No two organisms are alike, yet we name and sort them into species. Here is how: traits, breeding, chromosomes and whole genomes, and the keys and DNA barcodes that identify what lives in a habitat.
Twelve chapters, the last four for higher level. Read one, play its figure through or tap its parts, and make each prediction before you open the answer.
Look along a row of beetles, or round a classroom. No two individuals are identical in every trait. Even identical twins differ, in their fingerprints, their scars and their tans. This variation is a defining feature of life, and it has patterns. Eight cards are enough to find them.
Shuffle the cards in the figure, then sort them by colour and by body shape. Colour pairs a round beetle with a long one every time. Body shape gives two tidy groups: every beetle in one is round and every beetle in the other is long, although inside each group the colours, sizes and spots all differ. The trait to sort by is the one that stays the same inside a group.
Patterns like these are the basis for naming and classifying organisms. In the 1700s the Swedish naturalist Carl Linnaeus sorted living things by the traits they share. To him a species was a group of organisms with the same body form and structure, its morphology. This is the morphological species concept. It is a working definition: a tool for grouping what biologists find, which can be improved or replaced.
Watch out Variation inside a species is not a flaw in the definition. If every member were identical, natural selection would have nothing to act on. A species is a group of similar organisms, never identical ones.
Example Some traits vary in steps, like human blood group (A, B, AB or O). Others vary smoothly, like height. Both kinds of pattern help to describe a species.
No two individuals are identical in every trait.
Patterns in variation are the basis for naming and classifying organisms.
Linnaeus grouped organisms by shared physical traits: the morphological species concept.
Identical twins share all their DNA. Are they identical in every trait?
No. Twins share their DNA, but traits are shaped by the environment too: a scar or a tan is picked up during life, and even fingerprints differ. Organisms are never alike in every trait, even when their genes match.
Sort the same eight cards three ways. Look at what is alike inside each group, and at what still differs.
A3.1.3 Binomial naming
A name with two parts
Common names are slippery. “Robin” means one bird in Britain and a different bird in North America, and many species have a dozen names between them. Scientists need one name that means the same thing in every language.
The answer, from Linnaeus, is the binomial system: every species gets a two-part name. The first part is the genus. The second part picks out one species within it. We are Homo sapiens: genus Homo, species sapiens.
Species in the same genus have similar traits. The lion (Panthera leo), the tiger (P. tigris) and the leopard (P. pardus) are all big cats in one genus, while the domestic cat (Felis catus) belongs to another. Even before you meet a name, its first word tells you what kind of organism to expect.
The rules are short. The genus has a capital letter and the species has none. The whole name is in italics when typed and underlined when handwritten. After the first full use, the genus may be shortened to its initial: H. sapiens.
Watch out The species name never stands alone: leo means nothing without its genus, because other genera have species with the same second word. And the capital belongs to the genus only: Homo sapiens, never Homo Sapiens.
Remember it The four rules of a binomial name
GreenGenus firstwith a capital letter
SlothsSpecies secondall in lower case
InchItalicsunderlined if handwritten
AlongAbbreviatethe genus to its initial after first use
Write it right Tap the correct way to write each name.
The human, typed in an essay
The lion
The common chimpanzee
The gut bacterium Escherichia coli, shortened after its first use
The fruit fly Drosophila melanogaster, shortened
Two of these animals share a genus. Which two?
Two animals are named Panthera tigris and Panthera onca. What can you say about them?
Two species in one genus. The genus is the same (Panthera) and the species names differ (tigris, the tiger, and onca, the jaguar), so they are separate species that share similar traits.
A binomial name has two words, written in italics.
The first word is the genus, with a capital letter. The second names the species, in lower case.
Species in one genus have similar traits: lion, tiger and leopard share the genus Panthera.
After the first full use you may shorten the genus to its initial: P. leo.
A3.1.4A3.1.5 The biological species concept
Can they breed?
Looks can mislead: males and females of one species may differ, and two species can look almost the same. Many biologists use a different test. The biological species concept says a species is a group of organisms that can breed and produce fertile offspring.
“Fertile” matters. A horse and a donkey can mate and have a healthy mule, but a mule cannot have young of its own, so horses and donkeys count as two species. The figure runs the test on imagined beetles.
