Invertebrate Zoology

Lecture Notes - Evolution

Notes: This is an outline of my class notes - details and visuals will be given in class!

Read: Chapter 6 in Hickman, Roberts & Larson.

Lamarck, Darwin and Wallace
1. With some exceptions, most people held the view that species were unchanging through the 1700's
  1. This included Linnaeus
2. Lamarck - first complete theory of evolution, 1809
  1. others, including Darwin's grandfather had other, incomplete theories
  2. Lamarck - inheritance of acquired characteristics
    1. Giraffes stretch their necks to reach leaves, their necks grow longer and this is passed on to their offspring....
    2. a transformational theory - organisms transform themselves vs. Darwin's variational theory.
3. Lyell
  1. Geologist, originated theory of uniformitarianism
  2. Uniformitarianism
    1. laws of physics and chemistry remain constant through the history of the Earth
    2. past geological events are similar to current ones
    3. leads to a conclusion that the Earth is millions of years old
    4. book, Principles of Geology, first published in 1830 just before Darwin sailed on the Beagle
4. Darwin
  1. naturalist on the Beagle (1831-1836)
  2. stops in South America, Galapagos Islands, Australia, etc.
  3. key observations:
    1. fauna of different islands often unique
    2. fauna of islands resembles, yet differs from nearby mainland
    3. fossils at high elevations obviously once were on sea floor
  4. return to England
    1. reads Malthus - tendency of populations to overpopulate
    2. works on gathering evidence for evolution
    3. receives letter from Wallace (1858)
      1. Wallace had independently arrived at theory of evolution similar to Darwin's
    4. joint publication in Journal of the Linnean Society - 1858
    5. Origin of Species - 1859
      1. abstract - 460 pages long!
Darwin's Theory:
1. Evolution by Natural Selection
2. 5 observations; 3 conclusions:
  Observation 1 - populations can grow exponentially (Malthus)
  
  Observation 2 - in nature, population sizes remain constant
  
  Observation 3 - Natural resources are limited (Malthus)
  
  Conclusion 1 - A struggle for existence occurs among organisms (Malthus)
  
  Observation 4 - variation occurs among individuals in a population
  
  Observation 5 - such variation is heritable
  
  Conclusion 2 - some variations are helpful to the organism's survival and reproduction, others hurt it (Darwin).
  
  Conclusion 3 - with enough time, the survival and differential reproduction of individuals with favored traits will create new species
Evidence for evolution:
1. Fossils and perpetual change:
  1. an examination of the fossil record shows many instances of changes from one form to another
  2. fossil record is incomplete, many transitional fossils are missing because:
    1. soft bodied organisms not preserved
    2. many organisms rot before being fossilized
    3. fossilization only takes place under particular circumstances
    4. geologic processes may destroy fossils
  3. still, the fossil record shows many examples which support evolutionary theories
  4. the fossil record has been dated by separate, independent techniques
2. Common descent
  1. Darwin saw the pattern of life as a phylogeny - a branching tree
  2. homology is evidence for common descent
    1. homology - the same organ, though it may be formed and used in different ways by related organisms
    2. example - bones of vertebrate limbs
    3. tracking homologies can lead to nested hierarchies or phylogenies
3. Ontogeny
  1. The developmental stages of some organisms often resemble the adult stages of other, more "primitive" organisms.
    1. Shared early stages may be homologies (and thus evidence of relationship) but the overall picture is much murkier.
  2. paedomorphosis - retention of juvenile characteristics by adult organisms
    1. example - axolotyls
    2. paedomorphosis is an example of heterochrony, a change in the timing of development
4. Speciation
  1. if Darwin's theory is correct, we should see evidence of it as new species are formed
  2. speciation depends on the formation of reproductive barriers between populations, then subsequent divergence of the populations.
  3. reproductive barriers:
    1. premating
      1. habitat isolation
      2. temporal isolation
      3. behavioral isolation
      4. mechanical isolation
    2. postmating
      1. gametic isolation
      2. zygote mortality
      3. hybrid sterility
      4. fitness of offspring
  4. allopatric speciation
    1. geographic isolation of populations and subsequent change, either in response to natural selection or randomly
    2. may occur is habitat is split (vicariant speciation) or if a small population is separated from its native habitat (founder event)
  5. sympatric speciation
    1. no apparent physical separation
    2. certain organisms within a population select a particular microhabitat and spend most of their time there
    3. over time, variations accumulate
    4. more common with plants, since individuals may reproduce vegetatively
  6. adaptive radiation
    1. explosion of species from a small initial group
    2. no competitors to stop diversification into different niches
    3. common on new islands, lakes, etc.
Modern Refinements of Evolution:
1. Rate of evolution:
  1. Darwin viewed evolution as always occurring in small steps over long periods of time.
  2. Eldredge and Gould proposed punctuated equilibrium - species form rapidly (fast change), then remain unchanged (equilibrium) over long periods of time
  3. the fossil record, in rare cases where it is "complete" often supports puctuated equilibria
2. Mendelian genetics:
  1. Darwin knew nothing about how traits were passed on - Mendel, a contemporary, did. Darwin could have saved himself a lot of trouble by doing a full library search and digging up Mendel's work!
  Remainder of evolution to be covered in the spring!

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08/27/00