Big Things Come in Small Packages: a Macroevolutionary Adventure
*Hemidactylus*
so-called microevolutionary challenges that are supposed to make
evolutionists shake in their boots, because such challenges are
perceived by the person issuing the challenge as a means of shattering
the foundations of evolutionary biology and ruining the lives of its
adherents. This is sheer hyperbole and polemic, for sure, on the part of
the creationist, be they a Paleyian immutabilist or whatnot. But let’s
shift gears and instead concentrate on another bugbear of the
creationist, that of macroevolution. This is supposed to be far harder
for proponents of evolutionary theory to come up with examples and
demonstrate them using the tools of evolutionary research. There is
definitely going to be more difficulty inherent when leaving the
standard approach of looking at small scale changes, such as wolf
ancestors giving rise to the breeds of domesticated dog or research on
populations that show mundane shifts in allelic frequency from time
period to time period, the textbook definition of evolution, based upon
the genocentric or bean-bag approach (Mayr, 1982, p. 67. 558)
What if we could evaluate an apparent prima facie case of macroevolution
instead? We might have the typical limitations of a not all too
cooperative fossil record to supply us with a perfect stepwise
transitional sequence and the inability to point at a particular gene or
set of genes and say that part of the DNA is responsible, though this
might be possible in principle.
The not too old field of evolutionary developmental biology might have
provided us with a very interesting case where we have some natural
history of a group that shares a derived morphological feature and we
have some developmental research that shows how this character state
came about ontogenetically. It’s not a complete picture, especially
since I have limited access to sources and have no expertise in the
fields I am attempting to address in a very amateurish and most likely
error-prone manner. hopefully those with deeper knowledge can elaborate
further on what I present here.
What I will look at in this essay is a group of rodents known as pocket
gophers. They are nested within a larger grouping known as the Geomyoids.
For an overview see this wikipedia article:
http://en.wikipedia.org/wiki/Geomyoidea
In this wikipedia article they are considered a rodentian superfamily.
A more formal representation can be found at the Paleobiology Database:
http://paleodb.org/cgi-bin/bridge.pl?a=basicTaxonInfo&taxon_no=41370
where they are nested within Rodentia. Drilling further into this
database we have the superfamily Geomyoidea:
http://paleodb.org/cgi-bin/bridge.pl?a=basicTaxonInfo&taxon_no=41611
So Geomyoidea is given as a superfamily and contains subtaxa Geomyidae
(pocket gophers) and Heteromyidae (pocket mice and kangaroo rats)
http://paleodb.org/cgi-bin/bridge.pl?a=basicTaxonInfo&taxon_no=41651
http://paleodb.org/cgi-bin/bridge.pl?a=basicTaxonInfo&taxon_no=41673
There’s some variation on how these groupings can be classified. On the
following site from University of Michigan Museum of Zoology beavers,
pocket gophers and kangaroo rats are placed within suborder Castorimorpha:
http://animaldiversity.ummz.umich.edu/site/accounts/information/Castorimorpha.html
But a reference to Carroll 1988 in the Paleobiology Database puts
Geomyoidea in Myomorpha:
http://paleodb.org/cgi-bin/bridge.pl?a=displayTaxonomicNamesAndOpinions&reference_no=4783&display=opinions
Same opinion in Nowak 1999
http://paleodb.org/cgi-bin/bridge.pl?a=displayTaxonomicNamesAndOpinions&reference_no=14219&display=opinions
Myomorpha typically includes mice, hamsters, gerbils, etc:
http://en.wikipedia.org/wiki/Myomorpha
Geomyoidea is a sister taxon of Castorimorpha on the Paleobiology
Database. Geomyoidea is nested within Rodentia instead:
http://paleodb.org/cgi-bin/bridge.pl?a=basicTaxonInfo&taxon_no=41370
So if not just nested in Rodentia but deeper, these geomyoids could be
related more closely to castorimorphs like beavers or myomorphs like mice.
A classic, but maybe outdated, description of pocket gophers by Russell
(1968) goes into details on osteology and the then available fossil
record. Since the developmental study by Hall and Brylski (1988) focuses
on development of *Thomomys bottae* along with some kangaroo rat
species, I will primarily focus on Russell's treatment of *Thomomys*.
