Evolution, Variation, and Continuity of Life

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Evolution and Variation — Key Facts for Sri Lanka A/L Examination

Evidence for Evolution:

Evidence	Example	What it Shows
Fossil record	Archaeopteryx (reptile-bird link)	Sequential change over time
Comparative anatomy	Pentadactyl limb (human, whale, bat wing)	Common ancestry
Comparative biochemistry	Cytochrome c sequence similarity	Molecular relatedness
Biogeography	Marsupials in Australia	Geographic isolation
Embryology	Vertebrate embryos look similar early	Common ancestry

Key Evolutionary Concepts:

Concept	Definition	Example
Natural selection	Differential survival/reproduction of individuals	Peppered moth (industrial melanism)
Adaptation	Trait that increases fitness	Camel’s water conservation
Speciation	Formation of new species	Darwin’s finches
Common ancestor	Single species from which all life descended	LUCA (Last Universal Common Ancestor)

Mendel’s Contribution:

• Inheritance patterns explain variation
• Genes (alleles) are units of heredity
• Variation arises from: mutation, recombination (crossing over), independent assortment

⚡ A/L Exam Tip: Natural selection acts on phenotype, not genotype — the environment “selects” individuals best adapted to survive and reproduce!

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Evolution and Variation — Detailed Study Guide

Darwin’s Theory of Natural Selection

Historical Context:

• Charles Darwin (1809-1882): Naturalist on HMS Beagle (1831-1836)
• Alfred Russel Wallace (1823-1913): Independently conceived natural selection (1858)
• Jean-Baptiste Lamarck (1744-1829): Early evolutionary theory (inheritance of acquired characteristics — now rejected)

Darwin’s Four Postulates:

1. Variation: Individuals in a population vary in traits
2. Heritability: Some variation is inherited (passed to offspring)
3. Differential survival: Not all individuals survive to reproduce
4. Differential reproduction: Survivors reproduce more — traits are passed on

Natural Selection Examples:

Example	Mechanism	Outcome
Industrial melanism (peppered moth)	Dark moths survived on dark trees (predation)	Biston betularia shifted to dark morph
Antibiotic resistance	Bacteria with resistance gene survive antibiotics	MRSA, antibiotic-resistant TB
Darwin’s finches	Different beak sizes for different seeds	Adaptive radiation on Galápagos
Sickle cell anaemia	Heterozygotes (HbS/HbA) resist malaria	Balanced polymorphism in malarial regions

⚡ A/L PYQ: “Explain why overuse of antibiotics leads to antibiotic-resistant bacteria.” Answer: Antibiotics kill susceptible bacteria, leaving resistant ones (random mutation) to survive and reproduce. These resistant bacteria pass the trait to offspring, so the population becomes dominated by resistant strains — this is natural selection in action.

Sources of Variation

Genetic Variation:

Source	Mechanism	Result
Mutation	Changes in DNA sequence	New alleles (ultimate source)
Crossing over	Exchange between homologues	New allele combinations
Independent assortment	Homologues separate randomly at meiosis I	Unlinked genes sorted independently
Random fertilisation	Any sperm + any egg	Genetic uniqueness

Mutation as the Ultimate Source:

• Gene mutations: Point mutations (substitution), frameshift (insertion/deletion)
• Chromosomal mutations: Deletion, duplication, inversion, translocation
• Genomic mutations: Changes in chromosome number (polyploidy, aneuploidy)

Examples of Plant Mutations:

Mutation	Effect	Significance
Sickle cell (humans)	Substitution — Glu→Val in haemoglobin	Malaria resistance (heterozygote advantage)
Dwarfism in pea	Gibberellin biosynthesis defect	Study of plant hormones
Polyploidy in wheat	Whole genome duplication	Larger, more vigorous plants — agronomically important

Phenotypic Variation:

• Continuous variation: Many genes, small effects, bell curve distribution (e.g., plant height, seed weight)
• Discontinuous variation: Few genes, large effects, distinct categories (e.g., flower colour, seed shape)

⚡ A/L Important: Environmental factors also cause variation — even genetically identical plants (clones) vary in height depending on light, water, nutrients. Phenotype = genotype + environment.

