Variation and Evolution

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Variation and Evolution — Quick Facts

Key Definitions:

• Variation: Differences between individuals of the same species
• Evolution: Change in the inherited characteristics of a population over successive generations
• Mutation: Sudden, heritable change in DNA sequence
• Gene: A unit of heredity found at a specific locus on a chromosome
• Allele: Alternative forms of a gene controlling different traits

Types of Variation:

Type	Cause	Example
Continuous	Polygenes (multiple genes)	Height, skin colour
Discontinuous	Single gene with major effect	Blood groups, cystic fibrosis

Sources of Variation:

• Gene mutations: Changes in DNA sequence
• Chromosomal mutations: Changes in chromosome structure or number
• Genetic recombination: Mixing of genes during meiosis (crossing over)
• Independent assortment: Random separation of homologous chromosomes

⚡ Exam Tip (NABTEB): NABTEB frequently asks about the difference between continuous and discontinuous variation. Remember: continuous = bell curve with intermediate forms; discontinuous = distinct categories with no intermediate forms.

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🟡 Standard — Regular Study (2d–2mo)

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Variation and Evolution — Study Guide

Understanding Variation

Variation refers to the differences observed among individuals within a population. These differences can be morphological (physical appearance), physiological (body functions), or behavioural.

Continuous Variation:

Continuous variation shows a bell-shaped distribution when plotted on a graph. Characteristics include:

• Controlled by many genes (polygenic inheritance)
• Affected by environmental factors
• Shows all intermediate forms between two extremes
• Examples: Height in humans (range 150–190cm), seed size in plants, milk yield in cattle

The normal distribution curve applies — most individuals cluster around the mean, with fewer at extremes.

Discontinuous Variation:

Discontinuous variation produces distinct categories with no intermediate forms:

• Controlled by single genes or a few major genes
• Largely unaffected by environment
• Examples: ABO blood groups, gender, fingerprint patterns, seed shape (round vs. wrinkled in peas)

Environmental Effects on Variation:

While genetic constitution (genotype) sets the potential, environment influences expression:

• Modifications: Changes in phenotype due to environment, not inherited (e.g., tanning from sun exposure)
• Norm of reaction: The range of phenotypes a genotype can produce in different environments
• Example: Identical twins with same genes can have different weights depending on diet and exercise

Sources of Genetic Variation:

1. Mutation: Permanent changes in DNA

   • Point mutations: Single base pair changes (e.g., sickle cell anaemia)
   • Chromosomal mutations: Deletions, duplications, inversions, translocations
   • Polyploidy: Addition of complete chromosome sets (common in plants)

2. Sexual Reproduction:

   • Crossing over: Exchange of genetic material between homologous chromosomes during meiosis
   • Independent assortment: Random distribution of maternal and paternal chromosomes to gametes
   • Random fertilisation: Any sperm can fuse with any egg, creating unique combinations

Evolution: Darwin’s Theory

Charles Darwin proposed natural selection as the mechanism of evolution in “On the Origin of Species” (1859).

Darwin’s Postulates:

1. Individuals within a population show variation
2. Some variations are heritable
3. More offspring are produced than can survive (struggle for existence)
4. Individuals with favourable variations are more likely to survive and reproduce (survival of the fittest)
5. Over time, favourable traits accumulate in the population

Evidence for Evolution:

1. Fossil record: Shows changes in organisms over geological time
2. Comparative anatomy: Homologous structures (common ancestry), analogous structures (common function, different ancestry)
3. Comparative biochemistry: Similar DNA sequences, protein structures
4. Biogeography: Distribution of species across continents
5. Natural selection experiments: Darwin’s finches, peppered moth

Common NABTEB Mistakes:

• Confusing Lamarck’s inheritance of acquired characteristics with Darwin’s natural selection
• Forgetting that natural selection acts on existing variation, not creating new traits
• Mixing up gene and chromosome mutations

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🔴 Extended — Deep Study (3mo+)

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Variation and Evolution — Comprehensive Notes

Mendelian Genetics and Variation

Gregor Mendel’s experiments with pea plants established the fundamental laws of inheritance:

Law of Segregation: During gamete formation, paired alleles separate so each gamete receives one allele from each pair.

