Genetics and Evolution

This topic covers DNA structure, protein synthesis, inheritance, genetic crosses, genetic Engineering, the Hardy-Weinberg principle, and the theory of evolution by natural selection.

DNA Structure (OL/HL)

The Double Helix (OL/HL)

DNA is a double-stranded helix composed of nucleotides.

Nucleotide structure: phosphate group + deoxyribose sugar + nitrogenous base.

Bases:

• Adenine (A) pairs with Thymine (T) — two hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) — three hydrogen bonds.

Chargaff’s rule: [\mathrm{A] = [\mathrm{T] and [\mathrm{C] = [\mathrm{G].

DNA Replication (HL)

Semi-conservative replication: each new DNA molecule consists of one original strand and one Newly synthesised strand.

Process:

1. Helicase unwinds and separates the double helix.
2. DNA polymerase synthesises the new strand in the 5 prime to 3 prime direction.
3. The leading strand is synthesised continuously.
4. The lagging strand is synthesised in short fragments (Okazaki fragments), joined by DNA ligase.

Worked Example: DNA replication fork.

At a replication fork, the two strands of DNA are separated. DNA polymerase can only synthesise in The 5’ to 3’ direction. On the leading strand (oriented 3’ to 5’), synthesis is continuous. On the Lagging strand (oriented 5’ to 3’), synthesis must be discontinuous because DNA polymerase can only Move along the template in one direction.

The lagging strand is synthesised in short fragments called Okazaki fragments, each initiated by an RNA primer. DNA ligase then joins these fragments together.

Protein Synthesis (HL)

Transcription

DNA is used as a template to make messenger RNA (mRNA) in the nucleus.

1. RNA polymerase binds to the promoter region.
2. DNA unwinds; RNA polymerase synthesises mRNA using the DNA template.
3. MRNA is processed (introns removed, 5 prime cap and poly-A tail added).
4. MRNA exits the nucleus through nuclear pores.

Translation

MRNA is decoded by ribosomes to produce a polypeptide chain.

1. MRNA attaches to a ribosome.
2. The ribosome reads codons (groups of three bases).
3. Transfer RNA (tRNA) molecules bring amino acids, matching their anticodon to the mRNA codon.
4. Peptide bonds form between adjacent amino acids.
5. The polypeptide chain folds into a functional protein.

Start codon: AUG (methionine).

Stop codons: UAA, UAG, UGA.

Genetic Code

• Degenerate: most amino acids are coded by more than one codon.
• Universal: the same code is used by nearly all organisms.
• Non-overlapping: codons are read sequentially without overlap.

Inheritance

Mendel’s Laws (OL/HL)

1. Law of Segregation: each organism has two alleles for each gene, which segregate during gamete formation.
2. Law of Independent Assortment: alleles of different genes assort independently (unless linked).

Genetic Terminology (OL/HL)

Term	Definition
Gene	Section of DNA that codes for a protein
Allele	Different version of a gene
Genotype	The alleles an organism has (e.g., Bb)
Phenotype	The physical expression of the genotype
Homozygous	Two identical alleles (BB or bb)
Heterozygous	Two different alleles (Bb)
Dominant	Expressed in the phenotype when present (B)
Recessive	Only expressed when homozygous (bb)
Codominant	Both alleles are expressed in the phenotype

Monohybrid Crosses (OL/HL)

Example (OL): In pea plants, tall (T) is dominant over short (t). Cross two heterozygous tall Plants.

Parents: Tt x Tt

Punnett square:

	T	t
T	TT	Tt
t	Tt	tt

Genotype ratio: 1 TT : 2 Tt : 1 tt.

Phenotype ratio: 3 tall : 1 short.

Dihybrid Crosses (HL)

Example (HL): In pea plants, round seeds (R) are dominant over wrinkled (r), and yellow seeds (Y) are dominant over green (y). Cross RrYy x RrYy.

Genotypic ratio: 9 R_Y_ : 3 R_yy : 3 rrY_ : 1 rryy.

