Evolution

I. The Process of Evolution

A. Definition — any heritable change in a lineage of organisms.

The simple occurrence of change in the genetic makeup of an organism is referred to as mutation. Point mutations refer specifically to cases where the sequence of base pairs which male up the genetic material are altered. Mutations that are lethal are not passed on to the next generation and therefore do not constitute heritable change that can be acted upon by Natural Selection. A lineage refers to the sequence of organisms related to each other by descent.

Evolution appears to be a two step process. The first is the generation of variation in morphology, behavior or function which has its basis ultimately in the genetic code. The second step is Natural Selection, the differental reproductive outcome of the manifestation of that genetic variation. Evolution to a great extent causes an increase in complexity, that is over time, as a system it tends to produce more connected elements.

B. Generation of genetic change

  1. Mutation—Change in the DNA genetic sequence.
  2. Hybridization—Two closely related organism interbreed to produce viable offspring that are fertile (examples in the plant kingdom where chromosome doublin works to create fertile hybrids)
  3. Chromosomal changes—Epigenetic effects, changes occur during cell division at the chromosomal stage may have permanent evolutionary effects.
  4. Endosymbiosis—Completely unrelated forms at the cellular level may form permanent associations.

C. The Quantification of genetic change (How do we measure evolution?)

  1. Multidimensional space model
  2. Characters are morphology + molecular/ behavioral
  3. Frequency of alleles
  4. Wright’s genetic adaptive landscape

the trick to quantifying evolutionary change is not to measure the morphology or other aspect of phenotype, but, instead, to count the distribution of pieces of the genetic code, the different alleles that occupy positions in the genes.

D. Natural Selection

  1. Notion of fitness — related to successful production of offspring
    1. Numbers don’t matter
    2. Includes sexual selection, mating behavior (in higher organisms)
  2. Form and function
  3. Adaptation
    1. Produced progressively as selection refines morphology (finches’ beaks)
    2. Pre-adaption:
      • e.g. secondary compounds in plants
      • fishes to amphibian: swim bladder to lung

       

  4. Lamarckism

E. Vicariance

  1. Random factors in geography affect outcome
    1. Seal-level changes (Gulf vs. Pacific Ocean)
    2. Mt uplift
    3. Glacial advance in Europe vs. NE America
    4. Continental drift (long term)
  2. island biogeography
    1. founder effect — Darwin’s finches
    2. population bottlenecks may remove variation
    3. genetic isolation
    4. N= f(Area)
  3. Genetic clines
    1. Variation w/in population
    2. Incipient speciation
    3. East coast frogs

II. Species

  1. Species Concepts
    1. Species are natural groups of organisms related to each other that exhibit similar morphology: non-overlaping morphological space (typological)
    2. With their own evolutionary role and tendencies (Simpson)
    3. Populations capable of interbreeding and isolated from other such groups (Mayr)
  2. Speciation
    1. allopatry vs. sympatry
    2. Gradual vs punctuated
    3. Structural vs. developmental genes
    4. Heterochrony in development (e.g. neoteny)
    5. Vicariance Biogeography

Patterns in evolution

  1. Use of morphology in establishing relationships
    1. Homologous vs. analogous characters
    2. Cuvier's correlation of parts - the recognition of functional anatomy and adaptation
    3. Modern recognition of common gene complexes — homeobox

Web Resources

update 24 October 2000