You wish your neurons were this pretty
Greg Dunn, having a Ph.D in neuroscience, was inspired to do this artwork by seeing neurons treated with certain stains. The neurons he saw in his microscope reminded him of the Asian art he loved so much already and went on to paint neurons in the Asian sumi-e style.
Exploring the fusion of art and science through designs in gold leaf
*drools*
Punctuated Equilibrium
This post is in response to a question asked by a follower: What is punctuated equilibrium?
To begin, this isn’t a critique of the validity of punctuated equilibrium (also called punctuated equilibria). I do not support this hypothesis and a discussion of why it is incorrect can be reserved for another post. This post only aims to explain what punctuated equilibrium is and where it came from.
Most evolutionary biologists favour phyletic gradualism, the theory of evolution as a gradual process, which is supported by data found in the fossil record and by genetic studies that provide evidence for the slow transition from one species to descendent species. However, although we have evidence for the gradual evolution of many species such as that evidenced by molars of arvicolids (e.g., species of Mimomys) during the late Pliocene and Pleistocene*, there are nevertheless gaps in the fossil record for other species. The most common and most likely explanation for these gaps contends that the fossil record is inherently incomplete because of factors such as preservation and population size. Almost all individuals that die do not fossilise. Eldridge and Gould (see below) saw little evidence for speciation at the species level but saw a great deal of evidence for speciation between larger groups. This is important to note.
A minority of scientists prefers the controversial hypothesis referred to as punctuated equilibrium, which is less of a process and more of an event. Punctuated equilibrium was first proposed in the early 1970s by Niles Eldredge and Stephen Jay Gould. It is a hypothesis that asserts that there is a high rate of evolution at times of speciation but a low rate or nil rate of evolution between these speciation events (cladogenesis). It concerns the sudden appearance of closely related taxa but does not apply to higher taxa. It “refers to both a pattern of change in the fossil record and a hypothesis about evolutionary processes” (Futuyma 2009:93).
Punctuated equilibrium sees stasis in the fossil record at points wherein taxa do not display gradual change, a period that is then punctuated by rapid evolution. Accordingly, phenotypes (and genotypes) evolve in tandem with speciation. That is to say, change does not occur slowly over a long span of time; changes in a species that lead to speciation event occur concomitantly with that speciation event. This is in direct contrast to the slow accumulation of changes that phyletic gradualism proposes. To boot, the hypothesis is yet more controversial. Punctuated equilibrium also asserts that barring populations that are undergoing speciation, phenotypes cannot evolve because of a genetic constraint. The reason this is controversial will be explained in the following paragraphs.
To understand why this hypothesis was proposed we need to understand where it came from. Punctuated equilibrium was based on Ernst Mayr’s model of “peripheral isolate speciation,” also called “founder-effect speciation,” and “peripatric speciation,” which is a kind of allopatric speciation that Mayr believed was the dominant manner of speciation. It also had roots in I. Michael Lerner’s hypotheses about developmental and genetic homeostasis. According to Mayr’s model, a new taxon (the peripheral isolate), which is at the edge of its species range, will abruptly emerge in the fossil record because the taxon evolved in a small population isolated from its ancestral species. When fully formed as a new species, it migrated to wherever it is the fossils were found. Because the speciating population was small, it is statistically less likely specimens will fossilise and furthermore, specimens are not found in the geographic range of the parent population. To be clear, founder effect, according to Mayr, is the:
“establishment of a new population by a few original founders (in an extreme case, by a single fertilized female) that carry only a small fraction of the total genetic variation of the parental population” (Mayr 1942:237).
This model maintains its foundation in phyletic gradualism but suggests that the appearance of a species in the fossil record is rapid because we simply do not have the fossils from their place of origin to prove it. In Futuyma’s words, the evolution occurred slowly “off stage” (2009:95).