Law of Independent Assortment: Alleles for different genes assort independently during gamete formation (applies to genes on different chromosomes).

Monohybrid Cross: For a trait controlled by a single gene with two alleles (dominant allele = T, recessive allele = t):

	Parent 1 genotype	Parent 2 genotype
	Tt	Tt
Offspring genotypes:	TT : Tt : tt
Offspring phenotypes:	Tall : Short
Ratio:	3 : 1 (if complete dominance)

Dihybrid Cross: For two traits segregating independently (YyRr × YyRr):

• 9:3:3:1 phenotypic ratio in F2 generation
• 16 genotypic combinations

Codominance and Multiple Alleles:

Codominance: Both alleles are expressed equally in heterozygotes.

• Example: MN blood group system (L^M and L^N are codominant)
• AB blood group: I^A and I^B are codominant

Multiple Alleles: A gene with three or more allele forms.

• Example: ABO blood group — three alleles (I^A, I^B, i) with six genotypes but four phenotypes

Blood Group Genetics:

Genotype	Blood Group
I^A I^A or I^A i	A
I^B I^B or I^B i	B
I^A I^B	AB
ii	O

The Hardy-Weinberg Principle:

This principle provides a mathematical model for studying evolution. Allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary influences.

Equilibrium equation: p² + 2pq + q² = 1

Where:

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

Factors disrupting Hardy-Weinberg equilibrium:

• Mutation
• Genetic drift (random changes in small populations)
• Migration (gene flow)
• Natural selection
• Non-random mating

Natural Selection in Detail:

Types of Natural Selection:

1. Directional selection: One extreme phenotype is favoured (e.g., long necks in giraffes)

2. Stabilising selection: Intermediate phenotype is favoured (e.g., human birth weight — too small or too large increases mortality)

3. Disruptive selection: Both extreme phenotypes are favoured over intermediate forms (e.g., African seed-cracking finches)

4. Sexual selection: Traits increasing mating success, even if reducing survival (e.g., peacock’s tail, deer antlers)

Mechanisms of Evolution:

Gene Flow: Transfer of genetic material between populations through migration. Increases genetic diversity within populations but makes populations more similar to each other.

Genetic Drift: Random change in allele frequencies, most significant in small populations. Two types:

• Bottleneck effect: Population drastically reduced, surviving alleles become fixed or lost
• Founder effect: New population established by very few individuals

Speciation:

Allopatric speciation: Geographic separation of a population (mountains, rivers, islands) leading to reproductive isolation.

Sympatric speciation: New species arise without geographic separation (common in plants through polyploidy).

Reproductive Isolation Mechanisms:

• Pre-zygotic: Habitat isolation, temporal isolation, behavioural isolation, mechanical isolation, gametic isolation
• Post-zygotic: Hybrid inviability, hybrid sterility, hybrid breakdown

Human Evolution:

Evidence for human evolution:

• Fossil record (Australopithecus → Homo habilis → Homo erectus → Homo sapiens)
• DNA analysis showing common ancestry with chimpanzees (~98.6% shared genes)
• Vestigial structures (appendix, wisdom teeth, tailbone)
• Atavisms (occasional throwbacks like tail formation in babies)

Homo sapiens characteristics:

• Bipedal locomotion
• Large brain (average 1400 cm³)
• Reduced brow ridges
• Chin presence
• Reduced body hair
• Extended post-natal development

⚡ NABTEB Exam Pattern: NABTEB questions on evolution often require:

• Explaining Darwin’s theory with examples
• Distinguishing between different types of selection
• Calculating expected offspring ratios
• Identifying evidence supporting evolution

Expect 5-8 marks questions on: sources of variation, Darwin’s natural selection, difference between Lamarckism and Darwinism, and the Hardy-Weinberg equation.

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