It is tempting for science teachers to dive into each topic as it comes and teach its ideas as discrete lessons. Learning about Punnett squares? Let’s just focus on teaching how to draw them for a few lessons and then move onto selective breeding in the following week’s lessons, which is the next topic on the scheme of work. Discrete units of teaching make sense in this way, because pupils need to master ideas as they meet them.

I used to teach in this way. It is easier to plan one lesson or a few lessons at a time. You feel confident that you are covering the specification, perhaps in order, so that nothing is left out. If each of these lessons are delivered well with clear teacher explanations, then pupils will no doubt learn a lot.

However, I think there is a huge opportunity missed here. I’ve been thinking a lot about curriculum coherence over the last few years. A sequence of lessons has the power to tell a story. The links between the lessons – how they join together – are fundamental opportunities to help pupils develop an emergent understanding of a subject beyond discrete lesson chunks. If we set our curricular sights on deep understanding of our subjects, then a series of lessons must reveal something important about our subject that helps pupils build a well-connected schema, not just a series of separate schemas.

Take the topic of Inheritance, Variation and Evolution on the AQA Combined Science Trilogy GCSE specification. I have put the sequence in the specification on the left and my own sequence on the right in the table below:

Specification sequence	My sequence
Sexual reproduction	The tree of life
Asexual reproduction	Variation
Meiosis	DNA, chromosomes, genes and genome
DNA, chromosomes, genes and genome	Sexual reproduction
Alleles, genotype, phenotype, Punnett squares	Meiosis
Inherited disorders	Alleles, genotype, phenotype, Punnett squares
Variation & mutation	Asexual reproduction & mutations
Evolution	Species and adaptations
Natural selection	Selective breeding
Selective breeding	Natural selection
Genetic engineering	Evidence for evolution & evolutionary trees
Evidence for evolution & evolutionary trees	Genetic engineering

Notice how different the two sequences are!

If I would like my pupils to have a coherent understanding of this topic, then I as the teacher, need to have clarity about how the ideas of the topic fit together and why. It is only when I make the curricular narrative explicit for myself and for the teachers in my department, that my pupils have any hope of making links between otherwise discrete chunks of content with any coherence.

Why have I changed the order so dramatically? What follows, is the narrative – the story –  that I want to tell when teaching this unit. How this story manifests in the classroom, will be the topic of another post. For now, this is how I plan to write my curriculum on this topic.

My curricular narrative for Inheritance, Variation & Evolution:

The Tree of Life
Let’s start with one key idea: ‘All living organisms are related, sharing a single common ancestor that lived approximately 3.5 billion years ago.’ In the same way that you are more closely related to your siblings and more distantly related to your cousins, so too are all organisms ultimately related. Go back enough generations and you find that all humans are related. Go back even further and you find that all animals are related. Even further and all of the kingdoms are related to a single common ancestor.

This is a tricky idea to understand. To help us get our heads around it, we will start by looking at the idea of variation – differences between individuals. This is the most fundamental idea that will help answer the question: how are all organisms related?

Variation
Different humans and different organisms have different characteristics because of genetic differences, environmental differences or a combination of the two. Environmental differences are relatively easy to understand (one person chooses to get a tattoo, but the other person does not; one cheetah injures its paw, the other does not), but to understand genetic differences we need to understand what a gene is. We’ve used the term ‘genetic information’ and ‘DNA’ before, but what is genetic information?

DNA, chromosomes, genes and genome
A DNA molecule is very long, and we have 46 of them in every nucleus of every body cell. A gene is a section of DNA that codes for a particular characteristic. It ‘codes’ for a characteristic by specifying the sequence of amino acids that make up a protein. In short, our genome (all of the DNA code we have) decides which proteins we make, which influences most of our characteristics. Enzymes are examples of proteins. Lots of proteins we will not study about, help an embryo develop – they help shape our bodies.

Different organisms between species and within a species (rudimentary understanding of species for now, though this will have been covered in KS3) have different DNA codes. For example, different humans have different DNA codes so there is variation between humans that we describe as ‘genetic’. However, a surprisingly large amount of DNA is shared between organisms of a species. More closely related species have more similar DNA.

Sexual Reproduction (gametes, fertilisation, embryo development) & Meiosis
Where does each human get its DNA from? From our parents – we call this inheritance. But if siblings have the same parents, why do siblings look different? It is not just because of differences in the environment – siblings have similar environments. Even when we are born, some features differ greatly between siblings such as the shape of the nose. Why? In other words, why do closely related organisms differ from each other?

The answer is that humans, like lots of eukaryotic organisms, reproduce sexually. Two sex cells (gametes), containing DNA from each of our parents, come together to give each child its characteristics. The process which makes gametes – meiosis – results in the formation of genetically different gametes. This means that every sperm cell and every egg cell has a different mix of the 46 chromosomes a person inherited in turn from their father and mother respectively. Although every sperm and egg cell an individual of a particular species makes has exactly the same genes, the alleles may differ.

