Biology - Genetics and Evolution

Height in humans does not follow a simple dominant-recessive pattern. Explain why height shows continuous variation and discuss how both genes and environment contribute.

Model response: Height is a polygenic trait — it is controlled by many genes (estimates suggest over 700 genes contribute), each with a small additive effect. Because there are so many possible combinations of alleles across these genes, height does not fall into distinct categories (tall or short) but shows continuous variation — a smooth range of values that follows a normal distribution in a population. However, genes alone do not determine height. Environmental factors including nutrition, health during childhood, and socioeconomic conditions also have a significant effect. Identical twins (same DNA) raised in different environments can end up different heights, demonstrating the environmental contribution. Conversely, genetically similar populations in different nutritional environments show different average heights — average height in many countries has increased over generations due to improved nutrition, not genetic change. This gene-environment interaction applies to many human characteristics. The 'nature versus nurture' debate is often a false dichotomy — for most traits, the answer is both, interacting in complex ways. Heritability estimates for height (~80%) do not mean 80% of your height is genetic — they mean 80% of the variation in height within a population is attributable to genetic differences.

The Human Genome Project mapped all 3 billion base pairs in human DNA. Scientists expected to find about 100,000 genes but found only about 20,000. What does this tell us about DNA?

Model response: The finding that humans have far fewer genes than expected reveals that our model of DNA as simply 'a sequence of genes coding for proteins' is incomplete. Only about 1-2% of human DNA codes for proteins. The remaining 98% was once called 'junk DNA', but we now know much of it has important regulatory functions — it controls when, where, and how much of each gene is expressed. Some non-coding DNA contains regulatory sequences (promoters, enhancers) that switch genes on and off in different cell types — which is why a liver cell and a brain cell have the same DNA but behave very differently. Some non-coding regions are involved in chromosome structure and stability. Others are remnants of ancient viral DNA or repetitive sequences whose functions are still being investigated. The small number of human genes (similar to a fruit fly's ~14,000) suggests that complexity comes not from the number of genes but from how genes are regulated and how proteins interact. This has implications for medicine: understanding gene regulation could be more important than knowing gene sequences for treating genetic diseases. The genome project also revealed that humans share 99.9% of their DNA with each other — the 0.1% difference accounts for all human genetic diversity.

How does the DNA structure discovery illustrate the process by which scientific knowledge is developed and refined over time?

Model response: The DNA discovery is a textbook example of how scientific knowledge progresses. The process involved: (1) accumulation of evidence from multiple sources — Miescher identified DNA in 1869, Avery showed it carried genetic information in 1944, Chargaff established base ratios, Franklin and Wilkins provided structural data through X-ray crystallography; (2) model building — Watson and Crick synthesised the evidence into a testable model (the double helix) that explained both the structure and the mechanism of replication; (3) predictions — the model predicted that DNA replication would be semi-conservative, which was confirmed experimentally by Meselson and Stahl in 1958; (4) peer review and publication — the model was published in Nature and scrutinised by the scientific community; (5) refinement — the original model has been refined as new evidence emerged (e.g., different forms of DNA, epigenetics, gene regulation). This demonstrates that scientific knowledge is not discovered in a single eureka moment but is built incrementally through evidence, debate, and revision. The legacy is immense: the structure directly led to understanding DNA replication, the genetic code, genetic engineering, DNA fingerprinting, the Human Genome Project, gene therapy, and CRISPR gene editing. Watson and Crick's model became the foundation of molecular biology — arguably the most impactful scientific discovery of the twentieth century.

Explain why genetic variation is essential for the survival of a species, and discuss what would happen to a species with very low genetic variation if its environment changed suddenly.

Model response: Genetic variation is essential because it provides the raw material for natural selection. In a genetically diverse population, when the environment changes, some individuals will have alleles that make them better suited to the new conditions — they survive and reproduce, passing on those advantageous alleles. This is how populations adapt over time. The ultimate source of new genetic variation is mutation — random changes in DNA sequence. Most mutations are neutral or harmful, but occasionally one provides an advantage in a particular environment. Sexual reproduction shuffles existing alleles into new combinations (through independent assortment and crossing over during meiosis), creating diversity in each generation. A species with very low genetic variation (such as cheetahs, which went through a population bottleneck) is extremely vulnerable. If a new disease emerged, all individuals would be similarly susceptible because they lack the genetic diversity for some to have resistance. The Irish potato famine (1845-1852) is a historical example: the potato crop had very low genetic diversity (clonal reproduction), so when the blight Phytophthora infestans arrived, virtually all plants were affected, causing catastrophic crop failure and the deaths of over a million people. This is why conservation efforts focus on maintaining genetic diversity (gene banks, avoiding inbreeding in captive breeding programmes), not just population numbers.

