Classification

Chapter 10

Biological Classification

A system used to organise living organisms into groups based on shared characteristics.

Taxonomic hierarchy

• Domain – largest group; based on cell type and genetics

• Kingdom – broad groups like animals, plants, fungi

• Phylum – organisms with similar body plans

• Class – more specific grouping within a phylum

• Order – groups of related families

• Family – groups of related genera

• Genus – closely related species

• Species – organisms that can interbreed to produce fertile offspring

Binomial system of naming species

A system that gives every species a two-part scientific name using Latin or Latinised words (in italics).

• Genus name (capital letter)

• Species name (lowercase)

Advantages of the binomial system

• Provides a universal name used worldwide, avoiding language confusion

• Each species has a unique name, preventing mix-ups

• Shows relationships between species through shared genus names

• Helps scientists communicate clearly and accurately about organisms

Classification into the Five Kingdoms

Organisms are classified based on similarities in observable features such as cell structure, nutrition, and organisation.

Prokaryotae, protocista, fungi, plantae and animalia

Prokaryotae

• Cells have no nucleus (DNA free in cytoplasm)

• Usually unicellular

• No membrane-bound organelles

• Example: bacteria

Protoctista

• Mostly unicellular

• Cells have a nucleus

• Can be plant-like (photosynthetic) or animal-like (heterotrophic)

• Example: algae, amoeba

Fungi

• Usually multicellular (yeast is unicellular)

• Cells have a nucleus

• Do not photosynthesise; feed by decomposition

• Cell walls made of chitin

Plantae

• Multicellular

• Cells have a nucleus and chloroplasts

• Photosynthesise (autotrophic)

• Cell walls made of cellulose

Animalia

• Multicellular

• Cells have a nucleus

• No cell walls or chloroplasts

• Heterotrophic (feed on other organisms)

Classification changed

5 kingdoms changed to 3 domain system of classification

Kingdom system (older):

• Based on observable features (cell type, nutrition, structure)

• 5 kingdoms: Prokaryotae, Protoctista, Fungi, Plantae, Animalia

• Grouped all prokaryotes together

Domain system (modern):

• Based on genetic evidence and molecular data

• Splits prokaryotes into Bacteria and Archaea

• More accurate reflection of evolutionary relationships

The three domains of life

• Bacteria

  • Prokaryotic (no nucleus) or membrane bound organelles

  • Cell walls contain peptidoglycan

• Archaea

  • Prokaryotic but genetically different from bacteria

  • Often live in extreme environments

  • Cell wall structure differs from bacteria

• Eukarya

  • Eukaryotic cells (have nucleus) and membrane bound organelles

  • Includes animals, plants, fungi, and protoctists

Evidence for 3 domain system

• DNA (genomic) sequencing → compares base sequences

• rRNA (ribosomal RNA) → shows evolutionary relationships

• Protein comparisons (e.g. amino acid sequences)

• Conserved molecules like histones (similar in Archaea and Eukarya)

Classification and phylogeny

Classification:
The system of grouping organisms based on similarities.

Phylogeny:
The study of the evolutionary history and relationships between organism

Relationship between classification and phylogeny

• Modern classification is based on phylogeny (evolutionary relationships)

• Organisms are grouped according to how closely they are evolutionarily related

• The more similarities in DNA or structure, the more recent the common ancestor

Natural selection

• Charles Darwin and Alfred Russel Wallace

• They independently proposed that organisms evolve through natural selection, where advantageous traits are passed on to offspring.

Fossil evidence for natural selection

• Fossils show organisms changing over time in rock layers

• Older fossils are usually simpler; newer fossils show more complex or modern forms

• Transitional fossils (e.g. Archaeopteryx) show links between groups

• Supports the idea of gradual evolution over long periods

DNA (genomic) evidence for natural selection

• Comparing DNA base sequences shows how closely related species are

• More similar DNA = more recent common ancestor

• Mutations accumulate over time, increasing differences between species

Molecular evidence for natural selection

• Comparison of protein sequences (e.g. amino acids in cytochrome c which is protein in nearly all mitchondria)

• Fewer differences = closer evolutionary relationship

• Some molecules are highly conserved across species, showing shared ancestry

Intraspecific vs interspecific variation

• Intraspecific variation: differences within the same species

  • Example: differences in height, weight, or eye colour in humans

• Interspecific variation: differences between different species

  • Example: differences between a cat and a dog

Continuous vs discontinuous variation

• Continuous variation: shows a range of values with no clear categories

  • Usually measured

  • Example: height in humans, mass, leaf length in plants

• Discontinuous variation: falls into distinct categories with no overlap

  • Usually counted

  • Example: blood groups in humans, flower colour in some plants, antibiotic resistance in bacteria (present/absent)

Causes of variation

• Genetic causes:

  • Differences in DNA and alleles inherited from parents

  • Mutation creates new alleles

• Environmental causes:

  • Factors such as diet, climate, and lifestyle

  • Example: plant growth affected by light or water availability

• Combined causes:

  • Most characteristics are influenced by both genes and environment (e.g. height in humans)

Anatomical adaptations

Physical features of an organism’s body that help survival.

Examples:

• Thick fur in polar bears (insulation)

• Cactus spines instead of leaves (reduce water loss)

• Streamlined body in fish (efficient swimming)

Physiological adaptations

Internal body processes that help survival.

Examples:

• Production of insulin to control blood sugar

• Antifreeze proteins in Arctic fish

• Water conservation in desert animals (concentrated urine)

Behavioural adaptations

Actions or behaviours that increase survival or reproduction.

Examples:

• Migration of birds to warmer climates

• Nocturnal activity in desert animals

• Huddling in penguins for warmth

Convergent evolution

Unrelated organisms evolve similar features because they live in similar environments and face similar selection pressures

Mechanism of natural selection

• Genetic variation exists in a population (individuals have different alleles/characteristics).

• Selection pressure (e.g. predators, disease, climate, competition) acts on the population.

• Individuals with advantageous characteristics are more likely to survive.

• These individuals are more likely to reproduce successfully, passing on their alleles.

• Individuals with less advantageous traits are more likely to die or reproduce less.

• Over many generations, the frequency of advantageous alleles increases in the population.

Pesticide resistance in insects

• Some insects naturally have alleles that give resistance

• Pesticide kills non-resistant insects, leaving resistant ones

• Resistant insects survive and reproduce

• Over time, population becomes mostly resistant

• Causes harder pest control, more chemical use needed

Drug resistance in microorganisms:

• Some bacteria have mutations giving antibiotic resistance

• Antibiotics kill non-resistant bacteria

• Resistant bacteria survive and reproduce

• Resistant strains spread over time

• Happens if course is not finished fully or the wrong dosage is taken

• Infections become harder to treat