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Something there is that doesn't love a wall,
That sends the frozen groundswell under it, And spills the upper boulders in the sun; And makes gaps even two can pass abreast. —Robert Frost
Mending Wall |
If stabilization theory is accepted as a working hypothesis (and the evidence presented thus far suggests that it should be), then the intellectual structure based on Darwin’s description of evolution will have to be reevaluated. In particular, if the production of new types of organisms via stabilization processes is assumed typical, then a question arises — How do stabilization processes produce distinct groups of related forms (e.g., vertebrates, mollusks, mammals, insects, etc.)? Thus far, we have spoken of organismal types treated as subspecies or species, but we have not considered higher categories (i.e., genera, families, orders, classes, etc.). But a higher category is a type of organism, too. For example, a mammal is a type of organism. Mammalia, the order to which mammals belong, is also a category of the taxonomic hierarchy. Thus, in the language of stabilization theory, the present section discusses the questions of how forms treated as higher categories arise and how they become distinct from each other.
The answer to this question would be simpler if it were necessary only to describe an explanatory evolutionary mechanism. But there is some question whether such "higher categories" have any real existence, or, if they do, whether biologists have defined them correctly. When we observe nature, we don’t see higher categories. We see individual organisms. Taxonomists and tradition have created the categories into which organisms are sorted. The validity of many, perhaps even most, higher categories is, or has been, a point of dispute among taxonomists themselves. Systems of classification fall in and out of favor. In the 1960s there were two accepted kingdoms (plant and animal). Then a five-kingdom system became popular (animals, plants, fungi, protoctists, and bacteria). More recently, this number has been reduced from five to three (Archaea, Bacteria, and Eukarya). Who knows how long this latest arrangement will continue to hold sway?
Thus, explaining evolution is more than a simple matter of accounting for the origin of a particular set of categories through some sort of natural process. A full explanation has to consider not only evolutionary mechanisms, but also the predilections and prejudices of biologists themselves. We have to distinguish between the artificial and natural aspects of the phenomenon of higher categories. We also need to consider how, under stabilization theory, the simple, microscopic organisms found in the earliest known terrestrial strata could have evolved into the wide array of complex organisms inhabiting the earth today.
The Indiscretion of Discreteness. In the old, two-kingdom system of classification, bacteria were classified as plants. More recently, however, many biologists have claimed that the filing status of bacteria should be changed. In particular, in a system first proposed by Whittaker in 1959, cellular organisms were divided into five kingdoms, one for bacteria (Monera) and four eukaryotic ones (Protoctista, Fungi, Plantae, Animalia). Recall from section 3 that (1) eukaryotes are organisms with cells having a set of linear chromosomes enclosed in a nuclear membrane; and (2) bacteria are single-celled organisms in which the chromosome is single, circular, and not so enclosed. Kingdom Protoctista comprises unicellular eukaryotes and their immediate multicellular descendants. In general, Protoctista has been used as a catchall category for any simple eukaryote that doesn't fit well in one of the other three eukaryotic categories (animals, plants, and fungi).
The five-kingdom system of classification made it seem as if all bacteria shared a more recent ancestor than does any bacterium with any eukaryote. But it is certainly questionable whether such a belief is justified. For example, cyanobacteria employ chlorophyll a in photosynthesis, as do plants and a wide variety of protoctists. It seems improbable this same, complex molecule evolved independently in three unrelated groups (Monera, Protoctista, and Plantae), especially when it is known that various other types of chlorophyll are capable of carrying out the job of photosynthesis. Those bacteria, protoctists, and plants that contain this molecule must therefore have inherited chlorophyll a from some common ancestor, and so must be more closely related than this system of classification suggests. The simple photosynthetic organs of cyanobacteria (thylakoids) are similar in structure to those in the more complex photosynthetic organs (chloroplasts) of eukaryotes. As Minelli (1993: 134) points out, some biologists regard cyanobacteria as true plants, while others regard them as true bacteria. Some bacteria have traits that link them with fungi and funguslike protoctists. Thus, Golubic and Knoll (1992: 55) note that
These organisms have numerous traits characteristic of fungi. Is it a good idea to assign them to a kingdom entirely separate from fungi? Whether such a classification is reasonable or not, it is clearly plausible to suppose early simple eukaryotes with funguslike traits first evolved from bacteria with funguslike traits.
