Biogeography

Question 1

biogeography definition

Answer 1

Biogeographers aim to understand how geographical and geological factors, in combination with species’ traits and their evolutionary history, influence the spatial distributions of species that we observe today.

Biogeography plays a pivotal role in our understanding of evolutionary biology

(e.g. Darwins Finches and Alfred Russel Wallace’s discoveries in Indonesia)

“Zoogeographic Regions” are regions characterized by having evolutionarily similar species
What processes lead to these patterns? This is the subject of biogeography

Question 2

Dynamic processes relating to biogeography

Answer 2

*Movement (e.g., dispersal, migration)

*Spatial connectivity (e.g., sea level changes, plate tectonics)

*Diversification (e.g., speciation – extinction)

Question 3

Dispersal ability

Answer 3

“Vagility”, or the degree to which an organism can (or does) move or spread within an environment (synonymous with ‘dispersal ability’)

*Taxa that are more vagile should be less confined, more widespread than less vagile taxa.

(e.g. bats more vagile than snails)

*More widespread species are more resilient to local mortality, more likely to split off into multiple species. E.g. migratory bird species show higher speciation rates than non-migratory birds

Question 4

Migration

Answer 4

Migratory movements of organisms have important impacts on distributions:

*Seasonal migration –regular movement

*Chance events (rare) via ‘sweepstakes routes’ (e.g., arrival of lemurs to Madagascar)

*Filter routes (e.g., Beringia, where only spp. adapted to cold could migrate across ice bridge)

*High impacts on speciation & extinction dynamics

Question 5

Vicariance

Answer 5

the original population is divided by a geographic barrier resulting in reproductive isolation and speciation over time

*Vicariance is a process by which a lineages’ geographical range is split into two or more different populations by some extrinsic barrier (e.g., mountain uplift, ocean level rise, river formation).

*If split populations remain viable, can lead to allopatric speciation

Question 6

Vicariance example: sea level changes

Answer 6

Sea levels have fluctuated dramatically over time

*Owing to changes in the volume of ocean basins, global temperature fluctuations, and the amount of water stored in other forms (e.g., ice caps), the sea-level has fluctuated over time.

e.g. Wallace’s Line: Travelling between Bali and Lombok Alfred Russell Wallace noticed the diversity of bird communities between these two nearby islands was distinct. This was the same for mammals, insects and plant species.

There was a distinct line now referred to as the Huxley line – fluctuations in sea level had lead to biogeographical clustering

Question 7

Plate tectonics

Answer 7

plate tectonic theory was not confirmed and accepted until mid-twentieth century

240MYA land bridges allowed terrestrial animals to disperse widely as they broke up division by oceans resulted in reduced distribution. Movement of the plates drive up mountains and affect ocean circulation and therefore the climate

Question 8

Topographical changes

Answer 8

e.g. current changes in the gulf stream resulted in climate changing

Consequences for: Ocean circulation
Atmospheric circulation
-> Climate

Question 9

The great American biotic interchange (GABI)

Answer 9

Land bridge connecting North and South America connected terrestrial communities and divided marine communities.

*North American mammals that moved south: Carnivores (e.g., cats, canids, bears), ungulates (e.g., camelids, tapirs, deer)
*South American mammals that moved north: Sloths, glyptodons, primates, bats
*In general, North American taxa fared better in South America than vice-versa. Many mammals that were in South America prior to the GABI went extinct shortly thereafter.

4 possible reasons:

A: North American species disperse better
*From fossil data, they compared the estimated proportion of the total pool of genera that dispersed per million year.
No difference in rates from North to South and South to North – ruling out A

B: dispersal rate north to south and south to north equal but more speciation in South America
*From fossil data, they estimated origination rate (new genera per unit time) minus extinction rate.
No difference between diversification rates of migrant taxa in North and South America
-ruling out B

Meaning it must be C or D:

C: as North American animals arrive in South America native animals were more likely to go extinct

D: if there were more taxa in North America more will end up in South America

Question 10

Biogeography in the phylogenetics era: Modelling

Answer 10

*There is information in the arrangement of species’ ranges along a phylogeny
*Several models try to use this information to shed light on the processes that produced range distributions like the ones we see today.

