IMAT Biology · Free notes

Evolution and Ecology: Selection, Species and Ecosystems

Evolution and Ecology has 9 past questions since 2011, and the ministry papers from 2023 on have asked 1 of them. Most of them are about how natural selection works, usually as a set of statements where you must pick every one that is true. The difficulty is precision, not recall: the wrong options sound reasonable but break one rule, such as an acquired trait being inherited, a body-cell mutation being passed on, or selection needing a changing environment. Ecology is rarely asked, so learn its core ideas well and spend most of your time on the mechanisms of evolution.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

IMAT home All formulas on one page

Subtopic notes

Formula & revision sheet

2 formulas · 9 reference tables · 14 gotchas across all subtopics — the exam-eve cheat-sheet

Natural Selection, Drift and Hardy-Weinberg

Formulas (1)

The Hardy-Weinberg principle for allele and genotype frequencies · Hardy-Weinberg equations $p + q = 1 \qquad p^2 + 2pq + q^2 = 1$

Reference tables (3)

Darwin's natural selection, and why Lamarck's idea fails5 rows

Step	What happens	Bacteria and an antibiotic
Variation	Individuals differ; much of it is genetic	A few cells already carry a resistance allele (from a random mutation)
Overproduction	More offspring are made than can survive	Bacteria multiply very fast
Selection	Some variants survive and reproduce better	The antibiotic kills sensitive cells; resistant cells survive
Inheritance	Survivors pass their alleles on	Resistant cells divide and their offspring are resistant
Change over generations	Favoured alleles become more frequent	The infection becomes resistant; overuse and unfinished courses speed this up

Sources of variation and the forces that change allele frequencies5 rows

Force	Random?	Main effect
Mutation	Yes: not directed by need	Adds new alleles
Genetic drift	Yes: chance decides who breeds	Can fix or lose alleles, even useful ones, in small populations
Gene flow	Not driven by fitness; depends on who migrates	Brings in alleles; makes populations more similar
Natural selection	No: better-suited variants reproduce more	Adapts the population to its environment
Artificial selection	No: humans pick the parents	Exaggerates traits humans want

Directional, stabilising and disruptive selection4 rows

Type	Favoured	Effect on the curve	Example
Directional	One extreme	Mean shifts that way	Dark peppered moths on sooty trees in industrial England
Stabilising	The middle	Narrower, same mean	Human birth weight: very light and very heavy babies fared worst
Disruptive	Both extremes	Splits into two peaks	Finches with large or small beaks where only large or small seeds exist
Sexual	Traits that attract mates	Showy traits spread	The peacock's tail

Watch out for (6)

Bacteria become resistant, not the patient
→ Darwin's natural selection, and why Lamarck's idea fails
Selection works in stable environments and on asexual organisms
→ Darwin's natural selection, and why Lamarck's idea fails
Artificial selection is not a random process
→ Sources of variation and the forces that change allele frequencies
Only germ-line mutations matter to evolution
→ Sources of variation and the forces that change allele frequencies
Directional selection can favour a rare or a recessive phenotype
→ Directional, stabilising and disruptive selection
The recessive phenotype gives q squared, not q
→ The Hardy-Weinberg principle for allele and genotype frequencies

Speciation, Evidence for Evolution and Classification

Reference tables (3)

Species and speciation: isolation barriers, allopatric and sympatric6 rows

Barrier	Before or after the zygote	Example
Habitat	Before (prezygotic)	Two species live in the same area but in different places, such as water and land
Temporal	Before (prezygotic)	Breeding seasons or flowering times differ
Behavioural	Before (prezygotic)	Different courtship songs or displays, so mates are not recognised
Mechanical or gametic	Before (prezygotic)	Reproductive parts do not fit, or sperm cannot fertilise the egg
Hybrid inviability	After (postzygotic)	The hybrid embryo dies early
Hybrid sterility	After (postzygotic)	The mule, from a horse and a donkey, is sterile

Evidence for evolution: fossils, anatomy, embryos and molecules5 rows

Evidence	What it shows	Example
Homologous structures	Common ancestry	Pentadactyl limb: human arm, bat wing, whale flipper
Analogous structures	Similar pressures, separate origins	Wings of birds and of insects
Vestigial structures	Ancestors used the organ	Human coccyx, hip bones inside whales
Comparative embryology	Shared developmental plan	Vertebrate embryos all have pharyngeal pouches and a tail
DNA and protein sequences	Degree of relationship	Human and chimpanzee DNA differ by only a few per cent