Phenotypic ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

Incomplete Dominance and Codominance (HL)

Incomplete dominance: the heterozygote has an intermediate phenotype.

Example: Red (RR) x White (rr) = Pink (Rr).

Codominance: both alleles are fully expressed.

Example: Blood group AB (I$^A$I$^B$) — both A and B antigens are present.

Sex-Linked Inheritance (HL)

Genes carried on the X chromosome show sex-linked inheritance.

Example (HL): Haemophilia is caused by a recessive allele on the X chromosome.

A carrier female (X$^H$X$^h$) x normal male (X$^H$Y):

	X$^H$	Y
X$^H$	X$^H$X$^H$ (normal female)	X$^H$Y (normal male)
X$^h$	X$^H$X$^h$ (carrier female)	X$^h$Y (haemophiliac male)

Males are more likely to be affected because they have only one X chromosome.

Genetic Engineering (HL)

Recombinant DNA Technology

1. Isolation: the desired gene is cut from DNA using restriction enzymes.
2. Insertion: the gene is inserted into a vector (e.g., plasmid) using DNA ligase.
3. Transformation: the vector is introduced into a host cell (e.g., bacterium).
4. Expression: the host cell produces the protein coded by the inserted gene.

Applications

• Insulin production: human insulin gene inserted into bacteria for mass production.
• GM crops: pest-resistant or herbicide-resistant crops.
• Gene therapy: inserting functional genes to treat genetic disorders.

PCR (Polymerase Chain Reaction) (HL)

Amplifies a segment of DNA in vitro.

Steps:

1. Denaturation: DNA is heated to 95$^\circ$C to separate strands.
2. Annealing: primers bind to the target sequence at 55—65$^\circ$C.
3. Extension: DNA polymerase (Taq polymerase) synthesises new strands at 72$^\circ$C.

Hardy-Weinberg Principle (HL)

The Principle

For a large, randomly mating population with no mutation, migration, or natural selection, allele And genotype frequencies remain constant from generation to generation.

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

Where:

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

Conditions

1. Large population size (no genetic drift).
2. Random mating.
3. No mutation.
4. No migration (no gene flow).
5. No natural selection.

Example (HL)

In a population, 16% of individuals show the recessive phenotype (attached earlobes). Find the Allele frequencies and genotype frequencies.

$$Q^2 = 0.16 \implies q = 0.4$$ $$P = 1 - 0.4 = 0.6$$ P^2 = 0.36 \quad (\mathrm{homozygous dominant) 2pq = 2(0.6)(0.4) = 0.48 \quad (\mathrm{heterozygous) Q^2 = 0.16 \quad (\mathrm{homozygous recessive)

So 36% are homozygous dominant, 48% are heterozygous, and 16% are homozygous recessive.

Testing for Equilibrium (HL)

Use the chi-squared ($\chi^2$) test to determine whether observed genotype frequencies differ Significantly from expected Hardy-Weinberg frequencies.

$$\chi^2 = \sum \frac{(O - E)^2}{E}$$

Compare to the critical value at the appropriate degrees of freedom (\mathrm{df = \mathrm{number of genotypes - \mathrm{number of alleles).

Worked Example: Hardy-Weinberg with multiple alleles.

The ABO blood group system has three alleles: $I^A$, $I^B$And $i$. In a population, the allele Frequencies are $p$, $q$And $r$ respectively, with $p + q + r = 1$.

Genotype frequencies are:

$(I^A I^A) = p^2, \quad (I^A i) = 2pr, \quad (I^B I^B) = q^2, \quad (I^B i) = 2qr, \quad (ii) = r^2$

If the blood type O frequency is 36%, then $r^2 = 0.36$So $r = 0.6$.

If the blood type AB frequency is 10%, then $q^2 = 0.10$So $q = 0.316$.

$p + q + r = 1$So $p = 1 - 0.6 - 0.316 = 0.084$.

Carrier frequency for blood type A (I$^A$I$):$2pr = 2(0.084)(0.6) = 0.101 = 10.1%$.

