Chapter 17 Notes

Evolution as Genetic Change


Natural selection on single-gene traits can lead to changes in allelic frequency and thus to evolution.


Example: Plumage coloration in turkeys:






Organisms of one color may produce fewer offspring than organisms of another color because they might be more visible to predators and less likely to survive and reproduce. 


However, when traits are controlled by more than one gene, such as in polygenic traits, the effects of natural selection become more complex.


Natural selection can affect the distribution of phenotypes in any of three ways:

1.           directional selection

2.           stabilizing selection

3.           disruptive selection




Stabilizing Selection: When the average individuals in the population have higher fitness and are favored.



Directional Selection: When individuals at one end of the curve have higher fitness and are favored more than individuals in the middle or at the other end.


Example:  Environmentalists spray DDT pesticide on Mosquitoes to stop the spread of malaria and the West Nile virus.  Those mosquitoes with a high resistance to DDT will be favored over those who have no or little resistance to DDT.





Disruptive Selection: When individuals at both of the extremes (upper and lower ends) of a trait are both more fit and are favored over the average individuals in the population.


Natural selection is not the only source of evolutionary change. 


Genetic drift can also cause evolution to occur in a population.



Genetic Drift:

·    Occurs in small populations that are isolated.

·     Individuals that carry a particular allele may leave more descendants than other individuals, just by chance.  

·    Over time, this can cause an allele to become more common in a population. 

Genetic drift can be caused by the bottleneck effect. A change in the allelic frequency is caused by a dramatic reduction in the size of the population.
Genetic drift can also be caused by the founder effect. A change in the allelic frequency is caused by the migration of a small subgroup of a population.



Hardy-Weinberg principle: 


Allelic frequencies in a population will remain constant unless one or more factors cause those frequencies to change. 

·    The situation in which allele frequencies remain constant (the same) is called genetic equilibrium.


Ř                      Five conditions are required to maintain genetic equilibrium from generation to generation:


1.     Random Mating: all members of the population must have an equal opportunity to produce offspring. 

·    In nature, random mating rarely occurs. 

·    Example: Lions select their mates based on size or strength.  Peacocks select their mates based on the length of the male’s feathers.  This is considered non-random mating which means that those traits are NOT in equilibrium but under strong selection pressure. 


2.     Large Population: a large population size is also important in maintaining genetic equilibrium so genetic drift cannot occur. 


3.     No Movement Into or Out of the Population: Movement of individuals between different populations might bring new alleles into a population. 


·    Gene pools must be kept separate.


4.     No Mutations: most mutations are random and most are harmful; but sometimes the mutation can be beneficial and allow the organism to become better adapted to its environment.

·    A better adapted individual is more fit for that environment and will be selected for.


5.     No Natural Selection: All genotypes in the population must have equal probabilities of survival and reproduction. 

·    No phenotype can have a selective advantage over another. 

·    The allelic frequency CANNOT be changing. 




If any one of these 5 mechanisms is occurring (not true), then the Hardy-Weinberg principle says evolution is occurring.


One factor that drives these five conditions to happen is competition.


Competition is caused by overpopulation or environmental changes which changes the food availability.