The Geography of Evolution

THE GEOGRAPHY OF EVOLUTION

Chapter 6

Summary of Chapter 6: EVOLUTION, D. Futuyma, 2nd Ed., 2009.

Biogeography is the study of the distribution of organism over the earth.

·  Zoogeography is concerned with the distribution of animals.

·  Phytogeography is concerned with the distribution of plants.

·  Both are subdivisions of biogeography.

The evolutionary study of organisms’ distribution is related to geology, paleontology, systematics and ecology.

Geological studies of the history of the distributions of land masses and climates shed light on the causes of organisms’ distributions.

·  Historical biogeography studies past geological events in order to understand the present distribution of organisms, e.g. connection between South America and Africa explains the distribution of some taxa in those two continents.

·  Ecological biogeography uses present ecological factors like climate and soil to explain the distribution of taxa.

BIOGEOGRAPHIC EVIDENCE OF EVOLUTION

Darwin drew on the distribution of organisms to support the idea of descent with modification.

Darwin emphasized the following three principles about the distribution of organisms:

1. Climate and physical conditions alone cannot explain the differences or similarities that exist in organisms living in similar habitats, e.g. cactus family (Cactaceae) is restricted to the New World, but cactus-like plants belonging to different families (Euphorbiaceae, Apocynaceae) are found in arid regions of the Old World.

1. Barriers and obstacles to dispersal are related to the differences that exist between organisms on both side of the barrier; there is a correlation between the degree of difference and ability of organisms to disperse. Example: fishes on the east and west coast of South America are very different; S America is a great barrier and fish cannot disperse from one side to another, therefore, the difference is great.

1. Inhabitants of the same continent or same sea have greater affinity, and vary greatly from those on other continents and seas; the species themselves vary from place to place. Example: aquatic rodents of S America are related to mountain rodents of S America and not to aquatic rodents of N America.

Darwin believed that species had a single place of origin and dispersed from there. He drew evidence from his study of oceanic islands like Hawaii:

1. Oceanic islands have organisms that are adapted to long distance dispersal and lack those that are poorly adapted to dispersal, e.g. bats are the only native mammals in Hawaii and lack all other mammals.
2. Many continental species have been transported by humans to oceanic islands and have flourished there showing that some of the best adapted species to oceanic islands originated on continents.
3. Species on islands are related to species of the closest mainland, e.g. the finches.
4. The proportion of endemic species on an island is particularly high when the opportunity for dispersal to the island is low, e.g. the finches again.
5. Island species often bear marks of their continental ancestry, e.g. hooks on seed are an adaptation for dispersal by mammals, but in Hawaii where there are no mammals to aid in the dispersal of seeds, the seeds have hooks.

Interesting sites:

http://www.nyu.edu/projects/fitch/courses/evolution/html/geographic_distribution.html

http://www.ucmp.berkeley.edu/fosrec/Filson.html

MAJOR PATTERNS OF DISTRIBUTION

The geographic distribution of almost every species and higher taxa is limited to some extent (endemic).

Many higher taxa are equally restricted.

Wallace and early biogeographers recognized that many higher taxa have roughly similar distributions, and that the taxonomic composition of the biota is more uniform within certain regions than between them.

Wallace recognized several biogeographic realms for terrestrial and freshwater organisms that are still accepted today:

·  Palearctic: temperate Eurasia and northern Africa.

·  Nearctic: North America; Aleutian Islands.

·  Neotropical: Central and South America; Caribbean Islands; Galapagos Islands.

·  Ethiopian, Sub-Saharan Africa; Madagascar; Mascarene Islands.

·  Oriental: India and Southeast Asia.

·  Australian: Australia, New Guinea, New Zealand and nearby islands.

Each biogeographic realm is inhabited by many higher taxa that are much more diverse in that realm than elsewhere, or are even restricted to that realm.

These realms are the result of the Earth’s history (plate tectonics) rather than present day climatic conditions and land distribution over the Earth.

The World Wildlife Fund added to these six realms two more: Oceania and Antarctica, and called them ecozones.

“An Ecozone is the largest scale biogeographic division of the earth's surface based on the historic and evolutionary distribution patterns of plants and animals. Ecozones represent large areas of the earth's surface where plants and animals developed in relative isolation over long periods of time, and are separated from one another by geologic features, such as oceans, broad deserts, or high mountain ranges, that formed barriers to plant and animal migration. Ecozones correspond to the floristic kingdoms of botany or zoogeographic regions of mammal zoology.” http://en.wikipedia.org/wiki/Ecozone

Some species have restricted distribution within the realm while others are more widely distributed.