No definition is the last word. Other species concepts group organisms by how alike their DNA is, or by the way of life they share. A species concept is a working definition, a tool that helps scientists group what they find, and it is replaced when a better tool comes along.
The breeding test has problems. Fossils cannot be tested. Populations that live far apart may never meet, so nobody knows whether they could breed. Some groups that are called separate species do have fertile young where they meet, as many oaks do. And a few species form a ring around a barrier, where each population breeds with its neighbour but the two ends of the chain do not.
One more problem comes from how species form. Speciation, the splitting of one species into two or more, usually happens gradually (you saw it in A4.1). Populations that stop interbreeding grow apart a little at a time. At first they could still breed if they met, and much later they could not. There is no single day when one species becomes two, so deciding whether two populations are one species or two can be arbitrary.
Watch out The test is whether they could produce fertile offspring, not whether they do. Two populations that never meet may be one species, and two that never meet may be two. Looking at their traits and DNA is often the best that can be done.
Biological species concept: organisms that can breed and produce fertile offspring.
Competing definitions exist, and any concept is a working definition.
Speciation is gradual, so the line between populations and species can be arbitrary.
Two kinds of snail live on islands 500 km apart, and none has ever crossed the sea. Can the biological species concept say whether they are one species?
Not directly. The test asks whether they could breed and have fertile young, and that has never been tried. Scientists use other evidence: how alike the snails’ bodies and DNA are, and how long ago the islands were separated. Never having bred is not the same as being unable to.
Two populations of one species stop interbreeding and slowly drift apart over a million years. On which day does one species become two?
On no single day. The populations grow apart a little each generation, so the point where they count as two species is a judgement, and experts can reasonably disagree. That is why the line between populations and species can be arbitrary.
A breeding test: two beetles from the same group, and a pair with one from each (imagined beetles).
Both pairs have young. Mating alone does not decide it.
Only the first pair’s young are fertile. The mixed pair’s young cannot have young of their own.
By the biological species concept, A and B are separate species.
Switch on gene flow and run it again: a few animals crossing each generation keeps them one species.
A3.1.6 Chromosome numbers
Every species has its own count
A chromosome is a molecule of DNA wrapped up with proteins. The body cells of each species of plant or animal carry a characteristic number of them, and the numbers vary widely. A human body cell has 46 chromosomes. A chimpanzee’s has 48. Those are the only two numbers the syllabus asks you to know.
The figure adds a few more, only to show the range: a fruit fly has 8, a garden pea 14 and a dog 78. A bigger number does not mean a bigger or more advanced organism.
Look at every number: each is even. A body cell is diploid, with two copies of each chromosome, one inherited from each parent. The two that match are homologous chromosomes, so chromosomes come in pairs: 23 pairs in a human, 24 in a chimpanzee. The sex cells are haploid, with one from each pair: 23 in a human egg or sperm.
Watch out Chromosome number is not the number of genes, and it is not the amount of DNA. A chromosome can be long or short and can carry many genes or few. A dog has more chromosomes than a human, which tells you nothing about which is the more complex.
Each species has a characteristic number of chromosomes: 46 in humans, 48 in chimpanzees.
Diploid cells have an even number, because chromosomes come in homologous pairs.
Why does a diploid cell always have an even number of chromosomes?
Each parent supplies one chromosome of every kind, so the chromosomes match up in pairs. Two sets always make an even number: 23 + 23 = 46 in a human, 24 + 24 = 48 in a chimpanzee.
A fruit fly has 8 chromosomes in its body cells: 4 pairs. A garden pea has 14: 7 pairs. Different species, different numbers.
A human has 46: 23 pairs.
A chimpanzee has 48: 24 pairs, one more pair than a human.
A dog has 78. More chromosomes does not mean a bigger or more complex animal. Every number is even, because chromosomes come in pairs.
A3.1.7 Karyotyping and karyograms
Laying out the chromosomes
How do biologists compare chromosomes? They make a karyotype: the full set of one cell’s chromosomes. Photographed and arranged in pairs, the picture is a karyogram. The cell is stopped part-way through dividing, when the chromosomes are short and thick, and stained so dark bands show.
