Population Genetics

I. Mechanisms of Evolution

  • Mutation - Inheritable changes in the genetic material, resulting in phenotypic variability.

Point mutations:

 

Chromosomal Rearrangements:

  • Gene Flow - Emigration and immigration of individuals will affect gene frequencies within a population 

  • Genetic drift - Random events, particularly in small populations, may profoundly alter gene frequencies in each subsequent generation. An example is the "founder" effect, in which the characteristics of a small population colonizing a new habitat may be very different than the characteristics of the larger population from which the founders are derived.

Two examples of genetic drift

Founder Effect

Low reproductive rate results in changes from one generation to the next

Genetic drift: When the beetles reproduced, just by random luck more brown genes than green genes ended up in the offspring. In the diagram at right, brown genes occur slightly more frequently in the offspring (29%) than in the parent generation (25%).

 

 

 

  • Natural selection - Certain phenotypes are favored for survival or for higher reproduction rates than others. Genes coding for disadvantaged phenotypes will decrease in frequency through time.  

http://www.darwinawards.com/darwin/

II. An Example of Microevolution

  • Prior to 1750, most specimens of the peppered moth, Biston betularia, found in the entomology collections of Oxford University were the peppered form. Following 1750, most specimens, especially those found in forests polluted with soot from coal-fired manufacturing plants, were a melanic form.

Peppered and melanic forms of Biston betularia.  a) Against the bark of a tree in an unpolluted forest; b)Against the bark of a tree in a polluted forest.

Composition of various populations of the peppered moth in the British Isles

 

 

  • Peppered moths, being nocturnal, rest on tree trunks during daylight hours. In unpolluted forests, the peppered form is camouflaged against the background of lichen-encrusted tree trunks. In polluted forests, soot smothers the lichens and blackens the tree trunks, thus allowing peppered moths to become conspicuous to visual predators, i.e. birds. Predation pressure favors the melanic phenotype in polluted forests. This phenomenon, in which air pollution from industrial sources has induced predation pressure resulting in darker morphology is called industrial melanism.
  • In the 1960's H.B.D. Kettlewell conducted a series of experiments which demonstrated the results of industrial melanism in the peppered moth:

Experiment 1: Light traps were set up in polluted and unpolluted forests to collect resident peppered and melanic forms.

 

Unpolluted forest

Polluted forest

Peppered form

99.1%

9.3%

Melanic form

0.9%

90.7%

 

Experiment 2a: Peppered and melanic forms marked with nail polish were released into a polluted forest on Day 1. On Day 2, light traps were set up the number of peppered and melanic forms could be recaptured.

 

# Released

# Recaptured

% Recaptured

Peppered form

137

18

13.1

Melanic form

447

123

27.5

 

Experiment 2b: Peppered and melanic forms marked with nail polish were released into an unpolluted forest on Day 1. On Day 2, light traps were set up the number of peppered and melanic forms could be recaptured.

 

# Released

# Recaptured

% Recaptured

Peppered form

393

54

13.7

Melanic form

406

19

4.7

 

Experiment 3: Kettlewell set up camera blinds to record the behavior of birds as they approached nearby tree trunks. He was able to observe and record the actual capture of moths by a variety of birds living in unpolluted and polluted forests. The following is typical data:

Type of forest

Type of predator

# of Melanic forms eaten

# of Peppered forms eaten

Unpolluted

Yellow hammer

20

0

Polluted

Redstart

15

43

Data adapted from "The Peppered Moth: A Population Study" developed by Biological Sciences Curriculum Study - 1972, based on data collected by Dr. H.B.D. Kettlewell.