The borders between the realms are not sharply defined because some taxa have dispersed into the neighboring realms.

Some taxa have disjunct distribution; their distribution has gaps

Interesting website: http://evolution.berkeley.edu/evolibrary/article/_0/history_16

HISTORICAL FACTORS AFFECTING GEOGRAPHIC DISTRIBUTIONS

Contemporary and historical factors affect the geographic distribution of taxa.

Geological barriers and ecological conditions set limits to the distribution of a taxon.

Historical processes that have led to the current distribution of taxa are extinction, dispersal and vicariance.

1. Extinction of some populations can cause disjunct distribution.

·  Horses originated in North America dispersed to Asia, and then became extinct in North America. Only the Asian horses and asses, and zebras in Africa have survived.

2. Dispersal is the one-way movement of organisms from the site of origin to new habitats thus expanding the range of the species.

·  Range expansion: continuous movement across expanses of favorable habitats, e.g. opossums and armadillos; starlings and European sparrows.

·  Jump dispersal: movement across barriers, e.g. cattle egret.

If major barriers to dispersal break down, many species may disperse together.

3. Vicariance is the separation of a taxon by a geographic barrier, e.g. geological, climate, and habitat.

Vicariance may account for the presence of related taxa in disjunct areas.

Separate populations often become different species, subspecies or higher taxa.

The distribution of a taxon may have several explanations and may not be attributed to only one factor.

Vicariance, extinction and dispersal may all play a role in explaining the present distribution of a taxon.

Example of vicariance is the many species of fish, shrimp and other animal groups on the Pacific and Atlantic sides of the Isthmus of Panama.

Often the three phenomena are involved in the distribution of the species.

TESTING HYPOTHESES IN HISTORICAL BIOGEOGRAPHY

Some guidelines used to explain the distribution of a taxon are:

·  The present distribution of a taxon cannot be explained by an event that occurred before the origin of the taxon.

·  A taxon originated in the region where it is more diverse. There are exceptions to this, e.g. the Equidae, horse family.

Vicariance and dispersal are the major hypotheses accounting for the distribution of a taxon.

·  Phylogenetic analysis of morphological and molecular data plays an important role in evaluating these hypotheses. They are the foundation of most modern studies of historical biogeography.

·  An area is often suspected of having been colonized by dispersal if it has an unbalanced biota in which some major group is absent, e.g. absence of mammals and amphibians from oceanic islands.

·  Paleontological record may show that a taxon proliferated in one area before appearing in another, e.g. fossil armadillos appear in North America after the Pliocene when the Isthmus of Panama was formed.

All these methods use parsimony in their analysis.

Dispersal-Vicariance Analysis (DIVA).

·  Developed by Ronquist (1997).

·  Assumes that Vicariance is the “null-hypothesis”; it is considered true until proven wrong.

·  Assigns costs to processes such as extinction, dispersal or sympatric speciation in order to arrive at hypotheses that represent histories of areas and process explanations for the distribution of taxa in these areas.

·  The species distribution with the lowest cost is considered the most parsimonious or best hypothesis.

·  This method is well supported by the principle that states that new species are generally formed during geographic isolation.

·  This method most fully accounts for the importance of dispersal and is the most biologically realistic.

·  Speciation is assumed to subdivide the ranges of widespread species into vicariant components.

·  The optimal ancestral distributions are those that minimize the number of implied dispersal and extinction events.

·  Here is an example of the application of DIVA: http://www.ib.usp.br/~silvionihei/pdf/Sanmartin%20et%20al%202001%20-%20Holarctic.pdf

Vicariance hypothesis

·  Monophyletic groups occupy different areas.

·  Continuous distribution is broken by some happening, e.g. breakup of Gondwanaland.

·  Extinction also causes the split into two populations that eventually diverge into new taxa.

·  Population of many taxa should be expected.

·  Some biogeographers hold that vicariance should separate many taxa simultaneously, so the taxa should demonstrate a common pattern of distribution.

An example: Postglacial recolonization of western hemlock (Tsuga heterophylla [Raf.] Sarg.). Abstract.

