Our conception of how life on earth is organised is influenced by the mythconceptions we inherit. This affects how we view both the development of species and the boundaries between them. And of course it affects how we view not just their lives but our own.
In “Hybrid Vigour and Inbreeding” [Survival] I pointed out that theories new species arise from the expansion of small populations are flawed. But many cultures have myths about animals or humans descending from a single individual or pair of individuals. As a result, even if we accept species vary in space and time, we might still look for a series of missing links and beginnings. Even scientists who would consider themselves to be objective often unconsciously look for a beginning or single point of origin for each species.
All species vary through time and space. You don’t necessarily look the same as any one of your ancestors. Therefore “like begets like” but each individual can be a bit different. It is impossible for a sperm whale to evolve from a bowl of petunias, or a duck from a dandelion, as some creationist and Intelligent Design supporters accuse evolutionists of believing. But over time it is possible for a cow to evolve from something like a camel, or a human to evolve from something like an ape. Evolution doesn’t proceed in a straight line though, especially not a straight line with a human at the end.
A related problem is that some scientists might believe separate species are always discrete, genetically isolated populations.
If two individuals of the opposite sex are not capable of producing fertile offspring they are considered to be members of separate species. Great. This separates lions from tigers and horses, zebras and donkeys from each other. Animals within these two groups all produce sterile offspring when crossed although the offspring of a horse and a donkey are not always infertile. Horses and donkeys each have a different number of chromosomes and so simple chromosome number cannot be used to define separate species or kinds.
Domestic cattle and yaks can also breed together in captivity but a permanent crossbreed cannot be created. Male offspring are sterile and the females have to be bred back to one or other parent. It is not possible to form a new breed with the most desirable characteristics of each type. Domestic cattle seem to be more closely related to bison than they are to yaks. They can be fairly easily crossed to produce “beefalo”.
Should we therefore regard cattle and bison as being the same species or kind? What about bonobos (pygmy chimps) and chimpanzees? They can form fertile hybrids (Jobling et al 2004). And wolves and coyotes? Wolves and coyotes have fertile offspring together in captivity but are kept from breeding in the wild partly because large predators always attack and try to kill smaller ones.
What is a “kind”? I will use a group of ducks, the dabbling ducks to illustrate the problems. They are all classified together into the Anas genus. Up next is a diagram of the relationships within the genus. Names of several individual species and subspecies in the genus are provided. The diagram is based on various sources but mainly the “cladogram” of Bradley Livezey (1991). A cladogram is a classification tool devised by comparing similarities and differences within any kind of objects. It doesn’t necessarily represent a family tree. Other slightly different duck cladograms have been proposed (for example see Johnson and Sorenson 1999).
All the ducks in the cladogram are usually classified within the Anas genus. On the other hand the ducks at the right hand side of the diagram form a fairly well defined sub-group within the genus. We should call it the mallard “subgenus” or “superspecies”. The distribution of this superspecies and drawings of some of them are shown in map
I’ve added drawings of the northern pintail (Anas acuta acuta) and the African black duck (Anas sparsa). Although they are members of the Anas genus they are not members of the mallard superspecies. The African black duck shares most of its geographic range with the yellowbilled branch of the superspecies though. It is in turn divided into three subspecies. The northern pintail’s range largely coincides with that of the mallard. There are several other species and subspecies of pintail scattered around the world. You can see in the cladogram that various teal and shoveler superspecies are also included in the Anas genus. Each of these has spread around the world and some are equally as varied as the mallard superspecies.
Therefore all these other groups within the genus share at least part of their geographic ranges with members of the mallard superspecies but they are separate from it. But you can see from map 9 that the mallard superspecies is not represented in South America or in the Sahara desert. Other members of the genus, especially pintails along with spotted and cinnamon teal, replace the superspecies in South America. Dabbling ducks are not at all common in the Sahara Desert for some reason or other although some northern ducks do winter along the Nile.
