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Lecture 12

BIOL 215 Lecture Notes - Lecture 12: Ordovician, Macroevolution, Leigh Van Valen

Biology (Sci)
Course Code
BIOL 215
Neil Price

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BIOL215 Lecture 12 Notes
Sympatric speciation - speciation that occurs in the same place
Geographic isolation does not always precede speciation - in these cases speciation is sympatric
Sympatric speciation arises without geographic isolation
This means that a biological barrier to gene exchange has to arise within the confines of a
randomly mating population without any spatial segregation of the species
Very controversial! Many theoretical difficulties
Only one example of sympatric speciation not controversial - speciation by polyploidy in plants
A single instantaneous change caused by polyploidy doubling of chromosome # -
reproductively isolates a new polyploid from its ancestors
Cichlid species (fish)
Mechanisms facilitating reproductive isolation
Normally, however, the barriers separating species are not caused by a single isolating mechanism
Reproductive isolating mechanisms:
Are obstacles to interbreeding between genetically distinct species
Hybrids are not well adapted to environment
Low fitness
Reproductively costly for the parent
There are 2 types of reproductive isolating mechanisms:
Those that act before fertilization of the egg (prezygotic isolating mechanisms)
Geographic isolation
Ecological isolation
Behavioural isolation
Temporal isolation
Those that act after fertilization of the egg (postzygotic isolating mechanisms)
Hybrid embryos might not develop properly
Hybrid adults do not survive in nature
Hybrid adults are sterile or have reduced fertility
Those species that live much closer to each other will have much stronger prezygotic and
postzygotic isolating mechanism because hybrids are very bad for them
This modern synthesis gave rise to "NeoDarwinism"
The view that mutation, recombination, natural selection and other processes operating within
species account for the major, long-term features of evolution
Natural selection, through gradual changes over a long period of time, has produced the variety
we see today: microevolution
Paleontological view:
Paleontologists, however, recognized that:
1. Species appear "abruptly" in the fossil record
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