CH 6: The Geography of Evolution

Disjunct Distributions

Disjunct Distributions-Taxa that have gaps in their distributions Closely related organisms are located at a far distance from one another

Historical factors that influence biogeographical distribution (3)

Extinction Dispersal Vicariance

Multi-Regional Hypothesis (5)

-About 2 million years ago, Homo erectus, Orrorin and Ardipithecus ancient humans lived in Africa. Homo erectus left Africa but the other ancient humans remained there. -During the Pleistocene sea levels dropped, which exposed the land between Africa and the Middle East. The exposed tract of land is called the Gateway of Tears. Homo erectus crossed this land bridge to disperse into the Middle East. Homo erectus ends up in Europe and Asia in addition to being in Africa. -In these locations, Homo erectus evolves into Homo sapiens. -So, Homo sapiens originated in Europe, Asia and Africa. Due to migration and interbreeding, the various Homo sapien populations were maintained as a single species. -The estimated time of divergence is 2 million years

Replacement Hypothesis (5)

-About 2 million years ago, Homo erectus, Orrorin and Ardipithecus ancient humans lived in Africa. Homo erectus left Africa but the other ancient humans remained there. -The interbreeding between Homo erectus from Africa and Neanderthals from Europe resulted in the evolution of Homo sapiens 200,000 years ago. These Homo sapiens were only present in African land. -A second glacial advance occurs, which re-exposes land bridges. A second migration occurred 150,000 years ago as Homo sapiens left Africa. These Homo sapiens replaced the Homo erectus populations in Asia and Europe as well as the Homo neanderthalensis populations in Europe. -By about 35,000 years ago, Neanderthals had disappeared completely and in effect Homo sapiens replaced Homo neanderthalensis. -Note: All of the basal haplotypes from genetic studies using mtDNA are found in African populations. This means that Africans are ancestral and Europeans and Asians are more derived. Aforesaid, phylogenetic trees based on sequencing suggest that the replacement hypothesis is correct.

Biogeographic Realms (5)

-Biogeographic Realms-A spatial region characterized by similar flora and fauna. These realms explain the fact that the composition of biota is more uniform within certain regions than between them. -Realms are more of a result of Earth's history than its climate or land mass distribution -Realms can be divided into faunal and floral provinces, which are regions of endemism. High levels of provinciality increase biodiversity. -Range Expansion: The borders between geographic realms and provinces can't be sharply drawn because some taxa infiltrate neighboring realms to varying degrees. -With more information, especially genetic data, the number of biogeographic realms has increased

Biogeography (3)

-Biogeography-the study of the geographic distribution of organisms -Historical Biogeography-geographic distribution of a taxon may be explained best by historical circumstances. A species may exist at a given location because it has a history of existing there and has persisted there. -Ecological Biogeography-ecological factors operating at the present time are the best explanation for the distribution of organisms. A species may exist at a given location because the conditions are right.

Disjunct Taxa (2)

-Disjunct Taxa-taxa that are related but are located far apart. -Ex: The Pleistocene snail is found only on the algific talus slopes in Indiana and Illinois. Its presence in those locations but no other locations can be explained by historical biogeography. During the time of the ice age, these snails were probably located all over the world. But as the ice melted, they couldn't expand beyond their current location because they required very cold temperatures that only the presence of underground caves could provide.

Dispersal facts (5)

-Dispersal is another factor that influences biogeographical distribution -Species expand their ranges by dispersal -Some species can expand their ranges very rapidly -If a major barrier to dispersal breaks down, many species may expand their ranges more or less together. This results in correlated patterns of dispersal -An area is suspected as being colonized by dispersal if it has a highly unbalanced biota

Extinction (4)

-Extinction is a historical factor that can reduce the distribution of species -If there were previously many populations of the species in different locations and now only a few populations exist, it is likely that extinction is culpable for the reduction in the distribution of species. -Extinction causes disjunct taxa -Higher taxons can be influenced by extinction of constituent species

Summary of Multiregional vs. Replacement Hypotheses (4)

-Multiregional: place of modern human origin was Africa, Europe and Asia -Multiregional: one major migration of Homo erectus, approximately two million years ago -Replacement: place of modern human origin was Africa -Replacement: There were two major waves of migration from Africa Homo erectus migrated out approximately 2 mya Homo sapient humans migrated out of Africa and replaced populations in Europe and Asia 500,000 to 200,000 years ago

Modern Human Origins (3)

-Orrorin, Ardiptihecus and Homo erectus are all found in Africa. It is widely agreed that human life began in Africa and both the multiregional and the replacement models begin with ancient human life in Africa. -How these ancient human populations are related to one another is highly controversial -The two models differ on their premise of the location of origin of modern humans, not ancient humans

Phylogeography (4)

