Variation and evolution: WJEC A level Biology cram notes

Lesson 1 of 19

Causes of variation

Fast revision of topic 4.4, and , in WJEC A level Biology Unit 4. It follows the WJEC specification.

is the differences between individuals of the same . It comes from , from the environment, and usually from both.

Genetic sources: (the only source of new ), in prophase I, at metaphase I and II, random fertilisation, random mating.
Environmental sources: diet, light, temperature, water, minerals, disease, exercise.
is caused by and can be passed on; is caused only by the environment.

Lesson 2 of 19

Continuous and discontinuous variation

: a few distinct categories with no intermediates, usually one , little environmental effect, shown as a bar chart. : a complete of values, , strongly affected by the environment, shown as a .

The photographs below show the speckled (left) and melanic (right) forms of the peppered moth.

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.

Every moth is one form or the other, with no intermediates: .
One controls it; the melanic is .

The below shows adult height.

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.

A complete of values with a single peak: , close to a .
Height is and affected by diet.

Lesson 3 of 19

Averages and spread

: total ÷ number of values. : the middle value. : the most common value or class.
: largest minus smallest.
: spread around the , s = √[Σ(x − x̄)² ÷ (n − 1)].
In a , = = , and about 95% of values lie within two standard deviations of the .

The graph below shows two samples with the same .

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.

Narrow, tall curve: small .
Wide, low curve: large .

Lesson 4 of 19

Competition

More offspring are produced than can survive, and resources are limited, so individuals compete. is between members of the same ; is between different .

Competition affects survival and breeding success: plants compete for light, water, minerals and space; animals for food, water, , nest sites and mates. Winners pass on their .

Lesson 5 of 19

Selective agencies

A is an environmental factor that makes some more likely to survive and reproduce: food supply, breeding sites, climate, predation, disease and human impact such as pollution, and poaching.

The chart below shows peppered moth forms in two woodlands.

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.

Melanic moths dominate on soot-darkened bark, speckled moths on pale bark.
Birds eat the moths that stand out: predation is the .

Lesson 6 of 19

Natural selection

1) gives .
2) A acts.
3) Individuals with an advantageous have a and survive.
4) They reproduce and pass on the .
5) The increases over many generations.

The diagram below shows in the peppered moth.

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 dark existed before the bark darkened.
Pale moths are eaten; dark moths survive, breed and pass on the dark .

Lesson 7 of 19

Types of selection

Three types of selection
Type	What is selected against	Effect on the curve	Example
Stabilising	Both extremes	Same , narrower	Human birth mass
Directional	One extreme	shifts	, copper tolerance
Disruptive	The middle	Two peaks	Large and small male coho salmon

The graph below 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.

One extreme is favoured, so the whole curve and its shift.
It happens when the environment changes.

Lesson 8 of 19

Sickle cell and malaria

Where is common, HbᴬHbˢ are protected from severe , HbᴬHbᴬ people are more likely to die of , and HbˢHbˢ people have . have a , so both stay common. Without , the has no advantage and Hbˢ falls.

Lesson 9 of 19

Gene pool and genetic drift

The is all the of all the in a at one time. is the proportion of one . is a change in .

is a random change in , strongest in small , where can be lost or . The : a few founders carry an unusual sample of into a new . A : a crashes and loses .

The diagram below shows the .

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 founders carry a chance sample of the mainland .
The island inherits their unusual frequencies, and no keeps it different.

Lesson 10 of 19

The Hardy-Weinberg equation

p + q = 1, where p is the of the and q of the .
p² + 2pq + q² = 1: AA = p², Aa = 2pq, aa = q².
Start from the : q², then q, then p, then the asked for.

Lesson 11 of 19

Hardy-Weinberg conditions

Large (no ).
Random mating.
No .
No migration: no or .
No selection.

If observed frequencies differ from those predicted, selection, , or migration is changing the .

Lesson 12 of 19

Species and isolation

A is a group of similar organisms that can interbreed to produce offspring. needs , which stops between .

: geographical, seasonal, behavioural, morphological, gametic.
: , , .
: different places. : the same place.

Lesson 13 of 19

Allopatric speciation

The diagram below shows .

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.

A geographical barrier splits a and stops .
Different , and change each .
Eventually the groups cannot interbreed to produce offspring: two .

Example: polar bears evolved from brown bears isolated by ice in the far north.

