THE GEOGRAPHY OF EVOLUTION Chapter 6

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.

2. 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.

3. 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.

Darwin believed that species had a single place of origin. 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 others.

2. Many continental species have been transported 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

Related sites:

http://www.uwsp.edu/gEo/faculty/ritter/geog101/textbook/biogeography/biogeography_fundamentals.htm

l

http://cmsdata.iucn.org/downloads/udvardy.pdf

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 organism 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:

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.

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

islands.

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

Phylogeography relies strongly on phylogenetic analysis on variant genes within species.

It constructs genealogies of genes.

Phylogeography provides insights into the past movements of species. It depends on population genetics.

Example:

Phylogeographic analysis shows that the African elephant is in reality two species, the forest

elephant named Loxodonta cyclotis, and savannah elephant, Loxodonta africana.

The European grasshopper Chorthippus parallelus shows that the haplotypes from central and

northern Europe are more related to those in the Balkans. Those haplotypes found in Spain and

Portugal never crossed the Pyrenees, as never did those in Italy, which never crosses the Alps

after the retreat of the glaciers.

The Out-of-Africa hypothesis of human migration is supported by phylogeographic studies.

Most know the two hypotheses: Replacement hypothesis and Multiregional hypothesis.

See pages 147-150.

http://www.geocities.com/palaeoanthropology/OutofAfrica.html

http://www.actionbioscience.org/evolution/johanson.html

Interesting websites: http://biology.st-and.ac.uk/supplemental/ritchie/papers/PIS.pdf

GEOGRAPHIC RANGE LIMITS: ECOLOGY AND EVOLUTION

A species can persist where the growth rate is greater or equal to zero.

The organisms should be able to tolerate a range of several environmental conditions.

Law of Tolerance

Fundamental ecological niche is the set of environmental conditions in which a species can maintain a

stable population size.

Realized ecological niche is the actual niche occupied by the species due to the influence of competitors,

predators, etc.

Related species often have similar ecological requirements presumably due to their common ancestor.

This is referred to as phylogenetic niche conservatism. Species tend to retain their ancestral niche

characteristics.

e.g. many lineages of herbivorous insects have remained associated with the same genus or

family of food plants.

Niche conservatism contributes to our understanding of the geographic distribution of many clades.

“Abstract

Ecologists are increasingly adopting an evolutionary perspective, and in recent years, the idea that closely related species are ecologically

similar has become widespread. In this regard, phylogenetic signal must be distinguished from phylogenetic niche conservatism. Phylogenetic niche conservatism results when closely related species are more ecologically similar that would be expected based on their phylogenetic

relationships; its occurrence suggests that some process is constraining divergence among closely related species. In contrast, phylogenetic

signal refers to the situation in which ecological similarity between species is related to phylogenetic relatedness; this is the expected outcome of Brownian motion divergence and thus is necessary, but not sufficient, evidence for the existence of phylogenetic niche conservatism. Although

many workers consider phylogenetic niche conservatism to be common, a review of case studies indicates that ecological and phylogenetic

similarities often are not related. Consequently, ecologists should not assume that phylogenetic niche conservatism exists, but rather should empirically examine the extent to which it occurs.”

Phylogenetic niche conservatism, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity

among species. Jonathan B. Losos. Ecology Letters, Volume 11, Issue 10, pages 995–1003, October 2008

Range Limits: an evolutionary problem

Whether a species border is caused by climatic conditions, another species influence, etc. is puzzling.

Two hypotheses have been proposed:

1. Populations may lack the genetic characteristics necessary for adaptation.

2. Incursion of genes from adjacent populations in favorable environment could prevent recipient

populations from adapting to unfavorable environment at the range margin, because this process

counteracts natural selection for local adaptation.

Interesting readings:

http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2008.01229.x/full

http://www.jstor.org/stable/10.2307/30033815

http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2010.01515.x/full

EVOLUTION OF GEOGRAPHIC PATTERNS OF DIVERSITY

The field of community ecology is concerned with explaining the species diversity, species composition,

and trophic structure of assemblages of coexisting species.

The chief factor presumed to produce consistent community structure is interactions –especially

competition- among species.

Competition tends to prevent the overlap of niches – Gause’s Law of Competitive Exclusion.

- Two species with identical ecological requirements cannot occupy the same environment.

