Chapter 13 – Evolutionary History
Primordial earth
Earth was extremely hot. The heat was generated by asteroid impacts, internal
compression and radioactivity. Host gases made up the atmosphere, then the asteroid
bombardment stopped and the earth cooled, causing torrential rains which formed the
oceans. Gases such as N(g), CO2(g), CO(g), H2O(g), HS(g), NH4(g) and CH4(g) formed
the new atmosphere. Earth was subject to radioactivity, UV light, cosmic radiation,
lightening storms (electrical energy), and volcanic activity during this time.
Primary abiogenisis, a theory suggesting living organisms sprang from non-living
material was believed at first. It was reasoned that complex chemicals were
spontaneously, by the self-arrangement of cell-like structures, (I.e. thermal proteinoids).
If this was to happen than living things must show catalytic activity, in order to self
replicate.
Protocells;
For metabolic processes to occur the chemicals must remain concentrated and separate
from their environment.
Prokaryotes;
 Fossilization of prokaryotic cells proves they were the first true cells.
 Would have been chemoautotrophic.
Endosymbiosis;
 Prokaryote cells are believed to have given rise to archaebacteria and eubacteria.
 Eukaryotic cells are important to the evolutionary history complex organisms.
 The mitochondria and chloroplast are believed to have originated from the
process of endosymbiosis.
Rates of Evolution
There are two theories to examine the pace at which evolution proceeds. They’re;
Theory of Gradualism;
 When a new species evolve they are similar to the “original” species, and
gradually become distinct as their environment dictates.
 Slow incremental changes would be seen.
 Transitional forms could be seen through fossils.
Theory of Punctuated Equilibrium;
 Rapid evolution of species.
 Intermediate species are rare.
 Speciation is isolated.
 Initial spurts of evolution followed by periods of no/little change.
Both theories are accepted but neither can be proved due to gaps in the fossil record and
both theories account for patterns of evolution seen in earth’s species.
Evolution Pathways
Species derive from originating species through the process of evolution. There are four
major patterns/pathways that evolution follows. They’re;
Divergent Evolution;
 Disruptive selection may cause divergent evolution.
 Species begin to develop increasingly different traits.
Convergent Evolution;
 Due to similar selective pressures on species within the same environment,
species begin to develop similar phenotypes.
Parallel Evolution;
 Similar to convergent evolution, species evolve similarly because they’re subject
to the same pressures.
Coevolution;
 Two or more groups of species act as selective agents to drive the evolution of the
other species.
 If species are dependant of each other then coevolution will be experienced.
Evolution Pattern;
Adaptive Radiation;
 An originating species gave rise to a number of succeeding species through
evolution.
 Divergent evolution occurs simultaneously.
Phyletic Evolution;
 Through gradual change in a certain lineage speciation occurs.
Divergence or convergence can be seen in two species as a result of
selective pressures.
Adaptive radiation is common in habitats that favor diversification.
Clad Diagrams & Phylogeny;
Phylogeny and cladograms are used to group organisms related to common lineage.
 Phylogeny – theoretical evolutionary history of a species.
 Cladogram (phylogenic tree) – diagram showing proposed evolutionary
relationships between species.
Organisms sharing traits, suggest a common ancestor. Phylogenic relationships can be
aided by DNA comparisons.