Module 8 Lesson 3 - 5

Lesson Summary

---

The five causes of changes in frequency of alleles in the gene pool (micro-evolution) are the following:

1. Genetic drift - changes due to small populations (Typically, this decreases diversity; examples are founder and bottleneck effects.)
   
   
2. Natural selection - organisms with certain phenotypes or genotypes have selective advantage over others (Typically, this decreases diversity, especially in very competitive environments. Heterozygotes typically have the same advantage as the homozygous dominant genotype unless a heterozygote advantage is present.)
   
   
3. Change in mutation rates - a greater rate of mutation from one allele to the other (This increases the frequency of one allele over the other. Typically, this decreases diversity. Mutations resulting in new alleles increase diversity.)
   
   
4. Non-random mating - choosing one allele over another in preferred mates known as sexual selection (This increases frequency of one allele over the other, typically decreasing diversity.)
   
   
5. Gene flow - immigration and emigration increase diversity within populations, but decrease diversity among populations

Technologies developed to meet human needs often have consequences for gene pools of natural populations - some intended, some not.

Intended consequences

• Use of antibiotics increases natural selection and frequency of resistant alleles in bacterial populations. 
  
  
• Widespread and effective medical/pharmaceutical/surgical technologies increase survivability of individuals with disadvantageous alleles.
  
  
• Transfer of advantageous genes into crops and livestock improve yields and profitability, but this changes gene pools and evolutionary paths.
  
  
• Cloning of endangered or possibly extinct organisms is done to preserve rare alleles. 
  
  
• Crops and livestock with advantageous qualities are cloned to develop uniform and economically profitable products, but this reduces diversity in the gene pool. 
  
  
• Provision of wild-life corridors increases gene flow and maintains diversity in populations.
  
  
• Development of wildlife preserves can lead to genetic drift if either the founder effect or the bottleneck effect occurs. Wildlife preserves lead to inbreeding, which can result in genetic drift that reduces diversity substantially.

Unintended consequences

• Agriculture, construction of dams and roads, urban sprawl, logging, and industrialization result in habitat destruction and fragmentation leading to rapid selection and reduced genetic diversity.
  
  
• Over-hunting can reduce diversity by removing 'desirable' alleles from the gene pool. (The biggest and strongest animals are valued more by hunters.)
  
  
• Introducing genes into crops and livestock can have unintended effects on the expression of other genes, leading to reduced survivability and loss of diversity.
  
  
• Genes introduced into domestic species can 'jump' to wild species, changing the gene pool substantially. (The gene for herbicide resistance has jumped from corn to weed species.)

Genetic variation is ammunition against extinction. As environmental conditions change, genetic variation increases the probability that at least some individuals will survive in the new environment and continue the species.

The new skills you have learned allow you to determine allele and genotype frequencies in a population and to predict whether a population is evolving or is in equilibrium.  Having studied the mechanisms of genetic change (evolution), you can infer now from case histories whether a population is susceptible to change and what factors have led to it.

Human activity is notorious for causing habitat change and disruption in natural populations, both because of intended actions and unintended actions. Your studies have showed you how our actions impinge on gene pools and genetic diversity. You should have now a better understanding of our role in causing genetic change, and what we can do to reduce our effects on natural populations.