Evolution

Darwin’s Theory of Natural Selection (OL/HL)

1. Variation: individuals in a population vary in their traits.
2. Competition: resources are limited; individuals compete for survival.
3. Survival of the fittest: individuals with advantageous traits are more likely to survive and reproduce.
4. Inheritance: advantageous traits are passed to offspring.
5. Gradual change: over many generations, the frequency of advantageous alleles increases.

Evidence for Evolution (OL/HL)

1. Fossil record: shows gradual changes over time.
2. Comparative anatomy: homologous structures (similar origin, different function, e.g., pentadactyl limb).
3. Comparative embryology: early embryos of different species are similar.
4. Molecular evidence: similar DNA sequences in related species.
5. Biogeography: distribution of species reflects evolutionary history.

Speciation (HL)

Allopatric speciation: populations are geographically separated and evolve independently until They can no longer interbreed.

Sympatric speciation: new species arise within the same geographic area (e.g., polyploidy in Plants).

Types of Selection (HL)

• Directional selection: favours one extreme.
• Stabilising selection: favours the average, against extremes.
• Disruptive selection: favours both extremes, against the average.

Worked Examples

See the examples integrated throughout the sections above.

Common Pitfalls

1. DNA replication — leading strand is continuous, lagging strand uses Okazaki fragments.
2. Transcription vs translation — transcription occurs in the nucleus, translation at ribosomes.
3. Codominance vs incomplete dominance — codominance: both expressed; incomplete: intermediate.
4. Hardy-Weinberg — the equations apply to allele frequencies, not actual numbers of individuals.
5. Natural selection — acts on the phenotype, but evolution is a change in allele frequencies.
6. Sex-linked genes — males express X-linked recessive traits more frequently.
7. Confusing directional and stabilising selection. Directional shifts the mean; stabilising reduces variation around the mean.
8. Forgetting the conditions for Hardy-Weinberg. If any condition is violated, the population is evolving.

Practice Questions

Ordinary Level

1. Describe the structure of DNA.
2. A tall pea plant (Tt) is crossed with a short pea plant (tt). Show the Punnett square and give the phenotype ratio.
3. State three pieces of evidence for evolution.
4. Explain why males are more likely to show X-linked recessive conditions.

Higher Level

1. Describe the process of translation, including the roles of mRNA, tRNA, and ribosomes.

2. In a population, the frequency of cystic fibrosis (autosomal recessive) is 1 in 2500. Calculate the carrier frequency using Hardy-Weinberg.

3. Describe the process of DNA replication, naming the key enzymes involved.

4. Explain the difference between directional and stabilising selection with examples.

5. A population has the following blood group frequencies: O = 36%, A = 48%, B = 16%. Assuming a three-allele system (I$^A$I$^B$I), calculate the allele frequencies.

6. Explain how genetic engineering could be used to treat cystic fibrosis using gene therapy.

7. Use a chi-squared test to determine whether the following results fit a 9:3:3:1 ratio: 560, 180, 190, 70.

8. Describe three examples of molecular evidence that support the theory of evolution.

9. Explain the role of mutations in providing the genetic variation on which natural selection acts.

10. Explain why sympatric speciation is more common in plants than in animals.

Review: DNA Structure and Replication in Detail

The double helix: DNA consists of two antiparallel strands wound into a double helix. The Sugar-phosphate backbone is on the outside, and the nitrogenous bases project inward. The two Strands are held together by hydrogen bonds between complementary base pairs: A pairs with T (2 Hydrogen bonds) and G pairs with C (3 hydrogen bonds).

Chargaff’s rules: In any sample of DNA, the amount of adenine equals the amount of thymine ($A = T$), and the amount of guanine equals the amount of cytosine ($G = C$). Therefore, $A + G = T + C$ (purines = pyrimidines).

Worked Example: Applying Chargaff’s rules.

If a DNA molecule is 28% adenine, then it is also 28% thymine. Since $A + T + G + C = 100\%$: $G + C = 100 - 28 - 28 = 44\%$. Since $G = C$Both $G$ and $C$ are 22%.