Since biologists generally accept the assertion (based on fossil evidence) that bacteria existed long before any eukaryote, they also accept the idea that simple eukaryotes (protoctists) first evolved from bacteria. But protoctists cannot have evolved from all the different bacterial types that preceded them in time. Presumably, they evolved from some subset. Therefore, types of bacteria that are also descended from this subset share more recent common ancestors with protoctists than with other bacteria not descended from that subset. So Kingdom Monera, the category in which all bacteria are often lumped, does not constitute what many biologists term a “natural” taxonomic category (that is, its composition is not based on degree of evolutionary relationship).
Likewise, the more complex eukaryotes (fungi, plants, and animals) are widely assumed to be the descendants of early protoctists. But, again, they cannot be the descendants of all early protoctists. Some protoctists (e.g., Myxomycota, Acrasiomycota) are more similar to fungi than to plants or animals. Others resemble plants (e.g., they engage in photosynthesis). Still others have animal traits (e.g., eyespots, motility, jaws). It is thus reasonable to suppose some protoctists share more recent ancestors with fungi than do other protoctists (and others, with plants; still others, with animals). It seems, then, that Kingdom Protoctista is no more a “natural” category than is Kingdom Monera. This logical inconsistency has prompted many biologists to embrace contradictory claims. In particular, many accept both
Which is it? Are they profoundly different or are they direct descendants? Clearly, the latter of these two ideas is the more plausible: the characteristics distinguishing each of the various major types of protoctists probably arose from bacterial types with such characteristics. Similarly, one might expect that the various types of protoctists gave rise to more complex eukaryotes with corresponding traits. For example, it seems likely that funguslike protoctists arose from bacteria with funguslike traits, and fungi arose from funguslike protoctists (and, in fact, most botanists do think certain members of one large and varied protoctist group, Chlorophyta, must be similar to the ancestors of plants).
Efforts have been made to partition Kingdom Protoctista and allot its contents to the three categories of fungus, plant, and animal. But many organisms seem to fall between the types deemed typical of these three categories. The various euglenids engage in photosynthesis, but they also have eyespots and feed like animals. Cryptomonads also combine the characteristics of plants and animals, as does the chlorophyte Chlamydomonas. Water molds (Oomycota) have many characteristics in common with fungi, but have cell walls composed of cellulose (as do plants) instead of chitin (as do ordinary fungi). They also have undulipodia, another trait not usually considered characteristic of fungi (an undulipodium is a long, threadlike appendage used in cell locomotion as seen, for example, in spermatozoa).
In truth, it appears the long-standing desire to have a strictly discrete system of classification has imposed artificial boundaries between many taxa. The system that taxonomists employ is a hierarchical bucket sort, the same system used by the scholastics centuries ago. There are various kingdoms, containing various states, containing various cities, containing various houses, containing various rooms, containing various buckets. We have placed each organism's name on a strip of paper and have decided each strip of paper should be placed in one and only one bucket. We have agreed that no strip can be left out on the floor between buckets or on the frontier between kingdoms.
But nature isn’t listening. Anyone who is willing to take the time can find innumerable examples of organisms that don’t fit into any particular bucket, city, or kingdom (see Table 8.1). Biologists often disagree on the category to which a given form should be assigned. Indeed, Pearse et al. (1987: 454) note “Animals with characters resembling those of two or more otherwise discrete groups exist at every taxonomic level and present difficulties in classification.” There are, of course, distinct types. The typical bacterium is, in fact, radically different from the typical eukaryote. But typical types are more than typical—they are stereotypical. They represent a category in the naturalist’s mind. In biology, as in any other realm, stereotypic thinking saves trouble in the short run, but in the long run it leads to trouble. In the present case, the prominence of the stereotype obscures a fact of basic evolutionary significance: many types of organisms don’t fit the category to which they have been assigned (thus, even Darwin said, “intermediate and troublesome forms often destroy our definitions”).