Lesson 14 of 19

Sympatric speciation

The share an area but stop interbreeding because of behaviour (grasshopper songs), breeding season (copper-tolerant grass flowering earlier), body or flower shape (snail coiling, a long orchid spur) or gametes that cannot fuse. makes a new in one step.

Lesson 15 of 19

Hybrid sterility and polyploidy

The diagram below shows why a mule is sterile.

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.

Horse 64 and donkey 62 chromosomes give a mule 63.
The chromosomes are not , cannot pair in prophase I, and no gametes form.

The diagram below shows how doubling can make a .

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.

After doubling, each chromosome has a partner, so meiosis works.
The is a new , for example cord-grass and bread wheat (6n = 42).

Lesson 16 of 19

Darwin's theory

Observed: overproduction, stable sizes, limited resources, inherited .
Deduced: a struggle for existence; the best suited survive and reproduce; characteristics pass on; over many generations change.
Existing arose from ancestral by : .

The drawing below shows four Galápagos finches.

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.

Beak shapes suit different foods: large seeds, small seeds, insects.
They are thought to share a : .

Lesson 17 of 19

The t test

Compares the of two sets of , roughly normally distributed data, with enough values in each.
: no difference between the .
t = (x̄₁ − x̄₂) ÷ √(s₁²/n₁ + s₂²/n₂); = n₁ + n₂ − 2.
If t is greater than the at p = 0.05, reject the : the difference is , less than 5% probability it is due to chance.

Lesson 18 of 19

The specified practical

in beech leaves: measure 20 leaves from the sunny side and 20 from the shaded side of one hedge and compare the with a .

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 compare the leaf length on the sunny and shaded sides of one beech hedge with a .

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 measureMarks sampling squares.
Beech hedge
Beech hedgeOne hedge keeps genetic small.
Ruler
RulerMeasures to the nearest mm.
Calculator
Calculator, s² and t.

Safety

Traffic, thorns and nettlesRiskInjury, scratches, stings.ControlSample away from roads, wear gloves.

Variables

Independent variableSide of the hedge

Sunny or shaded.

Dependent variableLeaf blade length

mm, stalk excluded.

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

Plants sampled

One hedge.

Height above the ground

Same .

Age of the leaves

Fully expanded.

Time of year

Same day.

Method of measuring

Blade only.

Sample size

20 per side.

Method, step by step

1)
Choose 20 leaves at random from each side, at the same heights.
Why?
avoids bias.
Sampling leaves at random from the two faces of one beech hedge. Original BioWelsh diagram.
Open full size
2)
Measure each blade, without the stalk, to the nearest mm.
3)
Calculate each and s².
4)
Calculate t and compare it with 2.02 (38 , p = 0.05).

Results and analysis

Example data

Summary of beech leaf lengths

Statistic	Sunny side	Shaded side
Number of leaves (n)	20	20
length / mm	62.2	76.3
s²	22.48	26.33

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

t = 9.03 with 38 ; the at p = 0.05 is 2.02.

9.03 > 2.02, so reject the : shaded leaves are significantly longer, most likely because of the environment.

Worked calculations

t = 14.1 ÷ √(1.317 + 1.124) = 9.03

df = 20 + 20 − 2 = 38

Evaluation

Light was not measured.

Effect on resultsAnother factor might differ between the sides.

ImprovementMeasure light intensity with a light meter.

Only one hedge.

Effect on resultsMay not apply to beech in general.

ImprovementSample several .

Going further

Other ways

Limpet or dog whelk shells on sheltered and exposed shores.
Ivy or holly leaves in sun and shade.
Student hand span using a spreadsheet.

Model answer: describe the method

Describe how you would compare leaf length on the two sides of a hedge.

Original practice response · 6 indicative marks

at the same heights on each side.
20 leaves per side from one hedge.
Measure blade length to the nearest mm.
Calculate and standard deviations.
: no difference between the .
t compared with the for 38 at p = 0.05.

What the answer needs to cover

20 per side
One hedge
Measure to nearest mm
t, 38 df, p = 0.05

Exam practice

Why use a t test?Show answer

It compares the of two sets of , normally distributed data.

data
Compares two

Lesson 19 of 19

Summary

Only makes new .
: , . : , bar chart.
: , , survive, reproduce, pass on , rises.
: chance, small , .
: q² first; five conditions.
: isolation, no , diverge, no offspring.
: difference between two .