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Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Allopatric speciation

Leaving the mouse cursor over some words in green (in the text, not necessarily the headings) should reveal more detail.
Allopatric speciation differs from sympatric speciation in that:
The end result of this process is that the species in a particular area appear to be perfectly adapted to live there. This adaptation is the result of a protracted process involving chance variation and a metaphorical struggle for survival, not a simple choice.

Furthermore, it would be wrong to think that the evolution process has been focussed on producing the species as it exists today. Evolution is a continuous ongoing process, and (minor?) changes can be expected in the future, depending on developments.

Californian pupfish

Lake_Manley_2010 (514K) There was once a population of a small species of pupfish (Cyprinodon sp) in a widespread interconnecting lake system in Southern California.

This is a fleeting glimpse of a section of this: Lake Manly in Death Valley after heavy rain in 2010. The hexagonal cracking of the saltflats can be seen in the shallow water.
ash_meadows_pupfish (16K) Ash Meadows Amargosa pupfish (Cyprinodon nevadensis mionectes)
Salt Creek Pupfish (Cyprinodon salinus) (63K) Salt Creek Pupfish (Cyprinodon salinus)
Over thousands of years and changes in climate - notably rise in temperature - this pupfish population became separated into distinct populations in the smaller lakes, rivers and increasingly saline streams which still remained. The two main pupfish species are Cyprinodon salinus and Cyprinodon nevadensis, but sections of the river now have their own distinct sub-species of these pupfish.


devils-hole-pupfish (98K)
Cyprinodon diabolus - the Devil's Hole pupfish
Near Death Valley there is a small and isolated population of highly specialised species Cyprinodon diabolus - the Devil's Hole pupfish - which is able to tolerate water of a higher temperature and salinity. It has been shown that this species often lacks pelvic fins - a consequence of reduced thyroid hormone levels.

These fish can tolerate higher temperatures and salinity than others. They are small in size, so they do not need as much oxygen as larger fish, and their gills and kidneys have become efficient in excreting sodium and chloride ions.

As a result of falling population numbers, steps have been taken to protect this endangered species, including stopping the pumping of water out of the aquifer that feeds it. Species native to other hotsprings have recently been reported to be extinct, presumably as a result of increasingly hostile conditions.

Fauna of the Galapagos Islands

galapagos_map_topographic (52K) Following the writings of Charles Darwin, the different types of birds and reptiles on this isolated cluster of islands straddling the Equator 900km west of Ecuador (South America) have served as examples of evolution in action.

These 18 islands were formed as a result of volcanic action and have seen quite recent eruptions. The eastern islands are the oldest (Espanola formed 3.2 million years ago), and the western islands formed later (Fernandina, 700 thousand years ago).

Not shown on the map above: Wolf and Darwin islands - 160 km north-west of Isabela: home of the vampire finch.

Confusingly, each island has at least two names: of Spanish or English origin.

In fact Darwin was more interested in Geology than Biology when he visited but he preserved a number of specimens of biological material which were examined by others when he returned to England.

Darwin noticed that different islands had different mockingbird species, but there are different species of ground finch and tree finch as well as distinctive land and marine Iguanas, and giant tortoises.

Other bird species endemic to the Galapagos and worthy of note include the Galapagos Flightless Cormorant Phalacrocorax harrisi, the Galapagos Hawk Buteo galapagoensis and the Galapagos Penguin Spheniscus mendiculus - some of which are the only penguins to live in the northern hemisphere.

On the Galapagos there are also colonies of various wide-ranging seabirds known from other parts of the world: Blue-footed booby, Nazca booby

Darwin's finches

Tiaris_obscura,_Dull-colored_Grassquit (13K) Tiaris obscura, the Dull-colored Grassquit
These small birds are in fact not true finches but Tanagers. Their ancestors (nearest modern species the dull-coloured grassquit (Tiaris obscurus) were presumably blown from the mainland during extreme weather conditions.

Having arrived (as a flock, or at least a pair) on an Eastern Galapagos island 2–3 million years ago, these birds searched for food such as seeds of the local island plants, and insects. They then reproduced and became established as a population on the island. Mutations occurred, which produced a variety of different inherited conditions (genotypes) within the population. Some of these variations would have made the descendants of the initial finch more successful on the island than they would have been back on the mainland. Two separate gene pools had been established, with no interbreeding between them. At this stage (1.2 million years ago), allopatric speciation could be said to have occurred.