Two species cannot occupy the same ecological niche.

Complete competitors cannot coexist.

It results in the partitioning of resources: habitat partitioning.

This presumes that species have reached equilibrium.

Interesting reading: http://www.jstor.org/stable/10.2307/2265532

Community convergence

The diversity of species in a local region may or may not be at equilibrium.

1. Is the convergent evolution of taxa part of a pattern of community convergence?

2. If two regions present similar habitat, will species evolve to utilize and partition them in the same

way?

Study the example of the lizard genus Anolis in the Antilles, pages 153-154.

Study this abstract of an article on community convergence.

Intercontinental community convergence of ecology and morphology in desert lizards.

Melville J, Harmon LJ, Losos JB. Department of Natural Sciences, Washington University, St Louis, MO 63130, USA.

jmelv@museum.vic.gov.au Proc Biol Sci. 2006 Mar 7; 273(1586):557-63.

“Evolutionary ecologists have long debated the extent to which communities in similar environments but different geographic

regions exhibit convergence. On the one hand, if species' adaptations and community structure are determined by environmental

features, convergence would be expected. However, if historical contingencies have long-lasting effects convergence would be

unlikely. Most studies to date have emphasized the differences between communities in similar environments and little

quantitative evidence for convergence exists. The application of comparative phylogenetic methods to ecological studies provides

an opportunity to further investigate hypotheses of convergence. We compared the evolutionary patterns of structural ecology

and morphology of 42 species of iguanian lizards from deserts of Australia and North America. Using a comparative approach,

we found that evolutionary convergence of ecology and morphology occurs both in overall, community-wide patterns and in

terms of pairs of highly similar intercontinental pairs of species. This result indicates that in these desert lizards, deterministic

adaptive evolution shapes community patterns and overrides the historical contingencies unique to particular lineages.”

Interspecific interactions, especially competition, may limit species diversity and may result in different

communities with similar structure.

Partition of resources by similar species is a common phenomenon in communities. This suggests that

competition plays an important role in creating the community structure that exists in given location.

In some cases, sets of species have independently evolved to partition resources in similar ways.

Ecomorphs are species characteristic of a specific microhabitat and have similar morphological

characteristics.

“Species with the same structural habitat/niche, similar in morphology and behavior, but not necessarily close

phyletically.” Ernest Williams, The Origin of Faunas. A trial analysis.

http://www.anoleannals.org/wp-content/uploads/2011/08/williams-1972.pdf

Ecomorphs have been used as examples of convergent evolution.

Convergence of community structure is usually incomplete, suggesting that evolutionary history has had

an important impact on the ecological grouping of species.

Interesting readings:

http://sysbio.oxfordjournals.org/content/41/4/403.short

http://www.sciencedirect.com/science/article/pii/S0960982207015059

http://www.mapoflife.org/topics/topic_337_Anolis-lizard-ecomorphs/

EFFECTS OF HISTORY ON CONTEMPORARY DIVERSITY PATTERNS

Both current ecological conditions and long-term evolutionary events play a role in shaping the species

structure of the community.

There is great variation among geographic regions and among environments in the number of species of

plants and animals.

There is latitudinal diversity gradient of a decline in the number of species with an increase in latitude,

e.g. the tropics are more diverse than the polar regions.

The diversification rate hypothesis proposes that the rate of increase diversity has been greater in the

tropics for a long time because of higher origination rate, lower extinction rate, or both.

The time and area hypothesis holds that most lineages have originated in tropical environments

throughout the Cenozoic era and even before, simply because for about the first 40 million years of the

Cenozoic, the Earth was warmer than it is today.

This hypothesis is based on niche conservatism.

Tropical environments have occupied larger areas and have had longer time to accumulate

species than other environments.

Geographic patterns in the number and diversity of species may stem partly from current ecological

factors, but they probably cannot be understood without recourse to long-term evolutionary history.

Chapter 7 THE EVOLUTION OF BIODIVERSITY

On a scale of millions of years, extinction, adaptation, speciation, climate change, and geological change

create different assemblages of species.

Biodiversity can be studied from the points of view of ecology and evolution.

Ecologists focus primarily on factors that operate over short time scales to influence diversity

within local habitats or regions.

The evolution point of view focus more on the long term changes of climate, geology, extinction,

speciation, etc. that create different assemblages of species.

