BIOL1003 Lecture Notes - Lecture 9: Sympatric Speciation, Howea, Allopatric Speciation
Speciation
• History and diversity of life can be depicted as a phylogenetic tree
• Branches (lineages) split along the tree in response to speciation- the splitting of
one species into two
• Clade- All the species descended from any particular split
• Cladogenesis- The evolutionary change resulting from the splitting of lineages is
called
• Monophyletic- a natural clade contains all descendants resulting from a split
• The size of clade is determined by the balance between the rate of speciation
and the rate of extinction
• Sister groups- two clades that derive from a splitting event
• Sister species- if only split into two species
• Clades can persist for long periods of time without any speciation of other
evolutionary change (stasis), giving rise to living fossils
• Anagenesis- lineages which show evolutionary change without apparent
speciation
• When looking at fossils we can only detect morphological change so we do not
know whether or not speciation has occurred based on the biological species
concept (reproduce to produce fertile offspring)
• Speciation
oRing species are a reminder of the fact that if we moved back through
time there would be a point at which two lineages that we now call
species would have once been difficult to distinguish
oFor current cases we focus on the biological species concept so we
question why interspecific mating fails- isolating mechanisms
Prezygotic isolating mechanisms
- Mechanisms that prevent interspecific crosses- temporal
isolation, habitat isolation, pollinators, behavioural
isolation (mate choice and sexual selection
- Mechanisms that prevent fertilisation- genetic
incompatibility, gametic mortality
Postzygotic isolating mechanisms
- Mechanisms that prevent the spread of interspecific
hybrids- eggs are fertilised but the zygote dies, hybrid
inviability, hybrid sterility (e.g. mules), hybrids survive but
are unattractive or ecologically poor competitors
- Produce offspring but they don’t function well
oOne key problem to overcome- differentiation due to disruptive natural
selection is retarded by the spread of genetic information from one 'type'
or population to another (gene flow)
oMating between two types also breaks apart sets of genes that go well
together due to recombination of genes during sec (e.g. a big beak to eat
larges seeds but not the right enzymes and digestive chemicals to break
them down)
oSelection favours keeping genes for beak type and digestive enzyme type
when they are together in the same individual but gene flow and
recombination during sec breaks up sets of genes
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