Population Genetics

Gene Pool and Allele Frequencies:

Concept	Definition
Gene pool	All alleles in a population
Allele frequency	Proportion of each allele in gene pool
Genotype frequency	Proportion of each genotype

Hardy-Weinberg Equilibrium: $$p^2 + 2pq + q^2 = 1$$ $$p + q = 1$$

Where:

• p = frequency of dominant allele
• q = frequency of recessive allele
• p² = frequency of homozygous dominant
• 2pq = frequency of heterozygous
• q² = frequency of homozygous recessive

Conditions for H-W Equilibrium:

• No mutation
• No selection (random mating)
• Large population (no genetic drift)
• No migration
• No selection pressure

⚡ A/L Key Point: When H-W equilibrium is broken (small population, selection, mutation, migration), evolution is occurring!

Genetic Drift:

Type	Mechanism	Effect
Bottleneck effect	Population drastically reduced	Founder effect — loss of alleles
Founder effect	Few individuals colonise new area	Reduced genetic diversity

⚡ Sri Lankan Example: The Sri Lankan elephant population has experienced bottleneck effects due to habitat fragmentation — genetic diversity is reduced compared to historical populations.

Speciation

What is a Species?:

Species Concept	Definition	Limitation
Biological	Can interbreed and produce fertile offspring	Doesn’t apply to asexual organisms
Morphological	Similar physical characteristics	Arbitrary boundaries
Phylogenetic	Common evolutionary lineage	Requires molecular data
Ecological	Distinct ecological niches	Can be overlapping

Mechanisms of Speciation:

Mechanism	Description	Example
Allopatric	Geographic isolation	Darwin’s finches (island separation)
Sympatric	Same area, different ecology or genetics	Cichlid fish in African lakes
Parapatric	Adjacent areas, partial overlap	Grass species on metal-rich soils

Rates of Evolution:

Rate	Pattern	Example
Gradualism	Slow, steady change	Most lineages
Punctuated equilibrium	Long stasis, rapid change	Fossil record — Cambrian explosion

⚡ A/L Important: The Cambrian explosion (~541 million years ago) was a period of rapid diversification of animal body plans — most major animal phyla appear in the fossil record within ~20 million years.

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🔴 Extended — Deep Study (3mo+)

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Evolution and Variation — Complete Notes for A/L Sri Lanka

Molecular Evolution

Molecular Clock Hypothesis:

• Mutations accumulate at roughly constant rate in DNA
• Number of differences between two species ∝ time since divergence
• Used to estimate when species diverged
• More reliable for genes that are functionally constrained (less selection pressure)

Cytochrome c:

Organism	Amino Acid Differences from Humans
Chimpanzee	0
Rhesus monkey	1
Horse	12
Yeast	45

DNA Hybridisation:

• Denature (heat) DNA from two species → reanneal → measure stability
• More similar DNA = higher reannealing temperature = closer evolutionary relationship

Artificial Selection and Domestication

Principles: Same as natural selection but humans choose traits

Crop	Trait Selected	Sri Lankan Example
Rice (Oryza sativa)	Grain size, yield, disease resistance	Red rice, samba, kiri hetti
Coconut	Palm height, nut size, copra content	King coconut (Red dwarf)
Mango (Mangifera indica)	Fruit size, fibreless flesh	Willard mango, Alphonso
Banana	Parthenocarpy, seedlessness	Ambul seedling, Embul banana

⚡ A/L Important: Polyploidy in wheat (Triticum) — natural hybridisation followed by chromosome doubling created hexaploid bread wheat (AABBDD genome), which is agronomically superior. This is a form of sympatric speciation in plants.

