Directional vs Stabilising Selection, Species Diversity Index Formula & Courtship Behaviour
Genetic variation generated by random gene mutations, meiosis (crossing over and independent segregation), and random fertilisation provides the raw material upon which natural selection acts. Depending on whether the environment is changing or stable, selection pressures drive directional selection (shifting the population mean toward one advantageous extreme, such as antibiotic resistance in bacteria) or stabilising selection (eliminating both extremes around an optimal mean, such as human birth mass). Ecologists quantify the impact of human agriculture and conservation on ecosystems using Simpson's Index of Diversity, D = N(N - 1) / Σn(n - 1), which accounts for both species richness and relative abundance. Examiners test your mastery of the 5-step natural selection mark scheme template, phylogenetic hierarchy rules, and biodiversity calculations.
- Natural Selection 5-Step Mark Scheme Template
- 1. Pre-existing genetic variation exists within a population due to random mutations creating new alleles. 2. A selection pressure (e.g. predation, disease, antibiotic, climate change) acts on the population. 3. Individuals possessing the advantageous allele have a higher chance of survival and reproductive success (differential survival). 4. Surviving individuals reproduce and pass on the advantageous allele to their offspring. 5. Over many generations, the frequency of the advantageous allele increases in the gene pool.
- Directional vs Stabilising Selection
- Directional selection occurs when environmental conditions change, favouring individuals with phenotypes at one extreme of a normal distribution (e.g. penicillin resistance in bacteria); the mean shifts toward the favoured extreme. Stabilising selection occurs in a stable, unchanging environment, favouring phenotypes close to the mean and selecting against both extremes (e.g. human birth weight around ~3.2 kg); the mean stays constant while the standard deviation narrows.
- Species Concept & Courtship Behaviour
- Two organisms belong to the same biological species if they can interbreed to produce fertile offspring. Courtship behaviour is a genetically determined, species-specific sequence of visual, auditory, or chemical (pheromone) signals that enables individuals to: 1. Recognise members of their own species (ensuring fertile offspring), 2. Identify a mate capable of breeding (sexually mature/in season), 3. Form a pair bond, and 4. Synchronise mating so gametes are released at maximum probability of fertilisation.
Directional vs Stabilising Selection, Species Diversity Index Formula & Courtship Behaviour: Enzyme Collision, Active Site & Rate Curve Lab
Drag the temperature, pH, and substrate sliders to watch how kinetic collisions, active-site binding, and denaturation change the reaction rate in real time.
Key Concepts & Exam Definitions (6)
- Natural Selection 5-Step Mark Scheme Template
- 1. Pre-existing genetic variation exists within a population due to random mutations creating new alleles. 2. A selection pressure (e.g. predation, disease, antibiotic, climate change) acts on the population. 3. Individuals possessing the advantageous allele have a higher chance of survival and reproductive success (differential survival). 4. Surviving individuals reproduce and pass on the advantageous allele to their offspring. 5. Over many generations, the frequency of the advantageous allele increases in the gene pool.
- Directional vs Stabilising Selection
- Directional selection occurs when environmental conditions change, favouring individuals with phenotypes at one extreme of a normal distribution (e.g. penicillin resistance in bacteria); the mean shifts toward the favoured extreme. Stabilising selection occurs in a stable, unchanging environment, favouring phenotypes close to the mean and selecting against both extremes (e.g. human birth weight around ~3.2 kg); the mean stays constant while the standard deviation narrows.
- Species Concept & Courtship Behaviour
- Two organisms belong to the same biological species if they can interbreed to produce fertile offspring. Courtship behaviour is a genetically determined, species-specific sequence of visual, auditory, or chemical (pheromone) signals that enables individuals to: 1. Recognise members of their own species (ensuring fertile offspring), 2. Identify a mate capable of breeding (sexually mature/in season), 3. Form a pair bond, and 4. Synchronise mating so gametes are released at maximum probability of fertilisation.
- Phylogenetic Classification & Taxonomic Hierarchy
- Phylogenetic classification arranges species into evolutionary groups based on shared common ancestry, producing a hierarchy (taxa inside larger taxa with no overlap): Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species (Binomial name: Genus species). Modern taxonomy compares base sequences of DNA/mRNA, amino acid sequences of proteins (e.g. cytochrome c), and immunological serum protein precipitation.
- Simpson's Index of Diversity: D = N(N - 1) / Σn(n - 1)
- Species richness only counts the number of different species present, ignoring population sizes. Index of diversity (d or D) measures both species richness and the number of individuals of each species: d = N(N - 1) / Σn(n - 1), where N = total number of organisms of all species and n = total number of organisms of each individual species. A higher value of d indicates greater biodiversity and a more stable ecosystem resilient to environmental change.
- Genetic Diversity in Populations (Meiosis & Bottlenecks)
- Meiosis generates genetic variation via: 1. Crossing over between non-sister chromatids of homologous chromosomes in Prophase I (producing new combinations of linked alleles), and 2. Independent segregation of homologous chromosomes in Metaphase/Anaphase I (2ⁿ possible chromosome combinations in gametes, where n = haploid number, or (2ⁿ)² after random fertilisation). Genetic bottlenecks and founder effects sharply reduce allele diversity in a population.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
How do the normal distribution graphs of directional and stabilising selection differ over several generations?