Alleles, genotype, phenotype
Alleles are different versions of the same gene. All sexually reproducing organisms have two copies of every gene, one which they inherited from their mother and one from their father. These copies may or may not be the same allele.

Alleles were discovered by Mendel, a monk who experimented with pea plants. The characteristics of a pea plant are a good way to study the rules of inheritance: dominance, ratios and Punnett squares. This is a good opportunity to illustrate the disciplinary knowledge in science: how scientific knowledge is created.

Inherited disorders
Inherited disorders further provide concrete examples to explain how the complex interactions between alleles (the genotype) results in particular phenotypes. Polydactyly and cystic fibrosis are two such examples.

Understanding meiosis, fertilisation (sexual reproduction) and alleles explains variation amongst siblings.

So, we now know why even siblings can look different.

Asexual Reproduction, Comparing Reproduction & Mutations
But not all organisms reproduce sexually. Some organisms reproduce asexually. These organisms produce clones of themselves which are genetically identical. We can now contrast sexual and asexual reproduction to understand why one leads to variation in offspring, and the other does not.

However, mutations can still appear in asexually reproducing organisms, causing differences. In fact, mutations happen all of the time, even when sperm and egg cells are being made during meiosis. This is continuously introducing further variation between individuals.

We now know why there is variation in populations of organisms. How does this help us explain why all organisms are related?

Species & Adaptations
Let’s now look at the idea of species – it is the way we divide organisms which differ from each other by a significant amount. Individuals of different species may be able to reproduce, but the offspring are not fertile. In contrast, individuals of the same species can reproduce and the offspring will be fertile. We can have variation within and between species.

Within a species, organisms share lots of characteristics. These are usually characteristics that help organisms adapt to their environment, making it more likely for them to survive and reproduce. Adaptations may be structural, behavioural or functional. Chance to re-visit adaptations of a variety of organisms for a variety of ecosystems covered in KS3.

We have seen how organisms of the same species may be related. By saying all organisms are related, we are saying all species are related. How?

Selective Breeding
Thought experiment on selective breeding: imagine only selecting the frogs with the longest legs to breed. What would happen if you did this every generation? Eventually, the entire population would have far longer legs than the original generation. The same can be applied to a newt population – selecting for shorter tails and longer legs – one can imagine how newts could have evolved into frogs.

Note – it is easier to teach the idea of selective breeding before natural selection because the idea of humans deciding which organisms are allowed to breed is easier than thinking about the additional ideas of advantageous characteristics determining which organisms breed.

Natural Selection
The above is what happens in nature – but with one key difference. In the wild, it is nature which decides which individuals reproduce: the organisms who have the advantageous characteristics are the ones to survive and reproduce. This happens in the backdrop of competition for limited resources, predation and disease amongst other factors. The surviving organisms, having outcompeted others, and successfully evaded predation and disease, are the ones who can pass on the genes that help them be successful. This over billions of years has resulted in the gradual change of one species into another.

This is a good way to introduce some history of science (Darwin’s voyage on the Beagle and his publication of the Origin of Species), and disciplinary knowledge – how Darwin would make observations, come up with a theory and make predictions. Wallace’s similar observations and conclusions.

Evidence for evolution: fossils, chemical analyses, examples of evolution
Whilst you might accept the idea of evolution by natural selection theoretically, what evidence is there that this actually happened over the course of 3.5 billion years? What evidence do we have that it continues to happen? There is a lot of evidence for evolution: genetic analyses and chemical analyses of different species reveals the extent to which they are related. The fossil record shows how various species may have evolved. Evolution in bacteria is easily observable: evidence that this happens even now. Current animal examples e.g. anole lizards or guppies.

Evolutionary Trees
How do we represent how organisms are related to each other? We can piece together how organisms are related by constructing simple diagrams called evolutionary trees. Classification has changed over time: in the past, we grouped organisms by the similarity in their characteristics. But sometimes, distantly related organisms evolve similar characteristics. We now compare DNA sequences to measure how closely related species are.

Now that we understand how and why organisms are related, and we understand how each organism’s DNA provides instructions for making the bodies, which provide adaptations enabling them to survive, we have a powerful insight into how nature works. As humans, we can manipulate nature, as we do in genetic engineering.

Genetic Engineering
We can use our understanding of DNA and species to give organisms of one species the characteristics of another species. Who knows what the future will hold: we can now even edit the genes of embryos.

Whether any of this is ethical is not for scientists, but for society to decide.

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I’d love to hear what you think of this sequence and the narrative.

@Mr_Raichura

P.S. If you’d like to hear me discuss this sequence with four other excellent biology teachers, then watch out for details on the curriculum chat video by following #CogSciSciEvolution on Twitter.