10.3 - Gene Pools and Speciation

Gene Pool:- The total collection of different alleles in an interbreeding population.

Allele Frequency and Evolution

• Allele Frequency:- The proportion of all copies of a gene that is made up of a particular gene variant.
• New combinations of alleles lead to new phenotypes that can be selected for/against by the environment.
  • This leads to evolutionary change.

Patterns of Evolution and Selection

• Selective pressures need to change for evolution.
• Type of evolution depends on the phenotypes that experience the greatest pressure.

Directional Selection

• Population changes as one range of variation is better adapted.
• For example - bird beaks are better longer and harder, so they evolve till the population has longer and harder beaks.

Stabilising Selection

• Selection Pressures remove extreme varieties.
• For example - Human baby birth weight.
  • Too heavy = difficult to deliver through pelvis. (babies would die during delivery).
  • Too light = lose heat too quickly. (babies would die).

Disruptive Selection

• Selection pressures act to remove intermediate varieties.
• For example - Grass close to a quarry with metal contaminated soil.
  • One population will be metal tolerant and slow growing.
  • The other won’t be metal tolerant and will grow slower.

Sources for graphs

Reproductive Isolation

• Inability of a species to breed with a related species due to some factors (see below).
• Only promotes selection in sexually reproducing organisms

Temporal Isolation

• Mating seasons/times do not coincide
• For example - different pollination times for flowers

Ecological Isolation

• Organisms in the same area, but different habitats/conditions.
• For example - Plant A survives in alkaline soil vs. Plant B in acidic

Behavioural Isolation

• Organisms that mate based off of courting behaviour/pheromones will only mate with those who perform the best mating behaviour (eg dancing, fighting, etc.).
• So it can prevent two organisms from mating.

Rate of Speciation

Phyletic Gradualism

• Evolution occurring at a constant pace. Gradual Change.
• Due to accumulation of mutations.

Punctuated Equilibrium

• Long periods of stability followed by sudden changes
• Fossil record supports this
• Rapid evolution due to major environmental changes like a meteor

Polyploidy

• Non-disjunction can occur during meiosis in humans.
• Individual can end up with an extra chromosome or missing chromosomes.
• Total non-disjunction: One of the two cells produced during Meiosis I gets all of the chromosomes.
• Tetraploid offspring cannot mate with diploid organisms, so speciation has occurred
• More common in plants
  • increased size, resistance to disease and overall vigour in plants.

Sources

• Biology Class notes
• Biology - Course Companion - Andrew Allott and David Mindorff - Oxford 2014
• https://pioneerbiology.wordpress.com/2010/11/19/patterns-of-evolution/