Geography of Evolution.pdf

Darwin was the first to draw attention to the distribution oforganisms and their implications for understandingevolution.• Neither the similarity or dissimilarity of the inhabitantsof various regions can be wholly accounted for by climatalor other physical conditions.• Barriers of any kind, or obstacles to free migration, arerelated in a close and important manner to the differencesbetween the productions of various regions.• Inhabitants of the same continent or the same sea arerelated, although the species themselves differ from placeto place.

Darwin argued that the evidence against special creation wasapparent in biogeography.• Organisms found on oceanic islands are the kinds oforganisms that have the capacity for long-distance dispersal.• Organisms without dispersal abilities often do well on islandswhen they are transported there by humans.• Most of the species on islands are clearly related to specieson the nearest mainland.• The proportion of endemic species on an island is particularlyhigh when the opportunity for dispersal to the island is low.• Island species often bear marks of their continental ancestry.

Biogeographic PatternsThe distribution of higher taxa allow the designation of several“biogeographic realms. ” The biogeographic realms are defined bygroups of organisms that are unique to or highly diverse withinthose regions.

Eachbiogeographicrealm has somespecies orhigher taxa thatare endemic -found nowhereelse.The pattern of endemism is repeatedin many groups within abiogeographic realmArmadillos, anteaters, antshrikes,Armadillos, anteaters, antshrikes,and armored catfishes are SouthAmerican endemics.

Biogeographic g realms are largely the product of geologic g history- the movement of continental landmasses.A single interconnected landmass - “Pangaea” was present inthe Triassic. The climate was warm and many organismswere widely distributed.

During the Jurassic the northern continents were largely separatedDuring the Jurassic the northern continents were largely separated,but a single large southern continent “Gondwana” remained.

At the end of the Mesozoic, the southern continents hadseparated. Shallow oceans covered the interior North Americaand Eurasia. Some landmasses were in very different positionsthan today. India was a large island off the east coast of Africaand Australia was very close to Antarctica.

By the middle of the Tertiary the continents were positioned as weknow them today. India collided with Asia and the Himalayaswere built. Australia was isolated in the southern Indo-Pacific.North and South America were still separated by a seaway throughwhat is now Central America.

Within a biogeographic realm, the flora or fauna may be dividedinto provinces based on restricted distributions of species or groupsof species.

The borders between adjoining realms and provinces are notabsolute because some are relatively recent and thus some taxa areshared between them due to having a once widespread ancestor.For example, some closely related species of animals and plantsare shared between Europe and North America. Each is likely tohave a common ancestor that existed on both continents beforetheir separation.North AmericandaceEuropeanSycamorePlanetree

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Some higher taxa have disjunct distributions. They have uniquespecies found in nonadjoining regions. Examples include thelungfishes, with a single species in South America, a singlespecies in Australia, and three species in Africa.The few remainingilungfish species areevolutionary“relicts” of a timewhen lungfish werebroadly distributed.

The distribution of organisms or higher taxa today can beinfluenced by ecological requirements of the species, ,geologicalgbarriers that haven’t been crossed (e.g. mountains, deserts), or byhistorical factors - Extinction, Dispersal, and Vicariance.Extinction removes a taxon from a region where it was formerlypresent. (e.g. horses and relatives in North America, elephantrelatives throughout North America, and Northern Europe).Dispersal by individuals of a species expand the range of a taxon.Range expansion is the movement of a species across a favorablehabitat. Jump dispersal is movement across a barrier.In both starlings andcattle egrets an initialjump was followed by acontinuing rangeexpansion.

Historical BiogeographyStrict vicariance produces completelyconsistent patterns of phylogeneticdivergence and geographic separation.

Dispersal can be inferred when geologicevents precede cladogenic events.Dispersal can be concluded only when thepattern can’t be explained by vicariance.

Extinction complicates interpretations ofvicariance and dispersal.

The Hawaiian Islands have formed from a series ofvolcanic eruptions on the submerged Pacific plate as itmoved over a “hot spot.” Islands are progressivelyyounger to south and east. Islands were colonized byspecies from nearby islands as they formed.A phylogenetic analysis of cricketssuggests that the oldest species arefound on the oldest islands.

Several groups oforganisms are distributedfrom Africa toMadagascar, and toIndia. The distributionib tiof those organisms todaymay be explained bystrict vicariance - due towidespread distributionamong the southerncontinents beforeseparation.

Some groups have distributions consistent with an ancestralSome groups have distributions consistent with an ancestralGondwanan distribution with diversification after continentalseparation – a vicariant pattern.

The prosimians in Madgascar and Africa appear to havediverged long after the breakup of Gondwana. A pattern thatcan only be accounted for by dispersal.

The pattern of distribution amongspecies of chameleons is notconsistent with the breakup of thesouthern continents. It is bestexplained by origination inMadagascar (M) and dispersal toAfrica (A), the Seychelles Islands(SE) and to India (I).

Phylogeography is the description andanalysis of the processes that govern thedistribution of genetic lineages. Geneticanalysis is used to construct a geneticphylogeny and then the phylogeny is mappedonto geography.Classically there have been two hypothesesfor the evolution of humans, the multiregionalhypothesis - modern Homo sapiens evolvedsimultaneously throughout the old world fromarchaic Homo sapiens with exchange ofgenetic information by gene flow - and theout-of-Africa hypothesis - modern humansevolved in Africa and moved out replacingpreviously widely dispersed archaic humans.

Mitochondrial DNA analysis of modernhumans suggests that Asian, European,Australian and Indonesian populationsall share a common ancestor thatdispersed sed from Africa about 50,000years ago. Multiple dispersals out ofcentral Asia appear to account forEuropean populations.

Ecological BiogeographyThe current distribution of a species over broad geographic areasmay be the product of past dispersal or vicariance, but the localdistribution ib ti of a species may be better explained by its tolerance ofphysical variables (soil pH, salinity), its requirements forsustaining a population (oxygen concentration, soil nutrients), itscompetitors for resources, or factors that reduce its ability to livein an area (predators, parasites, disease).Ecologists ask what factors limit the distribution of a species: Whydoes a species live in one area but not another?

Ecologists and evolutionary biologists are both interested inexplanations for broad patterns of species diversity.iCommonly, within any group of organisms there are morespecies in the tropics than at higher latitudes.

There are several possible explanations for this pattern:• Time hypothesis: tropical regions have remained largelyundisturbed since the K/T boundary while temperate zones haveundergone repeated glaciations i recently. The tropics have hadmore time for the evolution of more species.• Niche partitioning hypothesis: species in the tropics have more• Niche partitioning hypothesis: species in the tropics have morespecialized life-styles. Thus, more species can exist with a givenrange of resources.

• Resource hypothesis: the greater productivity in the tropicsallows a greater range of resources for species to exploit. Thusallowing more species to coexist.• Predation hypothesis: greater rates of predation, parasitism,and disease in the tropics results in less competition forresources and thus more overlap in resource use.Each hypothesis hasEach hypothesis hassome evidence tosupport it.

How many species can an area support? On islands, there is ageneral relationship between species diversity i and total area.This suggests there many be a maximum number of speciesthat an area can support - an equilibrium number.

MacArthur and Wilson have proposed that the species diversityof an area is determined by a balance between rates of speciesimmigration from other areas and rates of extinction.Small islands are lesslikely to receive randomimmigrants from amainland sourcepopulation. Populationson small islands are alsomore likely to go extinctdue to chance events.Thus large islands close to the mainland should have the mostspecies and small islands far from the mainland should have thefewest species. This idea has been broadly supported and isknown as the Island Biogeographic Theory.