BIO120H1 Lecture Notes - Allele Frequency, Chromosome
Key Concepts/ Terms:
•Directional and disruptive selection
•Herbicide resistant weeds
1)Types of Selection: Artificial vs. natural, Modes of selection
2)Examples of selection: Evolution by pollution, Evolution in agriculture
Natural selection can be defined as the differential survival and reproduction of genotypes in
response to factors in the environment. It can only occur when genotypes differ in fitness. Fitness is
measured by genotype’s rate of increase relative to other genotypes in a population.
Artificial selection is sometimes used as a model for natural selection but there is a big difference,
natural selection has no purpose. For any given generation it results because of difference between
individuals with respect to their capacity to produce progeny. Natural selection is a self-generated
outcome of interactions between organisms and their environment. In contrast, artificial
selection is a purposeful process. It has a goal that can be visualized (eg. increase in yield) and a
selector – humans. Both processes depend on heritable genetic variation.
Three basic modes of natural selection on quantitative traits are generally recognized. These are
stabilizing selection, directional selection, and disruptive selection.
Pollution can act as a severe selection pressure and thus evolutionary responses can be quite rapid. Two
of the best study cases involved:
a)The evolution of industrial melanism in the Peppered Moth
b)The evolution of heavy metal tolerance in grasses
Some of the most spectacular examples of natural selection involve the evolution of mimicry in various
animal groups. Mimicry is the resemblance in form, colour, or behaviour of certain organisms (mimics) to
order more protected ones (models). This results in the mimics also being protected. (unusual form- weeds
resembling crops which they infest)
Natural selection in an agricultural context example: the evolution of herbicide resistance in annual weed
species. Genes that confer resistance to weeds sprayed with chemical herbicides have a strong selective
advantage and a rapid increase of resistant genotypes results. In populations not exposed to weed control
measures, the gene occur at low frequency implying that there is a “cost” to plants that are herbicide
resistant in normal environments.
A final example of natural selection involves the evolution of long floral tubes in moth-pollinated flowers.
Work by the Swedish pollination biologist L. Anders Nilsson has resolved a long-standing controversy
concerning the pollination biology of rare Madagascan Orchids with extremely long floral tubes.
Review and Study Questions:
1)What have studies on the evolution of heavy metal tolerance in grasses by A.D. Bradshaw and
colleagues shown us about the mechanisms of evolution?
2)Very large and very small human babies at birth suffer higher mortality than average-sized babies.
What type of natural selection is this an example of?
3)What is an adaptation?
4)Why does pollution often result in rapid evolutionary responses by population?
Scitable- Nature Education
Berkeley- Natural Selection
Testing natural selection (textbook, lecture 22)