Data -> model -> inferences

e.g traffic sensors use a model to make traffic predictions

Phylogeny and known ranges ->The Dispersal Extinction Cladogenesis (DEC) Model -> inferences of historical ranges

Question 11

The Dispersal Extinction Cladogenesis (DEC) Model

Answer 11

DEC Includes anagenetic events
(ie. Events that happen along a branch)
Example: distributing from Cuba to Jamaica and then becoming extinct in Cuba

DEC Includes cladogenetic events
(I.e. events that happen at speciation)

Then DEC asks: which set of anagenetic and cladogenetic events happening along a known phylogeny is most likely to produce the set of ranges we observe at the tips?

(see notes for diagrams)

Question 12

Ancestral range estimation in butterflies using DEC

Answer 12

~17500 butterfly species worldwide

Lack of historical record – leave no fossil record – therefore models are suited to their study

*Built time-calibrated phylogeny of nearly 2300 butterfly species (representative of all groups)
*Butterflies originated during the Cretaceous period

Ancestral range estimation in butterflies
- By plotting known ranges in DEC they generated ancestral range estimations
- Butterflies originated in the Americas ~90MYA reaching Europe 30-15MYA
- Originally radiation mostly from tropics and radiation continues from tropics and other areas

Conclusion on the ancestral range of butterflies:
*Armed with the phylogeny and knowledge of the distribution of butterflies, it is possible to estimate the historical biogeography of this clade using the DEC model.
*Butterflies originated somewhere in what is now the Americas, ~100 Ma
*Dispersal from the tropics into temperate regions is more common than the reverse.

Question 13

The vertebrate latitudinal diversity gradient

Answer 13

What explains the latitudinal diversity gradient?

*One prominent hypothesis posits that the net diversification rate (rate of speciation - rate of extinction) is higher in tropical regions

*This could result from:
- A. Faster speciation rates in the tropics (the ‘cradle’ hypothesis)
- B. Lower extinction rates in the tropics (the ‘museum’ hypothesis)

Question 14

Diversification models

Answer 14

*Diversification models are used to investigate the way that lineages accumulate through time.

These models ask: which speciation and extinction processes are likeliest to have produced a phylogeny like the one we see today?

Phylogeny -> diversification models -> estimates of speciation and extinction rates

Question 15

Birth - death model

Answer 15

Two types of events: speciation = birth, and death = extinction

Density of birth and death events calculated

These models ask: which speciation and extinction processes are likeliest to have produced a phylogeny like the one we see today?

Question 16

The Tetrapod Latitudinal Diversity Gradient
Do speciation and/or extinction rates differ in the tropics?

Answer 16

Findings suggest extinction rates lower in tropics than extra-tropics suggesting Museum Theory correct

Museum theory: that the tropics may act as a “museum” where older lineages persist through evolutionary time

However: Other factors that could generate patterns: Dispersal and Time

Vast majority of dispersal is from tropics to extratropics (as seen in butterfly study)

Time hypothesis: Lineage through time plot
lineages have diversified in extratropic far more than in tropics

Question 17

What explains the latitudinal diversity gradient?

Answer 17

*Phylogenetic analyses support a key role for lower extinction rates in the tropics (i.e., the ‘museum’ hypothesis)

*Dispersal into the tropics doesn’t explain the LDG: most dispersal is away from the tropics.

*The tropics haven’t been diversifying for a longer time, either: all four groups have a longer diversification history outside of the tropics.

Question 18

A bit more about models

Answer 18

*By fitting diversification models to tetrapod phylogenies, the authors were able to test hypotheses about the formation of the LDG

*Extinction rates are lower in the tropics, though speciation rates aren’t consistently higher (supports the “museum” hypothesis

(see notes)

Question 19

Summary

Answer 19

1.Biogeography refers to the study of the evolution of spatial diversity.

2.Biogeographical patterns arise from a combination of several processes: movement, connectivity, and diversification

3.The union of North & South America ~3 Mya drove a pronounced change in the mammals present in each region, though North American mammal lineages were more likely to persist in South America than vice versa.

4.Phylogenetic models of historical biogeography, such as the DEC model, can be used to infer ancestral ranges from contemporary range data.

5.Using the DEC model on a phylogeny of butterflies, we can see that butterflies originated in the Americas and have tended to disperse out of the tropics throughout the history of the clade.

6.Latitudinal diversity gradients are a pervasive pattern across vertebrates, with many more species in tropical than in temperate regions.

7.Diversification models can be used to investigate variation in the way that lineages in a phylogeny accumulate through time (via changes in the speciation rate, extinction rate, and/or net diversification rate) .

8.The tetrapod latitudinal diversity gradient appears to have arisen from decreased extinction rates in the tropics.