The percentage of purines ($A + G$) = $28 + 22 = 50\%$. The percentage of pyrimidines ($T + C$) = $28 + 22 = 50\%$.

DNA replication (semiconservative):

Each strand of the original DNA molecule serves as a template for the synthesis of a new Complementary strand. After replication, each daughter DNA molecule contains one original strand and One new strand.

Key enzymes:

• Helicase: Unwinds the double helix by breaking hydrogen bonds between base pairs.
• DNA polymerase: Synthesises new DNA strands by adding nucleotides complementary to the template strand. Works in the 5’ to 3’ direction only.
• RNA primase: Synthesises short RNA primers to initiate DNA synthesis (DNA polymerase cannot start from scratch).
• DNA ligase: Joins Okazaki fragments on the lagging strand.

The leading strand is synthesised continuously. The lagging strand is synthesised in short Segments called Okazaki fragments, each requiring its own RNA primer. After synthesis, the RNA Primers are removed and replaced with DNA, and the fragments are joined by DNA ligase.

Review: Protein Synthesis in Detail

Transcription (in the nucleus):

1. RNA polymerase binds to the promoter region upstream of the gene.
2. The DNA double helix is unwound in the region to be transcribed.
3. RNA polymerase builds a complementary mRNA strand using the template strand of DNA. Uracil (U) replaces thymine (T) in RNA.
4. The mRNA is processed: a 5’ cap is added, a poly-A tail is added, and introns are removed by splicing.
5. The mature mRNA exits the nucleus through a nuclear pore.

Translation (at the ribosome):

1. The mRNA attaches to the ribosome.
2. The start codon (AUG) is read, and the first tRNA carries methionine to the ribosome.
3. The ribosome moves along the mRNA, reading one codon (three bases) at a time.
4. Each codon specifies a particular amino acid. The appropriate tRNA, carrying the correct amino acid, binds to the codon via its anticodon.
5. Peptide bonds form between adjacent amino acids.
6. Translation stops when a stop codon (UAA, UAG, or UGA) is reached.
7. The polypeptide chain is released and folds into its functional three-dimensional shape.

Worked Example: Reading the genetic code.

Given the DNA template strand: 3’-TAC GGA CTT CGA-5’

The mRNA (complementary to the template, with U replacing T): 5’-AUG CCU GAA GCU-3’

Reading the mRNA in triplets (codons):

• AUG = methionine (start)
• CCU = proline
• GAA = glutamic acid
• GCU = alanine

The polypeptide: Met-Pro-Glu-Ala

Review: Evolution — Natural Selection in Detail

Darwin’s theory of natural selection (1859, On the Origin of Species):

1. Variation: Individuals within a species show variation in their characteristics (due to genetic differences — mutations, meiosis, sexual reproduction).
2. Competition: There is competition for limited resources (food, mates, territory).
3. Survival of the fittest: Individuals with characteristics better suited to the environment are more likely to survive and reproduce.
4. Inheritance: Advantageous characteristics are passed to offspring (through genes).
5. Gradual change: Over many generations, the frequency of advantageous alleles increases in the population.

Key point: “Fitness” in biology means reproductive success, not physical strength. An organism That produces more offspring that survive to reproduce is more “fit” than one that produces fewer.

Worked Example: Antibiotic resistance as natural selection.

1. A bacterial population contains genetic variation. Most bacteria are susceptible to a particular antibiotic, but a few have a mutation that makes them resistant.
2. When the antibiotic is applied, susceptible bacteria are killed.
3. The resistant bacteria survive and reproduce, passing the resistance allele to their offspring.
4. Over many generations, the frequency of the resistance allele increases in the population.
5. The population has evolved antibiotic resistance through natural selection.

This is a classic example of directional selection: the antibiotic creates a strong selection Pressure that favours the resistant phenotype. The problem is worsened by the overuse of antibiotics In medicine and agriculture.

Review: Evidence for Evolution

Fossil record: Shows a sequence of changes in organisms over geological time. Older rock layers Contain simpler organisms; younger layers contain more complex organisms. Transitional fossils (e.g., Archaeopteryx, Tiktaalik) show intermediate forms between major groups.