The mallard superspecies varies over its geographic range and forms a sort of star in the same way most species (including most other species within this genus) and even humans do: a series of clines. The fact that there is not a completely gradual change of the superspecies over its range suggests the clines are not perfect. Gene flow has largely been confined within each geographical region. In fact the boundaries between these closely related species generally coincide with geographic features. To return to the idea of a star there are smaller stars within the bigger star.
You can see from the bottom of the cladogram and on the map that the New Zealand branch of the mallard superspecies is called the grey duck (Anas superciliosa superciliosa). The grey duck is found throughout the tropical South Pacific islands from Belau to Tahiti and on south through New Guinea, Australia and New Zealand. It is usually separated into the three subspecies shown and the Australian version is usually called the black duck, as are two other species in the genus (African and American). The grey ducks near the equator (Pelew grey duck) are the smallest and the New Zealand ducks are the largest and there are other subtle differences between the three types. This grading in size from the equator towards the poles is common for many species (Bergmann’s rule - pdf). Another example within the genus is that the Greenland mallard is larger than the common mallard. The rule species on islands tend to be smaller than their mainland relations is supported by the fact the Laysan teal and Hawai‘ian duck are small.
The common mallard has lived in the tundra and in human company for thousands of years and so it has become well adapted to open farmland. It was first introduced to New Zealand from Britain more than a hundred years ago. The mallard slowly expanded its range in New Zealand until about 1930 when mallards of North American origin were introduced. It then rapidly became well established (Williams 1998). It had started to replace the native grey duck both physically and by cross breeding with it even before that time. The fact these ducks form fertile hybrids shows they need not be regarded as separate species although experienced hunters can distinguish them reasonably easily. In fact Oustalet’s duck (Anas platyrhynchos oustaleti) from Guam and Saipan is today usually accepted to be the result of natural crossing between mallard and grey duck, a stabilised hybrid (although not so stabilised, quite variable in fact, and now extinct).
Was this actually a separate species? The common mallard also forms hybrids with the American members of the superspecies, especially the black duck (Livezey 1991). As far as we know, except for the muscovy, all domestic ducks descend from the mallard and they too regularly form hybrids with it. But because ducks seem to have been first domesticated in East Asia I would bet that at least some domestic breeds have Chinese spotbilled and possibly Philippine duck genes as well as mallard. Boundaries between members of the superspecies are blurred.
But this is not the end of it. Mallards occasionally form hybrids with the common, or northern, pintail. You can therefore see that the whole question of what is a separate species is very complicated. Different plumage markings in ducks may not necessarily indicate great genetic difference. In fact only one or two genes may control plumage marking and the ducks may be genetically very similar apart from this. This situation may be fairly common in birds (Gill 1998). Species within the mallard group may be only as different from each other as different races of humans are.
Unlike humans the female passes on both Y-chromosome and mitochondrial DNA. Rhymer et al (1994) have studied the mtDNA of mallard and grey ducks and their hybrids in New Zealand. Grey duck and mallard mtDNA generally clumps into the two separate kinds. Both types of mtDNA were found in the hybrids though. And one duck that looked perfectly like a grey duck had mallard mtDNA and one perfectly mallard-looking duck had grey duck mtDNA. In these two cases the mtDNA did not match the nuclear DNA. The researchers concluded nuclear DNA has flowed each way between the two species with successive crossbreeding.
They quote similar examples in other pairs of species such as between mule deer and white-tailed deer in West Texas, between coyote and North American gray wolf, between two species of vole and in several species of amphibians and fish. Many new species have almost certainly developed from hybrids between species. In fact, because of the geographic distribution of some of their colour patterns, I believe the phenomenon has been very common in ducks (see also Johnson and Sorenson 1999).
Therefore the process of human migration and mixing that has occurred throughout history probably accounts for the distribution and variation of the Anas genus. The process presumably carried on through our earlier evolution. Of course many evolution -deniers would claim humans obey a different set of biological rules. I do concede that modern human migration is at least sometimes a little bit organised.
