Category Archives: Systematics

How do new species form?

by Piter Kehoma Boll

A long, long time ago, I wrote two posts here about the definition of species, explaining briefly the most important horizontal and vertical species concepts. So we all agree that species exists, but how they emerge? How one species become two, or how one species become another?

The phenomenon by which it occurs is called speciation. Well, sort of… It all depends on how you define a species, actually (so be certain to have read the posts I mentioned above).

355px-simplified_sketch_of_a_speciation_event_-_journal-pone-0042970-g007

Model of a lineage splitting into two lineages that evolve independently and eventually become separated species. Extracted from Hawlitschek et al. (2012)*

Speciation is usually defined as the evolution of reproductive isolation, therefore it deals more with the concept of biological species, but also with the ecological concept and certainly needs some insights on the vertical concepts. If two populations are reproductively isolated, it means that the individuals of one of them are unable or unwilling to breed with those of the other. This usually arrives through genetic and ecological differences that lead to differences in behavior, morphology, physiology. And considering that, we can classify reproductive isolation into two groups: pre-zygotic and post-zygotic isolation.

In pre-zygotic isolation, the two species are reproductively isolated because they do not want or cannot mate and produce an zygote. This may happen simply because of different behaviors in which the two species occupy different places in the environment, mate at different times of the year or even because they are not sexually attracted to each other. There are several experiments using fruitflies that demonstrate how this may evolve pretty fast.

In the late 1980s, William R. Rice and George W. Salt separated individuals of Drosophila melanogaster depending on their preference for dark × light and wet × dry environments, allowing them to mate only with other specimens showing the same preferences. After several generations, the individuals of one group were unable to mate with those of other groups because of their strong habitat preferences, making them unlikely to interact. A similar experiment was performed by Diane Dodd using the species Drosophila pseudoobscura, in which one population was raised with starch as food and other with maltose as food. In this case, after several generations the flies showed a strong preference to mate with individuals of the same group and to reject those of the other group.

640px-drosophila_speciation_experiment-svg

Evolution of reproductive isolation in fruit flies of the species Drosophila pseudoobscura after several generations fed with different sugars.

Such speciation events are called ecological speciation and are also well-documented in the widl, especially regarding fish preferring different habitats, such as shallow × deep water or still × running water. Eventually the individuals will diverge into two groups that are ecologically isolated in the same environment and consequently become reproductively isolated as well.

Post-zygotic isolation is generally a more advanced form of isolation that indicates deep genetic divergences. This is more commonly associated with the notion of biological species and is based on the inability of the individuals of the two species to produce viable offspring. They may mate with each other and even produce a zygote, but this will be unable to developed into an embryo or the offspring will be sterile or otherwise unable to survive enough to breed. A classical example is the mule, the hybrid of a mare and a donkey that is usully sterile.

Equus

A mare, Equus ferus caballus (left), a donkey, Equus africanus asinus (right) and a mule (center). Photos by ‘Little Miss Muffit’ (flickr.com/people/42562654@N00)(mare), Adrian Pingstone (donkey) and Dario Urruty (mule).

In both forms of speciation mentioned above, reproductive isolation usually arises from the accumulation of small differences due to natural selection. This may be enhanced by two phenomena, pleiotropy and genetic hitchhiking.

Pleiotropy is the phenomen by which a single gene have influence over more than one phenotypic trait. For example, a gene that influences the shape of a bird’s bill may also make it change its diet and its song. Several human genetic diseases, such as phenylketonuria (PKU), are examples of pleiotropy.

579px-leghorn_frizzle_chicken

The frizzled trait in chickens, which makes the feather curl outward, also leads to delayed sexual maturity and decreased metabolism rate. Photo by flickr user Just chaos.*

Genetic hitchhiking, on the other hand, is the phenomenon by which a gene that is naturally selected carries neighbours genes that are in the same DNA chain with it. In fruitflies, for example, a gene that is linked to courtship behavior may be drawn with the gene linked to a digestive enzyme, so that flies that specialize in one kind of sugar have a different courtship behavior than others specialized in another sugar.