To arrange them, match each chromosome with its partner using three clues: its length, its banding pattern and the position of its centromere, the pinched point where its two arms meet. Pairs are lined up from the longest. A human karyogram has 22 numbered pairs, roughly from longest to shortest, plus two sex chromosomes: XX in a female, XY in a male. It shows the number, the sex, and any extra or missing chromosome: three copies of chromosome 21 cause Down syndrome.
Now a puzzle. Humans have 46 chromosomes, but chimpanzees, gorillas and orangutans have 48. If we share an ancestor with them, where did a pair go? The hypothesis: human chromosome 2 was made when two ape chromosomes fused end to end in an ancestor we share with chimpanzees. (The syllabus calls the two ape chromosomes 12 and 13, an older way of numbering them.)
Look at them in the second tool below. The two ape chromosomes, placed end to end, match human chromosome 2 in length and in banding. Human chromosome 2 has DNA typical of a chromosome end (a telomere) in the middle, where a join would be. And it carries the remains of a second centromere, switched off. Every other human chromosome matches one ape chromosome.
Testing a prediction. A good hypothesis predicts something that could turn out false. If two chromosomes fused, chromosome ends should be found in the middle of the new one, and that can be checked. A claim such as “it fused so that humans could think” predicts nothing anyone can check. It is not testable, however interesting. Try sorting a few statements.
Testable or not? Decide whether evidence could ever show each statement wrong.
Chromosome 2 fused so that humans could become intelligent.
Human chromosome 2 contains the remains of a second, switched-off centromere.
The best human chromosome is chromosome 1.
If two ape chromosomes fused end to end, telomere DNA should be found in the middle of the new one.
Human chromosome 2 has the same banding as two ape chromosomes placed end to end.
Watch out The evidence supports the hypothesis from several directions, but nobody watched the fusion. And it does not mean humans came from chimpanzees: both lines come from a shared ancestor, which was neither.
A karyogram arranges chromosomes in pairs by length, banding pattern and centromere position.
Human chromosome 2 matches two ape chromosomes end to end, has telomere DNA in its middle and a remnant second centromere.
A testable hypothesis makes a prediction that evidence could contradict.
If two chromosomes had fused end to end, what would you expect in the middle of the new, longer chromosome?
Telomere DNA. Telomeres cap the ends of chromosomes, so two chromosomes joined end to end would leave two caps in the middle. Human chromosome 2 has such a stretch exactly where the join would be.
A cell is stopped part-way through dividing, when its chromosomes are short and thick.
The chromosomes are stained, so dark bands show, and photographed.
Each is cut out and matched with its partner by length, banding pattern and centromere position.
The pairs are lined up with the centromeres in a row and numbered from the longest: a karyogram. (Simplified: six pairs.)
Chromosomes from one cell, condensed and doubled. Pair them by length, centromere position and bands.
A pair matches on all three. A karyogram puts the pairs in order, longest first.
Human chromosome 2 beside two chimpanzee chromosomes, 2A and 2B. Do they fit together?
Drag each chromosome next to its partner, using length, bands and centromere position. The tool shows what matched.
Chromosomes from one cell, condensed and doubled. Pair them by length, centromere position and bands.
A pair matches on all three. A karyogram puts the pairs in order, longest first.
Human chromosome 2 beside two chimpanzee chromosomes, 2A and 2B. Do they fit together?
Human chromosome 2 beside two ape chromosomes. Look at the three pieces of evidence and judge how well each supports the hypothesis.
A3.1.8A3.1.9 Genomes within and between species
One genome, many versions
Your genome is all the genetic information you carry: your genes, the DNA between them, and the small loops of DNA in your mitochondria. Almost every cell in your body holds a full copy.
Two people share most of their genome: the same genes, in the same places. What differs is small and scattered. At about one place in a thousand, one base differs: a single nucleotide polymorphism, or SNP (said “snip”). Any two people share about 99.9% of their genome. Many SNPs lie in DNA that codes for nothing and have no known effect. Others alter a protein, or how much of it is made.
Between species the differences are far greater. Genomes differ in their overall size, which is the total amount of DNA, and in base sequence. Variation between species is much larger than variation within a species. That is why a stretch of DNA can show whose it is, and why a species can be recognised from its genome.
