Classification, Fossils, Extinction & Antibiotic Resistance

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Linnaean Classification & Binomial Naming

Developed by Carl Linnaeus: Kingdom -> Phylum -> Class -> Order -> Family -> Genus -> Species (Mnemonic: King Philip Came Over For Good Soup). Organisms are named by the binomial system using their Genus (capitalised) and species (lowercase) in italics, e.g., Homo sapiens.

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Three-Domain System (Carl Woese)

Based on chemical and RNA sequence analysis, Woese divided organisms into three domains above kingdoms: 1. Archaea (primitive bacteria, often extremophiles living in hot springs or salt lakes). 2. Bacteria (true prokaryotic bacteria). 3. Eukaryota (organisms with a nucleus: protists, fungi, plants, and animals).

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How fossils are formed

Fossils are the remains of organisms from millions of years ago found in rocks. Formed in three ways: 1. From parts of organisms that have not decayed because one or more decay conditions (oxygen, moisture, warmth) are absent (e.g., mammoths in ice, insects in amber, peat bogs). 2. Hard parts (bones/shells) are gradually replaced by minerals as they decay. 3. Preserved traces of organisms such as footprints, burrows, and rootlet traces.

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Extinction Causes

Extinction occurs when no individuals of a species remain alive. Caused by: 1. New predators evolving or migrating into a habitat. 2. New diseases/pathogens. 3. New, more successful competitors for food or territory. 4. Changes to the environment over geological time (climate change). 5. Single catastrophic events (e.g., asteroid impact or massive volcanic eruption).

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Antibiotic Resistance (MRSA) & Prevention

Bacteria evolve quickly because they reproduce at a fast rate. Random mutations produce antibiotic-resistant strains (like MRSA). To slow down the rate of development of resistant strains: 1. Doctors should not prescribe antibiotics for viral infections (like colds/flu) or mild bacterial infections. 2. Patients must complete the full course of antibiotics so all bacteria are killed and none survive to mutate. 3. Restrict agricultural use of antibiotics in livestock.

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Speciation (Triple Science: Wallace)

The formation of a new species when populations of the same species become so genetically different they can no longer interbreed to produce fertile offspring. Steps: 1. Geographical isolation separates two populations (e.g., river, mountain, island). 2. Different environmental conditions create different selection pressures. 3. Genetic variation/mutations occur in each population. 4. Natural selection favours different advantageous alleles in each population over many generations until they can no longer interbreed to produce fertile offspring.

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1. Q1

   Explain how a population of bacteria can evolve resistance to an antibiotic such as penicillin. [4 marks]

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   Click “Check Mark Scheme” to compare your answer, 1. Within a population of bacteria, a random DNA mutation occurs that gives one bacterium resistance to penicillin [1 mark]. 2. When a patient takes penicillin, the non-resistant bacteria are killed, while the resistant mutant bacterium survives (natural selection) [1 mark]. 3. With no competition for nutrients or space from other bacteria, the resistant bacterium reproduces rapidly by binary fission [1 mark]. 4. It passes the allele for penicillin resistance on to its offspring, forming a large population of antibiotic-resistant bacteria [1 mark].

2. Q2

   Describe three different ways in which fossils can be formed, and explain why the fossil record of early life on Earth is incomplete. [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Way 1: Hard parts of an organism (such as bones, teeth, or shells) do not decay easily and are gradually replaced by minerals over millions of years, forming a rock-like cast [1 mark]. 2. Way 2: Whole organisms are preserved without decaying because one or more conditions needed for microbial decay (warmth, moisture, or oxygen) are absent, such as in glaciers, amber, or acidic peat bogs [1 mark]. 3. Way 3: Traces of organisms (such as footprints, burrows, or rootlet marks) become buried in soft sediment that later hardens into rock [1 mark]. 4. Incomplete record 1: The earliest organisms were soft-bodied, so they decayed completely before they could fossilise [1 mark]. 5. Incomplete record 2: Many fossils that did form have been destroyed by geological activity (earthquakes, volcanoes, tectonic subduction) over billions of years [1 mark].

3. Q3

   A population of squirrels is split in two when a wide river forms across a valley. Over thousands of years, the two populations evolve into two separate species. Explain how this speciation occurs. (Triple Science) [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. The river acts as a physical barrier causing geographical isolation between the two squirrel populations, preventing them from interbreeding [1 mark]. 2. Each side of the river has slightly different environmental conditions (e.g., different predators, climate, or food sources), creating different selection pressures [1 mark]. 3. Within each population, there is genetic variation caused by random mutations in DNA [1 mark]. 4. Through natural selection, individuals with alleles better adapted to their local environment are more likely to survive, reproduce, and pass on their advantageous alleles to their offspring [1 mark]. 5. Over many generations, the allele frequencies and phenotypes diverge so much that even if brought back together, the two populations can no longer interbreed to produce fertile offspring [1 mark].

4. Q4

   Explain why Carl Woese proposed the three-domain system in 1990 to replace the traditional five-kingdom system. [3 marks]

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   Click “Check Mark Scheme” to compare your answer, 1. Improvements in electron microscopes allowed scientists to examine internal cellular structures in much greater detail [1 mark]. 2. Advances in biochemistry and genetics allowed Woese to compare ribosomal RNA (rRNA) and DNA base sequences across species [1 mark]. 3. This evidence revealed that some prokaryotes previously grouped together as 'bacteria' (now called Archaea, often living in extreme environments) are biochemically and genetically more closely related to eukaryotes than to true Bacteria [1 mark].

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