Patterns of Speciation

Diversify in Bursts(punctuational)

Diversify slowly(gradual)

Tim

e

No diversification(living fossil)

Evolutionary Changes

XX X

X

X = extinction

Adaptive Radiation

Adaptive Radiation

An adaptive radiation is a clade (group of related species) that has evolved into numerous, diverse forms

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 TrendsConvergences are common - independent

radiations come to resemble each other Australia, South America & North America

evolved similar grazers, browsers & saber-toothed ‘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. carnivoresgrass vs. horses, crabs vs. snails

(2) Mutualisms: flowering plants + pollinators + seed dispersers