Evolution Biol 4801

Lecture 19, Chapter 12


Topics for today

    1. Forms of selection
    2. Gene flow and selection
    3. Frequency-dependent selection


Forms of selection

  1. Directional selection
    • Moves the mean
    • Reduces the variance
  1. Stabilizing selection, balancing selection, (single loci - heterozygote advantage)
    • Doesn’t move the mean (unless asymmetrical)
    • Reduces the variance in a polygenetic trait (maintains p and q at a single loci)
  1. Disruptive selection, diversifying selection, (single loci - heterozygote disadvantage, underdominance)
    • Doesn’t move the mean (unless asymmetrical)
    • Increases the variance in a polygenic trait (maintains p and q at a single loci)


Real life examples

  1. Directional selection
    • Drug-resistant TB
    • Antibiotic resistance

Evolution of resistance happens fast

·         Insecticide resistance

Fig. 12.9 old and new

·         Intense natural selection caused by invasive species

·         Flat periwinkle

·         Native to New England rocky coastline

·         European green crab arrived in early 19th century

·         Feeds on flat periwinkle

·         Causing natural selection on shell thickness?

EvoBeaker: Snails


2.      Stabilizing selection

    • Goldenrod galls size

Solidago altissima Tall goldenrod

o   Fly attacks plant in early spring

o   Plant develops gall

o   Both plant and fly genes are involved in gall formation

o   Selection from biotic community shapes gall size and shape

o   Parasitoids oviposit on small galls and kill larvae

o   Selection against small galls

o   Birds peck out and consume larvae

1.      Selection against large galls

2.      Selection for intermediate gall size

3.      Disruptive selection

o   Beak dimensions in black-bellied seed cracker

o   Bill size and survivorship measure on > 200 juvenile birds

o   Birds with large and small bills specialize on different seeds

o   Intermediate sizes die

~Fig. 12.4 old and new


Gene flow can counter the effects of selection

Fig. 12.10 old and new

·         Osmotic balance different in salt and fresh water

·         Amimopeptidase I enzyme cleaves terminal amino acids from proteins increasing free amino acid concentration

·         ap94 allele more active

·         Mussels with ap94 in Long Island Sound have greater mortality

·         Influx of genes as larvae each year

Fig. 12-11 old and new


Gene flow can cause maladpatation

·         Larvae preyed on by fish

·         In ponds with fish

o   Paler coloration

o   Fish odor reduces feeding activity

o   The less gene flow from fishless populations, the more adaptive trait change

Fig. 12.12 old and new


4.      Frequency dependent selection – two kinds (Inverse and Positive)

1.      Inverse frequency dependent selection

·         Rare genotype is favored

·         Increases in frequency until it is not rare anymore

·         Fitness declines

Example 1: Self incompatibility loci in plants

Example 2: Scale-eating cichlid

·         Attacks other fish from behind

·         Feeds on scales

·         Right- and left-mouthed morphs

2.      Positive frequency dependent selection

·         Common genotype is favored

·         Whichever allele is initially more fit will rapidly be fixed

Example 1: two butterfly species

·         Two species of unpalatable butterfly

·         Parallel geographic races

·         Müllerian mimcry

·         Birds avoid butterflies with similar warning coloration

Fig. 12.19 old and new

Mallet and Barton 1989 (link on web)

·         Positive frequency dependent selection maintains separation of morphs

·         If postman invades rayed territory, birds don’t recognize it as poisonous and prey upon it