Variation and Evolution

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Variation refers to the differences between individuals in a population. It arises from genetic differences (heritable) and environmental influences (non-heritable). Understanding variation is fundamental to evolution.

Types of Variation:

1. Continuous Variation: Shows a range of phenotypes with no sharp boundaries. Examples: height, weight, blood pressure, skin colour. Determined by many genes (polygenic) and environmental factors.

2. Discontinuous Variation: Shows distinct categories with no intermediate forms. Examples: blood groups (A, B, AB, O), ability to roll tongue, gender. Determined by single genes (monogenic).

Causes of Variation:

• Genetic factors: Different alleles inherited from parents (gene recombination, mutations)
• Environmental factors: Nutrition, climate, disease exposure
• Combined: Both genetic and environmental (e.g., height — genes set potential, nutrition determines achievement)

Key Definitions:

• Gene: A section of DNA that codes for a specific protein/characteristic
• Allele: Different versions of a gene (e.g., allele for brown eyes vs blue eyes)
• Dominant allele: Always expressed in phenotype (capital letter, e.g., B)
• Recessive allele: Only expressed when homozygous (lowercase, e.g., b)
• Homozygous: Both alleles the same (BB or bb)
• Heterozygous: Different alleles (Bb)
• Genotype: Genetic makeup (BB, Bb, bb)
• Phenotype: Physical appearance (brown eyes or blue eyes)

Monohybrid Cross Example: Cross: Pure-breeding tall (TT) × Pure-breeding short (tt)

• P generation: TT × tt
• F₁ generation: All Tt (tall)
• F₂ generation: TT : Tt : tt = 1 : 2 : 1 (phenotypically: 3 tall : 1 short)

⚡ WAEC Tip: In monohybrid cross questions, always show your gametes. Remember that recessive traits only show in phenotype when homozygous. Draw a Punnett square to visualise offspring ratios.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of variation and evolution.

DNA and Inheritance:

Structure of DNA:

• Double helix (Watson and Crick, 1953)
• Sugar-phosphate backbone (deoxyribose + phosphate)
• Complementary base pairs: A-T and G-C
• 3 billion base pairs in human genome
• Genes are sequences of DNA that code for proteins

DNA Replication:

• Semi-conservative (each new DNA molecule has one old and one new strand)
• Enzyme: DNA polymerase
• Occurs during S-phase of cell division

Meiosis and Genetic Variation: Meiosis (gamete production) creates genetic variation through:

1. Crossing over: Homologous chromosomes exchange genetic material
2. Independent assortment: Random alignment of chromosome pairs during meiosis I
3. Random fertilisation: Any sperm can fuse with any egg

Mendel’s Laws:

1. Law of Dominance: In a heterozygote, the dominant allele masks the recessive allele
2. Law of Segregation: Alleles separate during gamete formation
3. Law of Independent Assortment: Alleles of different genes segregate independently (true for genes on different chromosomes)

Codominance: Both alleles are fully expressed in the phenotype. Example: Flower colour — red (R) × white (r) → RW = roan (pink) flowers.

Multiple Alleles: More than two alleles exist for a gene. Example: Human blood groups have three alleles: Iᴬ, Iᴮ, i. Four phenotypes: A (IᴬIᴬ or Iᴬi), B (IᴮIᴮ or Iᴮi), AB (IᴬIᴮ), O (ii).

Blood Group Genetics in Nigeria: Blood group O is the most common in Nigeria (~50% of population). This is important for blood transfusions — knowing ABO and Rhesus (Rh) factors is critical.

Sex Determination:

• Human: 46 chromosomes (23 pairs)
• 22 pairs: Autosomes (same in both sexes)
• 1 pair: Sex chromosomes (XX = female, XY = male)
• Father determines sex of child — he contributes either X or Y

Evolution: Definition: Change in allele frequencies in a population over time.