Within the population of colonising species there would be a range of beak sizes. Some, with (genes/alleles for) larger beaks could crack open larger seeds and cacti; others with smaller beaks confined themselves to small seeds.

As a result of adaptive radiation, the colonising species then developed into at least 15 species, each with a slightly different ecological niche, but nevertheless competing with the other species during critical weather conditions, e.g. drought. This situation is evidently rather flexible, as it has been noted that beak sizes vary between islands, depending on competition with other species, and that different species can occasionally interbreed successfully.
Darwins4finches (242K)

Click to see/ hide finch species listed by name


Darwin commented on the basic similarity in body shape and colour in his finches and he also noted a "perfect gradation in the size of the beaks in the different species". Curiously he did not mention Galapagos finches in any edition of Origin of Species.

Since Darwin, many scientists have visited Galapagos and made observations about the finches; none more so than Peter and Rosemary Grant who for years have logged the effect of favourable and unfavourable weather conditions on population numbers and body dimensions, especially beak sizes, and recorded hybridisation between different 'species', following the offspring for several generations.

It has been shown that the regulatory gene ALX1 has a major effect on beak shape in finches, together with another gene HMGA2. This has two alleles: one is common in finches with small beaks, while the other is common in finches with large beaks. There was a major change in allele frequency after 80% of medium ground finches perished following drought in 2004-6 and only small-beaked individuals of this species survived.

Galapagos giant tortoises

These were the origin for the name of the islands in Spanish. Passing ships from the 16th century onwards would pick them up as source of meat, as they could survive for long periods of time without food. This caused a major reduction in population, as did animals introduced by Man, including rats, which were predatory on eggs and young, and goats, which ate much of the vegetation on some islands. Luckily these tortoises have a lifespan of at least 100 years, so it has been possible to rectify this somewhat by protected breeding schemes and pest removal programs.

It seems likely that tortoises reached the islands by sea, presumably washed down rivers after strong rainfall, as their shell gives then buoyancy and they can raise their heads above water to breathe. They can survive for long periods without food. The most likely ancestral species was smaller, but it is commonly observed that island populations are larger than their mainland relatives, possibly as a result of reduced predation and competition. If an individual had a mutated gene or a combination of polygenes which increased its size, or prevented it stopping growth at a particular size, that could give it a survival advantage and increase its genetic fitness.

domed_tortoise (88K)
Domed shell
saddleback_tortoise (80K)
Saddle shell
Darwin was told that each island had tortoises with different features which could identify their origin. On islands with plenty of short vegetation the shell (carapace) shape is domed and the neck is short; on other drier islands the shell has a distinct saddle-like shape and the tortoise's neck is longer, allowing access to taller vegetation including tree cacti.


There are currently 11 varieties of Galapagos giant tortoise, each given either species or subspecies status within the genus Chelonoidis. Mitochondrial DNA analysis shows that some larger islands have several geographically distinct populations which differ significantly from one another but resemble those on other islands, suggesting there were 3 waves of colonisation. There are doubts about some populations which might have been brought by Man from other islands. This might prove to be fortuitous in re-establishing colonies of tortoises back on their presumed islands of origin, e.g Floreana.

lonesome-george_2406159b (64K)
Lonesome George - the last of his species
'Lonesome George' was the only remaining tortoise from Pinta Island (Chelonoidis abingdonii) and he died in 2012 aged about 100, having spent 40 years in captivity with females of a closely related species (Chelonoidis becki). He mated with them and they laid eggs but these proved to be infertile.

Historically, other giant tortoises (unrelated to Chelonoidis spp) were known to exist on many of the western Indian Ocean islands, as well as Madagascar. The Aldabra giant tortoise (Aldabrachelys gigantea), from the islands of the Aldabra Atoll in the Seychelles is the only remaining species today.

Anolis lizards in the Caribbean and USA

caribbean-islands-map-29392983 (47K) The islands of the Caribbean were produced as a result of the interaction of two or more tectonic plates, and volcanic activity along the eastern edge.

There are almost 150 species of lizards belonging to the genus Anolis on islands in the Caribbean.


