Evol. Lecture 4: More on Selection

Evol. Lecture 4: More on Selection

1. Polygenic Inheritance

   • Phenotype influenced by several genes (alleles at several loci) p.293
     • ~ Smooth range of phenotypes (Quantitative character/quantitative traits)

2. Directional Selection

   • One end of distribution selected against
   • Classic response to a changing environment
     • e.g. pesticide/drug treatments; climate change etc.
   • E.g. Mice with darker coats have a greater chance of surviving, so the whiter mice have GREATER SELECTIVE PRESSURES and are more quickly eliminated. So we expect to see that there is a shift in the average coat color of the population, as the average "shifts" to the right. This is called evolution by natural selection, specifically, directional selection.
   • Soapberry bugs
     • Shorter beaks favoured = average beak length in population falls
       • When flatter fruit becomes more common, shorter beaks are more favoured, so the average beak length gets shorter
         • p. 461 of textbook
   • Oscillating directional selection in fluctuating environments
     • When the environment changes, the average phenotype in the population for a trait moves in consequences
     • If the environment is not constant, it is changing, so different traits become favoured under certain conditions
     • Doesn't have to be in the same direction each time
     • NON-CONSTANT environments

3. Stabilizing selection

   • EXTREME phenotypes selected against
   • Often due to different, opposing selective forces
   • Greatest selective pressure falls upon BOTH ends of the distribution at the same time
     • e.g. both light AND dark mice have LOW reproductive success, grey have the highest
   • Natural selection NARROWS the range, but the AVERAGE DOES NOT CHANGE

4. Disruptive selection

   • Less common than the others
   • If there's polygenic inheritance, we might still have grey mice when white and black mice mate
   • INTERMEDIATE phenotypes selected against
   • Role in some speciation events?
   • CONSTANT environments

5. Fitness is more than survival

6. Sexual selection

   • A special case of natural selection
   • Competition for mating opportunities
   • Results in adaptations that INCREASE MATING SUCCESS
     • But can actually REDUCE chance of survival
   • Intrasexual selection
     • Competition WITHIN one sex (usually males) for mating opportunities
   • Intersexual selection
     • One sex (usually females) CHOOSES mate from (competing members of) other sex
   • Adaptations & sexual selection
     • Sexual dimorphism common
       • Armaments
         • Features of organisms that make them better fit to winning male combat/intrasexual selection
       • Ornaments
         • Things that make you more likely to be chosen