Watch out A SNP is a place in the genome where individuals differ, not a gene and not a disease. Having a SNP is normal: we all carry thousands, and many have no known effect.
A genome is all the genetic information of an organism.
Members of one species share most of their genome; SNPs give some diversity.
Variation between species is much larger than variation within a species.
Two unrelated people have their whole genomes read and compared. About how different are they?
About one base in a thousand. Humans share nearly all of their genome, and the small differences, mostly SNPs, are what make each genome unique. Between species the differences are far larger.
One short stretch of the genome in three people. Bases that differ from person 1 are pink: very few.
A difference at a single base is a single nucleotide polymorphism, or SNP. Together, SNPs make each person’s genome their own.
The same stretch in another species: far more differences. (Simplified: real differences are much rarer in both cases, but the pattern holds.)
A3.1.10 Comparing genome sizes
Big genome, small organism
A genome’s size is the total amount of DNA, counted in base pairs. A bacterium such as E. coli has a few million. A human has about three billion. You might expect more complex organisms to need more DNA, and the figure shows why that fails.
The Australian lungfish has a genome more than ten times as large as ours, and the flowering plant Paris japonica nearly fifty times. Neither is the most complex organism alive. Genome size is not a valid measure of how complex an organism is.
Why? A bigger genome does not always hold more genes. Much of the extra DNA does not code for a protein. It lies between genes, or is short sequences repeated many times. Some plants have also copied whole sets of chromosomes, which adds copies of the same genes and multiplies their DNA.
In the dry lab you will do what scientists do with a genome database: pick groups of organisms, find the genome sizes, and compare them with how complex the organisms are.
Watch out A bigger genome is not a more advanced one. When you see a big number, ask what the DNA is made of, and count genes if you can, not just base pairs.
Genome size varies enormously between groups of organisms.
Genome size is not a measure of complexity: much DNA is non-coding or repeated.
A student says: “The organism with the biggest genome must be the most complex.” Which fact shows this is wrong?
A fish and a flowering plant have genomes bigger than ours, and neither is more complex than a human. If size measured complexity, a lungfish would beat a mammal. The extra DNA is mostly non-coding or repeated.
A bacterium’s genome is millions of base pairs. A human’s is about three billion. (Mb is millions, Gb billions of base pairs.)
The Australian lungfish’s genome is more than ten times a human’s.
The flowering plant Paris japonica has one of the biggest genomes known. Size is not a measure of how complex an organism is.
The same four genes can sit in a small genome or a large one. The difference is mostly DNA that does not code for a protein.
A3.1.11 Whole genome sequencing
Reading a whole genome
Whole genome sequencing reads the order of every base in an organism’s genome. The first human genome took an international project more than ten years and billions of dollars. Machines have improved so much that a genome can now be read in days, for a small fraction of the cost.
Faster and cheaper sequencing matters because it brings genomes within reach of ordinary research. Today, comparing whole genomes of different species shows how closely related they are and which changes separated their lines. Genomes read from ancient remains have been compared with ours in this way.
In the future, a doctor may use a patient’s own genome to choose the medicine and dose most likely to work for them: personalised medicine. A genome can also reveal SNPs that raise the risk of a disease, so that it can be watched for or prevented.
Watch out A genome is a text, not an explanation. Reading every base does not tell you what every gene does, and a genome does not fix your future: many traits depend on the environment too. It also raises questions of privacy. Who should be allowed to see yours?
Whole genome sequencing is getting faster and cheaper.
Current use: research into evolutionary relationships. Future use: personalised medicine.
How has the cost of sequencing a human genome changed since the first one was done?
It has fallen steeply, and the time has fallen too, from years to days. That is why genome sequencing has moved from a few big projects to something used across research and medicine.
The first human genome was a huge international effort: years of work and billions of dollars.
Sequencing machines improved, and the cost and the time fell steeply. (The shape is drawn to show the trend, not exact values.)
Cheap, fast genomes bring uses within reach: comparing species now, and personalised medicine in the future.
A3.1.12 Asexual species and horizontal gene transfer
When the species rule breaks
The biological species concept needs breeding. But a great many organisms never breed with a partner. A bacterium reproduces by splitting in two, and each new cell is a copy of the first. There are no matings to watch and no offspring that are anything but clones, so the question “can they produce fertile offspring?” has no answer. Some plants, fungi and animals reproduce asexually too. Biologists group these organisms in other ways: by shape and structure, by how they get their energy and, increasingly, by how alike their DNA is.
