HISTORICAL BIOGEOGRAPHY

Transcription

1 HISTORICAL BIOGEOGRAPHY How do we reconstruct the origin, dispersal, and extinction of taxa? How historical events have affect the biology on the planet?

2 Changing climate and physical conditions Rearrangements of the continents and ocean basins Extinctions Catastrophic collisions with asteroids Proliferation of fish, mollusks, & crustaceans Proliferation of flowering plants ➋ ➊ ➌ ➍ ➎ Diversity has generally increased over the past several hundred million years

3 An exclusive focus on local environmental conditions will yield an incomplete understanding of diversity Where or how to find basic explanations for geographic patterns in species distributions and communities? Related species: different places How to explain? Creation theory: Multiple centers of creation Evolution theory: common center for ancestor?

4 There are several fundamental processes in biogeography Isolation and Evolution Center of origin Isolation and Evolution Evolution Speciation Extinction Dispersal These are the processes by which organisms respond to changes in the geographic template. Isolation and Evolution The relative importance of movement, or dispersal, has been the subject of great debate. The early dispersalists included Darwin, Alfred Russel Wallace and Asa Gray. Wallace Darwin They argued that disjunctions (a situation in which two closely related populations are separated by a wide geographic distance) could be best explained as the result of long distance dispersal. Gray

5 The dispersalists were opposed by the extremists, who believed that disjunctions had resulted from movement along ancient corridors that had disappeared. Lyell Among the leaders of the extensionist movement were Charles Lyell and Joseph Hooker. Hooker

6 No evidence was ever discovered for the lost corridors proposed by the extensionists. However, evidence surfaced in the 20 th Century for a new, powerful means of dispersal: continental drift. The movement of continents could raft populations away from each other and separate them in vicariant events (tectonic, climatic, or oceanographic occurrences that isolate previously connected populations). What is dispersal? Simply, the movement of organisms away from their birthplace. Often, confined to a particular life history stage. As a result, the debate between dispersalists and extensionists has been replaced by a debate between dispersalists and vicariance biogeographers Don t confuse with dispersion, which refers to the position of individual organisms with respect to others in the population.

7 Dispersal Species A Species B Ancestral population Geographic isolation Speciation The role of dispersal in biogeography is different. Biogeographers are interested in those dispersal events in which species change their range by dispersing over long distances. These events are rare, and largely random. They are, however, critical to understanding the distribution of organisms. Dispersal and Range Expansion Dispersal and Range Expansion In order to expand its range through dispersal, an organism must be able to: Reach a new area. Survive the potentially harsh conditions occurring during the passage. Survive and reproduce in the new area to the extent that a new population is established. Biogeographers often distinguish three types of dispersal events that can accomplish this: 1. Jump dispersal ( sweepstakes ) 2. Diffusion. 3. Secular migration. How do they operate?

8 Jump dispersal Jump dispersal Definition: Movement of individual organisms across great distances, followed by the successful establishment of a population of the original disperser's descendants at the destination. This usually takes place over a time period less than the life span of the individual and often over inhospitable terrain. Species skips over area outside its range to new location Island colonisation Some species lacking from islands limited ability to disperse (mammals, amphibians, freshwater fishes) Also occurs across continents The sheepshead minnow (Cyprinodon variegatus) Estuaries and mangrove swamps throughout the Caribbean It has been able to colonize these habitats by dispersing many hundreds of miles across ocean water. It s ability to tolerate wide ranges of salinity makes this possible.

9 Contrast Jump dispersal is simply the colonisation of new areas over long distance. An example can be seen in the rapid recolonisation of Krakatau after all life was wiped out by the volcanic explosion of 1883 The many species of sunfish (Lepomis spp.) of the southeastern United States, lacking this tolerance, were unable to disperse to Caribbean islands. We can see the same thing over longer distances and greater time periods for many other archipelagoes. The Galapagos lie 800 km west of Ecuador in the Pacific Ocean. The Hawaiian Islands lie 4000 km west of Mexico. In both cases, there is indisputable evidence of many groups of organisms reaching the islands by dispersal. Diffusion Definition: Diffusion is the gradual movement of populations across hospitable terrain for a period of many generations. Species that steadily expand their ranges can be said to be diffusing. Gradual spread of of individuals outward from the margins of a species range. It is a slower form of range expansion involving not just individuals, but populations.

10 The cnidarians Velella and Physalia have sails or floats that allow them to drift across the surface of the ocean. In both cases, the orientation of the sail causes them to drift either to the right or left. Range expansion Dispersal Dispersal of foraminifera Movement of species - may be active (flying) or passive (wind-transport, drifting on a log) Secular migration Definition: Secular migration is diffusion taking place so slowly that the diffusing species undergoes appreciable evolutionary change during the process. The range of the species expands or shifts over long time intervals (thousands or millions of years). The environments themselves may change and natural selection acts on the descendant populations. Evolutionary divergence through range expansion. Evolutionary time scale.