Comparative anatomy: Homologous structures (same basic structure, different function, e.g., Pentadactyl limb) suggest common ancestry. Analogous structures (different structure, similar Function, e.g., wings of birds and insects) suggest convergent evolution. Vestigial structures (e.g., human appendix, pelvic bones in whales) are remnants of structures that were functional in Ancestors.

Comparative embryology: Early embryos of all vertebrates are remarkably similar, with features Such as pharyngeal pouches and a post-anal tail. This suggests a common ancestry.

Molecular evidence: DNA and protein sequence comparisons show that closely related species have More similar sequences. The genetic code is nearly universal (the same codons code for the same Amino acids in almost all organisms), which is strong evidence for a common origin of all life.

Biogeography: The distribution of species around the world reflects evolutionary history. Island Species are often similar to those on the nearest mainland but have evolved differences due to Isolation (e.g., Darwin’s finches on the Galapagos Islands).

Review: Genetic Engineering — Applications and Ethics

Insulin Production Using Recombinant DNA Technology

1. The human insulin gene is identified and cut from human DNA using the restriction enzyme EcoRI.
2. A plasmid vector is cut with the same enzyme, producing complementary sticky ends.
3. The insulin gene is inserted into the plasmid using DNA ligase.
4. The recombinant plasmid is introduced into E. Coli bacteria by transformation.
5. The bacteria are grown in large fermenters, producing human insulin.
6. The insulin is extracted, purified, and used to treat diabetes.

Advantages of recombinant insulin: Identical to human insulin; produced in large quantities; no Risk of transmitting animal diseases (unlike pig insulin, which was previously used).

Gene Therapy

Gene therapy involves inserting a functional copy of a gene into a patient’s cells to treat a Genetic disorder.

Somatic gene therapy: Targets body cells (not germ cells). Changes are not inherited.

Germline gene therapy: Targets egg or sperm cells. Changes can be passed to offspring. This is More controversial because it affects future generations.

Example: Cystic fibrosis is caused by a mutation in the CFTR gene. Gene therapy involves Delivering a normal copy of the CFTR gene to the patient’s lung cells using a viral vector (a Modified virus that carries the gene). This can potentially cure or alleviate the symptoms of cystic Fibrosis.

Ethical Concerns of Genetic Engineering

1. Safety: Unknown long-term effects of inserting foreign genes into organisms. Could GM crops have unforeseen effects on ecosystems?
2. GM crops: Could transfer herbicide resistance to weeds (superweeds). Could reduce biodiversity if monocultures replace diverse ecosystems.
3. Gene therapy: Could have unintended effects on other genes (off-target effects). Germline gene therapy raises ethical questions about modifying human embryos.
4. Social justice: GM crops and gene therapies are expensive. Could widen the gap between rich and poor countries?
5. Animal welfare: Genetic modification of animals raises animal welfare concerns.

Review: Types of Selection and Speciation in Detail

Directional Selection

Favours one extreme of the phenotype distribution, shifting the mean in that direction.

Example: The peppered moth (Biston betularia) in industrial England. Before industrialisation, The light-coloured form was well camouflaged against lichen-covered bark. During industrialisation, Soot darkened tree bark, and the dark-coloured form became better camouflaged. The frequency of the Dark allele increased through natural selection. After clean-air legislation, the frequency of the Light form increased again as bark lightened.

Stabilising Selection

Favours the intermediate phenotype and selects against both extremes. This reduces genetic Variation.

Example: Human birth weight. Babies with very low or very high birth weights have higher Mortality. The intermediate birth weight (approximately 3.5 kg) has the highest survival rate. This Is stabilising selection maintaining the average birth weight.

Disruptive Selection

Favours both extremes and selects against the intermediate. This can lead to the formation of two Distinct phenotypes in the population.

Example: In African seedcracker finches, birds with very large or very small beaks are favoured Because the available seeds are either very large or very small. Birds with intermediate beaks are Less efficient at handling either seed type and have lower fitness. Over time, disruptive selection Can lead to the population splitting into two distinct groups.

