the barrier remains, with blue circles on the left and red circles on the right. In the fourth, labelled barrier gone, blue and red circles are mixed across the panel. Headings read one population, a barrier forms, the groups diverge and two species, each with a short explanation. — Allopatric speciation: a population split by a geographical barrier. Original BioWelsh diagram.

At first all individuals can interbreed, and keeps one .
The barrier stops between the two groups.
Each group faces different , has different new and undergoes , so change differently in each .
After many generations the are so different that, if the groups meet again, they can no longer interbreed to produce offspring. They are two .

Polar bears evolved from a of brown bears that was cut off in the far north by advancing ice. In the Arctic, selection favoured white fur for camouflage on snow and that let the bears live on a diet very rich in seal fat. Brown and polar bears are now separate , although where their ranges overlap again because of , a few have been found, showing that isolation is not always complete.

African elephants were once treated as one . DNA studies showed that forest elephants of central Africa and savanna elephants have been separated for a long time and have diverged so far that they are now classed as two .

Lesson 14 of 19

How can new species form in the same place?

In the two live in the same area, but something other than geography stops them interbreeding. Once stops, their diverge in the same way as in .

The photograph below shows a meadow grasshopper, one of several of grasshopper found together in British grassland.

Side view of a grasshopper clinging to a grass flower head against a blurred green background. It has a yellow-green body, long thin antennae, a large brown eye, short brownish wings that do not reach the end of its abdomen, and long hind legs folded beside its body. — A meadow grasshopper on a grass flower head. Photo: Bernard Ladenthin, CC BY 4.0. Resized.

Males sing by rubbing a row of pegs on each hind leg against a wing.
Each has its own song, and a female only responds to the song of her own .
Some look almost identical and live in the same fields, but they are kept apart by this .

Examples of isolation without a geographical barrier
Mechanism	Example
Behavioural	Grasshopper that differ in song; birds with different displays
Seasonal	Copper-tolerant grass on old mine waste in Wales flowers earlier than the same on normal soil nearby, so the two groups rarely cross-pollinate
Morphological	Snails whose shells coil to the left cannot mate with snails that coil to the right; a Madagascan orchid with a very long spur can only be pollinated by a moth with an equally long tongue
Gametic	Corals and sea urchins release gametes into the same water, but proteins on the gametes only allow fusion within a
	Doubling of the chromosome number makes an instant new that cannot breed with its parents

Lesson 15 of 19

Why are hybrids such as mules sterile?

Sometimes two do mate and produce a . then keeps the apart. Some die as embryos (). Others grow into healthy adults but are sterile, so the of the two still never mix in later generations.

The photograph below shows a mule, the of a female horse and a male donkey.

Head and neck of a dark brown animal looking straight at the camera in front of a tree trunk and a grassy field. It has very long upright ears like a donkey, a short dark mane, a paler muzzle, and a long, horse-like face. — A mule, the hybrid offspring of a female horse and a male donkey. Photo: Sogospelman, CC BY 2.5. Resized.

A mule has long ears like a donkey and a larger body like a horse.
Mules are strong, healthy and long-lived, so they are .
Almost all mules are sterile, so every mule has to be bred again from a horse and a donkey.

The diagram below shows why. A horse has 64 chromosomes and a donkey 62.

On the left, a box labelled horse egg, n equals 32, above four blue chromosomes of different lengths, and a box labelled donkey sperm, n equals 31, above four gold chromosomes of different lengths and centromere positions. Arrows labelled fertilisation lead to a large rounded box labelled mule body cell, 2n equals 63, containing the same four blue and four gold chromosomes. An arrow labelled meiosis leads to text explaining that at prophase I no chromosome finds a partner of the same size, shape and gene sequence, so bivalents cannot form, meiosis fails and the mule is sterile although healthy. — Why a mule is sterile: its chromosomes cannot form homologous pairs. Original BioWelsh diagram.

The horse egg has 32 chromosomes and the donkey 31, so a mule's body cells have 63.
The two sets come from different , so they differ in number, size, shape and order.
In prophase I of meiosis the chromosomes cannot form , so meiosis fails and gametes are not made.

In plants, a sterile can become if its chromosome number doubles, for example when a cell copies its chromosomes but does not divide. The diagram below shows how this works.