That’s all for now. In a future post, I’ll talk about the geographic and genetic variables in species formation.

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References and further reading:

Bolnick, D. I., Snowberg, L. K., Patenia, C., Stutz, W. E., Ingram, T. & Lau, O. L. 2009. Phenotype-dependent native habitat preference
facilitates divergence between parapatric lake and stream stickleback. Evolution, 63(8): 2004-2016.

Hendry, A. P.2009. Ecological speciation! Or the lack thereof? Canadian Journal of Fisheries and Aquatic Sciences, 66: 1383-1398.

Hoskin, C. J. & Higgie, M. 2010. Speciation via species interactions: the divergence of mating traits within species. Ecology Letters, 13: 409-420.

Maan, M. E., Hofker, K. D., van Alphen, J. J. M. & Seehausen, O. 2006. Sensory drive in cichlid speciation. The American Naturalist, 167(6):
947-954.

Nosil, P. 2008. A century of evolution: Ernst Mayr (1904-2005). Ernst Mayr and the integration of geographic and ecological factors in
speciation. Biological Journal of the Linnean Society, 95: 26-46.

Turelli, M., Barton, N. H. & Coyne, J. A. 2001. Theory and speciation. TRENDS in Ecology and Evolution, 16(7): 330-343.

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Filed under Evolution, Systematics

Shaking dinosaur hips and messing with their heads

by Piter Kehoma Boll

This week brought astonishing news regarding the phylogeny of dinosaurus, as you perhaps have heard or read. New anatomical evidences have completely rebuilt the basis of the dinosaur family tree and I’m here to explain a little bit of what happened.

As we all know, Dinosaurs include a great variety of beasts, from the meat-eating theropods to the long-necked sauropods and from the horned ceratopsians to the armored ankylosaurs, among many others.

largestdinosaursbysuborder_scale

Silhouette of a human compared to the largest known dinosaurs of each major group. Picture by Matt Martyniuk.*

For more than a century now, dinosaurus have been divided into two groups, called Ornithischia and Saurischia. Ornithischia (“bird-hipped”) includes dinosaurus whose pelvic bones are more similar to what is found in birds, with a pubis directed backward. Saurischia (“lizard-hipped”), on the other hand, have a pubis directed forward, as in reptiles in general. This grouped the theropods and the sauropods in the same group as Saurischia while other dinosaurus were grouped as Ornithischia. But birds are actually theropods, thus being lizard-hipped dinosaurus and not bird-hipped dinosaurus! Confusing, isn’t it? So let’s take a look at their hips:

Pelvic_bones

Comparison of the hips of a crocodile (Crocodylus), a sauropod (Diplodocus), a non-avian theropod (Tyrannosaurus), a bird (Apteryx), a thyreophoran (Stegosaurus), and an ornithopod (Iguanodon). Red = pubis; Blue = ischium; Yellow = ilium. Picture by myself, Piter K. Boll.**

As you can see, the primitive state, found in crocodiles, sauropods and early theropods, is a pubis pointing forward. A backward-pointing pubis evolved at least twice independently, both in more advanced theropods (such as birds) and the ornithischian dinosaurus. But could we be so certain that Tyrannosaurus and Diplodocus are more closely related to each other (forming a clade Saurischia) just because of their hips? Afterall, this is a primitive hip, so it is very unlikely to be a synapomorphy (a shared derived character). Nevertheless, it continued to be used as a character uniting sauropods and theropods.