It is believed that these species evolved from (two?) species found on mainland USA. The green or Carolina anole Anolis carolinensis is the main one today.

green_anole (6K)
The Carolina anole (Anolis carolinensis) is an arboreal lizard found primarily in the southeastern United States and some Caribbean islands.
green-anole2-4dd50e78 (68K)
Male Carolina anoles have attractive throat pouches (dewlaps) which they use for territorial displays or when courting females.
Presumably these islands were colonised by lizards from the area which is currently mainland USA or other islands, probably clinging to floating vegetation, produced as a result of violent weather conditions for which the area is famous.

Each island would have different climatic conditions and vegetation, and different predators, so the lizards would be given opportunities to feed and breed, as well as being exposed to different selective pressures.

It has been found that anole lizards vary in the colour of their skin, the length of their legs and tail and the development of toepads which are useful for gripping onto leaves, and these features could be key to their survival. Within each island there will be different types of landscape and different (layers of) vegetation so different species can find their own specific habitat and ecological niche. This is another example of adaptive radiation.

Camouflage will be key to survival, but reproductive success depends on attracting a mate. The images below show some of the variation between different Anolis species in the dewlap - a section of skin under the throat which males use in courtship display.

anole_dewlaps (491K)

It is interesting to note that several of these 'island' species have been re-introduced to the USA and these have out-competed the native green anole, forcing it to change its lifestyle and effectively evolve in response to competition and predation.

Anole lizards are undoubtedly quite photogenic, and there is a large community of afficianados who document their appearance both on the islands and in the USA.

Not quite giants ...

st-kilda-map6 (74K)
720-1440-cropped-St-KIlda-Wren-AF2T8096 (406K)
The St Kilda Wren (Troglodytes troglodytes hirtensis) is a subspecies of the Common Wren found on the Scottish Isle of St Kilda - 64km west of the Outer Hebrides. It is somewhat larger than individuals of the mainland population, and more grey and less reddish-brown in coloration. Barring on the feathers is more noticeable. And it has a slightly different - and louder - song. See link below

The population on St Kilda has been estimated to be just over 200 pairs. They are undoubtedly quite geographically isolated, and must have a separate gene pool.

St Kilda_Field_Mouse (28K)
The St Kilda Field Mouse (Apodemus sylvaticus hirtensis) - originally introduced (by accident?) by the Vikings, 1000 years ago - is also larger than the mainland version.

Perhaps you might want to find what happened to the St Kilda House Mouse!

Zebras

Zebras are fairly widely distributed in southern Africa. Their striped appearance is their main feature, but each individual has its own pattern.

There is an extensive fossil record supporting the development of the ancestors of the horse in America, although they spread out to other parts of the world and died out there. The modern genus Equus contains horses, asses and zebras.

somali_wild_ass (234K) Somali Wild Ass
The African or Somali Wild Ass is the only other member of the genus with stripes (confined to the legs), so it is tempting to think that the alleles responsible for stripes developed as a result of a mutation as the ancestor of the zebras colonised Africa. The significance of stripes is debatable; possibly they distract predatory insects or confuse predators.

It seems that the distinctive stripes of zebras were advantageous in some way for living in herds on the African plains. Different zebra subspecies are found in different geographical areas and although they can migrate over quite an area in pursuit of more luscious grasses, they clearly reproduce with others of the same localised group.

220px-Bwindi_Impenetrable_National_Park-112410.jpg
There are said to be three species of zebra:
Mouseover text below for pictures and distribution map for each

The Plains Zebra, Equus quagga, of which 5 sub-species are recognised today, as well as the extinct Quagga Equus quagga quagga.


The Mountain Zebra, Equus zebra, of which 2 sub-species are recognised today,

and
Grevy's Zebra, Equus grevyi, which is considered to be an endangered species.



Giraffes

giraffe_distribution (142K) Giraffes - scientific name Giraffa camelopardalis - are even more widely distributed in Africa. Their long neck is their main feature, but each individual has its own pattern of skin markings. As a result, a number of subspecies have been identified. It has been suggested that up to six of these subspecies should be classified as different species.

The various giraffe populations are widely separated in different sections of the African continent, so they are both geographically isolated. Each population has its own fairly distinctive markings and coloration.
See right - (not the most recent categorisation).


