* D4.1 on IB Textbook

Natural selection is the theory that explains the process whereby organisms better adapted to their environment tend to survive and produce more offspring.

 A paradigm shift is a fundamental and dramatic change in Scientific understanding

•  Darwin's theory of natural selection provided a convincing mechanism and replaced Lamarck’s theory of evolution by acquired inheritance.

Mutation and sexual reproduction in genetic 

1. Variation in sexual reproduction during meioses and fertilization

• Random assortment - paternal and maternal chromosomes in meiosis.
• Crossing over certain segments of individual maternal and paternal homologous chromosomes.
• Random fusion of male and female gametes during fertilization.

1. Variation as mutations giving entirely new alleles
•  The individual offspring of parents are not identical showing variation in their characteristics.

Overproduction and competition

• Overproduction of offspring naturally leads to competition for resources. 
• Increases in population size don’t go on for long as they are usually limited by environmental factors. 
•  Carrying capacity - maximum population size that can be long-term supported in an environment.
• Environmental factors will limit carrying capacity.

Biotic vs Abiotic Factors

• Selection pressures - favor a particular phenotype in certain environmental conditions.
• Biotic factors: living factors that affect the survival of other organisms.
• Ex. Prey-predator, disease-pathogens, competition for resources.
• Abiotic factor: nonliving parts of an ecosystem that can affect the survival
  of organisms. 
  • Ex. temp, water availability, light intensity. 

Differences between individuals as the basis for natural selection

• Fitness- Survival value and reproductive potential of a genotype.
• Intraspecific competition - competition between individuals of the same species; Individuals will usually interact with more members of their species than of other species. 

Natural Selection in Pepper Moths

• 19th century, air pollution due to industrialization increased. 
• Lichens and algae on the tree barks (made branches look lighter in color) died and the tree branches darkened.
• Species of dark moths increased in their habitats.
• The black moths were found in the industrialized areas but peppered moths outgrew them in the countryside and low-polluted areas. 
• The black moth effectively camouflaged in polluted areas from predation and became the dominant species. 

Natural Selection in Bacteria with antibacterial resistance

• Selection pressures:  Antibiotic, penicillin
• Genetic variation: Antibiotic resistance allele and non-resistant allele.
• Intraspecific competition: Bacteria overproduce. They compete in the presence
  of antibiotics.
• Favorable adaptations: antibiotic-resistant are better adapted to the
  environment, as they can withstand antibiotics.
• Survive and reproduce: Antibiotic resistance survive and reproduce.
• Genetic inheritance: Antibiotic-resistant alleles will be likely passed down.
• Natural Selection: The frequency of antibiotic-resistant alleles will increase over time.

Differences between individuals as the basis for natural selection

• Fitness- Survival value and reproductive potential of a genotype.
• Intraspecific competition - competition between individuals of the same species; Individuals will usually interact with more members of their species than of other species.

Natural Selection in Pepper Moths

• 19th century, air pollution due to industrialization increased. 
• Lichens and algae on the tree barks (made branches look lighter in color) died and the tree branches darkened.
• Species of dark moths increased in their habitats.
• The black moths were found in the industrialized areas but peppered moths outgrew them in the countryside and low-polluted areas. 
• The black moth effectively camouflaged in polluted areas from predation and became the dominant species. 

Natural Selection in Bacteria with antibacterial resistance

• Selection pressures:  Antibiotic, penicillin
• Genetic variation: Antibiotic resistance allele and non-resistant allele.
• Intraspecific competition: Bacteria overproduce. They compete in the presence
  of antibiotics.
• Favorable adaptations: antibiotic-resistant are better adapted to the
  environment, as they can withstand antibiotics.
• Survive and reproduce: Antibiotic resistance survive and reproduce.
• Genetic inheritance: Antibiotic-resistant alleles will be likely passed down.
• Natural Selection: The frequency of antibiotic-resistant alleles will increase over time. 

Sexual Selection 

1. Natural selection arises through preference by one sex for certain physical or behavioral traits in individuals of the other sex is called sexual selection. 
2. Ex. Endler’s experiments with guppies
1. Fish tanks are set up with guppies and their predators
2. The bottom of the tank is covered with either coarse or fine gravel.
3. Guppies evolved a pattern of spots in its body to work as camouflage. 
4. Predator was the selection pressure, guppies with better camouflage were more likely to survive and reproduce, passing genes for camouflage.
5. Predators were taken out.
6. Guppies evolved into patterns that are most likely visible to females.
7. Visible male guppies are more likely to reproduce and pass on the genes.

Gene Pools

• Consists of all their genes and their different alleles in an interbreeding population. 
• Evolution is the change in gene frequencies in a population’s gene pool over time. 

Types of Natural Selection

*Note: Natural selection reduces variation in a population

• Stabilizing selection
• Extreme phenotypes are neglected and the middle range of phenotypes is highly favored. 
• Extreme allele frequency decreases and common phenotype increases. 
• Ex. Peacock tails, birth weight for babies.
• Directional Selection
• One extreme range of alleles is advantageous for survival
• Selective pressure - one phenotype will survive. 
• Frequency shifts to advantages type to occurring more often. 
• Ex. Peppered moth
• Disruptive Selection 
• Two extreme alleles are favored over the intermediate phenotype
• Intermediate alleles will less likely survive and extreme alleles will survive and reproduce. 
• Mainly occurs when intraspecific competition is high due to high competition for resources.

Hardy - Weinberg Equilibrium

• Model to predict allele frequencies in populations that are not evolving.
• p and q represent the frequency of two alleles of a gene in a population. 

                                              p + q = 1

                                    Hardy Weinberg equation: 

                                            P² + 2pq + Q²

p - frequency of dominant allele.

q - frequency of recessive alleles

^{P2 -} frequency of homologous dominant genotype

2pq - frequency of the heterozygous genotype

^Q2- frequency of the homozygous recessive genotype

• The following conditions are assumed for a population in Hardy-Weinberg equilibrium:
• Population is large
• No immigration/emigration
• Mating is random (NO sexual selection)
• No mutations
• No natural selection.

Artificial Selection

Artificial selection, also known as selective breeding, is carried out in crop plants and domesticated animals by choosing individuals for breeding that have desirable traits.

• Farmers and scientists have used artificial selection on a wide range of plants and animals.

• Artificial selection is an example of observed evolution, as the gene pool of a plant or animal changes over time.

• Corn (Zea mays) is an example of a plant that has been artificially selected over time.

• There is genetic variation in the size of corn seeds.

• Human farmers selected the corn with bigger seeds and allowed it to reproduce.

  • The offspring produced had bigger seeds.

  • The process was repeated over many generations to produce modern corn from teosinte.

  • The gene frequency for the size of corn has changed over the generations.

  • Corn has evolved by artificial selection.

• Unintended consequences of human actions, such as the evolution of resistance in bacteria when an antibiotic is used, are due to natural rather than artificial selection.

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