On the left, under a box labelled hybrid, one set from each parent, sterile, are three blue and three gold chromosomes of different lengths. An arrow labelled chromosomes copy but the cell does not divide leads to twelve chromosomes arranged as six identical pairs, three blue pairs and three gold pairs, under a box labelled after doubling, every chromosome has an identical partner. A final arrow leads to text saying homologous pairs form at prophase I and the plant is a fertile polyploid species. A box below gives the example of cord-grass: two species with 60 and 62 chromosomes gave a sterile hybrid with about 61, which doubled to a fertile species with about 122. — How doubling the chromosomes can turn a sterile hybrid into a fertile new species. Original BioWelsh diagram.

Before doubling, each chromosome has no partner, so the is sterile.
After doubling, every chromosome has an identical partner, so form in prophase I and meiosis works.
The new plant is but cannot breed successfully with either parent, so it is a new , formed in one step.
Cord-grass on the south coast of Britain formed in this way in the late 1800s.

Bread wheat is a , with six sets of seven chromosomes (6n = 42). It arose from two rounds of hybridisation between wheat and wild grasses, each followed by chromosome doubling. , in which the first are but their offspring are weak or sterile, is known in cotton and rice.

Lesson 16 of 19

What is Darwin's theory of evolution by natural selection?

Charles Darwin sailed round the world on HMS Beagle from 1831 to 1836. On the Galápagos Islands, off Ecuador, he saw that animals such as finches and giant tortoises differed from island to island. In 1859 he published On the of , proposing that existing have arisen from ancestral by . Alfred Russel Wallace reached the same idea independently.

Darwin's observations and what he deduced from them
Observations	Deductions
Organisms produce far more offspring than survive	There is a struggle for existence: individuals compete
sizes stay roughly the same, because resources are limited	Only some offspring survive to breed
Individuals in a vary, and much of this is inherited	Those best suited to the environment survive and reproduce, passing on their characteristics
Environments change over time	Over many generations change, and new form:

The drawing below shows the heads of four Galápagos finches, published after the Beagle voyage.

Black and white engraving of four bird heads in side view, numbered 1 to 4. Head 1 has a very deep, heavy, cone-shaped beak. Head 2 has a similar but smaller beak. Head 3 has a small, short, stubby beak. Head 4 has a thin, straight, pointed beak. Printed names below read Geospiza magnirostris, Geospiza fortis, Geospiza parvula and Certhidea olivacea. — Heads of four Galápagos finches, from an engraving published after the voyage of HMS Beagle. Photo: John Gould, Public domain.

Finch 1 has a very deep, heavy beak for cracking large, hard seeds.
Finches 2 and 3 have smaller, shorter beaks for smaller seeds or buds.
Finch 4 has a thin, pointed beak for picking insects.
The finches are thought to descend from one ancestral that reached the islands. Different food on different islands selected different beaks: an example of .

give some of the strongest evidence. The photograph below shows a museum display of skulls and feet of the horse family from about 55 million years ago (bottom) to recent times (top).

Black and white photograph of a museum wall display. Six rows each hold a hind foot on the left, a skull in the middle and a forefoot on the right, with a label under each skull. The skulls and feet at the bottom are tiny, and each row upwards is larger, ending with two large horse skulls with long jaws at the top. A strip on the right is divided into geological epochs, labelled from Eocene at the bottom through Oligocene, Miocene and Pliocene to Pleistocene at the top. — Museum display of fossil skulls and feet of the horse family, oldest at the bottom. Photo: Thomas Hunt Morgan, Public domain.

The oldest horses at the bottom were about the size of a small dog, with small skulls.
Moving up through younger rocks, the skulls become larger with longer jaws and more teeth for grinding.
The feet change from several toes to a single hoof, as forests gave way to open grassland where running fast was an advantage.
The bones follow the same basic pattern throughout, which suggests a modified by .

Darwin did not know how characteristics were inherited. In the twentieth century, genetics explained it: comes from and sexual reproduction, and is a change in . DNA and protein sequences now give further evidence, because closely related have more similar sequences.

Lesson 17 of 19

How do you use Student's t test to compare two means?

When you measure a variable in two samples, the will almost always differ a little by chance. tells you whether the difference between the two is large enough to be , or small enough to be due to chance.