Write down the formula for the Index of Diversity (d) and define N and n.
Why is calculating an Index of Diversity more informative than simply measuring species richness?
What is meant by a 'hierarchical' classification system?
Why is comparing mRNA or DNA base sequences more accurate than comparing observable physical characteristics when classifying organisms?
If an organism has a diploid number of 2n = 16, how many genetically different gametes can it produce by independent segregation alone?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Use your knowledge of natural selection to explain how a population of Staphylococcus aureus bacteria becomes resistant to the antibiotic methicillin, and identify the type of selection involved. (5 marks)
Click “Check Mark Scheme” to compare your answer, 1. This is an example of directional selection [1 mark]. 2. A random spontaneous gene mutation occurs in the DNA of a bacterium, producing a new allele that confers resistance to methicillin (e.g. coding for an altered target protein or penicillinase enzyme) [1 mark]. 3. When the bacterial population is exposed to methicillin (the selection pressure), non-resistant bacteria are killed while bacteria with the resistance allele survive [1 mark]. 4. The surviving resistant bacteria reproduce rapidly by binary fission (and pass the allele via plasmids in conjugation) to their offspring [1 mark]. 5. Over many generations, the frequency of the methicillin-resistance allele increases in the bacterial population until most of the population is resistant [1 mark].
- Q2
Human birth mass shows a mean around 3.2 kg, with infants born with very low (<1.5 kg) or very high (>4.5 kg) birth masses experiencing higher mortality rates. Identify and explain the type of selection operating on human birth mass. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Stabilising selection acts on human birth mass [1 mark]. 2. Infants at the two extremes (very low birth mass due to underdeveloped lungs/thermoregulation, and very high birth mass due to childbirth complications) have a lower chance of survival [1 mark]. 3. Infants with birth masses close to the optimum mean (~3.2 kg) have the highest survival rate, grow to adulthood, and pass on their alleles for intermediate birth mass to their offspring [1 mark]. 4. Over generations, alleles for extreme birth masses are selected against and decrease in frequency, preserving the mean while narrowing the range/standard deviation around the mean [1 mark].
- Q3
An ecologist sampled invertebrates in a woodland stream and counted 4 species with the following numbers of individuals: Species A = 12, Species B = 8, Species C = 5, Species D = 5. Calculate the Index of Diversity using d = N(N - 1) / Σn(n - 1). Show your working. (3 marks)
Click “Check Mark Scheme” to compare your answer, 1. Calculate N: N = 12 + 8 + 5 + 5 = 30; therefore N(N - 1) = 30 × 29 = 870 [1 mark]. 2. Calculate Σn(n - 1): (12 × 11) + (8 × 7) + (5 × 4) + (5 × 4) = 132 + 56 + 20 + 20 = 228 [1 mark]. 3. Calculate d: d = 870 ÷ 228 = 3.82 (to 3 s.f) [1 mark].
- Q4
Explain how intensive farming practices reduce species diversity in agricultural ecosystems and describe two conservation measures farmers can use to increase biodiversity. (4 marks)
Click “Check Mark Scheme” to compare your answer, 1. Clearing woodland/hedgerows and planting a single crop (monoculture) reduces plant diversity and removes varied habitats and nesting sites for animals [1 mark]. 2. Use of herbicides kills weeds (reducing plant species richness and removing food sources for primary consumers) and pesticides kills insect pests (starving insectivorous birds/predators) [1 mark]. 3. Farmers can maintain or replant native hedgerows and leave uncultivated wildflower field margins to provide corridors, shelter, and diverse pollen/nectar food sources [1 mark]. 4. Farmers can use biological pest control, crop rotation with nitrogen-fixing legumes, and intercropping instead of blanket chemical pesticides and inorganic fertilisers [1 mark].
Common Exam Questions (4)
Does a mutation happen because an organism needs to adapt to an antibiotic or cold climate?
Never write in an exam that bacteria 'mutate in order to survive the antibiotic'. Mutations occur randomly and spontaneously during DNA replication before or regardless of the antibiotic being present; the antibiotic simply acts as a selection pressure that kills non-mutated bacteria and allows pre-existing mutants to survive.
Why is comparing DNA base sequences slightly more informative than comparing amino acid sequences of a protein?
Because the genetic code is degenerate (most amino acids are coded for by more than one triplet codon), two closely related species might have accumulated several silent base-substitution mutations in their DNA or mRNA while still having the exact same amino acid sequence in the resulting protein.
How does immunological comparison (serum precipitation) measure evolutionary relatedness?
Human blood serum albumin (acting as an antigen) is injected into a rabbit to produce anti-human antibodies. When these antibodies are mixed with serum from other primates (e.g. chimpanzee, gorilla, lemur), a closer evolutionary relationship means more similar albumin tertiary structure, forming more antigen-antibody complexes and a heavier precipitate.
Why do mules prove that horses and donkeys are two separate biological species?
Horses have 64 chromosomes (n = 32) and donkeys have 62 chromosomes (n = 31). Their hybrid offspring (a mule) receives 63 chromosomes - an odd number that cannot pair up into homologous pairs (bivalents) during Prophase I of meiosis, rendering the mule infertile.
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