ESTIMATING CHANGES IN TAXONOMIC DIVERSITY

ESTIMATES OF DIVERSITY

The simplest expression of diversity is the simple count of species, species richness.

Geological periods and stages vary in duration, and more recent geological times are represented by

greater volumes and areas of fossiliferous rock.

Rare species are more likely to be included in larger than smaller samples; this is called

rarefaction.

The estimated number of species increases with the size of the sample.

The duration of a taxon is measured by determining its first and last appearance in the fossil record,

mostly 5 to 6 million years into which each geological period is divided.

The duration of a taxon is imprecise; the actual times of originating and extinction of a taxon may have

occurred earlier and later of the apparent times of origin and extinction.

Our count of living, recent, species is much more complete than our count of past species.

Extant taxa have apparently longer durations and lower extinction rates than they would if they had been

recorded only as fossils.

Diversity seems to increase as we approach the present: Pull of the Recent.

The most useful unit to estimate diversity is the number of originations per million years, mya or Myr.

TAXONOMIC DIVERSITY THROUGH THE PHANEROZOIC EON.

The Phanerozoic Eon is the current eon. It began with the Cambrian period at the beginning of the

Paleozoic era and extends to the present.

The most complete fossil record has been left by marine animals with hard parts: shells or skeletons.

Jack Sepkoski (1984, 1993) compiled data on the stratigraphic ranges of more than 4000 marine families

and 20,000 genera throughout the 542 Myr of the Phanerozoic. His plot shows…

Rapid increase in the Cambrian and Ordovician

A plateau throughout the rest of the Paleozoic

A steady increase throughout the Mesozoic and Cenozoic.

Diversity reaching its peak in the Tertiary.

This pattern is interrupted by mass extinctions.

On land, diversity has also increased.

Number of families of insects shows a steady increase since the Permian.

Flowering plants, birds and mammals have also steadily increased after the mid-Cretaceous.

Other studies differ from Sepkoski in that

There is a decline in diversity in the Devonian instead of a plateau in the Paleozoic,

A sharp increase in the Permian previous to the mass extinction, and

A much steep post Paleozoic increase in biodiversity, in the Mesozoic and Cenozoic.

The debate goes on …

RATES OF ORIGINATION AND EXTINCTION

There were episodes when an exceptionally high number of taxa became extinct are called mass

extinctions.

Times when the number of extinctions occurs at more gradual rate are referred to as background

extinctions.

Biodiversity in marine species has increased during the Mesozoic and Cenozoic due to an increase in

originations since the Triassic, but on the average it has declined.

The rate of origination of new families was highest in animal evolution in the Cambrian and Ordovician,

and in the early Triassic after the great Permian extinction.

Five mass extinctions are generally recognized:

1. Ordovician-Silurian: ~450-440 million years ago. 50% of animal families, including many trilobites.

2. Devonian: ~370-360 million years ago. 30% of animal families, including agnathans and placoderm

fishes and many trilobites.

3. Permian: ~250 million years ago. 50% of the marine families, 90% of the marine species, eight of

twenty-seven orders of insects. Many trees, amphibians, most brachiopods and bryozoans; all

trilobites.

4. Triassic-Jurassic: ~205 million years ago. 35% of the families including many reptiles and

ammonoids.

5. Cretaceous: ~65 million years ago. Dinosaurs, many foraminiferans, ammonites. This is called the

K/T extinction, which occurred in the Cretaceous-Tertiary boundary.

Site: http://en.wikipedia.org/wiki/Extinction_event#Major_extinction_events. Check the references

provided here.

http://www.geo.wvu.edu/~kammer/g231/readings/sepkoski.pdf

http://www.pnas.org/content/81/3/801.full.pdf

Extinction rates have declined over time

Studies suggest that the number of families becoming extinct decreases during the Phanerozoic eon.

Taxa with high rate of origination also have a high rate of extinction.

Extinction is caused by the failure of a species to adapt to changes in its environment.

When the environment deteriorates, populations become extinct, and the range of the species contracts.

Natural selection has no foresight and cannot prepare species for changes in the

environment.

Gilinsky and Bambach (1987) found that the rate of extinction within Orders commonly increased over

time, although the origination rate generally declined even faster.

Gilinsky (1994) showed that the rates of origination and extinction are highly correlated.