Looking at the dental characteristics of Geomyinae, Russell makes a
generalization about molars examined from Miocene through Pliocene
through Pleistocene through Recent. There’s an evolutionary trend “from
a bicolumnar to a monocolumnar pattern” (p 479). Basitemporal fossa are
described as unique to “advanced Geomyinae”(p. 481). This appears
alongside grooved incisors and are assumed to be adaptations allowing
for eating “coarse food”(p. 481). This appears only in the tribe
Geomyini. This fossa serves as a place for attaching temporal muscles
that aid in chewing food.
Geomyinae appear in the Miocene (p. 484).The fossil record improves in
the Pliocene. This history occurs in North America. Russell says of his
presentation (p. 485): “In the present account, *Dikkomys* is regarded
as the ancestor from which the Pliocene and modern geomyines were
derived.” Thus Russell sees this *Dikkomys* as the ur-geomyine, the
archetype. He contrasts the geomyines with gophers belonging to
Entoptychinae who are said to be doing better than geomyines in the
Miocene. Russell speculates on a divergence between these groups in the
Oligocene (p. 486).
My usage of the term “archetype” follows Darwin’s definition of
“archetypal” in the _Origin_. Darwin’s Glossary definition is: “Of or
belonging to the Archetype, or ideal primitive form upon which all
beings of a group seem to be organized.”
In the late Pliocene, groups such as *Geomys* and *Thomomys* occur in
the record (Russell, p 488). A member of *Thomomys* is used by Hall and
Brylski (1988) (along with kangaroo rats), to show the evolutionary
development biology of the macroevolutionary shift from an internal to
external pouch. Russell gives more details on *Thomomys* fossil record.
In the Pleistocene Russell says (page 490) “*Geomys* and *Thomomys* are
especially common” . This was published in 1968, so the evidence may
have changed, but at least we see that a group of organisms not
previously found in the fossil record has appeared and has become more
abundant. Was it specially created at this time or did it evolve within
the rodent group?
Figure 2 on page 491 shows the possible early Pleistocene range of
*Thomomys* in North America versus other groups. It extended over a good
portion of the Western United States. *Geomys* was more a Midwestern
rodent, though its range dips down into Texas back then. On page 492-5,
the Pleistocene record of *Thomomys* is elaborated. Work is based on
dental fragments. Some material found is “indistinguishable from
*Thomomys bottae*”(page 492). This species is used by Hall and Brylski
(1988) in developmental studies cited below.
On page 505 Russell talks of William Bartram’s brief 1791 allusion to
the pocket gopher. I can elaborate on this from Bartram’s book itself.
In southeast Georgia, William Bartram was describing the gopher tortoise
(Bartram, p. 13): “The dens, or caverns, dug in the sand hills, by the
great land-tortoise called here Gopher, present a very singular
appearance; these vast caves are the castles and diurnal retreats, from
whence they issue forth in the night, in search of prey. The little
mounds, or hillocks of fresh earth, thrown up in great numbers in the
night, have also a curious appearance.” In Harper’s commentary (Bartram,
p 338) this description of the “hillocks” by Bartram which he assumed
were from the tortoise, were actually that of a pocket
gopher(*Geomys*). Harper credits Bartram with what might be the first
mention of this genus in the United States.
In Table 1 (p. 508) Russell sets *Thomomys* apart in a tribe called
Thomomyini. On page 510 he says: “*Thomomys* is the least specialized of
the modern Geomyinae, and, consequently, shows the most resemblance to
the ancestral tribe.” But is has enough specialization to pull it away
from tribe Dikkomyini. The Thomomyini has experienced less adaptive
radiation than the Geomyini according to Russell.. On page 515 the
tribes are more formally distinguished according to Russell. One
interesting key characteristic is the basitemporal fossa, which occurs
in the Geomyini but not in the Thomomyini. On pages 518-520 he gets more
morphologically descriptive of tribe Thomomyni.
On page 536 Russell starts detailing Geomyidae phylogeny. He contrasts
the terms stufenreihe and ahnenreihe, which Gould (2002, page 1016)
defines as “collateral relatives” and “direct ancestors” respectively.