Allopatric Speciation

Speciation that occurs when populations are geographically separated. The key steps:

1. A geographical barrier separates a population into two subpopulations.
2. Each subpopulation experiences different selection pressures and accumulates different mutations.
3. Over time, genetic differences accumulate to the point where the populations can no longer interbreed to produce fertile offspring.
4. Even if the geographical barrier is removed, the populations remain reproductively isolated.

Example: The Kaibab squirrel (north rim of the Grand Canyon) and the Abert squirrel (south rim) Have been separated by the Grand Canyon for approximately 10,000 years and are now considered Separate species.

Sympatric Speciation

Speciation without geographic separation. This is much rarer in animals but common in plants, through polyploidy.

Polyploidy: An organism has more than two complete sets of chromosomes. Autopolyploidy involves Duplication of the same genome. Allopolloidy involves combining genomes from different species.

Example: Wheat (Triticum aestivum) is a hexaploid ($6n$) that arose through two allopolyploidy Events, combining genomes from three different wild grass species.

Review: Hardy-Weinberg — Worked Problems

Worked Example 1: Determining if a population is in equilibrium.

A population has the following genotype counts: AA = 360, Aa = 480, aa = 160. Total = 1000.

Calculate allele frequencies:

$p = \frac{2(360) + 480}{2(1000)} = \frac{1200}{2000} = 0.6$

$q = \frac{2(160) + 480}{2(1000)} = \frac{800}{2000} = 0.4$

Expected (under H-W): $p^2 = 0.36$ (AA = 360), $2pq = 0.48$ (Aa = 480), $q^2 = 0.16$ (aa = 160).

The observed frequencies exactly match the expected Hardy-Weinberg frequencies. The population is in Equilibrium.

Worked Example 2: Cystic fibrosis carrier frequency.

Cystic fibrosis is an autosomal recessive disorder. The incidence is approximately 1 in 2500 births In some populations.

$q^2 = 1/2500 = 0.0004$.

$q = \sqrt{0.0004} = 0.02$.

$p = 1 - 0.02 = 0.98$.

Carrier frequency ($2pq$) = $2 \times 0.98 \times 0.02 = 0.0392 = 3.92\%$.

Approximately 1 in 25 people in this population is a carrier for cystic fibrosis.

Worked Example 3: A population not in equilibrium.

A population has genotype counts: AA = 400, Aa = 200, aa = 400. Total = 1000.

$p = \frac{2(400) + 200}{2(1000)} = \frac{1000}{2000} = 0.5$.

$q = \frac{2(400) + 200}{2(1000)} = \frac{1000}{2000} = 0.5$.

Expected (under H-W): $p^2 = 0.25$ (AA = 250), $2pq = 0.50$ (Aa = 500), $q^2 = 0.25$ (aa = 250).

Observed: AA = 400, Aa = 200, aa = 400.

The observed frequencies do not match the expected Hardy-Weinberg frequencies. There is a Significant excess of homozygotes (AA and aa) and a deficit of heterozygotes. This pattern suggests Non-random mating, such as inbreeding or positive assortative mating (individuals with similar Genotypes preferentially mate with each other).

Using the chi-squared test:

$\chi^2 = \frac{(400-250)^2}{250} + \frac{(200-500)^2}{500} + \frac{(400-250)^2}{250} = \frac{22500}{250} + \frac{90000}{500} + \frac{22500}{250} = 90 + 180 + 90 = 360$

Degrees of freedom = $3 - 1 = 2$. Critical value at 5% for 2 df = 5.991.

Since $\chi^2 = 360 \gt 5.991$The population is significantly different from Hardy-Weinberg Equilibrium.

Types of natural selection:

Natural selection can act on phenotypic variation in different ways, depending on the relationship Between fitness and the trait:

1. Directional selection:

Favours individuals at one extreme of the phenotypic range. The mean value of the trait shifts in One direction over time. Example: Antibiotic resistance in bacteria — when antibiotics are applied, Bacteria with resistance genes have higher survival and reproduction, shifting the population Towards resistance.