Bacteria add a second problem. Genes normally pass down from parent to offspring. In horizontal gene transfer they pass sideways, between cells that are already alive. In conjugation two bacteria touch and a copy of a plasmid passes across. Cells can also take up DNA from their surroundings or receive it from a virus. A gene can even cross from one species to another, which is one reason antibiotic resistance spreads so fast.
A species concept that assumes genes stay inside one species’ gene pool breaks down when genes jump between species, so it does not work well for these organisms.
Watch out Horizontal gene transfer is not reproduction: no new cell is made. And asexual does not mean unclassifiable. Bacteria and other asexual organisms still fall into recognisable groups, just not groups defined by breeding.
Remember it Where the species concept struggles
FiveFossilsno breeding test is possible
ApesApartpopulations that never meet
HugHybridssome different species have fertile young
GrumpyGradualno single day when one species becomes two
AuntsAsexualno partners to breed with
HappilyHorizontal transfergenes jump between species
The biological species concept fits poorly where organisms reproduce asexually.
Horizontal gene transfer moves genes between species, so gene pools are not closed.
A bacterium carries a plasmid with an antibiotic-resistance gene. It touches a bacterium of a different species. What can happen?
A copy of the plasmid can pass across. This is horizontal gene transfer: the second bacterium now has the gene without being anyone’s offspring, and the gene has crossed a species line.
A bacterium reproduces by copying itself: two identical cells, with no partner and nothing to test.
Two bacteria of different species. One carries a plasmid, a small ring of DNA, with a gene on it.
The cells touch and a bridge forms. A copy of the plasmid passes across.
The second species now has the gene. No new cell was made: the gene moved sideways.
A3.1.13 Chromosome number as a shared trait
The number that has to match
Chromosome number is a trait that members of a species share, as their body form and their DNA are. It matters most when two species mate. Each parent gives a set of chromosomes to the offspring, and at meiosis every chromosome must find a partner before the sets can be shared out into gametes.
If the parents have the same number and similar chromosomes, they pair cleanly, like the matching pair in the figure. If the numbers differ, the hybrid has chromosomes with no equal partner. When these are shared out, some gametes receive part of the genome twice and part of it not at all. Gametes like that rarely work, so cross-breeding between closely related species is unlikely to produce fertile offspring when their chromosome numbers differ.
The mule shows it (you met it in A4.1). A horse gamete has 32 chromosomes and a donkey gamete 31, so a mule has 63, an odd number, and not every chromosome can have a partner. A mule is healthy but sterile. It is also why different chromosome numbers, such as 46 in humans and 48 in chimpanzees, are one sign that two groups are separate species.
Notice the word unlikely. Some closely related species with different numbers do produce fertile young, and equal numbers do not guarantee fertility. It is a strong pattern, not a law.
Watch out A hybrid’s chromosome number is the sum of the two gametes, not of the two parents’ body cells: 32 + 31 = 63 for the mule, not 64 + 62.
Chromosome number is a trait shared within a species.
Hybrids of parents with different numbers have chromosomes with no partner, so they are usually sterile.
A hybrid cell has 63 chromosomes. Could every one of them have an equal partner at meiosis?
No. An odd number of chromosomes cannot all form pairs, so at least one has no partner. When the unpaired chromosomes are shared out, gametes tend to be unbalanced, which is why the mule is sterile.
Two imagined species. Q has one long chromosome where P has two short ones, so they have different chromosome numbers.
A hybrid gets both sets: five chromosomes, an odd number. One cannot have an equal partner.
At meiosis each chromosome looks for its match. Two pair neatly, but the long one is left matching two at once.
Some ways of sharing them out give complete sets. Others give a gamete with part of the genome twice and part missing. With many such mismatches, most gametes fail.
A3.1.14 Dichotomous keys
Building a key
To name an organism you have not met, use a dichotomous key. “Dichotomous” means divided in two. At every step you choose between two statements, or answer a yes or no question, and each choice sends you to the next step or to a name. Follow the right path and you finish at one organism and no other.