“After the last glacial period, western hemlock recolonized its current range from refugia. Disjunct populations of some otherwise coastal mesic forest plants, such as western hemlock, are found inland in the northern Rocky Mountains. Two contrasting hypotheses have been proposed to explain these disjunct distributions of mesic forest species: vicariance and inland dispersal. Under the vicariance hypothesis, western hemlock populations remained in inland regions during recent Pleistocene glaciations. Thus, coastal and inland populations would have been separated for many thousands of years. Under the inland dispersal hypothesis the inland populations would have gone extinct during the Pleistocene and western hemlock would have survived the most recent glacial period only along the Pacific Coast. Then, after the glaciers receded western hemlock would have returned to the inland regions via dispersal. To differentiate between these two hypotheses for western hemlock, we are examining patterns of nucleotide variation within the chloroplast genome. Four non-coding regions of chloroplast DNA have been compared among populations from throughout the range of western hemlock but little variation has been found. Areas of increased genetic variation should indicate locations of refugia. Western hemlock populations in Queen Charlotte Island, southern Cascades of Oregon, and inland areas of Montana show modestly-increased amounts of sequence diversity, suggesting that the vicariance hypothesis may best explain western hemlock recolonization. However, coalescence calculations, based on our sequence data and estimating the time to the MRCA (most recent common ancestor), do not allow us to reject the inland dispersal hypothesis.”

Peery, Rhiannon,Raubeson, Linda A.

Central Washington State University, Department of Biological Sciences, Ellensburg, Washington, 98926-7537, USA

http://www.2005.botanyconference.org/engine/search/index.php?func=detail&aid=237

EXAMPLES OF HISTORICAL BIOGEOGRAPHIC ANALYSIS

ORGANISMS IN THE HAWAIIAN ISLANDS

·  Kauai at the northwestern end of the archipelago is the oldest of the major islands (5.1 million years old).

·  Hawaii, the Big Island, is the youngest at about 500,000 years of age.

·  The most basal lineages according to a molecular phylogeny occupy Kauai, and the youngest lineages Hawaii.

·  Species successively dispersed as new islands were formed:

Ø  Kauai → Oahu → Molokai/Maui/Lanai → Hawaii.

ANIMALS OF MADAGASCAR

·  Madagascar has a highly endemic biota.

·  Madagascar and India were the firs land masses to split from Gondwanaland 160-120 mya.

·  India and Madagascar separated about 88-63 mya.

·  India collide with south Asia about 50 mya.

·  Recent molecular phylogeny supports that dispersal was a major factor in the formation of the Malagasy biota, and not vicariance.

·  Most lineages of plants and animals are too young to have been originated by vicariance, the separation from Gondwana.

Ø  Chameleons originated in Madagascar and dispersed to Africa, India and the Indian Ocean islands.

Ø  Lemurs evolved in Madagascar and mongoose in India from ancestors that dispersed from Africa after the split of Gondwanaland.

See fig. 6.11 and 6.12, page. 143.

GONDWANAN DISTRIBUTION

·  Cichlids are freshwater fishes found in South America, Africa, Madagascar and India.

·  Indian-Madagascar split was over 120 mya.

·  DNA sequence difference supports the speciation of cichlids to have happened after the fragmentation of Gondwanaland, younger that 56 mya

·  They are part of a clade know from the late Cretaceous, also after the fragmentation of Gondwanaland.

·  Therefore, their distribution is probably by dispersal than vicariance.

·  Ratites are flightless birds found in New Guinea, Australia, South America, New Zealand and Africa.

·  A phylogenetic study using the complete sequences of the mitochondrial genome was made.

·  Except for the kiwi and ostrich, the branching date and sequence is in agreement with the fragmentation of Gondwanaland.

·  The kiwi and ostrich must have used some method of dispersal.

Most orders of birds are old enough to have been affected by the breakup of Gondwanaland according to molecular studies.

·  The basal lineages of the Galliformes (chickens, etc), Anseriformes (ducks, etc.) and Passeriformes (perching birds) are distributed among pieces of Gondwana.

·  See the example of the southern beeches, Nothofagus, in South America, Australia, New Zealand and New Caledonia.

o  The disjunction between the Australian and South American species may be the result of vicariance according to molecular data, but the presence of Nothofagus in New Zealand and New Caledonia appears to be the result of dispersal.

·  See fig. 6.13 A and B, page 144.

THE COMPOSITION OF REGIONAL BIOTAS

The composition of the regional biota is the result of a mixture of ancient and recent events.

·  Allochthonous taxa originated elsewhere, e.g. mountain lion and cattle egret is South America.

·  Autochthonous taxa evolved in the region, e.g. South American rheas and lungfishes.

PHYLOGEOGRAPHY

Phylogeography studies the processes and principles that control the geographic distribution of lineages of genes especially within species and closely related species.

See Avise, J.C. 1998. http://biology.fullerton.edu/baja/docs/avise_98.pdf

“Phylogeography is the study and understanding of the relationships found among living things and their location on Earth. It is also used to help investigate geological events and their resulting effect on and distribution of living things.” Kitt Volmer, http://tolweb.org/treehouses/?treehouse_id=4383