Darwin's Evidence:  Biogeography
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David H.A. Fitch
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Lecture notes

Darwin's evidence for evolution:  Biogeographic distributions

I.  Three important principles:  How do these principles support descent with modification?

A. Environment cannot account for either similarity or dissimilarity, since similar environments can harbor entirely different species groups
B.  "Affinity" (=similarity) of groups on the same continent (or sea) is closer than between continents (or seas)
C.  Geographical barriers usually divide these different groups, and there is a correlation between degree of difference and rate of migration or ability to disperse across the barriers
 

II.  Disjunct locations for the same extant species:  Good evidence for creation?
(Evolution proposes Single Centers for the origins of species, so Discontinuous Distributions need to be explained)

A.  Means of dispersal must be proposed
1.  Changes in climate or geology must have affected migration (i.e., by first allowing migration and then preventing migration)
2.  Darwin designed tests of a priori assumptions
3.  Although "accidental", dispersal is not really random (and thus allows very specific predictions about distributions in some cases)

B.  Case study:  Similarity of flora and fauna at mountain summits (great evidence for independent creations?)
1.  Evidence is clear for recent glaciation
2.  Migrations are easily visualized in the gradual advances and retreats of glaciers
3.  Because mountain tops retain a colder climate, some cold-adapted, northern species would be retained on mountain tops (and thus isolated during glacial retreat)
4.  Also explains why such mountain-top species are most closely related to species living due north
5.  Isolation poses an opportunity for change, esp. if it means a change in its interspecific associations
6.  Assumption of the scenario:  Circumpolar distribution is uniform (presently the case)
7.  Secondary assumption:  Similar situation for subarctic species

C.  Many difficulties remain to be solved, esp. the very distinct, but distantly related forms in the Southern hemisphere (e.g., marsupial versus placental mammals)
1.  These species are too distinct to be explained by the recent glaciation
2.  Darwin postulates an earlier glaciation, because he did not know about plate tectonics
3.  With plate tectonics, many (if not all) of these kinds of problems are soluble
 

III.  Fresh water distributions
Because freshwater is isolated, might expect restricted ranges; but this is not the case (in fact, they often have distributions even broader than terrestrials):  How is this explained?
A. Distribution of Fish
B. Distribution of Shells (molluscs)
C. Distribution of Plants (often very wide ranges)
In all cases, dispersal of freshwater organisms depends largely on animal (esp. bird) transport
 

IV. Distribution of species on oceanic islands
Darwin considered this evidence as especially strong in its support of descent with modification

A.  The total number of species on oceanic islands is small compared to the number on an equal area of continent
B.  Proportion of endemic species is very high
C.  Oceanic islands are missing entire Classes
D.  Endemic species often possess characters that are adaptive elsewhere, but are useless characters on the island
E.  Endemic species often show (new) adaptive traits not possessed by any of their relatives
F.  Batrachians are universally absent (except one frog in New Zealand)
G. Terrestrial mammals are not found on any island >300 miles from mainland
H.  But arial mammals are found on such islands, and many of these are endemic
I.  Also a correlation between the depth of the sea separating islands inhabited by mammals and the degree of "affinity" (classification) between these species
J.  "The most striking and important fact" (p. 397) is the affinity of these island species to those of the nearest mainland, without being actually the same species
K.  Within an archipelago, species are more closely related to each other than to those on the mainland (but still distinct from each other)
L.  The principle applies widely that island inhabitants are most closely related to the inhabitants of a region from which colonization is possible
M.  According to this principle, it must be the case that at some former time, a single parental species covered both ranges (i.e., the migration event itself)
N.  Darwin draws a parallel between Time and Space in the "Laws of Life" (p. 409)

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Exercises

  1. Why does Darwin need to carefully explain the broad distribution of freshwater species?
     
  2. How do Darwin's experiments test his hypotheses about biogeographic distributions of plants?  How can one possibly test evolution by experiment?
     
  3. Point by point, why are Darwin's observations of species distributions in the Galapagos archipelago so convincing as evidence for descent with modification?  Which of these observations are consistent with predictions that can be made by the other hypotheses (creation and transformism)?
     

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