Under punctuated equilibrium, speciation is an event in the geological sense. Here’s the trickier part: Presumed stasis for the duration of a taxon questions adaptation and variation in that taxon (intraspecifically) except when it occurs during the speciation event of the peripheral isolate (the descendent species). As stated above, this means that new morphologies will not evolve slowly because they are genetically constrained by gene flow**. This is a significant difference because it “decoupled intraspecific evolution from macroevolution, creating two levels of a new evolutionary hierarchy” (Erwin & Anstey 2004:188).
Again, I hope this was helpful. In truth I find this topic difficult to understand and even more difficult to explain. If anyone offers construct criticism this post can be edited. I’m here to learn too!
*Molars of taxa in the genus Mimomys are useful for dating archaeological and hominid-fossil bearing sites.
**According to allopatric speciation large populations are maintained in part by their volume and gene flow. In contrast, small populations of the peripheral isolate are not subject to the effects of gene flow that homogenise the larger population thus stymying gradual evolution.
Online references:
- Berkeley. Competing Hypotheses about the Pace of Evolution
- Etrilobite.com (first image)
- PBS. Punctuated Equilibrium (second image)
- Princeton. Punctuated Equilibrium
- Ridley, M. Online resource for Evolution (2004).
- Saylo, M.C., Escoton, C.C. and Saylo, M.M. 2011. “Punctuated Equilibrium vs. Phyletic Gradualism,” International Journal of Bio-Science and Bio-Technology 3(4):27-42.
- Templeton, A.R. 1980. “The Theory of Speciation via the Founder Principle,” Genetics 94:1011-1038.
- Tesakov, A.S. and van Kolfschoten, T. 2011. “The Early Pleistocene Mimomys hordijki (Arvicolinae, Rodentia) from Europe and the origin of modern nearctic sagebrush voles (Lemmiscus),” Paleontologia Electronica, article number: 14.3.39A (third image)
Main references:
- Erwin, D.H. and Anstey, R.L. 2004. “Speciation in the fossil record” in Mark Ridley (ed.), Evolution. Oxford: 185-197
- Futuyma, D.J. 2009. Evolution. Sinauer Associates
- Mayr, E. 1942. Systematics and the Origin of Species from the Viewpoint of a Zoologist. Harvard University Press.
Embryology of the Diaphragm
neural tube closure in salamander embryos
that sounds boring, but seriously, just watch this
you won’t regret it
credit: John Wallingford
The nineteen century German biologist Ernst Haeckel is famous for his fantastically illustrated book Artforms of Nature. The copyright for this book from 1904 has now expired and thanks to Wikimedia Commons it is available for everyone to appreciate.
neuromorphogenesis:
Happy Birthday to the Father of Modern Neuroscience, Who Wanted to Be an Artist
It took Santiago Ramón y Cajal quite a while to find his true calling in life. He tried his hand at cutting hair and at fixing shoes. As a boy in the mid-1800s, he planned for a career as an artist. But his father, an anatomy professor, shook his head and decided that young Ramón y Cajal would pursue medicine instead. The would-be artist went on to found the field of modern neuroscience, earning the Nobel Prize in Physiology or Medicine along the way. Born May 1, 1852, in Spain, Ramón y Cajal would have celebrated his 151st birthday today.
Before he began to stand out as a researcher, Ramón y Cajal had been an anatomy school assistant, a museum director and a professor and director of Spain’s National Institute of Hygiene. His most important work did not begin until around 1887, when he moved to the University of Barcelona and began investigating all of the brain’s different cell types. He discovered the axonal growth cone, which control the sensory and motor functions of nerve cells, and the interstitial cell of Cajal (later named after him), a nerve cell found in the smooth lining of the intestine. Perhaps most significantly, he developed the “neuron doctrine,” which demonstrated that nerve cells were individual rather than continuous cellular structures. Researchers consider this discovery the foundation of modern neuroscience.
In 1906, the Nobel committee awarded Ramón y Cajal and an Italian colleague the prize in Physiology or Medicine ”in recognition of their work on the structure of the nervous system.”
While Ramón y Cajal may have changed neuroscience forever, he maintained his original childhood passion. Throughout his career, he never gave up his art. He sketched hundreds of medical illustrations, and some of his drawings of brain cells are still used in classrooms today.