Anyone who thinks in terms of stereotypes will tend to overlook these types sitting on the edge of the bucket, those that might just as well be placed in a different category. This kind of mindset makes the accepted taxonomic topology (a dichotomously branching tree) seem more plausible and concurrently makes the accepted evolutionary topology (also a dichotomously branching tree) seem valid. The existence of intermediate forms therefore tends to undermine these orthodox views. Such forms are gaps in the intellectual walls we place around types. Often they go unrecognized because there is a human tendency to think in terms of rules instead of exceptions. Moreover, ordinary taxonomic practice tends to conceal them since it is usual to classify specimens as being of one type or the other, and not to leave them unclassified somewhere in between. However, a careful examination of nature reveals many such organisms do exist. As Robert Frost said of the gaps that each winter appeared in the wall dividing his own property from his neighbor's,
| Table 8.1: Just a few of the abundant counterexamples to the idea that taxonomic categories are discrete. Most of the intermediates listed here are extant. Some are extinct. The intermediate nature of these forms may in some cases intimate a hybrid origin. However, they are listed here only because they are morphologically intermediate between the types indicated in the left-hand column, not because they are thought to be hybrid. | |
| CONNECTED CATEGORIES | INTERMEDIATE FORMS |
| Snakes-Annelids | Caecilian amphibians, blind worm snakes (Typhlopidae), amphisbaenids |
| Annelids-Arthropods | Velvet worms (onychophorids), polychaete annelids |
| Annelids-Molluscs | Monoplacophorans |
| Annelids-Lophophorates | Polychaete worms, phoronid worms |
| Chordates-Arthropods | Nectocaris, placoderms |
| Mammals-Reptiles | Synapsids, pterosaurs, loricates (see section 9) |
| Reptiles-Amphibiansa | Caecilians, Softshell turtles (Trionychidae), Paleozoic reptiles (e.g., Seymouria, Diadectes) |
| Insects-Crustaceans | Rotifers; half-insects (Protura); doubletails (Diplura), bristletails (Archaeognatha) |
| Rotifers-Echinoderms (Crinoids) | Ectoprocts |
| Bivalved molluscs-Phoronids | Brachiopods |
| Birds-Mammalsb | Platypus |
| Platypus-Otter | Otter Civet (Cynogale) |
| Cats-Weaselsc | Jaguarundi (Felis yaguarundi) |
| Cats-Mongoosesd | Fossa (Cryptoprocta ferox) |
| Carnivores-Primatese | Kinkajou (Potos flavus) |
| Carnivores-Insectivores | Giant Otter Shrew (Potamogale) |
| Rodents-Insectivores | Shrew mice (Blarinomys, Coelomys); shrewlike rats (Rhynchomys); mouse shrews (Myosorex); shrew rats (Archboldomys, Echiothrix, Melasmothrix, Tateomys); mole-voles (Ellobius, Prometheomys); mole rats (Bathyergus, Cryptomys, Georychus, Heliophobius, Heterocephalus, Myospalax, Nannospalax, Spalax, Tachyoryctes) |
| Moles-Shrews | Mole shrews (Anurosorex, Solisorex); shrew moles (Neurotrichus, Uropsilus, Urotrichus) |
| Murids-Porcupines | Long-tailed Porcupine (Trichys); echimyids (esp. Chaetomys); Maned Rat (Lophiomys) |
| Porcupines-Insectivores | Hedgehogs (Erinaceidae); tenrecs (Tenrecidae) |
| Rats-Hamsters | Ratlike hamsters (esp. Tscherskia triton) |
| Murids-Jerboas | Kangaroo rats (Dipodomys); and kangaroo mice (Microdipodops); jumping mice (Zapodidae); pocket mice (Heteromys, Liomys, Perognathus) |
