lecture 15

Question 1

diversification of the cambrian explosion

Answer 1

1) climate and distribution of ocean and landmasses have changed through time.
2) taxonomic composition has changed through time as species originate others go extinct.
3) extinction rates have been particularly high at times.
4) diversification rates have also been particularly high at times, often after mass extinctions.
5) diversification brings new species and new morphologies and ecologies.
6) extinct taxa sometimes gets replaced by unrrelated but ecologically similar taxa.
7) over time, the composition of the biota increasingly resembles that of the present.

Question 2

for the first 150 million years of their evolutionary history (210-65 Ma)

Answer 2

mammals were mostly small-bodied, nocturnal insectivores.

Question 3

following the K-T extinction

Answer 3

placentals greatly diversified taxonomically, morphologically, and ecologically in less than 10 million years.

Question 4

evolutionary radiation

Answer 4

rapid burst of taxonomic, morphological, or ecological diversity.
- late branches are long and early branches are short.

Question 5

tests of mammalian radiation

Answer 5

• maps changes in species richness and body size in mammal fossil record.
• both are greatest right after the K-T boundary.
• supports ecological opportunity following extinction of dinosaurs, but fossil records may be incomplete.
• molecular phylogeny of living mammals suggests that most early divergences occur before the K-T boundary.
• timing corresponds to break up of continents into major landmasses, not extinction of dinosaurs.
• changes in feeding ecology, body size, and richness of multiuberculate mammals.
• dental complexity, taxonomic richness, body size increased 20 myr before K-T.
• radiation possibly due to rise of flowering plants, not extinction of dinosaurs.

Question 6

what triggers evolutionary radiation? 1)

Answer 6

1) ecological opportunity - unfilled or vacated ecological niches.
- unoccupied region with few competitors and a wide variety of resources to exploit.
- removal of incumbent provides release from competition/predation and open ecospace.
- affects many independent clades with broad geographic or ecologic distribution.

Question 7

what triggers evolutionary radiation? 2)

Answer 7

2) key innovation - evolution of highly beneficial adaptation.
- enables exploitation of new resource, ecological niche, and further subdivide ecologic space (e.g., aerial nocturnal insectivore).
- possibly co-evolutionary response to other groups.
- affects single clade, often referred to as adaptive radiation.

Question 8

ecological opporunity and the origin of adaptive radiation:

Answer 8

a) sources of ecological opportunity: key innovation, new habitat, antagonist extinction.
b) release from natural selection.
c) ecological release: increased trait variation, density compensation, broader habitat for resource use.
d) rapid speciation and morphological diversification.
* * adaptive radiation

Question 9

stages of evolutionary radiation

Answer 9

1) habitat (natural selection)
2) morphology (natural selection)
3) communication (sexual selection)

Question 10

lineage diversification rate studies

Answer 10

1) estimate diversification parameters.
- speciation rate, extinction rate, net diversification rate.
2) identify significant diversification rate shifts through time.
3) locate significant diversification rate shifts along branches.
4) evaluate correlations between diversification rates and traits.
5) detect significant diversification rate variation across tree.

Question 11

lineage diversification

Answer 11

the process by which the number of species in a lineage changes.

• speciation (lambda): the birth of new lineages.
• extinction (mu): the death of lineages.
• net diversification (v): lamda - mu

Question 12

diversification models

Answer 12

• constant pure birth: lambda, mu=0
• generalized pure birth: lambda(t), mu=0
• constant birth-death: lambda and mu, lambda > mu
• generalized birth-death: lambda(t) and mu(t), lambda > mu

Question 13

testing for adaptive radiation

Answer 13

• did the clade undergo adaptive radiation?
• expect initial diversification to be fast.
• gamma statistic (Y)

Question 14

constant rates model (testing for adaptive radiation)

Answer 14

• balanced node depths.

- gamma = 0.

Question 15

early and rapid branching model (testing for adaptive radiation)

Answer 15

• excess of old nodes.
• gamma will be negative.
• evidence for adaptive radiation.

Question 16

late branching (testing for adaptive radiation)

Answer 16

• excess of young nodes.
• gamma will be positive.
• could also be due to what?

Question 17

lineage through time plot (LTT)

Answer 17

accumulation of lineages through time.

• species increases exponentially when pure-birth (Yule) process (extinction = 0).
• the slope of the plot is V.
• lineage diversification is stochastic; there is variation.

Question 18

extinction unlikely to affect

Answer 18

young lineages (no chance to go extinct).

Question 19

increase in diversification towards the

Answer 19

present = “pull of the present.”

Question 20

decrease in diversification

Answer 20

over time.

Question 21

diversity-dependent selection:

Answer 21

• ecological interactions among lineages:
  > increase the probability of extinction.
  > decrease the opportunity for speciation.
  > speciation is inversely proportional to the number of species: r alpha (1/N)
• the fewer the species present, the greater the availability of niches and resources. this causes high speciation rates. tightly linked to adaptive radiation.

Question 22

empirical trends:

Answer 22

1) diversification often slows down with time.
2) diversification may (or may not) be diversity dependent.
3) diversification may (or may not) be adaptive.
4) many clades may be declining in diversity.
5) diversification is generally heterogeneous across lineages.
6) both biotic and abiotic factors influence long-term diversity dynamics.