Chap. 13

Microevolution

• Change within a population or species.
• Does not involve the generation of new species.
• Clinically significant because alleles/traits causing disease are subject to microevolution.

Lamarckian Evolution: Inheritance of Acquired Traits

• Definition: A model of evolution where traits acquired during an organism's lifetime are passed on to its offspring.
• Key Proponent: Jean Baptiste Lamarck.
• Core Idea: Populations or species have an inherent desire to evolve and adapt to their environment.
• Example using Instructor's Physique:
  • 15 years ago: Instructor was scrawny.
  • Present: Instructor gained some muscle by swimming and lifting weights.
  • Lamarckian Explanation: Changes in the instructor's biology (muscle gain) during his lifetime would be directly inherited by his offspring.
  • The offspring would be born more muscular, inheriting the traits acquired by the father.
• How Organisms Adapt (Lamarck's View):
  • Use certain features more, causing them to enlarge or strengthen.
  • These acquired enhancements are then passed to the next generation (inheritance of acquired traits).
• Popular Misconception: The idea that organisms actively adapt themselves to their environment over time (e.g., squirrels adapting to live in trees).
• Critique:
  • Implies a misunderstanding of how inheritance works.
  • Lamarck lived in the 1750s, before the discovery of genes or DNA.
  • No understanding of how traits are passed on.
  • People knew of gametes, but they didn't know what they contained or how they contributed to the next generation.
• Modern Relevance:
  • Dog Breeders: Some dog breeders still believe in this mode of inheritance.

Darwinian Evolution: Natural Selection

• Darwin's model is the accepted model of evolution.
• Darwin, like Lamarck, did not know what a gene was.
• Relies on natural selection.
• Based on two key observations:
  • Observation 1: Individuals have inheritable variations (alleles).
    • Individuals within a population differ from each other, and these differences can be inherited.
    • Modern understanding: Individuals have different alleles (variations of genes).
    • Alleles arise from random mutations.
      • Example: The first blue-eyed person resulted from a random mutation.
      • Mutations are not guided or intentional.
      • A mole did not mutate to have big strong claws to become a better digger.
    • Random mutations are often harmful (e.g., cancer).
    • The body has DNA editing and repair mechanisms to prevent these mutations.
  • Observation 2: Individuals reproduce more than the environment can support.
    • More individuals are born than can survive on available resources.
    • Example: More crows or rabbits are born than will survive.
    • Historical Context: Darwin was influenced by Thomas Malthus, a British economist.
      • Malthus studied human population growth and the limited resources available.
      • Malthusian: A term referring to radical overpopulation and a bare-knuckles fight for resources in a Mad Max-like scenario.
      • Often used humorously to describe an extreme struggle for survival in an overpopulated world.

Synthesis: Observations and Implications

• Individuals have alleles resulting from random mutation.
  • Mutation: Mutations are constantly injecting variation into the population.
• More offspring are produced than the environment can support.

The key point here is that most of the ones born won't make it to reproduce.