Transcript Patterns of Speciation

Patterns of Speciation
X = extinction
Evolutionary Changes
X
X
X
Time
X
Diversify in Bursts
(punctuational)
Diversify slowly
(gradual)
No diversification
(living fossil)
Adaptive Radiation
An adaptive radiation is a
clade (group of related
species) that has evolved
into numerous, diverse
forms
Adaptive
Radiation
Hawaii
Cichlids in the African Great Lakes
What causes adaptive radiations?
Pioneering a new habitat containing few
competing organisms
– remote islands, isolated large lakes
What causes adaptive radiations?
Mass extinction wipes out competitors
• There have been 5 major catastrophic mass extinctions
• The last is associated with the final extinction of
dinosaurs
• Surviving forms radiate to occupy ‘niches’ left vacant
by the extinct forms
• e.g. Radiation of mammals after extinction of
dinosaurs
What causes adaptive radiations?
Evolution of a key innovation
• birds radiate after the evolution of flapping
flight
• Angiosperms (flowering plants) radiate after
the evolution of reproduction via flowers
Why are there ‘living fossils’?
Organisms may be ‘well-adapted’, but there is
little room for evolutionary change because
of competitors or environmental constraints
Evolutionary Trends
• Little change in the
number of phyla
(higher taxonomic
levels) in the last 500
million years
• The number of
species has been
steadily increasing
(despite set-backs
caused by mass
extinctions)
Evolutionary Trends
Convergences are common - independent
radiations come to resemble each other
 Australia, South America & North America
evolved similar grazers, browsers & sabertoothed ‘cats….. All from different ancestors
Evolutionary Trends
Repeated patterns sometimes acquire names:
• Cope’s rule: Members of a clade tend to get larger
in evolutionary time
• Bergman’s rule: Organisms get larger at higher
latitudes (toward the poles)
• Islands: Large animals evolve to be smaller (dwarf
elephants), Small animals evolve to be larger (giant
tortoises)
Evolutionary Trends
Much of the diversification
of species seems to be
related to …..
(1) Arms Races (Red Queen):
plants vs. herbivores vs.
carnivores
grass vs. horses, crabs vs. snails
(2) Mutualisms: flowering
plants + pollinators + seed
dispersers