Some clades evolve more families and lost old ones over long periods of time than others.

These taxa have shorter life span before they become extinct.

Families with a longer life span and lower extinction rate remain.

This results in a decline in the average extinction rate of clades over the long time as long as

“highly volatile” clades do not evolve new ones.

Extinction and origination rates appear to be correlated.

Possible reasons for this correlation are:

Degree of ecological specialization. Specialized species are likely to be more vulnerable than

generalists; also more likely to originate new species

Population dynamics. Species with low or fluctuating population sizes are susceptible to

extinction. Some authors believe that small populations enhance speciation: this idea is

controversial.

Geographic range. Species with broad geographic range tend to have a lower risk of extinction

because the entire species is not found in one locality where a localized environmental change

would have a great effect.

A study conducted by Foote (2000) showed that biodiversity:

Increased when the rate of origination increased.

Decreased when the rate of extinction increased.

Extinction had a stronger effect on biodiversity than did origination in the Paleozoic.

Origination had a stronger effect during the Mesozoic and Cenozoic.

Do extinction rates change as clades age?

The rates of extinction of taxa in the fossil record can be analyzed by plotting the fraction of the

component taxa that survive for different lengths of time, e.g. some of the genera within a family and

their age at extinction.

Van Valen (1973) conducted a study that suggested that the probability of extinction is rather constant

over a long period of time.

Studies show that the probability of extinction is roughly constant because organisms are

continually assaulted by new environmental changes, each carrying the risk of extinction.

The rates of extinction do not always remain constant with age, and they have changed throughout the

Phanerozoic.

Other studies have suggested that the species within a genus became extinct at a lower rate as the family

aged.

If the number of species within a genus increases over time, genera will have lower extinction rates

because the genus persists until all its component species become extinct.

As other taxa evolved, the environment of an organism deteriorates and it must constantly evolve to

survive.

Background extinction has declined over time.

This may be the result of the increase in the average number of species per family, e.g. a large

family will take longer to become extinct than a smaller one.

Higher taxa that were more prone to extinction were eliminated early in the Phanerozoic.

Causes of extinctions

Habitat deterioration and destruction is the most frequent cause of extinction.

When populations become extinct, the geographic range of the species contracts unless former unsuitable

habitats become available to the species. Note that here we are talking about a “population” becoming

extinct and not the species!

Not all environmental changes cause the population to decline.

Declining populations depend on adaptive genetic changes to survive.

The survival of the population will depend on how rapid is the environmental change relative to the rate at

which characters evolve.

The rate of evolution may depend on the rate at which mutation supplies genetic variation and on

population size, because small populations will experience fewer mutations.

An environmental change that reduces population size reduces the chances of adapting to it.

The change in one environmental factor may bring about changes in other factors and the survival of a

species may require evolutionary change in several or many features.

A change in temperature may cause a change in species composition of a community.

Both biotic and abiotic changes have caused extinctions.

The role of competition in extinction is controversial.

Mass extinctions

Of the five mass extinctions generally recognized, the one at the end of the Permian (~250 m.y.a.) was the

most severe eliminating about 54% of the marine families, 80% of the genera, and 80-90% of the species.

On land, several orders of insects became extinct, and the dominant amphibian and therapsids

were replaced by new groups of therapsids that eventually gave rise to mammals and dinosaurs.

The second most severe mass extinction in terms of taxa affected occurred at the end of the Ordovician

between 440-450 mya.

Over 100 families of marine invertebrates disappeared.

The end of the Cretaceous extinction, K/T (65 m.y.a.), eliminated the dinosaurs except for birds; it

occurred about 65 mya.

http://park.org/Canada/Museum/extinction/extincmenu.html

Causes of mass extinctions

Raup and Sepkoski (1984) have suggested that extinctions occur at rough intervals of 26 million years.

Check the conclusions and implications at the end of this article.

http://www.pnas.org/content/81/3/801.full.pdf

Most paleontologists agree that the cause of the K/T extinction was the impact of an asteroid.

The Chicxulub crater, off the coast of Yucatan is the prime suspect.

Proposed by Alvarez et al.

Some scientists think that there were other causes because the extinction of various taxa was too spread

out in time to be caused by one catastrophe.

Several causes have been proposed for the extinction at the end of the Permian.

