Evolution and Biodiversity (IB)

Fossil Record

• Shows progressive changes in organisms over geological time
• Transitional fossils: organisms with features of two different groups (e.g., Archaeopteryx — features of both dinosaurs and birds)
• Limitation: incomplete record (not all organisms fossilise)

Homologous Structures

• Similar structures in different species inherited from a common ancestor
• Example: pentadactyl limb (five-fingered limb) in humans, whales, bats, dogs
• Same basic bone structure, but adapted for different functions (walking, swimming, flying)
• Shows divergent evolution

Selective Breeding

• Artificial selection demonstrates that characteristics can change over generations
• Dogs bred from wolves; many crop varieties from wild ancestors
• Shows that variation exists and selection can change populations

Comparative Biochemistry

• DNA/protein sequences compared between species
• More similar sequences → more closely related → more recent common ancestor
• Universal genetic code supports common ancestry of all life

Darwin's theory:

1. Variation exists within a population (genetic)
2. Overproduction: more offspring are produced than can survive
3. Struggle for existence: competition for limited resources
4. Survival of the fittest: individuals with advantageous traits are more likely to survive and reproduce
5. Inheritance: advantageous alleles are passed to the next generation
6. Over time, the frequency of advantageous alleles increases in the population

Types of Selection

• Stabilising selection: Extreme phenotypes selected against; intermediate phenotypes favoured (e.g., birth weight in humans)
• Directional selection: One extreme phenotype favoured (e.g., antibiotic resistance; peppered moths)
• Disruptive selection: Both extreme phenotypes favoured; intermediate selected against (e.g., beak sizes in finches)

Speciation

• Geographical isolation separates populations
• Different selection pressures lead to different adaptations
• Over time, populations become so different they can no longer interbreed → new species
• This is allopatric speciation

Binomial Nomenclature

• System devised by Linnaeus
• Each species has a two-part Latin name: Genus species (e.g., Homo sapiens)
• Genus is capitalised; species is lowercase; both italicised

Taxonomic Hierarchy

$\text{Domain} → \text{Kingdom} → \text{Phylum} → \text{Class} → \text{Order} → \text{Family} → \text{Genus} → \text{Species}$

Three-Domain System

• Bacteria: prokaryotes with peptidoglycan cell walls
• Archaea: prokaryotes without peptidoglycan; often extremophiles; distinct rRNA
• Eukarya: organisms with membrane-bound nuclei (protists, fungi, plants, animals)

Based on rRNA sequence comparisons (Carl Woese)

Cladistics

• Classification based on evolutionary relationships (shared derived characteristics)
• Cladograms: branching diagrams showing evolutionary relationships
• Each branch point (node) represents a common ancestor
• Based on molecular evidence (DNA/protein sequences) and morphological evidence
• Clade: a group of organisms sharing a common ancestor and ALL its descendants

Analogous vs Homologous Structures

• Homologous: Same origin, different function (e.g., bat wing and human arm) → evidence of divergent evolution from common ancestor
• Analogous: Different origin, similar function (e.g., bird wing and insect wing) → convergent evolution, NOT evidence of close relationship

What Is Biodiversity?

• Species diversity: variety of species in an ecosystem
• Genetic diversity: variation within a species
• Ecosystem diversity: variety of habitats in a region

Importance of Biodiversity

• Ecosystem services: pollination, water purification, carbon sequestration, soil formation
• Economic value: food, medicines, raw materials
• Ethical: all species have intrinsic value; we have a responsibility to protect them
• Aesthetic/cultural: recreation, spiritual significance

Threats to Biodiversity

• Habitat destruction (deforestation, urbanisation)
• Climate change
• Pollution
• Overexploitation (overfishing, poaching)
• Invasive species

Conservation

• In situ: protecting species in their natural habitat (national parks, reserves)
• Ex situ: protecting species outside their habitat (zoos, seed banks, captive breeding)
• International agreements: CITES, Convention on Biological Diversity

For a population in equilibrium (no evolution occurring):

$p + q = 1$ $p^2 + 2pq + q^2 = 1$

Where:

• $p$ = frequency of dominant allele
• $q$ = frequency of recessive allele
• $p^2$ = frequency of homozygous dominant
• $2pq$ = frequency of heterozygous
• $q^2$ = frequency of homozygous recessive

Conditions for Hardy-Weinberg Equilibrium

• No mutation, no migration, no natural selection, large population, random mating
• If these conditions are NOT met → allele frequencies change → evolution occurs

Question: In a population, 16% have the homozygous recessive phenotype. Calculate the percentage of heterozygous carriers. (3 marks)

Solution:

$q^2 = 0.16$, so $q = \sqrt{0.16} = 0.4$

$p = 1 - q = 1 - 0.4 = 0.6$

$2pq = 2 \times 0.6 \times 0.4 = 0.48 = 48\%$

48% of the population are heterozygous carriers.

• Evidence for evolution: fossils, homologous structures, selective breeding, comparative DNA/protein sequences.
• Natural selection: variation → competition → survival of the fittest → inheritance → evolution.
• Classification: binomial nomenclature; three-domain system (Bacteria, Archaea, Eukarya); cladistics uses molecular data.
• Biodiversity: species, genetic, and ecosystem diversity; threatened by human activities; conserved in situ and ex situ.
• Hardy-Weinberg: $p^2 + 2pq + q^2 = 1$; used to calculate allele and genotype frequencies.