Use it to compare the of two sets of data, such as leaf length on two sides of a hedge.
The data should be roughly normally distributed.
Each sample needs enough values: 15 to 20 is sensible, but 4 is far too few.
Use instead for counts in categories compared with an expected ratio.

Start with a : there is no difference between the leaf length on the sunny and shaded sides. The test then works out the probability that a difference as large as the one you found could arise by chance if the were true.

Critical values of t
Degrees of freedom	p = 0.05	p = 0.01
10	2.23	3.17
18	2.10	2.88
20	2.09	2.85
28	2.05	2.76
38	2.02	2.71

With 38 the at p = 0.05 is 2.02. The calculated value, 9.03, is far greater, so the is rejected. There is a difference between the lengths of the leaves on the two sides: the probability that a difference this large is due to chance is less than 5%. It is even beyond 2.71, so the probability is below 1%.

Lesson 18 of 19

The specified practical work for this topic

WJEC lists one piece of specified practical work for this topic: an investigation of in a , including the use of . It is set out in full below, comparing the length of beech leaves on the sunny and shaded sides of one hedge, with other ways of carrying it out in the teaching notes.

Specified practical

Investigating continuous variation in beech leaves, with Student's t test

Unit 4, section 4, specified practical 1 (BioWelsh entry number)

Aim To find out whether there is a difference between the length of leaves on the sunny side and the shaded side of one beech hedge, using .

Five rounded panels joined by arrows. The first shows a tape measure, captioned mark sampling points at random on each side. The second shows a green leaf, captioned pick 20 leaves from the sunny side and 20 from the shaded side. The third shows a pencil and ruler, captioned measure each leaf blade to the nearest millimetre. The fourth shows a clipboard with a table, captioned record results in a table. The fifth shows a calculator, captioned find each mean and standard deviation, then t. — Continuous variation in beech leaves and the t test, at a glance. Original BioWelsh diagram.

Equipment

Tape measure
Tape measure10 mMarks out the sampling squares at the same heights on both sides.
Beech hedge with a sunny and a shaded side
Beech hedge with a sunny and a shaded side1All the leaves come from plants in one hedge, so genetic differences are small.
Ruler
Ruler30 cm, mm scaleMeasures each leaf blade to the nearest millimetre.
Clipboard and results table
Clipboard and results tableRecords each length as it is measured.
Calculator or spreadsheet
Calculator or spreadsheetWorks out , standard deviations and t.

Safety

Roadside RiskBeing hit by traffic.ControlOnly sample away from roads, or on the field side, with a teacher present.
Thorns and nettles in the hedge baseRiskScratches and stings.ControlWear gloves and long sleeves, and do not reach deep into the hedge.
Plant sap and soilRiskMinor skin irritation.ControlWash hands after the practical.

Variables

Independent variableSide of the hedge

Sunny south-facing side or shaded north-facing side.

The two sides differ mainly in the light the leaves receive.

Dependent variableLength of the leaf blade

Measured with a ruler to the nearest mm, stalk excluded.

Length varies continuously, so a on the is suitable.

Example: 62 mm

Controlled variablesPlants sampled, Height above the ground, Age of the leaves, Time of year, Method of measuring, Sample size

Plants sampled

All leaves from the same hedge, planted at the same time.

Keeps genetic to a minimum, so any difference is likely to be caused by the environment.

Height above the ground

Sample the same of heights on both sides.

Leaves higher up may receive more light and wind.

Age of the leaves

Fully expanded leaves from this year's growth only.

Young leaves have not reached full size.

Time of year

Sample both sides on the same day in late summer.

Leaves grow through spring and early summer.

Method of measuring

Same person, same ruler, blade only.

Different people may start and stop the measurement in different places.

Sample size

20 leaves from each side.

Equal, reasonably large samples make the reliable.