-Phylogeography-the description and analysis of the processes that govern the geographic distribution of lineages of genes, especially within species and among closely related species -These processes include the dispersal of organisms that carry the genes. So, phylogeography provides insight into past movements of the species and history by which they attained their present distributions -Initially, phylogeography used phylogenetic analysis of polymorphic genes within species (gene trees) to infer population history but the genealogy of any on genetic locus may not accurately reflect the history of populations Reasons for inaccuracy: incomplete lineage sorting, changes in population size, natural selection -Currently, phylogeography integrates information from multiple loci and compares it with the results of models based in population genetics

Why don't species enlarge their ranges indefinitely, by incrementally adapting to conditions farther and farther away? (2)

-Proposed Explanation #1: Populations may simply lack genetic variation in one or more characteristics necessary for adaptation Ex: populations of two rain forest dwelling Drosophila have no detectable genetic variation in one or more characteristics necessary for desiccation tolerance and this prevents them from expanding into drier habitats -Proposed Explanation #2: Incursion of genes from populations in favorable environments could prevent recipient populations from adapting to unfavorable environment at the range margin because the process of gene flow would counteract the natural selection for local adaptation

How do biologists determine if a given distribution is due to dispersal or vicariance? (4)

-Sometimes it is hard to determine if the distribution of a species is due to vicariance or dispersal. In these cases, phylogenetic relationships are used to reconstruct biogeographical history. Species phylogeny reflects biogeographical distribution -If the event is due to vicariance, then a tree reflecting the separation of the landmasses will resemble a tree of the phylogeny of a species on the land. The same concept is true of divergence. -With that being said, evolutionary biologists can construct two different trees: one that reflects vicariance and an event involving physical separation and another that reflects colonization by the dispersal of species. A species phylogeny is then analyzed and compared to the two hypothetical phylogenies. The known species phylogenetic tree will match either the tree hypothesized by dispersal or that hypothesized by vicariance and will lend information about the biogeographical history of the area. -This principle is true of all the species in a given area. The phylogeny of all species in an area should reflect the biogeographical area

Implications of Replacement Hypothesis (4)

-The archaic sapiens went extinct as a result of competition or climate change -Most genes in today's populations should be descended from those carried by the modern sapien populations that spread from Africa -The modern sapiens that evolved from archaic sapiens in Africa were reproductively isolated from archaic species like Neanderthals. They were separate species. -Closely related species do interbreed to some extent so it is possible that humans acquired some Neanderthal genes, even if the two forms didn't interbreed extensively. About 2-4% of the human genome is descended from Neanderthals

Mechanisms for Dispersal (3)

-There are two mechanisms for dispersal: range expansion; jump dispersal -Range Expansion refers to a gradual increase in specie's range over time. -Ex: juniper plants -Jump Dispersal- Occurs when animals disperse over long distances and inhospitable habitats -Ex: The Krakatoa exploded due to large volcanic eruptions and only a small island remained from the ash and pumice that landed everywhere + the remnants from an island called Rakata. After about 9 months, scientists travelled to Rakata and found a baby spider. The spider had undergone jump dispersal and travelled from the nearest mainland across the sea. It did this by ballooning. In spider ballooning, they tilt their abdomens and release a thread of silk. This silk catches the breeze and they can travel long distances, they have even been recorded ballooning as high in the altitude as airplanes.

Vicariance (5)

-Vicariance is another factor that influences biogeographical distribution -Vicariance-the separation of populations of a widespread species by barriers arising from geology, climate and habitat -The separated populations diverge and often become different subspecies, species or higher taxa -Vicariance sometimes accounts for the presence of related taxa in disjunct areas -Ex: The isthmus of Panama developed during Pliocene. It formed as two plates collided and were pushed upwards to create a land bridge. It facilitated what is known as the Great American Exchange because it allowed for the exchange of North and South American flora and fauna. This was great for animals living on the land because it enabled them to migrate. However, its formation was detrimental to marine organisms because it provided a physical barrier to their populations. Ex: the snapping shrimp is found to have sister taxa present on either side of the Isthmus of Panama. The presence of closely related shrimp on either side of the isthmus can be explained by vicariance. The shrimp species would have been distributed evenly throughout the ocean but when the isthmus formed, it split the population in two.

Signatures of Vicariance (3)

-Vicariance leaves two major signatures: patterns of geological breakup mirrors phylogeny (1); there are congruent tree topologies amongst many taxa (2) -1: Species phylogeny reflects biogeographical distribution -2: The vicariant event will affect the distribution of various populations in a similar manner

Wallace's Line (3)

-Wallace's Line-a faunal break in the taxonomic composition of animal species among the islands that lie between southern Asia and Australia -This is different from other areas where the flora and fauna located in the middle region between two realms are a mix of the characteristic flora and fauna of each realm. This makes sense because animals will migrate north or south so there will be extremes on both ends and a gray-area in the middle. -The unique taxa difference found on either side of Wallace's line can be explained by historical biogeography. Australia didn't reach its current position until the Cenozoic era. Previously, the Australian and Oriental realms were far apart and it wasn't until Australia moved into its current position that the two realms were located near one another. With that being said, when Australia moved into its current position, the flora and fauna on that landmass were already established.