2. Stabilising selection:

Favours individuals with intermediate phenotypes and selects against both extremes. Reduces Variation and maintains the status quo. Example: Human birth weight — very low and very high birth Weights have higher mortality, so the intermediate range is favoured.

3. Disruptive selection:

Favours individuals at both extremes and selects against the intermediate phenotype. Can lead to Speciation if the two extremes become reproductively isolated. Example: African seedcracker finches — birds with either large or small beaks have higher survival (feeding on large or small seeds Respectively), while birds with intermediate beak sizes are less efficient at both.

Worked Example: Identifying the type of selection.

A population of snakes varies in colour from light to dark. In a forest habitat, medium-coloured Snakes are best camouflaged against the bark. Light and dark snakes are more visible to predators.

This is stabilising selection: the intermediate phenotype (medium colour) has the highest Fitness, and both extremes are selected against. Over time, the variance in colour decreases.

If the forest is replaced by a dark lava field, dark snakes become best camouflaged. This shifts to directional selection: the dark extreme is now favoured, and the mean colour darkens over Generations.

Speciation:

Speciation is the formation of new species. The biological species concept defines a species as a Group of organisms that can interbreed to produce fertile offspring. Speciation requires Reproductive isolation, which can occur through:

• Geographic (allopatric) speciation: Populations are physically separated by a geographic barrier (mountain range, river, ocean). Over time, genetic differences accumulate until the populations can no longer interbreed even if brought back together.
• Sympatric speciation: New species form without geographic isolation, often through ecological specialisation or polyploidy (common in plants).
• Pre-zygotic barriers: Prevent mating or fertilisation (e.g., differences in mating behaviour, breeding season, genital morphology, or gamete incompatibility).
• Post-zygotic barriers: Reduce the fitness of hybrid offspring (e.g., hybrid inviability, hybrid sterility as in mules, or hybrid breakdown in subsequent generations).

Evidence for evolution:

In addition to molecular evidence, there are several lines of evidence supporting the theory of Evolution by natural selection:

1. The fossil record:

Fossils provide a record of organisms that lived in the past. The fossil record shows:

• A progression from simple to more complex organisms over geological time.
• Species appearing and disappearing at different times (extinction).
• Transitional forms that show intermediate characteristics between groups (e.g., Archaeopteryx, which has features of both reptiles and birds; Tiktaalik, which shows features intermediate between fish and tetrapods).

Limitations of the fossil record:

• Fossilisation is rare and requires specific conditions (rapid burial, absence of oxygen, presence of hard parts).
• Many organisms have no hard parts and leave no fossils.
• The fossil record is biased towards organisms that lived in certain environments (marine, tropical).
• Only a tiny fraction of organisms that have ever lived are represented as fossils.

2. Comparative anatomy:

Homologous structures (structures with a common evolutionary origin) provide evidence for common Descent. The pentadactyl limb (five-digit limb) is found in mammals, birds, reptiles, and Amphibians, with the same basic bone arrangement (humerus, radius, ulna, carpals, metacarpals, Phalanges) adapted for different functions (grasping, swimming, flying, running).

3. Biogeography:

The geographic distribution of species provides evidence for evolution. Species on oceanic islands Are often more similar to species on the nearest mainland than to species on other islands, even if The other islands have similar environments. This is consistent with species colonising islands from The mainland and then diverging through natural selection.

4. Embryology:

Early embryonic stages of vertebrates are remarkably similar, suggesting a common ancestry. For Example, all vertebrate embryos have pharyngeal pouches (which develop into gills in fish and into Parts of the ear and throat in mammals) and a post-anal tail.

Summary

This topic covers the biological principles of genetics and evolution, including key concepts, experimental evidence, and real-world applications.

Key concepts include:

• Mendelian inheritance
• gene expression and regulation
• mutations and genetic variation
• genetic engineering (PCR, gel electrophoresis)
• genome projects

Success requires the ability to recall specific factual content, apply knowledge to novel scenarios, and evaluate experimental evidence critically.