| Murids-Squirrels | Rat-squirrel Laonastes (Diatomyidae) |
| Hystrichognathids-Sciurognathids | Gundis (Ctenodactylidae) |
| Reptiles-Fish | Broadbanded Moray, Channomuraena vittata; Tiger Snake Moray, Scuticaria tigrina; Large-spotted Snake Moray, Uropterygius polyspilus. The ancient Roman naturalist Pliny (9.39) says morays come out on land to be impregnated by snakes. Certainly, some kinds of morays leave the water, sometimes for extended periods (Chave and Randall 1971). |
| Amphibians-Fish | Crested newts (Triturus); eel-newts (Amphiuma); Frogfishes; Paleozoic tetrapods (e.g., Acanthostega; Ichthyostega); Article |
| Teleosts-Elasmobranchsf | Ratfish (Hydrolagus colliei) |
| Bryozoans-Brachiopods | Phoronid worms |
| Snakes-Lizards[g] | Ophiognomon, Chamaesaura, Panasepsis,. Acontias, Acontophiops, Typhlosaurus, Procelotes, Scelotes, Typhlacontias, Tetradactylus, Chalcides, Leptophylophidae, Bipes, Pachyrhachis |
| Bees-Moths | Clear-winged moths (Sesiidae), bee hawk-moths (Hemaris) |
| Bees-Flies (Hymenoptera-Diptera) | Hover flies (Syrphidae); social wasps (Vespidae) |
| Nerve-winged insects-Mantids | Mantispids; e.g., Styrian Praying Lacewing (Mantispa styriaca) |
| Birds-Reptiles | Archaeopterygids, avimimids, ornithomimids, garudimimids |
| Sphenisciforms-Procellariforms[h] | Eudyptulid penguins (Eudyptula) |
| Hawks/Eagles-Storks/Herons[i] | Secretary Bird |
| Ratites-Carinates | Tinamous (Tinamiformes), lithornithids |
| Galliforms-Anseriformsj | Screamers, Magpie Goose |
| Tyrant Flycatchers-Manakins | Cinnamon Tyrant-manakin (Neopipo cinnamomea)k |
| Tyrant Flycatchers-Cotingasl | Tytyras (Tytyra), becards (Pachyramphus) |
| Todies-Motmots | Tody motmot (Hylomanes momotula)m |
| Owls-Nightjars | Oilbird (Steatornis caripensis) |
| Owls-Hawks | Hawk Owl (Surnia ulula)n |
| Hawks-Cuckooso | Hawk-cuckoos (Hierococcyx) |
| Songbirds-Nonpasserinesp | Sunbirds, Shrikes |
| Crows-New World Blackbirdsq | Tamaulipas Crow (Corvus imperatus) |
| Crows-Starlingsr | Stresemann’s Bush-Crow (Zavattariornis stresemanni); Piapiac (Ptilostomus afer ) |
| Plovers-Sandpiperss | Diademed Sandpiper-plover (Phegornis mitchellii) |
| Vultures-Eaglest | Palm-nut Vulture (Gypohierax angolensis) |
| Bats-primates | Flying Lemurs (Dermoptera) |
| Molluscs-Annelids-Chordates | Chaetognaths, caecilians, Pikaia |
| Gastropods-Bivalves | Bivalved gastropods (Juliidae) |
| Animals-Protoctists | Zoomastigota, Acrasiomycota, mesozoans (Dicyema, Dicyemmerea, Conocyema) |
| Plants-Animals | Euglenoids, cryptomonads, chlorophytes (e.g., Chlamydomonas), zoomastigotes |
| Plants-Fungi | Water molds (Oomycota) |
| Fungi-Protoctistsu | Acrasiomycota, Chitridiomycota, Labyrinthulata, Myxomycota, Plasmodiophora, Hyphochytriomycota |
| Bacteria-Eukaryotesv | Myxobacteria, cyanobacteria, pelobiontids, dinoflagellates, rhodophytes,archaebacteria, microsporans |
| Ferns/mosses-Seedplants | Cycads, seed ferns, ginkoaleans, progymnosperms |
| Gymnosperms-Angiosperms | Gnetophytes |
| Plants-Protoctists | Chlorophyta, Acrasiomycota |
Notes:
a. Romer (1966: 102) states that "Primitive Paleozoic reptiles and some of the earliest amphibians were so similar in their skeletons that (as was the case with Seymouria and Diadectes) it is almost impossible to tell when we have crossed the boundary between the two classes."
