Darwinian Evolution Review Flashcards

7.1 Evolution

• Definition: Genetic changes in a population over time.
• Historical Context: The concept of species changing over time has existed for centuries.
• The BIG QUESTION: What process explains why species have characteristics that suit their environments?

7.2 Natural Selection

• Publication: In 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection.
• Darwin's Influences:
  • Charles Lyell: Suggested geological forces gradually shape the Earth.
  • Jean-Baptiste Lamarck: Suggested living species might change over time.
  • Georges Cuvier: Documented extinction through fossils.
  • Georges Buffon: Suggested the Earth is older than previously thought.

The Voyage of the Beagle

• Darwin spent years observing nature and comparing species from various regions during his travels (1831-1836).

The Logic of Natural Selection

• Observation #1: Overproduction: More individuals are born than the environment can support.
• Observation #2: Limited resources: Resources remain constant despite overproduction.
• Conclusion #1: Competition arises because not all individuals survive and reproduce.
• Observation #3: Variation: There is variability among individuals in resource acquisition.
• Observation #4: Heritability: Certain traits are passed from parents to offspring.
• Overall Conclusion: Individuals with favorable traits are more likely to survive and reproduce, leading to the perpetuation of these traits in the population.
• Natural Selection vs. Artificial Selection: Both processes favor specific traits.

Examples of Natural Selection

• Antibiotic Resistance in Bacteria:
  1. Random mutation occurs for antibiotic resistance.
  2. Antibiotic is introduced.
  3. Individuals without the trait are eliminated.
  4. Survivors reproduce, passing on the resistance trait.
  5. Population changes over generations.

Important Points About Evolution

• Individuals don’t evolve: Natural selection acts on individuals; evolution involves populations.
• Natural selection and Heritability: Only heritable traits are subject to natural selection.
• No Goal in Evolution: Evolution occurs due to current environmental conditions, not future needs.

Lines of Evidence for Evolution

1. Fossil Record: Fossils indicate different species existed in the past, often simpler organisms in older layers.
2. Radiometric Dating: Used to determine the age of fossils and the Earth (e.g., uranium-238, half-life = 4.5 billion years).
3. Biogeography: The distribution of species reflects historical continental movements (e.g., marsupials).
4. Comparative Anatomy: Similar structures in different species indicate common ancestry (e.g., vertebrate forelimbs).
5. Molecular Evidence: DNA similarities among closely related species.

Mechanisms of Evolution

1. Mutation: The original source of variation; changes in gene structure.
2. Gene Flow: Genetic exchange among populations which can reduce differences.
3. Genetic Drift: Random changes in allele frequencies, particularly in small populations; includes the bottleneck effect and founder effect.
4. Sexual Selection: Differential mating success can influence evolution.
5. Natural Selection: Favors the survival of beneficial traits over generations.

Genetic Drift

• Bottleneck Effect: A catastrophic event drastically reduces population size, altering allele frequencies.
• Founder Effect: A small group starts a new population, leading to a loss of genetic variation.

Case Study: Pingelap Island

• After a typhoon in 1775, a severe bottleneck left few survivors on Pingelap Island, resulting in a high rate of color-blindness due to genetic drift from the few surviving individuals.

Summary of Evolutionary Mechanisms

• Gene Flow: Transfers genes between populations, making them more genetically similar over time.
• Founder and Bottleneck Effects: Both lead to decreased genetic diversity and unique genetic features in new or surviving populations.
• Natural Selection: Continues to dictate which traits are advantageous based on environmental pressures.