11 Diffusion and Secular migration Shift happens Range extension: the species is at first found only in area A. It later gradually extends its range of distribution into the neighbouring area. However, in the absence of any barrier between the two areas, it cannot differentiate into a new separate species. Cox, C. B. & Moore, P. D. (2005). Biogeography. An ecological and evolutionary approach. Oxford: Blackwell Scientific Publications, London. Vicariance Ancestral population Geographic isolation Speciation

12 VICARIANCE? Vicariance : Species distributions are determined by geological and climatological events: continental drift, Pleistocene glaciation etc VICARIANCE? Isthmus of Panama closed ~ 3.1 MYA Split ~150 geminate (twin) species Allopatric speciation in snapping shrimp A. formosus A. nuttingi Atlantic Ocean Isthmus of Panama Pacific Ocean Knowlton et al.(1993) created a phylogeny of Pacific (P) and Caribean (C) species pairs of Alpheus In 6 out of 7 cases, the closest relative of a species was on the other side of the Isthmus P7 P6 P6 C6 P3 C3 C3 P5 C5 P4 C4 P1 C1 P2 C2 Alpheus sibling species A. panamensis A. millsae 53 P7

13 VICARIANCE? Vicariance: a species originally occupies the whole of area A and B, but these two areas become separated from one another by a barrier. The original species then differentiates, by gradual genetic change, into two separate species that are separated by the barrier. Jump dispersal: the species is at first restricted to area A by a barrier that separates if from area B, and only later disperses across the barrier. The two populations of the species now each diferentiate into separate species. Cox, C. B. & Moore, P. D. (2005). Biogeography. An ecological and evolutionary approach. Oxford: Blackwell Scientific Publications, London. The final results of vicariant speciation and of jump dispersal are identical. BARRIERS The nature of long-distance dispersal means that organisms often have to survive for periods of time in environments that are hostile to them. These environments constitute physical and biological barriers to dispersal. The effectiveness of such barriers in preventing dispersal depends not only on the nature of the barrier, but also on the organism dispersing.

14 Pacific Pacific Pacific Rise of the Isthmus of Panama Atlantic Atlantic Atlantic Molecular phylogeny and geographical distribution of the intertidal snails, Cerithideopsis californica and C. pliculosa. A maximum-likelihood tree was constructed based on 873 bp of the CO1 gene. The major clades are categorized as clades A, B and the detailed subclades are within. Numbers near nodes are the support values for the clade from the different analyses (ML/BI). The scale bar represents the phylogenetic distances expressed as units of expected nucleotide substitutions per site. The distributions of the major genetic clades are shown at the right side of the figure. Numbers near the geographical points indicate sample size. Letters indicate sampling sites. VICARIANCE / DISPERSAL Bannerfishes (Chaetodontidae) IAA: Indo-Australian Archipelago Peter F. Cowman

15 BIOTIC EXCHANGE AND DISPERSAL ROUTES Corridors Filters Sweepstake routes Biogeographers often distinguish three kinds of dispersal routes based on how they effect biotic interchange. 1. Corridors. Allow dispersal by permitting movement Contemporary examples. Historical account for related of different species or even same species in widely separated regions 2. Filters. Conditions fall outside range of physiological tolerance. Restrictive dispersal pathway. Conditions restrictive to some species, not others. Can be biotic or abiotic 3. Sweepstakes routes. Hazardous or accidental dispersal mechanisms by which animals move from place to place. The standard examples are island hopping and natural rafts. Many land vertebrates live in the Caribbean Islands, and (if their biogeography is correctly explained by dispersal) they might have moved from one island to other, perhaps being carried on a log or some other sort of raft. DISPERSAL: OVERCOMING BARRIERS dispersal corridor A B corridor habitats wide variety A to B dispersal easy Corridors are dispersal routes that allow movement of most taxa from one region to another. They do not selectively discriminate against one form, but rather allow a balanced assemblage of plants and animals to cross them. The areas at the two ends of a corridor should contain a fairly similar assemblage of organisms. dispersal filter A B limited array (oases) difficult; only certain organisms make it sweepstakes dispersal A B none occasional migrants The Bering Land Bridge which existed some 20,000 years ago likely functioned as a corridor which allowed organisms to pass from northern Eurasia to North America with very little selection of the types that could pass. Conditions along the corridor would have differed little from those on either end.

16 A filter is a dispersal route that exercises some selection over the types of organisms that can pass through it. As a result, the colonists are a somewhat biased subsets of those that could potentially pass. The Arabian subcontinent acts as a filter in that only certain mammals, reptiles and ground birds can disperse between northern Africa and central Asia. 1. species originate in centers of origin The Lesser Sunda Islands form a twoway filter for the reptilian fauna of southeastern Asia and Australia. This region is sometimes known as Wallacea, and is bisected by Wallace s Line. 1. species originate in centers of origin 2. speciation follows 1. species originate in centers of origin 2. speciation follows 3. new species evolve and disperse, thus displacing more primitive species to the periphery of the center of origin

17 1. species originate in centers of origin 2. speciation 3. new species evolve and disperse, thus displacing more primitive species to the periphery of the center of origin. 4. organisms disperse as far away as permitted by their abilities and their physical conditions. Burgeoning info from Paleontology Origin, dispersal, radiation, and decline of terrestrial vertebrates Development of phylogenies New groups rise Increase in number of species Radiate to fill niches and expand range Still question of how dispersed Relationships between geological and ecological properties of environment Patterns of morphological variation Coincided with Modern Synthesis Linking lead to formulation of biological species concept (Ernst Mayr) Also identified allopatric speciation Earth s crust believed fixed until 1960s Continental drift 1 st proposed 1858 Resurfaced by Wegener and Taylor using geological and ecological evidence Criticized and denied until evidence became overwhelming Changed dispersal from land bridges to movements of continents

18 Continental drift lead to study of disjunctions Vicariance biogeography Examination and revision of phylogenies Computers Geographic information systems Geostatistics Approaches to investigating historical biogeographic patterns Phylogeography or historical biogeography 1. Emphasizes hypothesis testing 2. Uses phylogenies (many of which are developed with molecular data) 3. Uses area cladograms 4. Uses fossil and geological data HISTORICAL BIOGEOGRAPHY VICARIANCE DISPERSAL END