b. Calder (1978: 142). Grützner et al. (2004) showed that the platypus (Ornithorhynchus anatinus) genome shares genes with the bird Z and mammal X chromosomes. There are two obvious hypotheses that might account for this finding: (1) both bird Z and mammal X genes primitively existed in a single genome, but bird Z genes were later lost in mammals and mammal X genes were later lost in birds; or (2) these two distinct gene complexes came into being separately, one in birds and one in mammals, but were united by an exceptionally distant hybridization producing viable, fertile offspring.
c. Although classified as a cat, the jaguarundi is intermediate in appearance between mustelids and cats. Known in Central America as the "otter cat" or "weasel cat," it is like a mustelid in having short legs, a slender elongate body, a very long tail, and small and flattened head, particularly the nasal region (members.aol.com/cattrust/jagundi.htm).
d. Although the fossa is usually grouped with the mongooses in family Herpestidae, Nowak (1999: 785) notes that it has sometimes been placed in the cat family (Felidae).
e. Though treated taxonomically as a carnivore, the kinkajou is more primatelike than many primates. Judging by its arboreal habits, its rounded head, its short face, its long, fully prehensile tail, its large, forward-facing eyes, and its largely frugivorous diet, taxonomists once classified it as a primate, calling it Lemur flavus (Kays 2001, 2003; Kaysand Gittleman 1995: 300).
f. Ohno et al. (1969) remark that "the ratfish [Hydrolagus colliei] is an evolutionary oddity. It has a cartilaginous skeleton and fertilizes internally as do sharks and rays of the class Elasmobranchii, yet it wears gill covers (opercula) like bony fish (Osteichthyes), and it belongs to a class of its own, Bradyodonti."
g. If the difference between a lizard and a snake is measured in terms of the presence/absence of legs, then various intermediate taxa represent a continuum of variation between the extreme of having four legs and having no legs. For example, all modern pythons have rudimentary rear limbs, small claws at the base of the tail. Bipes has two small front limbs. Indeed, some organisms classed as lizards lack legs.
h. According to Martinez (1992: 140) the penguin genus Eudyptula “apparently links penguins and Procellariformes.” An Internet site (neaq.org/penguins/littleblue.html) states that Little Blue Penguins (Eudyptula minor) are "Far more reminiscent of their flying cousins the Procellariformes, shearwaters, petrels and albatross than the other 16 penguin species.
i. Kemp (1994) notes that the secretary bird is always placed in its own monotypic family and often in its own suborder (Sagittarii), sometimes even in its own order (Sagittariformes). It shares anatomy of skull and head (except long upper lashes) with eagles (which belong to order Falconiformes), but aspects of its breeding behavior are most similar to storks (Ciconiiformes). DNA-DNA studies indicate close affinity to storks and birds of prey."
j. Screamers (Family Anhimidae) have been described as links between Galliformes and Anseriformes (Carboneras 1992a: 528). Also Marchant and Higgins (1990: 1114) say many osteological features of the Magpie Goose (Anseranas semipalmata), of New Guinea and northern Australia, “resemble screamers rather than Anatidae.”
k. Genus Neopipo contains a single aberrant “manakin” N. cinnamomea found in Amazonia. According to Ridgely and Tudor (1994: 697), it resembles the Ruddy-tailed Flycatcher (Terenotriccus) to a remarkable degree, though it has long been considered a manakin. They propose this bird be called the Cinnamon Tyrant-Manakin and say “this species so resembles Ruddy-tailed Flycatcher that it could easily be passed over in the field.”
l. Fitzpatrick (2004: 449, 453) says various authors treat two South American genera, Tytyra (tytyras) and Pachyramphus (becards), either as tyrant flycatchers (Tyrannidae) or as cotingas (Cotingidae), or as a separate family (Tityridae). They share many characters with both tyrant flycatchers and cotingas.