Volcanic eruption that produced the Siberian Traps.

The eruptions caused a global warming that depleted the oceans of oxygen by altering oceanic

currents.

Global warming may have caused the release of large amount of methane that further enhanced

warming in a positive feedback spiral.

The concept of periodicity has important implications for determining which factors cause extinction. Hypotheses

invoking catastrophism have particularly been advanced utilizing this concept, which imply extra-terrestrial forces

as extinction-causing agents. This is because only astronomical forces are known to operate on such a precise

periodic time schedule. Contrary to catastrophism are hypotheses, which focus on gradualism. These gradualistic

hypotheses invoke various terrestrial extinction mechanisms including volcanism, glaciation, global climatic

change, and changes in sea level. Most recently hypotheses centered on the new non-linear science of complexity

have emerged. Under these hypotheses species-species interactions lead to occasional instability resulting in

cascades which may ripple through entire ecosystems, with potentially devastating results.

http://park.org/Canada/Museum/extinction/patterns.html

Extinctions exerted a selective pressure by eliminating some taxa and allowing others to survive.

Species of gastropods with wide range and ecological distribution, and genera with many species survived

better.

Extinctions appear to have been random with respect to other characteristics such as mode of feeding.

The same pattern appears to be the same in periods of background extinctions.

These characteristics correlated with survival seem to be different from those that allowed survival at the

end of the Cretaceous.

Physical and environmental conditions were probably very different after mass extinctions than

before.

This appears to be the reason for the diversification of some taxa after the extinction, and the slow

dwindling of other taxa.

According to Stephen J. Gould, there are tiers of evolutionary change that must be understood in order to

comprehend the full history of evolution.

1. Microevolutionary change within the population and species.

2. Differential proliferation and extinction of species during normal geological times, which affect

the relative diversity of lineages.

3. The shaping of the biota by mass extinctions, which can eliminate diverse taxa and reset the stage

for new evolutionary radiations, initiating evolutionary histories that are largely decoupled from

earlier ones.

The extinction of one group permitted the flourishing of others allowing the emergence of new

community structures.

No truly massive mass extinction has occurred in the last 65 million years.

The course of biodiversity has been altered for the foreseeable future by human domination of the Earth.

DIVERSIFICATION

Modeling rates of change in diversity

The rate of change in diversity depends on the rates at which taxa originate and become extinct.

The number of taxa, N, changes over time by origination and extinction.

These events are analogous to the births or deaths of individual organisms in a population.

Models of population growth can be adapted to describe changes in taxonomic diversity.

N = the number of taxa.

S = the number of originations per original taxon in the time interval.

E = the number of extinctions per original taxon in the time interval.

ΔN = the change in N; equals the number of “births” (SN) minus the number of “deaths” (EN).

Δt = the time interval.

SN = ΔN, the change in N due to births (speciation).

EN = ΔN, the change in N due to deaths (extinction).

The diversification rate is the rate of change in diversity. It is calculated by the formula below.

ΔN = SN - EN or ΔN = RN

Δt Δt

Where R is the per capita rate of increase, R = S-E.

S= new taxa originated per taxon; absolute number of origination/total number of taxa present at the start

of the period.

E= taxa that became extinct per taxon; absolute number of extinctions/total number of taxa present at the

start of the period.

R is the difference between the per capita rate of speciation and the per capita rate of extinction.

Per capita means the number of species produced or eliminated per unit of time.

If R > 1 the growth number of taxa is positive.

A population grows in a time interval beginning at t0 and ending at t1. This is calculated by multiplying

the original size of the population, N0, by the per capita rate of increase, R.

N1 = N0R

If S and E remain constant, then the population will be at the end of the next interval…

N2 = N1R = N0R2

In general, after t time intervals, the number of taxa will be Nt = N0Rt as long as the per capita

origination and extinction rates remain the same.

Nt = N0Rt describes exponential growth at discrete time intervals but not continuous growth.

For continuous growth rather than growth in discrete intervals:

Population size at various times during exponential growth can be projected with the formula:

Nt = Noert Nt = number at time t

No = number at time 0

e = base of natural log, 2.71828...

t = period of time being studied

r = biotic potential or instantaneous per capita rate of increase; the rate of

change at a particular moment. Highest birthrate ad lowest mortality rate.