Method, step by step

1)
Choose a beech hedge running east to west, so one face gets sun all day and the other is shaded.
Why?
Light is the environmental factor being compared, so the two sides must differ in light.
2)
Mark out a grid of numbered squares on each face, covering the same of heights.
Why?
Height above the ground also affects leaves, so both samples must come from the same heights.
3)
Use to choose a square on the sunny face and pick the leaf nearest its centre.
Why?
removes bias, such as choosing the biggest leaves.
Sampling leaves at random from the two faces of one beech hedge. Original BioWelsh diagram.
Open full size
4)
Repeat until you have 20 leaves from the sunny face, then do the same on the shaded face.
Why?
Twenty leaves in each sample gives enough values for a and reduces the effect of any unusual leaf.
5)
Only take fully expanded, undamaged leaves from the current year's growth.
Why?
Young or damaged leaves are shorter for reasons that have nothing to do with light.
6)
Measure the length of each leaf blade from the base to the tip, not including the stalk, to the nearest millimetre.
Why?
The stalk length varies separately, so including it would add extra .
7)
Calculate the and the of each sample.
Why?
The compares the , taking the spread of each sample into account.
8)
State the , calculate t and compare it with the for n₁ + n₂ − 2 at p = 0.05.
Why?
This decides whether the difference between the is or could be due to chance.

Results and analysis

Example data

Lengths of 20 beech leaves from each side of one hedge, in order of size, with summary statistics

Leaf	Sunny side length / mm	Shaded side length / mm
1	54	68
2	55	69
3	57	70
4	58	71
5	58	72
6	59	73
7	60	74
8	60	74
9	61	75
10	62	76
11	62	76
12	63	77
13	64	78
14	64	79
15	65	79
16	66	80
17	67	81
18	68	83
19	70	85
20	71	86
(x̄)	62.2	76.3
squared (s²)	22.48	26.33
(s)	4.74	5.13

Bar chart of number of leaves from 0 to 8 against leaf length in 4 millimetre classes from 54 to 57 up to 86 to 89. Gold bars for the sunny side, mean 62.2 millimetres, read 3, 6, 6, 3 and 2 in the classes from 54 to 57 to 70 to 73. Green bars for the shaded side, mean 76.3 millimetres, read 2, 4, 6, 5, 2 and 1 in the classes from 66 to 69 to 86 to 89. The two sets overlap only in the 66 to 69 and 70 to 73 classes. — Length of beech leaves from the sunny and shaded sides of one hedge, using illustrative values. Original BioWelsh graph.

What the results show

The is that there is no difference between the length of leaves on the sunny and shaded sides of the hedge.

The length on the shaded side, 76.3 mm, is 14.1 mm greater than on the sunny side, 62.2 mm. The shows the two distributions barely overlap, and the standard deviations, 4.74 mm and 5.13 mm, are small compared with the difference.

The calculated value of t is 9.03. With 38 the at p = 0.05 is 2.02. Because 9.03 is greater than 2.02, the is rejected: there is a difference between the , and the probability that it is due to chance is less than 5%.

The leaves all came from one hedge, so genetic differences are small. The difference is therefore most likely caused by the environment: leaves in shade grow larger, which gives a bigger area for absorbing the dim light.

Worked calculations

, sunny side = 1244 ÷ 20 = 62.2 mm; shaded side = 1526 ÷ 20 = 76.3 mm

s² = Σ(x − x̄)² ÷ (n − 1): sunny 427.2 ÷ 19 = 22.48; shaded 500.2 ÷ 19 = 26.33

t = (76.3 − 62.2) ÷ √(26.33 ÷ 20 + 22.48 ÷ 20) = 14.1 ÷ 1.562 = 9.03

= 20 + 20 − 2 = 38; at p = 0.05 = 2.02

Evaluation

Leaves from one hedge may still differ genetically if the plants were grown from seed.

Effect on resultsSome of the difference could be genetic rather than environmental.

ImprovementUse a hedge grown from cuttings of one plant, or compare the two sides of a single tree.

Only one hedge was sampled.

Effect on resultsThe result may not apply to beech in general.

ImprovementRepeat on several and compare the results.

Light was assumed, not measured.

Effect on resultsThe difference might be caused by another factor that differs between the sides, such as temperature or wind.

ImprovementMeasure light intensity on each side with a light meter at each sampling point.

Deciding exactly where the blade meets the stalk is subjective.

Effect on resultsSmall measuring errors are added to every length.

ImprovementAgree a clear rule and have one person measure every leaf.

Going further

Why the t test?

Leaf length is data, and the question is whether two differ. The is designed to compare the of two samples of , roughly normally distributed data.