Classification: the three domains, the ranks and binomial names3 rows

Domain	Cell type	Key features	Examples
Bacteria	Prokaryote	No nucleus; cell wall of peptidoglycan; 70S ribosomes	E. coli, Streptococcus, cyanobacteria
Archaea	Prokaryote	No nucleus; no peptidoglycan; unusual membrane lipids; many live in extreme places	Methanogens, salt-loving and heat-loving archaea
Eukarya	Eukaryote	Nucleus and membrane-bound organelles; 80S ribosomes in the cytoplasm	Protists, fungi, plants, animals

Watch out for (3)

Producing offspring is not enough to be one species
→ Species and speciation: isolation barriers, allopatric and sympatric
Similar function does not mean close relatives
→ Evidence for evolution: fossils, anatomy, embryos and molecules
Archaea are not a kind of bacteria
→ Classification: the three domains, the ranks and binomial names

Ecosystems, Energy Flow, Nutrient Cycles and Interactions

Formulas (1)

Energy flow between trophic levels and the 10% rule · Energy passed up a food chain $E_{k+1} \approx 0.1 \times E_{k} \qquad \text{efficiency} = \frac{E_{k+1}}{E_k} \times 100\%$

Reference tables (3)

Levels of ecological organisation and trophic levels5 rows

Term	Meaning	Example
Population	All individuals of one species in one area	The perch in a lake
Community	All populations of all species in one area	Every plant, animal and microbe in the lake
Ecosystem	A community plus its abiotic environment	The lake with its water, mud, light and temperature
Habitat	The place where an organism lives	Shallow water among reeds
Niche	The role of a species: what it eats, when, where, and how it uses resources	A perch hunting small fish by day near the surface

The carbon cycle and the nitrogen cycle5 rows

Process	Change	Done by
Nitrogen fixation	$\mathrm{N_2} \rightarrow \mathrm{NH_3}$ or $\mathrm{NH_4^+}$	Rhizobium in legume root nodules, free-living Azotobacter, cyanobacteria; also lightning and the industrial Haber process
Ammonification	Proteins and urea in dead matter and waste $\rightarrow \mathrm{NH_4^+}$	Decomposers (bacteria and fungi)
Nitrification	$\mathrm{NH_4^+} \rightarrow \mathrm{NO_2^-} \rightarrow \mathrm{NO_3^-}$	Nitrosomonas (first step), Nitrobacter (second step)
Assimilation	$\mathrm{NO_3^-}$ and $\mathrm{NH_4^+}$ taken up into amino acids	Plant roots, then animals by eating plants
Denitrification	$\mathrm{NO_3^-} \rightarrow \mathrm{N_2}$ back to the air	Anaerobic bacteria such as Pseudomonas

Interactions between species and population growth5 rows

Interaction	Effect on each species	Example
Competition	Both harmed	Two plant species shading each other for light
Predation	Predator gains, prey harmed	Lynx and snowshoe hare
Parasitism	Parasite gains, host harmed	Tapeworm in a human gut
Mutualism	Both gain	Rhizobium in legume roots; the fungus and alga of a lichen
Commensalism	One gains, the other unaffected	Barnacles riding on a whale

Watch out for (5)

An ecosystem includes the non-living part
→ Levels of ecological organisation and trophic levels
Energy flows, matter cycles
→ Energy flow between trophic levels and the 10% rule
Most energy is lost as heat from respiration
→ Energy flow between trophic levels and the 10% rule
Nitrification is not nitrogen fixation
→ The carbon cycle and the nitrogen cycle
A change that harms one competitor helps the other
→ Interactions between species and population growth

PYQ weightage by concept

11 concepts · 0 PYQs — where the marks actually sit, so you know what to drill first

Natural Selection, Drift and Hardy-Weinberg0 PYQs · 0%

Concept	PYQs	Share
Darwin's natural selection, and why Lamarck's idea failsfoundation	—	—
Sources of variation and the forces that change allele frequenciesfoundation	—	—
Directional, stabilising and disruptive selectionfoundation	—	—
The Hardy-Weinberg principle for allele and genotype frequenciesfoundation	—	—

Speciation, Evidence for Evolution and Classification0 PYQs · 0%

Concept	PYQs	Share
Species and speciation: isolation barriers, allopatric and sympatricfoundation	—	—
Evidence for evolution: fossils, anatomy, embryos and moleculesfoundation	—	—
Classification: the three domains, the ranks and binomial namesfoundation	—	—

Ecosystems, Energy Flow, Nutrient Cycles and Interactions0 PYQs · 0%

Concept	PYQs	Share
Levels of ecological organisation and trophic levelsfoundation	—	—
Energy flow between trophic levels and the 10% rulefoundation	—	—
The carbon cycle and the nitrogen cyclefoundation	—	—
Interactions between species and population growthfoundation	—	—