m. The Tody Motmot (Hylomanes momotula) looks intermediate between todies (Todidae) and motmots (Momotidae) and differs from other birds assigned to Momotidae in its small size, facial pattern, lack of racquet tips on the tail and of serrated bill edges (Snow 2001: 279 and Plate 23).
n. Classified as an owl this bird is similar to falconiforms in its long tail, wing shape, and diurnal habits. Harrison and Greensmith (1993: 327); Perrins and Middleton (1998: 396).
o. Payne (2005: 470) says hawk-cuckoos (Hierococcyx) look much like Eurasian Sparrowhawks (Accipiter nisus), for example, the Large Hawk-Cuckoo (Hierococcyx sparverioides), is like A. nisus in wing and tail shape, broad body plumage pattern, and color, and flight pattern. Small birds respond to it as they do to sparrowhawks.
p. Classified as passerines, shrikes have the weak feet of a songbird, but they are birds of prey with the keen eyesight and sharp beak of a hawk, which are nonpasserines (Harris 2000; Lefranc 1997; Perrins and Middleton 1998). The Old World sunbirds are similar in appearance to New World hummingbirds. Like hummingbirds they have iridescent plumage and hovering flight on rapidly vibrating wings. Both are small to very small birds. Both probe flower tubes for nectar with long beaks. Both supplement their diets with insects and use spiders' webs in constructing their nests. Sunbirds, however, are classified as passerines and hummingbirds as nonpasserines. Cheke and Mann (2001); Perrins and Middleton (1998); Williamson (2001).
q. In his section on the Tamaulipas Crow (Corvus imparatus), Goodwin (1986: 66) notes that this "species, with its relatively small bill, small size, slender appearance and the rich gloss on its silky plumage, shows a remarkable convergence towards some of the American blackbirds in the family Icteridae.”
r. Of the Piapiac (Ptilostomus afer), Madge and Burn (1994: 135) say “this long-tailed black African corvid recalls both magpies and Corvus crows in outward appearance … It might well be that like Zavattariornis of Ethiopia it is perhaps a surviving relic of a long extinct group of crows that have no close link to the present-day corvids; indeed both of these African aberrant crows share a remarkable superficial resemblance to quite different groups of starlings." Elsewhere, they say (ibid: 123) Stresemann’s Bush-Crow (Zavattariornis stresemanni)recalls "a starling rather than a crow."
s. Regarding Phegornis mitchellii (Diademed Sandpiper-plover), Meyer de Schauensee (1966) noted that “whether this bird is a sandpiper or a plover is still uncertain.”
t. Thiollay (1994) says the Palmnut Vulture (Gypohierax angolensis) is intermediate between fish-eagles and vultures. Most similar to Neophron percnopterus (Egyptian Vulture), it also recalls juvenile Haliaeetus vocifer (African Fish Eagle), which is sympatric with N. percnopterus in North Africa (G. angolensis occupies an intermediate range).
u. Campbell (1987: 550) states that "Even with five kingdoms instead of only two, the slime molds are a taxonomic enigma. They resemble fungi in appearance and life style, but the similarities are beileved to be the result of convergence. In their cellular organization, reproduction, and life cycles, slime molds depart from the true fungi and probably have their closest relatives among the protoctists."
v. Doolittle (1999, 2000); Margulis and Schwartz (1982: 42, 62, 72, 74; 1998: 117-118); Martin (1999); Minelli (1993: 131, 134); Reichenbach (1984); Xiong et al. (1998). Because of their peculiar nuclear organization, dinoflagellates have been called mesokaryotic, that is, "between prokaryotic and eukaryotic" (see Margulis and Schwartz 1982: 74). According to Minelli (1993: 136), rhodophytes (Aconta) are recognized as eukaryotes “lacking both flagella and centrioles and possessing plastidia similar to the cyanobacteria, and pigments intermediate to those of the cyanobacteria and the green plants.”
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