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A theory as important as evolution needs strong evidence, and there is plenty: the fossil record preserved in the rocks, and the antibiotic resistance you can watch happening today. This lesson, part of Topic B5 of OCR Gateway Science A, examines that evidence, looks at extinction and its causes, and then turns to classification — how biologists sort the living world into groups, from the Linnaean system to the modern three-domain system (Higher), and how evolutionary (phylogenetic) trees show how species are related. It builds directly on the theory of natural selection from the previous lesson.
By the end of this lesson you should be able to describe the fossil and antibiotic-resistance evidence for evolution, explain extinction and its causes, describe the Linnaean classification system and binomial naming, explain (Higher) why the three-domain system was introduced, and interpret evolutionary trees.
A fossil is the preserved remains, or traces, of an organism that lived thousands or millions of years ago, found in rocks. By comparing fossils from different rock layers — older fossils are found in deeper layers — scientists can see how organisms have changed over time, which is powerful evidence for evolution.
Fossils form in several ways:
| How the fossil forms | What happens |
|---|---|
| Hard parts replaced by minerals | Hard parts such as bones, teeth or shells are slowly replaced by minerals as they decay, forming a rock-like copy of the original (this is mineralisation) |
| Casts and impressions | An organism leaves a mark — for example a footprint, a burrow or the impression of a leaf — which is then filled in or hardened into rock |
| Preservation in places where decay is prevented | Where conditions stop microbes from decaying the organism — too cold, too little oxygen, or too acidic — remains can be preserved, for example whole mammoths frozen in ice or insects trapped in amber |
A complication is that the fossil record is incomplete — there are gaps in it. This is because many early organisms were soft-bodied and decayed before they could fossilise, and because fossils only form in particular conditions and many have been destroyed by movements of the rocks or have simply not yet been found. The gaps are one reason it took time for the fossil evidence for evolution to become convincing.
The second key piece of evidence is the antibiotic resistance you met in the previous lesson. Because bacteria reproduce so quickly, we can actually observe evolution by natural selection happening over days and weeks: a random mutation makes a bacterium resistant, the antibiotic kills the non-resistant bacteria, and the resistant survivors reproduce so resistance spreads. This is direct, present-day evidence that natural selection really does change populations.
Exam Tip: When asked for evidence for evolution, the two reliable answers are the fossil record (showing change over time) and antibiotic-resistant bacteria (natural selection observed today). For fossils, be ready to explain the gaps: many early organisms were soft-bodied and decayed before fossilising.
Extinction is the permanent loss of all the members of a species — when the last individual of a species dies, the species is extinct and gone forever. Extinction is a natural part of the history of life (most species that have ever lived are now extinct), but it can be caused by several factors:
Often it is a combination of these factors. The common thread is that the species is unable to adapt quickly enough — through natural selection — to a change it faces, so it dies out.
Exam Tip: Extinction means the whole species is lost permanently. A species is at greater risk of extinction if it has little genetic variation (so it cannot adapt), a small population, a slow reproductive rate, or a very specialised habitat or diet. Linking extinction back to a failure to adapt earns marks.
There are millions of different species, so biologists classify them — sort them into groups — to make sense of the diversity and to show how organisms are related.
In the eighteenth century, Carl Linnaeus devised a system that arranges living things into a hierarchy of groups, from the largest and most general to the smallest and most specific. Each group is divided into smaller groups within it:
The order, from largest to smallest, is Kingdom → Phylum → Class → Order → Family → Genus → Species. As you move down the hierarchy, the groups get smaller and the organisms in them are more closely related and more similar. A species is the smallest group — a group of similar organisms that can reproduce together to produce fertile offspring.
Linnaeus also gave every species a two-word Latin name — the binomial system. The name is made of the organism's genus (first word, capital letter) and its species (second word, lower-case), and is written in italics. For example, humans are Homo sapiens (genus Homo, species sapiens).
The great advantage of binomial naming is that it gives every species a single, universal name used by scientists all over the world, avoiding the confusion of different common names in different languages or regions.
| Feature | Detail |
|---|---|
| Two words | Genus + species |
| Capital letters | Genus capitalised, species lower-case |
| Style | Written in italics (or underlined in handwriting) |
| Example | Homo sapiens, Panthera leo (lion) |
Exam Tip: A binomial name has the genus first (capital letter) and the species second (lower-case), in italics — for example Homo sapiens. The benefit is that it is the same worldwide, so scientists in any country know exactly which species is meant.
Higher tier only: You should know that classification has changed as scientific knowledge has improved, especially with the development of microscopes and the ability to study organisms' DNA and biochemistry.
Linnaeus could only classify organisms by features he could see. As techniques improved, scientists could compare the internal structures of cells and, crucially, the base sequences of organisms' DNA and the proteins they make. This revealed relationships that were not obvious from appearance alone — in particular, that a group of microorganisms once classed simply as "bacteria" were actually very different from true bacteria.
In response, in 1990 Carl Woese proposed the three-domain system, which divides all life into three large groups called domains, based largely on differences in their genes and biochemistry:
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