Images: 1. From “Structure of the Mammalian Retina” c.1900 By Santiago Ramon y Cajal.
2. Drawing of Purkinje cells and granule cells from pigeon cerebellum by Santiago Ramón y Cajal, 1899
A little bit of Ramon y Cajal in my life.
“Comparison of ancestral and existing horse”
The lower figure is a model of a full-sized Eohippus, placed beneath the skull of a modern horse, to show that the skull of the modern horse is larger than the entire body of its ancestor.
Despite being one of the most commonly cited “facts” in basic paleontology (as found in many elementary school science texts), this size comparison is actually incorrect. Geologist Henry Fairfield Osborn distributed (largely correct, aside from this point) educational pamphlets citing the size of Eohippus to be comparable to a “small fox terrier” to schoolhouses, to promote the science of paleontology, around the turn of the century.
His pamphlets reached so far and wide that the “fact” still persists to this day in many textbooks. Eohippus was about twice the size of a fox terrier, which is about 2.5 times the size of a modern horse skull.
Origin and History of the Horse. Address before the New York Farmers Metropolitan Club, 1905.
Cladistic analysis of extant and fossil African papionins using craniodental data
“This study examines African papionin phylogenetic history through a comprehensive cladistic analysis of extant and fossil craniodental morphology using both quantitative and qualitative characters. To account for the well-documented influence of allometry on the papionin skull, the general allometric coding method was applied to characters determined to be significantly affected by allometry. Results of the analyses suggest that Parapapio, Pliopapio, and Papio izodi are stem African papionin taxa. Crown Plio-Pleistocene African papionin taxa include Gorgopithecus, Lophocebus cf. albigena, Procercocebus, Soromandrillus (new genus defined herein) quadratirostris, and, most likely, Dinopithecus. Furthermore,S. quadratirostris is a member of a clade also containing Mandrillus, Cercocebus, and Procercocebus; ?Theropithecus baringensis is strongly supported as a primitive member of the genus Theropithecus; Gorgopithecus is closely related to Papio and Lophocebus; and Theropithecus is possibly the most primitive crown African papionin taxon. Finally, character transformation analyses identify a series of morphological transformations during the course of papionin evolution. The origin of crown African papionins is diagnosed, at least in part, by the appearance of definitive and well-developed male maxillary ridges and maxillary fossae. Among crown African papionins, Papio, Lophocebus, and Gorgopithecus are further united by the most extensive development of the maxillary fossae. The Soromandrillus/ Mandrillus/Cercocebus/Procercocebus clade is diagnosed by upturned nuchal crests (especially in males), widely divergent temporal lines (especially in males), medially oriented maxillary ridges in males, medially oriented inferior petrous processes, and a tendency to enlarge the premolars as an adaptation for hard-object food processing. The adaptive origins of the genus Theropithecus appear associated with a diet requiring an increase in size of the temporalis, the optimal placement of occlusal forces onto the molar battery, and an increase in the life of the posterior dentition. This shift is associated with the evolution of distinctive morphological features such as the anterior union of the temporal lines, increased enamel infoldings on the premolars and molars, a reversed curve of Spee, and delayed molar eruption” (read more/not open access).
(Source: Journal of Human Evolution 64(5):399-433, 2013)
She blinded me with science
A. Fore-limb of Monkey B. Fore-limb of Whale
WHAT IS MEANT BY HOMOLOGY? ESSENTIAL SIMILARITY OF ARCHITECTURE, THOUGH THE APPEARANCES MAY BE VERY DIFFERENT
This is seen in comparing these two fore-limbs, A, of Monkey, B, of Whale. They are as different as possible, yet they show the same bones, e.g. SC, the scapula or shoulder-blade; H, the humerus or upper arm; R and U, the radius and ulna of the fore-arm; CA, the wrist; MC, the palm; and then the fingers.
from The Outline of Science by J. Arthur Thomson; section THE GEOLOGICAL MIDDLE AGES
Another note which tends to be left out is that homologies are a result of constructional contraints/phylogenetic history limiting morphological change.