Evidence for Evolution:

1. Fossil record: Shows changes in organisms over geological time
2. Comparative anatomy: Homologous structures (common ancestry) vs analogous structures (convergent evolution)
3. Comparative biochemistry: Similar DNA sequences, proteins across species
4. Geographic distribution: Species distribution on continents and islands
5. Selective breeding: Dogs, cattle, crops — artificial selection

Natural Selection: Proposed by Charles Darwin (1859):

1. Within a population, individuals vary
2. Some variations are heritable
3. More offspring are produced than can survive (struggle for existence)
4. Individuals with advantageous traits are more likely to survive and reproduce (survival of the fittest)
5. Over generations, advantageous traits become more common

Speciation: Formation of new species:

• Allopatric speciation: Geographic isolation (e.g., separated by mountains, oceans)
• Sympatric speciation: Without geographic separation (e.g., different food sources, polyploidy in plants)

⚡ Common Student Mistakes: Confusing gene and allele. Confusing genotype and phenotype. Thinking evolution is “survival of the fittest” when it should be “survival of those best adapted to current conditions.” Thinking individual organisms evolve — WRONG, populations evolve.

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Hardy-Weinberg Equilibrium: A mathematical model for studying evolution. Allele frequencies remain constant if no evolutionary forces act.

p² + 2pq + q² = 1 Where:

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

Conditions for Hardy-Weinberg:

• No mutation
• No natural selection
• Large population (no genetic drift)
• Random mating
• No migration

Mutation:

• Gene mutations: Changes in DNA sequence (point mutations, frameshift mutations)
• Chromosomal mutations: Changes in chromosome structure (deletions, duplications, inversions, translocations)
• Chromosomal number mutations: Aneuploidy (extra/missing chromosome), polyploidy

Mutations and Evolution:

• Most mutations are neutral or harmful
• Rare beneficial mutations can spread through population by natural selection
• Mutation rate: ≈ 1 in 10⁶ gene replications (but some genes have higher rates)
• Ionising radiation and certain chemicals increase mutation rates

Genetic Drift: Random change in allele frequencies, especially in small populations. Examples:

• Bottleneck effect: Population drastically reduced → allele frequencies shift
• Founder effect: Small group colonises new area → different allele frequencies

Human Evolution:

• Primates: Lemurs, monkeys, apes, humans share common ancestor (~65 million years ago)
• Hominids: Humans and great apes (chimpanzees, gorillas, orangutans)
• Australopithecus: “Lucy” (3.2 million years ago) — walked upright, small brain
• Homo habilis: “Handy man” — used tools (2.4 million years ago)
• Homo erectus: Fire, complex tools, migrated out of Africa (1.9 million years ago)
• Homo neanderthalensis: Neanderthals — burial practices, art (200,000-30,000 years ago)
• Homo sapiens: Modern humans — language, agriculture, technology

** Lamarckian vs Darwinian Evolution:**

• Lamarck: “Inheritance of acquired characteristics” — organisms develop traits during lifetime and pass them on (e.g., giraffes stretching necks → longer necks in offspring) — DISPROVEN
• Darwin: Natural selection — variation exists, environment selects those best adapted — ACCEPTED

Adaptations:

Type	Example	Explanation
Structural	Camel’s hump	Stores fat for food/water
Behavioural	Hibernation	Reduces metabolic rate in winter
Physiological	Snake venom	Digests prey faster

Variation in Nigerian Populations: Nigeria has enormous genetic diversity due to its many ethnic groups (over 250). Different ethnic groups show variation in:

• Sickle cell allele frequency (highest in malaria zones)
• Lactase persistence (adult ability to digest milk)
• Skin colour gradation (darker in south, lighter in north — correlation with UV exposure)

Natural Selection Examples:

1. Peppered moth: Dark moths increased in industrial England (birds couldn’t see light moths on dark trees)
2. Antibiotic resistance: Bacteria with resistant alleles survive antibiotic treatment
3. Sickle cell anaemia: Heterozygotes (HbA HbS) resistant to malaria — balanced polymorphism

DNA Technology and Evolution:

• DNA sequencing: Compare gene sequences across species
• PCR (Polymerase Chain Reaction): Amplify tiny DNA samples for analysis
• DNA hybridisation: Measure evolutionary relationships
• Gel electrophoresis: Separate DNA fragments by size

Key Evolutionary Dates:

• Earth formed: 4.6 billion years ago
• First life: ~3.8 billion years ago
• First multicellular organisms: ~600 million years ago
• Cambrian explosion: 540 million years ago (rapid diversification)
• Dinosaurs: 252-66 million years ago
• Mammals diversified: after dinosaur extinction

⚡ WAEC Examination Patterns: Draw Punnett squares for monohybrid crosses and dihybrid crosses. Explain the difference between dominant and recessive alleles. Describe how natural selection leads to evolution. Explain the difference between continuous and discontinuous variation. Calculate expected offspring ratios.