Distribution by Vicariance
Distribution by Vicariance
Disjunct distributions of animals may be created by physical changes in the environment that cause formerly continuous habitats to become disjunct. Areas that were once joined may become separated by barriers that are effectively impenetrable for many animals that inhabit them. The study of fragmentation of biotas in this manner is called vicariance biogeography. At the species level, “vicariance” is often used as a synonym of “allopatry,” which is simply a distribution of populations in geographically separated areas. Lava flows from a volcano may cause a formerly continuous forest to become separated into geographically discontinuous patches, thereby breaking many species of plants and animals into geographically isolated populations.
Perhaps the most dramatic vicariant phenomenon in the earth’s history is continental drift, through which a once continuous landmass was sequentially broken into continents and islands separated by ocean (see the following text). All terrestrial and freshwater animal species that had spread across the initially continuous landmass became sequentially fragmented into many populations on different continents and islands separated by ocean. Vicariance by continental drift gives us another hypothesis for explaining the disjunct distribution of flightless birds; they may descend from an ancestral species that was widespread in the Southern Hemisphere when Africa, Australia, Madagascar, New Guinea, New Zealand, and South America were in closer contact than they are today. When these landmasses moved apart across the ocean, the ancestral species would have fragmented into disjunct populations that evolved independently, producing the diversity of forms that we observe today.
How do we test hypotheses of vicariance biogeography? Reconstructing past histories of environmental changes might seem impossible, but we do have a very powerful method for testing such hypotheses, and it is based on the systematic methods presented in Classification and Phylogeny of Animals.
Suppose that the different species of flightless birds evolved allopatrically as continental drifting sequentially broke their terrestrial environment into isolated pieces. If we construct a cladogram or phylogenetic tree of these birds as shown in Figure 39-14, the earliest divergence should correspond to the first vicariant event that fragmented their common ancestral species. All subsequent branching events on the tree should correspond sequentially to subsequent vicariant events that fragmented major lineages further. Our tree hypothetically reconstructs the history of vicariant events for the group. If we erase the names of the species from the terminal branches of the tree and replace them with the geographic areas in which each species is found, we have a hypothesis for the sequential separation of the different geographic areas. We can test this vicariant hypothesis further by identifying other groups of terrestrial organisms that have different species in each of the same geographic areas as flightless birds. If our hypothesis is correct, these groups were fragmented geographically by the same vicariant events that fragmented the flightless birds. We therefore predict that the cladogram or phylogenetic trees constructed for species in the other groups will show the same branching pattern as the flightless bird tree when we replace the species names with those of the areas they inhabit. If this hypothesis is confirmed, we have a general area cladogram that depicts the history of fragmentation of the different geographic areas studied. This general area cladogram can be investigated further using geological and climatic studies.
In many groups of organisms, it is likely that both vicariant and dispersal events have contributed to the evolution of disjunct distributional patterns. Methods of vicariance biogeography will be very useful for finding such cases. Indeed, the cladogram of flightless birds is not just a simple grouping of birds that inhabit nearby areas. We can ask whether any branches on a cladogram representing a particular group of species are inconsistent with the general area cladogram for geographic areas that the species inhabit. Suppose that the cladogram for a particular taxon is consistent with the area cladogram except for placement of a single branch. We explain most of the geographic disjunctions within the taxon by vicariance but look for dispersal to explain the single branch that is not compatible with the general area cladogram. In this way, we can focus our study of dispersal on specific cases in which it is most likely to have occurred.
Disjunct distributions of animals may be created by physical changes in the environment that cause formerly continuous habitats to become disjunct. Areas that were once joined may become separated by barriers that are effectively impenetrable for many animals that inhabit them. The study of fragmentation of biotas in this manner is called vicariance biogeography. At the species level, “vicariance” is often used as a synonym of “allopatry,” which is simply a distribution of populations in geographically separated areas. Lava flows from a volcano may cause a formerly continuous forest to become separated into geographically discontinuous patches, thereby breaking many species of plants and animals into geographically isolated populations.
Perhaps the most dramatic vicariant phenomenon in the earth’s history is continental drift, through which a once continuous landmass was sequentially broken into continents and islands separated by ocean (see the following text). All terrestrial and freshwater animal species that had spread across the initially continuous landmass became sequentially fragmented into many populations on different continents and islands separated by ocean. Vicariance by continental drift gives us another hypothesis for explaining the disjunct distribution of flightless birds; they may descend from an ancestral species that was widespread in the Southern Hemisphere when Africa, Australia, Madagascar, New Guinea, New Zealand, and South America were in closer contact than they are today. When these landmasses moved apart across the ocean, the ancestral species would have fragmented into disjunct populations that evolved independently, producing the diversity of forms that we observe today.
How do we test hypotheses of vicariance biogeography? Reconstructing past histories of environmental changes might seem impossible, but we do have a very powerful method for testing such hypotheses, and it is based on the systematic methods presented in Classification and Phylogeny of Animals.
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| Figure 39-14 The phylogenetic relationships inferred for flightless birds (Organic Evolution). Vicariance biogeography proposes that these flightless species descended from an ancestral species that was widespread in the Southern Hemisphere when Africa, Australia, adagascar, New Guinea, New Zealand, and South America were connected. By moving apart, these landmasses fragmented both themselves and the flightless populations they contained. If the vicariance hypothesis is correct, the phylogenetic branching sequence inferred for the allopatric flightless species reflects the sequence by which their landmasses broke apart from each other. This hypothesis is tested by looking for similar phylogenetic patterns in other groups of animals and plants whose ancestral populations would have been fragmented by the same geological events. The widespread geographic distribution of the elephant bird suggests that it has dispersed following the fragmentation of landmasses. |
Suppose that the different species of flightless birds evolved allopatrically as continental drifting sequentially broke their terrestrial environment into isolated pieces. If we construct a cladogram or phylogenetic tree of these birds as shown in Figure 39-14, the earliest divergence should correspond to the first vicariant event that fragmented their common ancestral species. All subsequent branching events on the tree should correspond sequentially to subsequent vicariant events that fragmented major lineages further. Our tree hypothetically reconstructs the history of vicariant events for the group. If we erase the names of the species from the terminal branches of the tree and replace them with the geographic areas in which each species is found, we have a hypothesis for the sequential separation of the different geographic areas. We can test this vicariant hypothesis further by identifying other groups of terrestrial organisms that have different species in each of the same geographic areas as flightless birds. If our hypothesis is correct, these groups were fragmented geographically by the same vicariant events that fragmented the flightless birds. We therefore predict that the cladogram or phylogenetic trees constructed for species in the other groups will show the same branching pattern as the flightless bird tree when we replace the species names with those of the areas they inhabit. If this hypothesis is confirmed, we have a general area cladogram that depicts the history of fragmentation of the different geographic areas studied. This general area cladogram can be investigated further using geological and climatic studies.
In many groups of organisms, it is likely that both vicariant and dispersal events have contributed to the evolution of disjunct distributional patterns. Methods of vicariance biogeography will be very useful for finding such cases. Indeed, the cladogram of flightless birds is not just a simple grouping of birds that inhabit nearby areas. We can ask whether any branches on a cladogram representing a particular group of species are inconsistent with the general area cladogram for geographic areas that the species inhabit. Suppose that the cladogram for a particular taxon is consistent with the area cladogram except for placement of a single branch. We explain most of the geographic disjunctions within the taxon by vicariance but look for dispersal to explain the single branch that is not compatible with the general area cladogram. In this way, we can focus our study of dispersal on specific cases in which it is most likely to have occurred.
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