Exponential growth rate:

dN = rN N = number of individuals;

dt r = biotic potential or instantaneous per capita rate of increase.

t = time

Exponential growth is not realistic. No population can grow indefinitely.

There are limitations presented by the environment (food, space, etc), and competition for available

resources increases. These are called density-dependent factors.

Increased density eventually increases mortality, decreases fecundity, and causes emigration.

Logistic growth or sigmoid growth is exponential at first then growth begins to slow down until it

reaches zero, when births balance deaths.

Slow, fast, slow, zero growth.

Environmental resistance modifies growth.

S-shaped curve.

It represents how populations respond to density.

The biotic potential and size of the population modified by the environmental resistance

determine the growth rate.

Environmental resistance refers to the limitations placed on the biotic potential due to unfavorable

environmental conditions, e.g. overcrowded, predation.

Carrying capacity is the maximum number of individuals an area can support, K. The density of

organisms is in equilibrium with the source supply, the environment.

Paleobiologists have suggested that changes in the number of species or higher taxa may similarly be

affected by diversity-dependent factors that might reduce the origination rates or increase extinction

rates as the number species increases.

Competition among species for resources may limit the number of species to some maximum

number, K.

Undercrowding can be detrimental to a population since cooperation between members may be necessary.

dN = rN (K - N) N = number of individuals; dN = instantaneous rate of change

dt K r = biotic potential dt

t = time

K= carrying capacity

(K - N) it is a measure of the environmental resistance or the effect of crowding.

K this represents the opportunity for further population growth.

As the population grows, this unutilized opportunity declines.

This expression slows population growth.

When N is small, K - N/K is close to 1.

As N increases and approaches K, the value of the expression decreases towards 0.

If N > K, then dN/dt is negative and the population N decreases toward K.

Logistic growth curve is theoretical. It is a mathematical model of how populations grow under

favorable conditions.

Natural populations although they appear to grow logistically, they rarely do.

Some reasons for this difference: age structure may not be stable, immigration or emigration,

birth and death rate changed.

The increase in numbers, ΔN, declines as N increases and (K – N) goes toward zero.

Δt

At equilibrium ΔN = 0, N = K.

Δt

The major factors that have promoted diversification are:

Release from competition (one competitor is removed then the other is “released”, no more

competition).

Ecological divergence.

Coevolution.

Provinciality (the restriction of the range to a specific region).

DOES SPECIES DIVERSITY REACH EQUILIBRIUM?

The question whether or not the number of coexisting species tends toward equilibrium remains

unresolved.

The space and energy organisms compete for is finite.

Interaction between species tend to limit species diversity, e.g. as suggested by competitive

exclusion

The fossil record shows that the per capita rate of increase in the number of species or higher taxa is

lowered as the number grows. See the explanation of the logistic curve above.

Studies suggest that …

The higher the diversity of genera at the beginning of a time interval, the lower the rate of

origination of new genera.

The higher diversity is at the beginning of a time interval, the higher the number of genera that

became extinct.

A system may shift from one equilibrium state to another when conditions change.

At least three kinds of changes have altered conditions for organisms:

1. Changes in the physical environments, e.g. climatic changes.

2. The taxa that became dominant after a mass extinction were different from those that prevailed

before, and would attain different equilibrium level of diversity because of new patterns of

competition, predation, etc.

3. Taxa have evolved to use new resources and habitats, changing the overall number of species that

the planet can support.

Release from competition

The rate of originating new taxa has been much greater at times when diversity was unusually low: the

Cambrian and after mass extinctions.

Lineages diversify rapidly when they are presented with vacant niches.

E.g. Adaptive radiation of cichlid fishes in the Great Lakes of Africa; the finches in the

Galapagos, and the honeycreepers of Hawaii.

Competition may play the important role. There are two hypotheses:

1. Competitive displacement: the younger group caused the extinction of the older group.

2. Incumbent replacement: the extinction of the older taxon allowed the younger taxon to radiate.

Both seem to have played a role.

How competition has affected diversity remains controversial.

A pattern of replacement is consistent with competitive displacement.

Competitive displacement requires that:

The two taxa lived in the same locality at the same time.

Competed for the same resources.

The older taxon was not decimated by an extinction event.

The diversity and abundance of the younger taxon increased as the older taxon decreased.

It probably rarely occurred.