Giraffe (42K)
The giraffe's neck has often been used in discussions about evolution. The Lamarckian concept of inheritance of acquired characteristics - tallness due to repeated attempts to stretch up for food from trees to browse on - has no connection with the reproductive system so it cannot be expected to give rise to an inheritable condition. This is not accepted nowadays, whereas Darwinian evolution as a result of natural selection is seen as a more acceptable explanation.

Since the skeleton of the giraffe has the same number of bones as its only close relative the okapi, it seems reasonable to assume that the dimensions of the giraffe's body are increased as a result of genes which cause each bone to elongate, or at least not to cease growth at the same point that lesser and potentially competitive creatures do. This was presumably a result of a mutation in genes controlling development. In other species such gigantism could be disadvantageous, but coupled with the advantage of being able to feed from the canopy of trees in fairly arid environments it is clearly a useful feature.

In fact the giraffe outreaches all of its competition, so it has been suggested that there must be another factor besides better browsing potential. Additionally extra height gives the advantage of better vision for the identification of predators, and it has even been suggested that females select their mates on the basis of neck length. Males occasionally fight by swinging their heads like hammers, so there may be a form of 'arms race' which causes this.

The genes responsible for the lanky body are clearly likely to be selected for, as shorter versions would be at a greater disadvantage. At the same time, random mutations in the genes controlling the body pigmentation could result in variations in the phenotype of body colour and markings. Presumably the brown background and irregular lighter network gives some form of camouflage, which would be especially important to young giraffes which have not yet gained the advantage of extra height. But the exact details of shade of colour might not be as important as the characteristic of height. This could lead to variation in both colour and reticulation, so that different races or varieties could evolve. Conceivably only a few giraffes entering a new area would populate it with offspring derived from the original colonisers, and due to genetic drift each population could have distinct markings different from others. It has also been suggested that female giraffes choose their mates on the basis of markings similar to their parents - a form of imprinting.

As a result giraffes with particular (local) markings are more likely to survive and breed. This greater (differential) reproductive success than those with other markings, means that the alleles responsible for the local variety's skin markings will increase in frequency within the population.

It is thought that giraffes entered Africa and spread out from Ethiopia about 7 million years ago, as vegetation changed in the African continent.

Other related topics on this site

(also accessible from the drop-down menu above)

Sympatric speciation

Web references

California Fish Species - scroll down for the various pupfish

Death valley, california - salt creek pup fish - great video of mating - and this behaviour is what gave them the name!

On The Trail: Devils Hole YouTube video trailer for a program shown in 2009!

Devils Hole pupfish Good background to the recovery programme

What makes devils hole pupfish special? Kathryn Phillips Journal of Experimental Biology 2006 209: i doi: 10.1242/jeb.02509

Galápagos Islands From Wikipedia, the free encyclopedia

Geomorphology of the Galapagos

Darwin's finches From Wikipedia, the free encyclopedia

Unpredictable Evolution in a 30-Year Study of Darwin's Finches Peter R. Grant, B. Rosemary Grant

The Galápagos Finches: Evidence for Evolution? by Richard Peachey - Alternative interpretation of the evidence

On the Origin of Darwin's Finches Peter and Rosemary Grant get some help with DNA

Gene behind ‘evolution in action’ in Darwin’s finches identified Simple comments on a paper in Nature

The calmodulin pathway and evolution of elongated beak morphology in Darwin's finches

A Darwin Finch, Crucial to Idea of Evolution, Fights for Survival One of the world's rarest birds, the mangrove finch has dwindled to a habitat the size of just 12 city blocks. Here's how scientists are trying to bring it back from near-extinction.

Woodpecker Finch Using a Tool - YouTube

Vampire Finch - YouTube

Extinct Floreana Tortoise Species is being Resurrected in the Galapagos Islands

On the Origin of Life in the Galápagos Islands

Hear the St Kilda Wren

Anole Annals Your source for the latest on Anolis lizards.

The Origin of Species: Lizards in an Evolutionary Tree - good video

Florida Lizards Evolve Rapidly, Within 15 Years and 20 Generations

Field identification guide for the anole lizards of Miami All but one are re-introduced species

Zebra From Wikipedia, the free encyclopedia

Is the endangered Grevy’s zebra threatened by hybridization?

Multi-locus Analyses Reveal Four Giraffe Species Instead of One

A new paper confidently claims that there are four giraffe species rather than one, but I’m not so sure - response to the above?

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