Thus Russell is saying that the material worked with might not exactly
portray the actual ancestors. He says (page 536): “From the established
record, several clearly defined lineages can be distinguished; in fact
the sequence of origin, pattern of evolution, and specializations, of
the principal lineages are reasonably well expressed.” On page 537 it is
said there’s no known common ancestor of pocket gophers and kangaroo
rats. Within the Geomyinae, he says (p. 543): “*Dikkomys*, the earliest
known genus of the tribe Dikkomyini, can be taken as a starting point of
evolution for the subfamily Geomyinae.” It is thus archetypal and
perhaps has the body plan of the group in rudiment. As for the geomyids,
Figure 3 on page 544, draws dashed lines lacking a fossil record for a
common ancestor.
Russell was clearly focused upon the groups of pocket gophers within the
geomyids and we leave off on his account, which continues on a course
not very relevant to the discussion here. While we focused mainly on
Russell's details relevant to the natural history of *Thomomys*, we must
now broaden our horizons and think of the developmental biology of a
larger grouping, the geomyoids. Years ago, I had gotten interested in a
field called evolutionary developmental biology (“evo-devo”). One of the
authors I had read back then was Brain Hall. His second edition of
_Evolutionary Developmental Biology_ has a section on developmental
patterns of cheek pouch development that have undergone a radical
phenotypic change in the geomyoids (p. 384-5). This includes work found
in Hall and Brylski (1988). Hall (1999) cites this research which used a
pocket gopher *Thomomys bottae* and several species of kangaroo rat.
An important morpological feature of the geomyoids is the external
pouches of the cheek. The cheek is more technically referred to as the
buccal region of the oral cavity. These pouches are are fur-lined and
separate from the mouth itself. External pouches could be an adaptation
to limit loss of water from the animal into the stored food material. In
various groups of rodents food is stored in cheek pouches, whether
internal or external. The potential for dehydration would occur when the
animal places the transported food somewhere (caching) for later
retrieval. Hall and Brylski (1988) contrast “fur lined cheek pouches
that open outside the mouth” of geomyoids and “internal cheek
pouches...[that] open into the oral cavity and are lined with buccal
epithelium” of other rodents. A geomyoid common ancestor had an internal
pouch condition, like some other rodents (squirrels and mice). Other
rodents have neither condition. In rodents the lack of any pouch is the
ancestral state. Both pouch states are derived. The pouch rudiment comes
from the buccal epithelium in the derived state. This pouch rudiment
does not develop in the ancestral state. It is inferred that the
internal pouch state is ancestral to the external pouch state.
Hall (1999) discusses this research in a section about heterotopy, which
is the spatial displacement of the way some feature develops (p.383).
Ernst Haeckel used this term to explain away a assumed difficulty with
the biogenetic law. Gould (1977) defines it so (p. 482): “Haeckel’s term
for a phyletic change in the location (in the germinal layers) from
which an organ differentiates in ontogeny- thus forming an exception to
recapitulation...” As Gould’s definition goes on to say and Hall
elaborates on it was mainly used in context of germ layers. It explained
away difficulties encountered with adherence to a strict germ layer
theory. Hall uses it in as a way of looking at ways tissues might
interact in development, such as epithelial-mesenchymal interaction.
As the apparent macroevolutionary shift was from internal state to
external state (Hall and Brylski, 1988). There’s no conceivable
intermediate stage between the primitive internal pouch condition and
derived external pouch condition.This shift is morphologically discrete
and of macroevolutionary significance. It seems almost saltational in
nature. One would wonder if the first offspring to have this derived
trait were indeed hopeful monsters, sensu Goldschmidt. The ability to
conserve water might indeed make survival and reproductive prospects
more likely for these offspring versus the individual in the population
lacking this trait. But is it too much a leap for evolution to explain?
Could an internal pouch beget an external pouch, via some simple
developmental process shift?
Hall (1999) covers this process in summary. Buccal epithelium evaginates
in the process of internal and external cheek pouch development. So this
evagination is the common thread. The evagination shifts anteriorally in
the case of the development of an external pouch.This is a heterotopic
shift. Thus Haeckel’s terminology sees a new usage to explain the
macroevolutionary process of discrete pouch divergence in morphological
outcome. It’s either A or B, not in between. This shift brings the pouch
primordium closer spatially to the mesenchyme with which it will
interact. The mesenchyme is the ticket to hair follicles allowing this
newly external pouch to be fur lined and able to store food without
direct contact with the mouth and saliva. The heterotopic shift combined
with interaction of epithelium and mesenchyme generates morphological
novelty. Hall (page 385) says: “Heterotopy initiates, differential
growth facilitates, and the developmental plasticity of buccal
epithelium permits the development of a novel organ, the external cheek
pouch.”
