BIO1022 Week 1 – Natural Selection & the Tree of Life

Learning Outcomes (Week 1)

• Explain the requirements for natural selection and compare them with artificial and sexual selection.

• Describe the biological species concept (BSC), its limitations, and alternative species concepts.

• Discuss how new species form and outline the two broad categories of reproductive isolation.

• Construct and interpret phylogenetic trees built from shared-derived characters; classify groups as monophyletic, paraphyletic, or polyphyletic.

LO1 Natural, Artificial & Sexual Selection

• Darwin & Wallace’s core insight

• “Descent with modification” driven by natural selection explains the fit of organisms to environment.
  • Three indispensable conditions for natural selection

• Variation – individuals differ in heritable traits.

• Inheritance – some of that variation is genetic (passed to offspring).

• Differential fitness – individuals with certain traits leave more viable offspring than others.
  • Malthusian influence

• Thomas Malthus noted that populations grow geometrically whereas resources grow arithmetically ⇒ inevitable competition → only some survive/reproduce.
  • Biological fitness

• Relative contribution of an individual’s genotype/phenotype to the next generation’s gene pool (often expressed as w, where 0 \le w \le 1).

Major Types of Genetic Selection

Sexual Selection (Subset of Natural Selection)

Artificial Selection

• Humans act as selective agent; intentional breeding.
• Examples: domestication of dogs, maize from teosinte, broccoli/cauliflower from wild mustard.

Genetic Framework

• Population’s gene pool = total alleles at all loci.
• Natural selection changes allele frequency through time.

• Hardy–Weinberg baseline: p + q = 1 and p^2 + 2pq + q^2 = 1 (no evolution).

• Deviations indicate evolutionary forces such as selection.
  • Mutations

• Advantageous (increase fitness), deleterious (decrease), neutral (no immediate effect).

LO2 Species Concepts & Their Limits

LO3 Speciation & Reproductive Isolation

• Reproductive isolation halts gene flow ⇒ divergence + independent evolution ⇒ new species.

Two Broad Categories

1. Pre-zygotic barriers – prevent fertilisation.

2. Post-zygotic barriers – fertilisation occurs but hybrids inviable/sterile.

Allopatric Speciation (Geographic Isolation)

• Definition: divergence of populations after physical separation; gene flow ≈0.

1. Dispersal (peripheral isolation)
   • Mainland → island colonisation (e.g., Galápagos finches).
   • Peripatric (small founder group) often faster: strong genetic drift + selection.

2. Vicariance
   • Barrier arises (mountain uplift, river change) splitting range; e.g., Isthmus of Panama dividing snapping shrimp.

3. Co-speciation
   • Host lineage splits, obligate parasite/symbiont cospeciates; e.g., pocket gophers & lice.

Sympatric Speciation (No Physical Barrier)

• Definition: reproductive isolation evolves within a single, continuous population.

1. Disruptive selection across ecological gradient → assortative mating (e.g., scale-eating cichlids left vs right-jawed).

2. Instantaneous speciation via hybridisation or polyploidy

   • Allopolyploid plants form fertile tetraploids in one generation (e.g., wheat).

   • Homoploid hybrid speciation in Heliconius butterflies.

LO4 Phylogenetic Trees & Character Analysis

• A phylogenetic tree depicts hypothesised evolutionary relationships; branch lengths may represent time or change.

Tree Anatomy

• Root – common ancestor of all taxa in tree.

• Node – divergence point; represents speciation.

• Branch – lineage through time.

• Clade – all descendants of a common ancestor (monophyletic group).

• Out-group – taxon outside group of interest, polarises character states.

Phyletic Groupings

1. Monophyletic – ancestor + all descendants (e.g., Mammalia).

2. Paraphyletic – ancestor + some but not all descendants (e.g., “Reptilia” excluding birds).

3. Polyphyletic – taxa lacking most recent common ancestor within group (e.g., warm-blooded animals: birds + mammals).

Characters & Character States

• Character = heritable trait; three main types

  1. Morphological (bone structure)

  2. Molecular (DNA/RNA/protein sequences)

  3. Behavioural/developmental (larval pattern)

• Character state = specific condition (e.g., presence/absence, nucleotide A/G/C/T).

Homology vs Analogy

• Homologous characters – derived from common ancestor; informative for trees.

• Analogous characters – result from convergent evolution (no shared ancestry); misleading (e.g., wings in bats vs birds).

Synapomorphies & Parsimony

• Synapomorphy – shared-derived character uniting a clade.

• Maximum parsimony – preferred tree minimises total evolutionary steps (simplest explanation).

• Algorithm: tabulate character matrix → identify synapomorphies → search tree space for least changes.

Molecular Phylogenetics

• DNA/RNA sequence alignment: count substitutions, indels.

• Genetic distance (e.g., Jukes–Cantor) converted to branch lengths; supports molecular clock D = \frac{3}{4} \left[ -\ln!\left(1 - \frac{4}{3}p \right) \right] where p = proportion differing sites.

• Large data sets (whole genomes) enable high-resolution trees & reveal deep divergences.

Key Term Glossary

• Allele frequency – proportion of a particular allele among all alleles at locus.

• Gene pool – sum of all genetic information in population.

• Convergent evolution – independent evolution of similar traits.

• Divergent evolution – accumulation of differences after lineage split.

• Out-group – reference lineage for polarity.

• Node – branching point denoting speciation.

• Root – ancestral lineage base of tree.

• Mutation – permanent change in DNA; ultimate source of variation.

These notes cover all guiding questions and supply examples, definitions, formulas, and conceptual links to provide a self-contained study resource for BIO1022 Week 1.