The Open Door Web Site
The origins of life
Evolution and Fixity
Systematics and Comparative Anatomy
Collections of animals and plants in museums increased from 17th century.
The need for systematic classification became apparent to organise organisms.
The binomial classification scheme was developed by Carl Linneus in 1735 to "put order into God's creation".
Classification led to comparisons of shape and form that gave rise to comparative anatomy. The "father" of comparative anatomy was Georges Cuvier (1769 - 1832) who was a prominent Catastrophist and worked at Le Jardin des Plantes in Paris.
Comparative anatomists noticed that different species have similar structures used for different functions (e.g. the pentadactyle limb of terrestrial vertebrates). These are called homologous structures.
Evolutionists argued that if species had been created independently by a creator then there was a great deal of coincidence in their design.
If all organisms evolved from a common ancestor, this could explain their common features. Occasionally a fossil is found close to the origin of a common ancestor; These are called missing links (e.g. Archaeopteryx a fossil dinosaur with bird-like features).
Classification led to phylogeny. This is the study of related groups as revealed by systematic classification. Put simply: closely related organisms are more similar than distantly related organisms.
Currently the systematic analysis of relatedness uses a technique called cladistics.
Comparative Embryology and Comparative Biochemistry
Early embryos of animals show surprisingly similar features, revealing a common ancestry.
Protein molecules essentially constitute the bottom line in studying the phenotypes of organisms.
Similarities and differences in the amino acid sequences of the same molecule (e.g. haemoglobin) taken from different species produce a phylogeny. The phylogeny revealed by studying protein structure reflects the same phylogeny as comparative anatomy and embryology, but with a much finer resolution.
The ultimate is the comparison of the base sequences of variable regions of DNA (in particular mitochondrial DNA) taken from different organisms. Here the genotype is being analysed.
Rates of mutations are assumed to be constant.
The analysis of DNA provides a molecular clock against which the geological clock can be compared.
Biogeography: The geographical distribution of species
Organisms are not found in every habitat that they could occupy.
Travelling around the world the distribution of organisms follows a simple pattern.
Two similar habitats that are close will contain species that are closely related.
Two similar habitats separated by a great distance (e.g. an ocean) will contain unrelated species e.g. Australian fauna and the fauna on the rest of the continents.
This distribution suggests that all organisms were not created at the same time or they would be evenly distributed all over the world.
Disjunct distributions of living or extinct organisms were difficult to explain e.g. some marsupials (possums) are found in South America.
The problem was resolved with the development of the theory of plate tectonics in geology.
Australia was once attached to Africa, Antarctica and South America. This large continent broke up before the placentals evolved. The marsupials of Antarctica died out as it froze over. Those of Africa suffered from competition when the placentals evolved and those of South America survived until the Panama Isthmus was formed, only the Possum survived the competition form North American placentals.
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