The is for counts in categories compared with expected numbers, so it would not suit these measurements.

Other ways to carry out this practical

Shell height of limpets or dog whelks on a sheltered shore and an exposed shore.
Length or width of ivy or holly leaves in sun and in shade.
Length of pine needles, or of bramble leaflets, from two habitats.
Hand span or height of two groups of students, where the data are collected and processed with a spreadsheet.

Genetic or environmental?

Sampling one hedge, or seedlings from one parent, keeps genetic small, so a difference points to the environment.
Comparing two separate , such as limpets on two shores, cannot separate genetic from environmental causes: selection may also be acting.

Model answer: describe the method

Describe how you would investigate whether leaf length differs between the sunny and shaded sides of a beech hedge, and how you would analyse the results.

Original practice response · 6 indicative marks

Mark out numbered squares at the same heights on each side of the hedge and choose squares using .
Collect 20 fully expanded leaves from each side, all from the same hedge so genetic is kept to a minimum.
Measure each leaf blade from base to tip, without the stalk, to the nearest mm.
Calculate the and for each side.
State the : there is no difference between the leaf lengths on the two sides.
Calculate t, use n₁ + n₂ − 2 = 38 and compare t with the at p = 0.05; if t is greater, reject the .

What the answer needs to cover

of leaves from each side
Large enough sample, for example 20 per side
Leaves from one hedge or genetically similar plants
Measurement of blade length to the nearest mm
of no difference between the
t compared with the for 38 at p = 0.05

Exam practice

Give two reasons why the t test is suitable for these results.Show answer

The data are measurements that are roughly normally distributed, and the investigation compares the of two samples.

, normally distributed data
Compares two

Write a suitable null hypothesis for this investigation.Show answer

There is no difference between the length of beech leaves on the sunny side and on the shaded side of the hedge.

No difference
Between the

A student measured only 4 leaves from each side. Explain why a t test should not be used.Show answer

Four values per sample are too few to show whether the data are normally distributed or to give a reliable , so a larger sample, such as 15 to 20, is needed.

Sample too small
Need a larger number of samples

Explain why all the leaves were taken from one hedge.Show answer

To keep genetic to a minimum, so that any difference in leaf length is likely to be caused by the environment, here the amount of light.

Minimise genetic
Difference due to environment

Related learning

Inheritance
, , , and the are the basis for genetics.
Cell division
Meiosis explains , , and why with unpaired chromosomes are sterile.
Populations & ecosystems
Competition, predation and size link ecology to .
Classification & biodiversity
Classification and the concept build on from common ancestors.

Specification & sources

https://www.wjec.co.uk/media/mxbh1zaf/wjec-gce-biology-spec-from-2015-e-101025.pdf
https://openstax.org/books/biology-2e/pages/12-2-characteristics-and-traits
https://openstax.org/books/biology-2e/pages/18-1-understanding-evolution
https://openstax.org/books/biology-2e/pages/18-2-formation-of-new-species
https://openstax.org/books/biology-2e/pages/19-1-population-evolution
https://openstax.org/books/biology-2e/pages/19-2-population-genetics
https://openstax.org/books/biology-2e/pages/19-3-adaptive-evolution
https://openstax.org/books/introductory-statistics-2e/pages/2-7-measures-of-the-spread-of-the-data
WJEC GCE A Level Biology Unit 4 mark schemes, Summer 2017 (Q1, Q4), 2018 (Q2), 2019 (Q2, Q5), 2022 (Q3, Q4, Q6), 2023 (Q4, Q5), 2024 (Q4) and 2025 (Q1, Q2, Q3), with examiner reports: depth, exam wording and rejected answers
WJEC GCE A Level Biology Unit 5 mark schemes, Summer 2017 (variant 1, Q1), 2019 (variant 2, Q1) and 2023 (variant 2, Q1): the t test, null hypotheses, sample size and controlled variables
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u4.variation-and-evolution__photo-finches.jpg · John Gould · Public domain · https://commons.wikimedia.org/wiki/File:Darwin's_finches_by_Gould.jpg
u4.variation-and-evolution__photo-horse-fossils.jpg · Thomas Hunt Morgan · Public domain · https://commons.wikimedia.org/wiki/File:Critique_of_the_Theory_of_Evolution_Fig_001.jpg
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