Hall and Brylski (1988) list three stages of development the of external
cheek:
1. Buccal evagination-There‘s an “evagination of oral epithelium into
the facial mesenchyme”. Epithelium is able now to interact with mesenchyme.
2. Pouch externalization and growth- “Evaginated epithelium” is the
“presumptive pouch”and is externalized around the same time the snout
appears. Snout and pouch development are related. Hall and Brylski say:
“the developing external pouch “escapes” the mouth cavity without
requiring any novel developmental events.”
As Jung (1905/1957) showed in his cryptomnesia essay, novelty is just
rearrangement of old parts. Nothing is truly new under the sun. Jung was
looking at this apparent novelty as actual recombination of elements in
the way unconscious memory works and how fragments left dormant to us
can emerge together in ways we think of as new. Material from disparate
sources can come together creatively. This is, in a sense, how evolution
works also. Old material gets put together in new ways. Things that
exist in the depths of a genetic “memory” (the phylogenetic mneme of the
old organic memory theorists like Semon that influenced Jung) that
ontogenetic processes derive from can be shifted in a manner that
generates apparent novelty. A difference in spatial location can
facilitate the same materials (epithelia and mesenchyme) producing a
different result.
3. Differentiation of hair follicles- Pouch epithelium has been induced
to form hair follicles and eventually fur, but this will not occur until
sometime after birth.
Hall and Brylski (1988) state: “We hypothesize that the external pouch
is a threshold character, a discontinuous character produced by
continuous variation in its developmental control parameters...” They go
on to say that a change in evagination that in itself is relatively
minor can have dramatic effects on developmental outcome. And once the
pouch is externalized and lined with fur, it can subsequently be subject
to adaptive shifts in size to increase the storage capacity for food
carried.
Hall and Brylski (1988) don’t speculate on the external pouch’s rooting
in the evolutionary emergence of these related rodent groups. How it
fits exactly with common ancestral predecessors is not given. They don’t
say the shift in character state came at the same exact time as the
taxon. They do rule out a need for an intermediate stage in character
state. No ancestors needed to have both internal and external pouches
for instance. The shift was a quantum leap, so to speak yet as Hall and
Brylski (1988) more cautiously state earlier in the article that this is
“a small developmental change with major phenotypic effect”.
So we have seen a little of the phylogeny and of the ontogeny of the
pocket gopher. This was an unpolished, amateur, and cursory attempt to
cover a very fascinating topic that shows the importance of development
in evolutionary events that superficially seem to be of profound
macroevolutionary significance. It would be nice to have detailed fossil
evidence of the ancestors of the geomyoids and it would be nice to have
genetic analysis of the factors contributing to the heterotopic shift
that is responsible for the character state of external pouches.
References:
Bartram W. 1998. The Travels of William Bartram (Francis Harper’s
Naturalist Edition). University of Georgia Press. Athens, Georgia
Darwin C. 1963 edition. On The Origin of Species. The Heritage Press.
New York
Gould SJ. 1977. Ontogeny and Phylogeny. The Belknap Press of Harvard
University Press. Cambridge, Massachusetts.
Gould SJ. 2002. The Structure of Evolutionary Theory. The Belknap Press
of Harvard University Press. Cambridge. Massachusetts
Hall BK. 1999. Evolutionary Developmental Biology (2nd ed). Kluwer
Academic Publishers
Hall BK and Brylski P. 1988. Ontogeny of a Macroevolutionary Phenotype:
the External Cheek Pouches of Geomyoid Rodents. Evolution. 42(2): 391-5
Jung CG. 1905/1957. Cryptomnesia. found in Psychiatric Studies (CW 1).
Pantheon Books
Mayr E. 1982, The Growth of Biological Thought. The Belknap Press of
Harvard University Press. Cambridge, Massachusetts
Russell RJ. 1968. Evolution and classification of the pocket gophers
of the subfamily Geomyinae. University of Kansas publications, Museum of
Natural History 16(6):473-579 http://biostor.org/reference/13665