A look at map 9, the distribution of the mallard superspecies, suggests drying of the Sahara Desert probably split the mallard from the African yellowbilled ducks. If we move one step to the left in the cladogram we see that an even earlier drying was probably responsible for the separation of the African black duck from the whole mallard superspecies in the first place. A further step to the left in the cladogram and we come to the shoveler, blue-winged and cinnamon teal group. Although widespread their greatest diversity is in South America. We could carry on in this way but even most creationists and Intelligent Design supporters might be prepared to concede evolution is responsible for all this variation within the Anas genus. In other words the cladogram may, in fact, be a family tree.
Evoluion-deniers often claim each kind or genus was created separately. But the boundaries between genera (the plural of genus) are also fairly arbitrary. Livezey (1991) believes wigeon and gadwall should be classified in a separate genus Mareca rather than Anas. Are they now suddenly a separate kind? Johnson and Sorenson (1999) on the other hand argue that species today classified in genera such as Lophonetta, Amazonetta and Tachyeres should be included in Anas. In fact we could easily extend the cladogram to the left and connect other duck groups such as shelducks (genus Tadorna), eider ducks (Somateria), pochards and diving ducks (Athya and Netta) and several other genera into one large group (the subfamily Anatinae) within the family Anatidae.
Which of all these genera were separate creations? And geese and swans (subfamily Anserinae) are also obviously part of the duck family (Anatidae). Ultimately, branching off first at the far left of an extended diagram of the duck family, we would find a bird called the magpie goose. This bird is found in Northern Australia and New Guinea and doesn’t even have webbed feet. The family Anatidae is grouped along with some South American birds called screamers (family Anhimidae) into the order Anseriformes.
Ultimately these families show connections to birds such as megapodes, pheasants, grouse, turkeys and common farmyard fowls (all in the order Galliformes). Do all these species descend from just one or two pairs on Noah’s ark? Virtually all groups of species on earth, including monkeys and apes, can be arranged in a similar manner and many groups of plants are especially fascinating. They don’t move around as much as birds and animals. Creationists and ID supporters are remarkably secretive as to exactly which types or kinds were created in a beginning.
People who classify life forms into species, genus, family, order etc. can be themselves classified into “splitters” and “lumpers” (Johanson and Edey 1982). You can understand the problems they face. Extreme lumpers could put all the members of the Anas genus that can interbreed into one very variable species with variable boundaries. On the other hand many splitters consider the Mexican and Florida ducks are not merely subspecies of the mallard but are separate species (i.e. Anas diazi and Anas fulvigula). Some then even split each of these into local geographical subspecies. Livezey (1991) for example regards Mexican and Florida ducks as distinct species and considers they show geographic or subspecies variation but he doesn’t separate off the Greenland mallard or divide the grey duck group into subspecies. I am from New Zealand and he is from North America and we each look more closely at our local species, the ones we are most interested in.
This all tends to show classification into species is not a simple process. The fact two individuals are capable of producing fertile offspring can’t be taken to prove they are members of the same species. But in general we can say separate species will not breed together in natural conditions, even if they will breed in captivity.
But it gets even more complicated. Study of Galapagos finches shows that during times of plenty boundaries can become porous, two species can mix a little. Conversely specialisation and speciation are greatest at times of environmental stress. The idea species are fixed entities is therefore simply a classification tool.
Classification into separate species is usually easy enough for any given region. Where the problem arises in a big way is deciding between populations separated through time or space. They may look fairly different but be able to interbreed readily, such as breeds of dog, or may look much the same as each other but not breed together at all. This last does happen, for example in species of gazelle, mice and birds (Tudge 1996).
The expression “parallel evolution” is used to explain the process that gives rise to species that look the same but have a separate origin. The same environmental processes acting on different populations leads to them looking much the same. The similarity in the shape of fish and whales is a good example. With living populations we can at least find out if they will breed but with fossils this is impossible to check. Many brave statements are made about the interbreeding capability of species, including ancient human species, known only from fossils.
I mentioned in “Hybrid Vigour and Inbreeding” [Wave Theory of Evolution] that all species spread over a large geographical range vary over that range in the same way humans and human languages do. In other words they form a series of clines. As I have just shown with dabbling ducks the clines are not perfect. Geographical boundaries isolate populations to some extent. But the various subspecies that form move around. The hybrid zones are very fluid and the balance of nature is constantly changing. There is biological selection within a species as well as selection between species. In practice the distinction between the two types of selection is blurred. In a way species tend to merge together. The gaps we see are sort of artificial and are usually caused by the extinction of intermediates or the interruption of a cline.