Incumbent replacement

Probably more common.

The radiation of groups only after the older taxa had disappeared, radiation of mammals after the

disappearance of non-avian dinosaurs may be the result of this.

Extinction and radiation occurred in different places and times.

All these support the release from competition.

The separation of the landmasses during the Mesozoic and Cenozoic provided greater latitudinal variation

in climate that might have helped to increase diversity by providing new ecological spaces.

The rates of origination and extinction have been diversity dependent. These observations suggest that

diversity tends toward equilibrium.

Equilibrium is affected by changes in climate and movement of continents, and because organisms evolve

new ways of using habitats and resources.

Ecological Divergence

Key adaptation: an adaptation that allows an organism to occupy a new ecological niche, often by using

a novel resource or habitat.

It implies the diversification of the group.

Air or swim bladder of fish allowed the occupation of land.

An adaptive zone is a set of ecological niches occupied by a group of species, a higher taxon that exploits

same resources in a similar way.

Insectivorous and fruit-eating nocturnal bats occupy different adaptive zones.

Insectivorous bats that are nocturnal occupy an adaptive zone different from that of diurnal bats.

It includes all nocturnal bats that belong to different genera and families but belong to the same

order, Chiroptera.

It represents an ecological pathway along which a taxon evolves.

At the species level is the same as the fundamental niche.

A set of adaptive zones is called an ecological space.

Expansion into new habitats and feeding habits accounts for the diversification of most families of

tetrapod vertebrates, such as frogs, snakes and birds.

Applying the method of Replicated Sister-Group Comparison gives support to the hypothesis that moving

into new, underutilized niches increases diversification.

Involves the comparison of sister clades that retain the ancestral character with those that have new

adaptation.

E. g. herbivorous beetles are more diverse than their sister taxa that retain the ancestral character

of feeding on detritus, fungi or animals.

Phylogenetic studies have to identify the ancestral and the evolved characters.

The partitioning of ecological niches has contributed to the diversification of species.

Related species reduce competition by subtle differences in resource utilization – occupation of

microhabitats, e.g. warblers feeding at different levels on the same tree.

Provinciality

The division of biota among geographic regions is called provinciality.

A faunal or floral province is a region containing high number of distinctive, localized taxa. See Chapter

6.

A trend from a cosmopolitan distribution of taxa to more localized distributions has persisted throughout

much of the Mesozoic and Cenozoic.

After the Permian extinction, paleontologists recognized only one worldwide province of marine

organisms. They were all cosmopolitan.

The number of provinces increased during the Jurassic, Cretaceous and Triassic. The marine

organisms were distributed among an increasing number of latitudinally arranged provinces in

both the Atlantic and the Pacific

The separation of the landmasses during the Mesozoic and Cenozoic over a greater latitudinal variation

has increased the diversity of habitats and climatic conditions.

Landmasses spread almost from pole to pole developing greater latitudinal variation in land

climate.

Two disjunct oceans were created: the Atlantic and the Indo-Pacific Oceans.

Fragmentation of landmasses prevented the interchange of species and increased isolation.

Other influences on diversification

Interaction among species affect changes in diversity that are not fully analyzed and understood, e.g.

predators.

The increase in the number of species in a clade almost surely results in an increase in affiliated species

such as mutualists and parasites.

Changes in climate have been associated with changes in the distribution of habitats and vegetation types,

which in turn have facilitated major changes in the distributions taxa, often leading to diversification.

Changes in the rates of origination and extinction of mammals during the Tertiary are not closely

correlated with changes in temperature, although some of the temperature changes match a decline

diversity.

The importance of climate change in the evolution of diversity, relative to other factors such as key

innovations and biotic interactions, is not yet well known.

Phylogenetic analyses of diversity trends

It is possible to make inferences about the rate of increase in the number species in a clade from a

molecular phylogeny of living species.

Some sister taxa show different diversification rate correlated with a key adaptation, e.g. herbivorous

beetles are more diverse than their sister taxa that retain the ancestral character of feeding on detritus,

fungi or animals.

McPeek and Brown (2007) compiled date from molecular phylogenies of 163 groups of animals and

found that, in general, species richness is correlated not with diversification rate but with the age of the

clade.

In conclusion: differences among taxa in species richness are attributable…

To different rates of diversification

To the age of the clade.