A key is only as good as its questions. Use features that are easy to see, that mean the same to anyone, and that do not change as the organism grows or with the season: the number of legs on an animal, or a leaf edge that is toothed or smooth. Avoid size, colour and behaviour. A leaf is green in summer and red in autumn, and a young animal is smaller than an adult.
A key can be drawn as a branching chart, as in the figure, or written as numbered pairs of statements, with the same result.
The same key, as numbered statements1a Leaf made of separate leaflets: go to 2. 1b Leaf in one piece: go to 3. 2a Leaflets spread from one point: horse chestnut. 2b Leaflets in pairs along a stalk: ash. 3a Edge deeply lobed: oak. 3b Edge not deeply lobed: go to 4. And so on.
To build your own, choose six to ten local organisms, such as the leaves of trees in your school grounds. List what tells them apart, find one feature that splits the set into two groups, then split each group again until every organism is alone. Then test the key on someone who does not know the answers. If an organism ends in the wrong place, or two end together, the key needs fixing. The two tools below let you practise on a card set of garden animals: build a key for them and test it, then use a key we give you.
Watch out Never harm what you study. Photograph organisms, take only a leaf from a plant that has many, and put animals back where you found them. Never taste or touch a plant or fungus you cannot name.
A dichotomous key uses paired choices that lead to one name.
Choose features that are visible, countable and do not change.
Test a key on someone else, then fix it where it fails.
Which feature is best for a key that tells the leaves of different trees apart?
The edge. It is easy to see and does not change much with the season or the leaf’s age. Colour changes through the year, and size depends on the age of the leaf and the tree’s conditions, so neither makes a reliable question.
Every step is a question with two answers. A leaf made of leaflets leads to one more question, and then to horse chestnut or ash.
A leaf in one piece leads to the next question. Deep lobes: oak.
Each remaining question uses an edge that is easy to see: spiny, toothed, or neither. Every leaf reaches exactly one name.
Check a key by following one leaf through it: holly is no to leaflets, no to deep lobes and yes to a spiny edge.
Eight animals from leaf litter and soil. A dichotomous key tells them apart with questions that each have two answers.
Each question splits the group in two; the next question splits each part again, until each animal is alone with its name.
Choose a feature at each fork. The tool sends every card down your key and tells you which organisms end together or in the wrong place.
Eight animals from leaf litter and soil. A dichotomous key tells them apart with questions that each have two answers.
Each question splits the group in two; the next question splits each part again, until each animal is alone with its name.
Use the key you are given to name each organism, one choice at a time.
A3.1.15 DNA barcodes and environmental DNA
Barcodes from the soil
How many species live in a pond? Netting them all takes weeks, and shy or tiny ones are missed. But every animal leaves traces of itself in the water: skin cells, mucus, droppings. Environmental DNA (eDNA) is the DNA collected from a sample of water, soil or air, not from an animal you have caught.
To tell whose DNA it is, scientists read a DNA barcode: a short stretch of DNA, a few hundred base pairs long, that is alike in members of one species and different between species. A good barcode has two ends that are the same in every species, so one pair of primers works on all of them, and a middle that varies. For animals it is usually part of a mitochondrial gene, for bacteria a ribosomal RNA gene, and plants use other genes.
The steps are always the same: collect a sample, extract the DNA, make millions of copies of the barcode region with PCR, read their sequences, and match each sequence against a reference library of barcodes from known species. Out comes a list of the species present, in a few days, including ones nobody saw.
Because it is so quick, barcoding with eDNA lets the biodiversity of a habitat be investigated rapidly.
Watch out eDNA tells you what has left DNA there, not how many, and not that it still lives there. And it can only name species that are in the reference library: a species missing from it gives no match.
Remember it Steps of an eDNA survey
CatsCollecta sample of water or soil
EatExtractthe DNA from it
AnyAmplifythe barcode region with PCR
SausageSequencethe copies
MostlyMatchthem to a reference library
A barcode is a short, variable stretch of DNA flanked by sequences shared by all species.
eDNA plus barcodes gives a rapid species list for a habitat.
A water sample from a pond is barcoded and matched. What can the result tell you?
Which species in the reference library have left DNA in the water. It does not give numbers or ages, and it cannot name a species the library lacks, but it can be done quickly from one bottle of water.