Extinction is an important process in “speciation”, the division of one species into two. Where intermediates survive we get the situation we have with the Anas group and with many other kinds of bird such as the herring gull (genus Larus) and the great tit (Parus). These last two groups vary markedly, but gradually, over their range and both have been divided into many species and subspecies. It is very difficult to define the actual boundaries between the species though, or even how many species there actually are. In such cases we can get really carried away and name species, superspecies, allospecies and subspecies and even subgenus, supergenus and infragenus. Often these variations are referred to as races and they are the result of the same processes that have led to races in the human species.
Ecology is the study of how living organisms interact with the earth and each other.
A basic rule of biology is that two separate species can’t occupy the same ecological or environmental niche in the same place and at the same time. In other words they can’t obtain the same food in the same way or share a way of making a living. Species that have very similar environmental requirements are usually separated geographically, although there is often a great deal of overlap. Rory Putman (1988), for example, discusses the complex competition between different species of deer and other grazers and browsers in various parts of the world.
But if two species are very similar ecologically and occupy the same space they will either interbreed over time, becoming one species as Oustalet's duck did or, if this is not an option, one species will become locally extinct. There is actually a third option. If they can maintain isolating mechanisms or tribalism the two species can in fact split the ecological niche, each specialising in one extreme. There is a tendency for the grey duck and the mallard to separate ecologically in New Zealand although each species now has genes from the other, gene flow (Williams 1998). The grey duck may survive as a separate species if it is able to exclude the mallard from inland, semi-forested areas. The situation with the mallard and the grey duck in New Zealand is artificial but the two species met naturally in the isolated Northern Marianas Islands.
Human groups also often split the environment ecologically. In parts of Africa groups of farmers, cattle breeders and hunters can all live in roughly the same region but they are separated ecologically. There is still gene flow of course. Such separations have probably been very common throughout our evolution.
The situation with the mallard superspecies is instructive because for most pairs of species, by definition, hybrids are at a disadvantage to either parent species for various reasons. For example several alpine Ranunculus (buttercup) species in Australia each specialise in exploiting slightly different environments. They are easily able to cross but hybrids are found only in very narrow zones between species. Each individual species has become especially well adapted to a particular environment and within that environment there is selection against the hybrids (Armstrong 2001).
Hooded crows and carrion crows also separate the environment between them where their distributions overlap, with hooded crows occupying the higher country. The two species do form hybrids in a narrow zone but the hybrids do not expand from this zone (Tudge 1996). In this case the hybrids may be less fertile than either parent species (“Hybrid Vigour and Inbreeding” [Hybrid Vigour]).
Hermit and Townsend’s warblers from the Pacific Coast of North America also form hybrids and again there is selection against these hybrids, this time possibly cultural (Gill 1998). Many other examples exist of pairs of species able to form hybrids in the wild.
On the other hand if the two populations are inbred we will have restored hybrid vigour. This may allow a hybrid population to replace both its parent populations. It seems to be happening with the duck genus Oxyura in Western Europe.
Ultimately ecology usually keeps species separate where they overlap. Any hybrid population can take over only if there is a change in the ecology.
See next :: Human Evolution On Trial - Evolution
Armstrong, Tristan (2001) Notice of a lecture. Auckland Botanical Society News-Sheet. July.
Gill, Frank B. (1998) Hybridization in Birds. The Auk, Vol. 115 No 2 April.
Johanson, Donald and Edey, Maitland (1982) Lucy. Warner Books, New York.
Rhymer et al (1994) Mitochondrial analysis of Gene Flow Between New Zealand Mallards (Anas platyrhynchos) and Grey Ducks (Anas superciliosa). Auk, 111:970-978.
Tudge, Colin (1996) The Time Before History. Scribner, New York.
Williams, Murray (1998) An Assessment of Grey Duck X Mallard Hybridisation. A report to the New Zealand Fish & Game Council.