IB Biology Sub-topic D4.1 Notes
Patterns in natural selection
In the HL syllabus, you need to know more detail about natural selection. Here, you are supposed to appreciate that many characteristics are polygenic and thus impact an organism's ability to survive and reproduce. In the context of natural selection, the term used for all these characteristics is gene pool. This is all the genes, and their different alleles, present in an interbreeding population.
Due to natural selection, some alleles may appear more frequently than others. This can be visualized by gene pool distribution, and there are three main patterns you need to be aware of:
Stabilizing natural selection - this occurs when selection pressures remove the extremes of a phenotype.
For example, the clutch size of eggs are mostly medium in number, as too few eggs decreases survival rate, whereas too many increases predation.
Disruptive natural selection - this occurs when selection pressures remove the intermediate phenotype.
For example, the beak of the red crossbill is either crossed left or right, but never straight as the crossing allows it to extract conifer cones and obtain food.
- Directional natural selection - this occurs when selection pressures select one extreme of a phenotype. For example, the Darwinian finches during El Nino were selected for longer beaks during times of drought.
Genetic equilibrium
The impact of natural selection is important, because generation upon generation it changes the population's gene pool by weeding out characteristics that are less well adapted. In periods of stagnation, all members have an equal change of reproducing and thus the population is said to have reached a state of genetic equilibrium.
Remember that long periods of no appreciable change like this support punctuated equilibrium, whereas gradual changes over time support gradualism.