IMAT Biology · Formula sheet
Evolution and Ecology formulas
2 formulas, 9 reference tables and 14 common traps for IMAT Biology Evolution and Ecology, grouped by subtopic.
Natural Selection, Drift and Hardy-Weinberg
Learn this subtopic in the notesThe Hardy-Weinberg principle for allele and genotype frequencies
Hardy-Weinberg equations
- frequency of the dominant allele
- frequency of the recessive allele
- frequency of homozygous recessive individuals (the visible recessive phenotype)
- frequency of heterozygotes (carriers)
Darwin's natural selection, and why Lamarck's idea fails
| 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 frequencies
| 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 selection
| 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 |
Common traps
Bacteria become resistant, not the patient
Antibiotic resistance is a property of the bacteria, selected when the drug kills their sensitive neighbours. People do not become resistant or immune to an antibiotic. Overuse and not finishing a course both help resistant bacteria take over.
Selection works in stable environments and on asexual organisms
In a stable environment, selection keeps removing the extremes (stabilising selection). Asexual organisms such as bacteria vary through mutation, so they are selected too. Any population with heritable variation is subject to natural selection.
Artificial selection is not a random process
Mutation and genetic drift are random. Artificial selection is the opposite: a breeder deliberately chooses which individuals reproduce. Natural selection is not random either, although the variation it acts on arises at random.
Only germ-line mutations matter to evolution
For a mutation to change the next generation it must be in a cell that makes gametes. A mutation in a body cell, dominant or recessive, however it was caused, is not passed on in animals.
Directional selection can favour a rare or a recessive phenotype
At the start, the favoured phenotype is often rare (dark moths were once uncommon). The allele behind it may be recessive, and its frequency still rises. Directional selection is defined by which end of the range is favoured, not by how common or how dominant it is.
The recessive phenotype gives q squared, not q
Individuals showing a recessive trait are aa, so their share is . Take the square root to get . Using the phenotype percentage directly as the allele frequency is the commonest mistake, and IMAT includes that number among the options.
Speciation, Evidence for Evolution and Classification
Learn this subtopic in the notesSpecies and speciation: isolation barriers, allopatric and sympatric
| 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 molecules
| 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 names
| 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 |
Common traps
Producing offspring is not enough to be one species
The test is fertile offspring in nature. Horses and donkeys produce mules, but mules are sterile, so horses and donkeys are separate species. A barrier that acts after the zygote forms is still reproductive isolation.
Similar function does not mean close relatives
Structures that do the same job but are built differently (analogous) evolved separately under similar conditions. Shared structure, not shared function, is what points to a common ancestor.
Archaea are not a kind of bacteria
Archaea look like bacteria under a microscope, but they form their own domain. They lack peptidoglycan and in several molecular features (such as how they copy and read DNA) they are closer to eukaryotes than to bacteria.
Ecosystems, Energy Flow, Nutrient Cycles and Interactions
Learn this subtopic in the notesEnergy flow between trophic levels and the 10% rule
Energy passed up a food chain
- energy at trophic level k (producers are level 1)
- energy passed to the next level
Levels of ecological organisation and trophic levels
| 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 cycle
| Process | Change | Done by |
|---|---|---|
| Nitrogen fixation | or | 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 | Decomposers (bacteria and fungi) |
| Nitrification | Nitrosomonas (first step), Nitrobacter (second step) | |
| Assimilation | and taken up into amino acids | Plant roots, then animals by eating plants |
| Denitrification | back to the air | Anaerobic bacteria such as Pseudomonas |
Interactions between species and population growth
| 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 |
Common traps
An ecosystem includes the non-living part
A community is only the living organisms. An ecosystem is the community together with its abiotic environment. Options that use the two words as if they meant the same are wrong.
Energy flows, matter cycles
Carbon and nitrogen atoms go round and round between organisms and the environment. Energy does not: it enters as light and leaves as heat. An option saying decomposers return energy to the producers is wrong; they return minerals.
Most energy is lost as heat from respiration
The roughly 90% that does not reach the next level is mainly used in respiration and lost as heat, with the rest in uneaten parts and faeces. It is not stored in the predator's body.
Nitrification is not nitrogen fixation
Fixation turns nitrogen gas into ammonia. Nitrification turns ammonium into nitrite and then nitrate. They are done by different bacteria, and only fixation brings new nitrogen in from the air.
A change that harms one competitor helps the other
If two species share a resource, anything that slows one of them (a temperature or pH it tolerates badly, or a chemical it is sensitive to) leaves more of the resource for the other, which then grows faster. The change does not need to act on the winner directly.