A barcode is a short stretch of DNA with two ends the same in every species and a middle that differs between species.
Because the ends match everywhere, one pair of primers can copy the barcode from any species in a mixed sample.
Each species in the reference library has its own barcode. Pink marks where a barcode differs from species A. (Letters invented.)
A barcode read from a sample is matched against the library. Here it is identical to one species’ barcode.
Follow the DNA from the water to a list of species, and look for the species nobody saw.
02Dry lab: Genome size explorer
Look up twenty organisms from a genome database, plot genome size against group and gene count, and decide whether a bigger genome means a more complex organism.
Your questionDoes a bigger genome mean a more complex organism, or even more genes? What does the pattern look like inside one group, and between groups?
What you can changeWhich organisms you look up (twenty are in the database, in five groups), whether the graph plots genome size against group or against gene count, and which two organisms you compare side by side.
What to recordFor each organism you look up: its group, genome size and gene count. After at least ten, note the pattern, the organisms that break it, and your judgement on the four statements. Write a one-sentence conclusion that names two organisms as evidence.
The bench: look up each organism’s genome size and gene count, and plot it against its group.
Find out
Which group has the biggest genomes? Look up at least ten organisms, with some from every group, and plot genome size against group. Do the groups with the biggest genomes hold the organisms you would call the most complex?
Does a bigger genome mean more genes? Switch the graph to gene count. Find two organisms with about the same number of genes but genomes of very different size.
How much can one group vary? Pick the group with the widest spread. Name its smallest and its largest genome, and say roughly how many times bigger one is than the other.
Compare two. Choose any two organisms and compare them side by side. Which is more complex, and which has the bigger genome? Do the two answers agree?
Draw a conclusion. Judge the four statements the lab gives you once you have looked up ten organisms, then write one sentence on whether genome size is a good measure of complexity, naming at least two organisms as evidence.
Record
For every organism you look up: its group, its genome size and its gene count, with units. Genome sizes are in millions of base pairs (Mb), and 1,000 Mb is 1 Gb.
Treat the gene counts as approximate: they are estimates, and they change as genomes are studied further. Some organisms have no gene count in the table.
The groups are in a rough order of complexity, and that order is arguable. Say where you would change it.
Download the results of your look-ups and plot your own graph, with axes labelled and a title. Use a log scale for genome size and say why.
A nematode worm and a mouse have about the same number of genes, around 20,000. The worm's genome is about 100 million base pairs. What do you expect of the mouse's?
Much larger: about 2,700 million base pairs, some 27 times the worm's, for about the same number of genes. Most of the difference is DNA that does not code for a protein, including long stretches between genes and repeated sequences. Look both up and see.
You compare the plants in the table, from thale cress to Paris japonica. How much do their genome sizes vary?
Enormously. Thale cress has a genome of about 135 million base pairs and Paris japonica about 149,000 million: roughly a thousand times larger, in the same group of organisms. Genome size varies as much inside one group as between groups, which is why it is a poor guide to complexity.
The lab shows a fish and plants with genomes much bigger than a human's. What can you conclude?
Genome size alone does not measure complexity. If it did, the fish and the plants would be more complex than we are. Size tells you the total amount of DNA, not how many genes there are, how they are used or how complicated the organism is.
03Check
A fresh paper from the exam bank on this guide's topics each time: past-paper style questions, marked as you go, and saved to your Exam practice progress when you are signed in. It opens on biologybybradford.com, so it needs the internet.
The guide as three one-hour lessons, and two more at higher level. Each has a plan to print (the timings, what to project and where to stop, the activities with what they need, what students get wrong) and slides to teach it from, in the same order, with the teacher's moves in the notes.
Lesson
Covers
Focus
Chapters
Lesson plan
Slides
1Variation, species and names
A3.1.1 to A3.1.560 min
Variation, the morphological and biological species concepts, and binomial names
How this was made. The course structure, the activities and the teaching choices come from my own classroom, my presentations and my years of teaching IB and AP Biology. I use Claude, an AI, to help write and build the interactive guides, lesson plans, slides and practice questions, and to draw the diagrams and code the interactives. I direct the work and review it against the IB and AP specifications before it goes live.