Human Evolution

Hominin Timeline:

Species	Approx. Age	Key Features
Sahelanthropus tchadensis	7-6 mya	Late Miocene, chimpanzee-sized brain
Australopithecus afarensis	3.9-2.9 mya	Lucy (AL 288-1), bipedal, small brain
Homo habilis	2.4-1.4 mya	Stone tools, larger brain
Homo erectus	1.9 mya-110 kya	Fire, erect posture, spread from Africa
Homo neanderthalensis	200-40 kya	Large brain, heavy brow ridges
Homo sapiens	300 kya-present	Fully modern, symbolic thought

Homo sapiens in South Asia:

• Early Homo sapiens in South Asia ~78,000 years ago (based on genetic evidence)
• The Vedda people of Sri Lanka — hunter-gatherers, considered the indigenous people
• Ancient Sri Lankan archaeological sites: Balangoda Man (Homo sapiens balangodensis) — late Pleistocene

Molecular Evidence for Human Evolution:

Evidence	What it Shows
Mitochondrial DNA (mtDNA)	All humans descended from one African woman (“Mitochondrial Eve”)
Y-chromosome studies	All humans descended from African man (Y-chromosome Adam)
>98% DNA similarity with chimpanzees	Close common ancestor (~6-7 mya)

⚡ A/L Common Mistake: Students confuse the timeline — humans did not evolve from monkeys. Humans and modern monkeys share a common monkey-like ancestor that lived ~25 million years ago. We share a more recent common ancestor with apes (chimpanzees ~6-7 mya).

####Adaptations in Plants

Xerophytes (dry adaptations):

Adaptation	Example	Function
Thick cuticle	Calotropis (Sri Lankan: wara)	Reduce water loss
Sunken stomata	Nerium (oleander)	Humid microclimate reduces transpiration
CAM photosynthesis	Pineapple, cactus	Stomata open at night — reduces water loss
Spines instead of leaves	Cactus	Reduce surface area
Deep taproot	Desert trees	Access water deep underground
Succulent stems	Cactus	Water storage

Hydrophytes (water adaptations):

Adaptation	Example	Function
Aerenchyma	Rice, water lily	Buoyancy, O₂ to roots
Floating leaves	Water lily	Receive light for photosynthesis
Submerged leaves	Vallisneria	Reduce water current resistance
Stomata on upper epidermis	Floating leaves	Gas exchange with air

Sri Lankan Xerophytes:

• Dry zone plants: Ziziphus (orphan tree), Salvadora persica (toothbrush tree)
• Coastal: Spinifex littoreus (sand dune grass), Casuarina (Australian pine, invasive)
• Montane: Strobilanthes species (Sri Lanka’s “mystery plant” — mass flowering every 7-12 years)

GCE A/L Sri Lanka Past Paper Tips

Common Structured Questions:

1. “State and explain Darwin’s theory of natural selection” (12 marks)
2. “What is meant by a species? Compare the biological and morphological species concepts” (10 marks)
3. “Explain the role of mutations in evolution” (8 marks)
4. “Describe the evidence for evolution from the fossil record” (10 marks)
5. “Using the Hardy-Weinberg principle, calculate genotype frequencies in a population” (10 marks)

Short Answer Questions:

1. “How does the fossil record provide evidence for evolution?”
2. “What is meant by adaptive radiation? Give an example.”
3. “Explain why antibiotic resistance in bacteria is an example of natural selection.”
4. “Distinguish between gene flow and genetic drift.”

Practical and Data-Based Questions:

• Hardy-Weinberg calculations (given allele frequencies, calculate genotype frequencies)
• Interpretation of phylogenetic trees
• Analysis of variation data (measuring seed weight, plant height — continuous variation)

⚡ A/L Strategy: Evolution questions require both knowledge AND application. Always link theory to specific examples (natural selection → peppered moth, antibiotic resistance; speciation → Darwin’s finches; human evolution → Australopithecus, Homo lineage).

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