Biology 41
Evolution

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This section focuses on an introduction to evolution and natural selection with an emphasis on macroevolution.

Alfred Wallace's work and travels; co-discovered the theory of natural selection (selections of his work). As a naturalist Wallace travelled extensively through the Neotropics as well as the Malayan Archipelago. These years are well described in the first part of David Quammen's book "The Song of the Dodo". He independently developed the mechanism for evolution based on his extensive observations of the natural world.
 
 
 
 

Evidence for Evolution:
1. Taxonomy and Systematics:
    Classification of organisms leads to "natural" groups. What degree of evolutionary relatedness can be associated with these groups?

2. The Fossil Record
definition
formation of fossils
record of past life
    some species dominate (linked to..
        hard parts present (bone/shell)
        habitat (oceans, close to wetlands, rivers)
        abundance (high population)
        distribution (large area)
        prevalence in time (all species go extinct - the longer a species exists the more likely they will be part of the record)
        (In addition: consider surviving geological destruction and being found)
The fossil record of horses
    change in size, toes, teeth
    climate changes?
Changes in the fossil record of animals over time
Role of Extinctions

3. Comparative Anatomy
Biologists compare different anatomical features of organisms.
Example: homologous structures
    e.g. limbs of birds, humans, porpoise, reptile and a bat
    Functions are different but the structures have the same evolutionary origin.
Analogous Structures
Vestigial Structures (e.g. in the skeletons of the Greenland whale and the python snake one can still find the rudiments of limbs).

4. Comparative Embryology
Comparing the early development of organisms.
Study the images of the early development of several classes of the vertebrates. These embryos are very similar at early stages but quite different as development proceeds.
E.g. gill arches present in all of them at an early stage. Why? In a fish these structures are part of the gills while in a human they become part of the middle ear. Why do they form during the early stages?
    These organisms seem to follow the same developmental sequence during the early stages but take their own unique path as time goes on. The developmental process has been modified by evolution. The similarities during the early stages point to common descent, i.e. the vertebrates have a common ancestor about 500 million years ago.

5. Biogeography
The study of the distribution and abundance of life forms. Understanding historical geography is crucial for being able to explain the current distribution of organisms as well as the presence of fossils of life from the past.
Plate tectonics
The continents move (continental drift)
Film showing the movement of the continents over the last 700 million years.
The global geography has shifted drastically over this time.
As a biologist it is important to consider the geography of the past to be able to understand the current distribution of species on the planet.

6. Molecular Biology
[Note: Darwin had no knowledge about genes and DNA even though he was aware of the fact that traits were passed down through time from parents to offspring).
The sequencing and comparison of DNA has become a powerful tool in terms of understanding the evolutionary relatedness between different species.
Certain important genes are relatively stable and change very slowly over time. This phenomena is referred to as the molecular clock. By comparing the changes that have occurred in the genes between different living species we gain knowledge about the time that has passed since these organisms had a common ancestor. The more similar the sequences the closer the connection in time.
Graphs showing the similarities between major taxa of organisms.

The Mechanism: Natural Selection
Definition
Discussion of artificial selection
    e.g. the mustard plant family
    e.g. dogs
Acts on the genetic variation in a population.
Differential reproductive success

Example of natural selection:
    Biston betularia (The Peppered Moth) - Industrial melanism in England.
    The link between lichens and air pollution
    Gray and Black Morphs in the population of moths.
    Normally the gray one dominates due to the camouflage provided by lichen covered tree trunks.
    Loss of lichen due to air pollution leads to a predominance of the black variety of the moth.
    The trend has reversed since air pollution has decreased and lichens once again grow on the tree trunks.

The population evolves and not the individual.

Review Questions:

• In the past it was thought that life was static and unchanging. Briefly describe how this view of the world was challenged, leading towards our current understanding of life as dynamic and ever-changing.
• How did Lamarck contribute towards our understanding of evolution?
• Use the finches and tortoises of the Galapagos islands as examples of how Darwin started to understand how life changes over time due to the process of evolution.
• Why are some species more common in the fossil record?
• How do you explain that fossils of certain reptiles are found in both West Africa as well as on the East coast of Brazil?
• Define the concept of homologous structures. Give an example!
• Why do the early stage embryos of a fish, a chicken, a mouse and a human have gill slits?
• How can the sequencing of genes from different organisms tell us something about evolutionary relationships?
• How can we learn something about evolution from artificial selection (e.g. dogs)?
• Populations are the units of evolution. Why cannot individuals evolve?
• Use the Peppered Moths as an example of evolution. Describe and explain the changes in the population of moths.

• Great Links to Evolution
• Biology Book - Evolution I : An excellent overview of evolution with lots of graphics. The page may take a while to load unless you have a fast connection, but it is definitely worth the wait.

• Biology Book - Evolution II : Further information focused on the mechanism of evolution.

Recommended Readings:
Richard Dawkin's "The Blind Watchmaker"
Richard Dawkin's "Climbing Mount Improbable"
David Quammen's "The Song of the Dodo"
Stephen Jay Gould's "Ever Since Darwin"
Jonathan Weiner's "The Beak of the Finch"
 
 

Page created by: Peter Svensson
Updated: August 24, 2002