Adaptive Radiation

Adaptive radiation - 'evolutionary divergence of members of single phyletic line into series of different niches or adaptive zones'

Distinguished from progressive adaptation by being more rapid.
 
 

Generalizations

1. Shift into new ecological range most likely to occur edge of species range

2. Lack of competitors facilitates adaptive radiation -- empty niches.

3. Islands provide many empty niches; continents provide few

4. Lack of predators -- Predators may prevent radiation (Hawaii has very few predators).

5. Occupation of new adaptive zone frequently followed by increase in rate of evolution;

6. Speciation & opportunities for adaptive radiation define size of taxonomic categories (more speciation -> more species/genus; more radiation -> monotypic genera) -- 'hollow curve'.

 Ammonite phylogeny and radiation. New order tended to radiate from small, unimpressive species in earlier period. Why? 1. Competitive interactions limited radiation? But no sign of equilibrium during period from Devonian to Cretaceous.

2. Strong element of stochasticity.

3. Nature of survival through extinction periods may play role.

 Other patterns in ammonites agree with Mayr's generalizations: 1. At any one time, a small number of large groupings and a large group of smaller taxa -> hollow curve.

2. Adaptive radiation rapid after extinction episode.

3. Opportunities for radiation coincided with availability of new space for colonization.

 Echinoid radiation -- similar to ammonoids

Radiation of brachiopods and foraminiferans. Same pattern -- one lineage radiates while others static;

Are radiations predictable?

Possibilities:

1. Deterministic

2. Random series of radiations and extinctions

3. Evolutionary sequences that are unpredictable, but in retrospect rational.

Supported by evidence from Burgess shale and elsewhere:

a) Wiwaxia and relatives: six genera in Cambrian. None now.

Clams had only 2 genera in Cambrian, but it was they that radiated.
 
 
 
 
 
 
 
 
 
 
 


 
 

b) Priapulids and polychaetes: same number of genera in Burgess shale, but priapulids abundant, polychaetes rare. However, polychaetes radiated.

c) Predaceous birds or mammals. When large carnivorous dinosaurs died out in Cretaceous, room for new radiation. In north felids and canids radiated; in south marsupials and large bird predators. Mammals then went on to 'win' the conquest -- but it could just as easily have been the birds.
 
 
 

Types of adaptive radiation

1. General adaptation -- New general adaptation opens up new adaptive zone, such as:

  a) evolution of flight in birds;

b) evolution of parasitic habit in book lice: went on to birds.

c) agromyzed flies -- acquired general adaptation of leaf mining and radiated across plants.

d) beetles on milkweed plants; potato beetles on solonaceous plants.

 
 
 
 
 
 
 
 
 
 
 
 
 

Adaptive zone is 'set of ecological niches that may be occupied by a guild of organisms'; guild is a 'group of species that exploits the same resources in a similar manner'.
 
 
 
 
 

2. Environmental change -- environmental change opens up new adaptive zones.

a) Ammonites radiated as continental shelves flooded.

b) Ruminants & equids radiated as savannas appeared with climate drying.
 
 
 
 
 
 
 

4. Combination of environmental change on archipelagoes -- combination of Type 2 & 3 required to explain radiation of cichlid fish in African lakes.

a) Lakes change dramatically in water level -> new open habitat;

b) Lakes decline -> isolation.

Also, lakes consist of many isolated rocky 'islands' for fish.

84% if fish are cichlids in Lake Victoria (only 8% in Nile, source river).
 
 
 
 

Models of Radiation:

1. Adaptive -- general pattern of mutation and natural selection.

2. Genetic drift -- founder events;

3. Sexual selection.

4. Drift plus selection model -- Kaneshiro hypothesis. Founder event causes shift from adaptive peak and sexual selection establishes new stabilized mating system different from parent.

3. Archipelagoes -- remote, geographically isolated locales with heterogeneity. May be oceanic island groups or habitat islands.

Hawaii in particular -- most isolated archipelago in world. Characterized by:

1. Isolation -- Isolated biotas with many families absent.

2. Volcanic activity repeatedly opens up new habitat.

3. Plentiful empty ecological niches.

4. Absence of competing species outside radiating taxa.

5. Importance of environmental heterogeneity.

6. Depauperate predators and parasites.

7. Importance of competition between radiating species & resultant character displacement.

Rapid divergent evolution