Darwin's finches on the Galapagos Islands have different beak shapes on different islands. Explain how natural selection could lead to one ancestral species becoming many different species.

Model response: The ancestral finch population colonised the Galapagos from the South American mainland. Different islands had different food sources — seeds of different sizes, insects, cacti, and fruit. On each island, natural selection favoured individuals with beak shapes best suited to the available food: large, strong beaks for cracking hard seeds; long, thin beaks for probing flowers or extracting insects; sharp beaks for pecking wood. Over many generations, the beak shapes on each island diverged as different alleles were favoured in different environments. Geographic isolation (separate islands) prevented interbreeding between populations, allowing genetic differences to accumulate. Eventually, the populations became so genetically different that they could no longer interbreed even if brought together — they had become separate species. This process is called speciation (specifically, allopatric speciation because geographic isolation was involved). The evidence for this includes: (1) anatomical — beak shapes correlate precisely with diet on each island; (2) genetic — DNA analysis shows all the species are closely related but with increasing divergence; (3) observational — Peter and Rosemary Grant documented natural selection acting on beak size in real time during droughts (birds with slightly larger beaks survived better when only hard seeds were available). Darwin's finches are one of the most powerful demonstrations that natural selection, given enough time and geographic isolation, can generate new species from a common ancestor.

Scientists debate whether we are currently in a sixth mass extinction. What defines a mass extinction, and what evidence supports or challenges this claim?

Model response: A mass extinction is defined as the loss of more than 75% of all species within a geologically short period (typically less than 2 million years). Earth has experienced five: the Ordovician (443 Mya, ~86% species lost), Devonian (372 Mya, ~75%), Permian (252 Mya, ~96% — the 'Great Dying'), Triassic (201 Mya, ~80%), and Cretaceous (66 Mya, ~76% — including dinosaurs). Evidence supporting a sixth mass extinction: current extinction rates are 100-1,000 times the background rate; vertebrate species are declining at rates consistent with previous mass extinctions; insect populations have declined by 40-75% in some studied regions; the IUCN Red List classifies over 40,000 species as threatened. Evidence that challenges the comparison: total species loss has not yet reached 75%; mass extinctions in the fossil record took thousands to millions of years, making rate comparisons difficult; we have not yet documented extinctions of entire higher taxonomic groups (families, orders) as in previous events; and our knowledge of total species numbers is incomplete (estimated 8-10 million, only ~1.5 million described). However, many biologists argue that waiting until 75% of species are lost before declaring a mass extinction would be scientifically irresponsible — by then, it would be too late to act. The current trajectory, if unchecked, projects losses consistent with a mass extinction within centuries. Conservation strategies must address root causes: protected areas, habitat corridors, reduced consumption, climate change mitigation, and sustainable agriculture are all necessary. Single-species conservation (saving pandas) is less effective than ecosystem-level protection because it addresses symptoms rather than causes.

Some people argue that conservation is too expensive and slows economic development. Construct a counter-argument using the concept of ecosystem services.

Model response: Ecosystem services are the benefits that humans obtain from functioning ecosystems — and their economic value far exceeds the cost of conservation. These services include: provisioning services (food, fresh water, timber, medicines — over 50% of modern medicines derive from natural compounds); regulating services (pollination of crops worth hundreds of billions of pounds annually, natural flood defences from wetlands and forests, carbon sequestration by forests and oceans, air and water purification by natural systems); supporting services (nutrient cycling, soil formation — it takes hundreds of years to form a few centimetres of topsoil); and cultural services (recreation, tourism, mental health benefits from natural environments). The economic argument for conservation is compelling: the cost of replacing natural pollination with technology is estimated at far more than the cost of protecting pollinator habitats. The cost of flood damage when wetlands are drained exceeds the cost of preserving them. Once biodiversity is lost, these services must be replaced artificially — at enormous cost — or they are lost entirely. Beyond economics, there are ethical arguments: other species have intrinsic value regardless of their usefulness to humans, and we have a responsibility to future generations not to deplete the natural world irreversibly. The ecological argument is that biodiversity underpins ecosystem resilience — diverse ecosystems are better able to withstand and recover from disturbances. Reducing biodiversity makes ecosystems (and thus human systems that depend on them) more fragile. Conservation is not an alternative to economic development — it is a prerequisite for sustainable development.