A new paper published by Nature this week, however, showed new evidences that point to a different relationship of the groups. After a detailed analysis of the bone anatomy, Matthew G. Baron, David B. Norman and Paul M. Barrett have found 20 characters that unite theropods with ornithischians and not with sauropods. Among those we can mention the presence of a foramen (a hole) at the anterior region of the premaxillary bone that is inside the narial fossa (the depression of the bone that surrounds the nostril’s opening) and a sharp longitudinal ridge along the maxilla.

skulls

The skulls of both ornithischians and theropods (above) show an anterior premaxillary foramen in the narial fossa (shown in yellow) and and a sharp ridge on the maxilla (shown in green), as well as other characters that are not present in sauropodomorphs and herrerasaurids (below). Composition using original pictures by Carol Abraczinskas and Paul C. Sereno (Heterodontosaurus), Wikimedia user Ghedoghedo (Eoraptor and Herrerasaurus), and flickr user philosophygeek (Plateosaurus).**

In his blog Tetrapod Zoology, Dr. Darren Naish comments the new classification and points out some problems that arise with this new view. One of them is the fact that both theropods and sauropodomorphs have pneumatic (hollow) bones, while ornithischians do not. If the new phylogeny is closer to the truth, that means that pneumacity evolved twice independently or evolved once and was lost in ornithischians.

He also mentions that both ornithischians and theropods had hair-like or quill-like structures on their skin. In theropods this eventually led to feathers. Could this be another synapomorphy uniting these groups? Maybe… but when we think that pterosaurs also had “hairs”, one could also conclude that a “hairy” integumentary structure was already presented in the common ancestor of dinosaurus. In this case, perhaps, we only had not found it yet on sauropods. Now imagine a giant Argentinosaurus covered with feathers!

One concern that appeared with this new organization is whether sauropodomorphs would still be considered dinosaurs. The term “dinosaur” was coined by Richard Owen in 1842 to refer to the remains of the three genera known at the time, Iguanodon, Hylaeosaurus and Megalosaurus, the first two being ornithischians and the latter a theropod. As a consequence, the original definition of dinosaur did not include sauropods. Similarly, the modern phylogenetic definition of dinosaur was “the least inclusive clade containing Passer domesticus (the house sparrow) and Triceratops horridus“. In order to allow Brachiosaurus and his friends to continue sitting  with the dinosaurs, Baron et al. suggested to expand the definition to include Diplodocus carnegii. So, dinosaurus would be the least inclusive clade containing P. domesticusT. horridus and D. carnegii.

In this new family tree, the name Saurischia would still be used, but to refer only to the sauropodomorphs and some primitive carnivores, the herrerasaurids. The new clade formed by uniting theropods and ornithischians was proposed to be called Ornithoscelida (“bird-legged”), a name coined in 1870 to refer to the bird-like hindlimbs of both theropods and ornithopods (the subgroup of ornithischians that includes dinosaurs such as Iguanodon and the duck-billed dinosaurs).

What can we conclude with all that? Nothing will change if you are just a dinosaur enthusiast and do not care about what’s an ornithischian and a saurischian. Now if you are a phylogeny fan, as I am, you are used to sudden changes in the branches. Most fossils of basal dinosaurs are incomplete, thus increasing the problem to know how they are related to each other. Perhaps this new view will last, perhaps new evidence will change all over again the next week.

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ResearchBlogging.orgReferences and further reading:

Baron, M., Norman, D., & Barrett, P. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution Nature, 543 (7646), 501-506 DOI: 10.1038/nature21700

Naish, D. (2017). Ornithoscelida Rises: A New Family Tree for DinosaursTetrapod Zoology.

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Filed under Evolution, Extinction, Paleontology, Systematics

New Species: March 11 to 20, 2017

by Piter Kehoma Boll

Here is a list of species described from March 11 to March 20. It certainly does not include all described species. Most information comes from the journals Mycokeys, Phytokeys, Zookeys, Phytotaxa, Zootaxa, International Journal of Systematic and Evolutionary Microbiology, and Systematic and Applied Microbiology, as well as journals restricted to certain taxa.

Cherax_warsamsonicus

Cherax warsamsonicus is a new crayfish from Indonesia.

SARs

Plants

Fungi

Flatworms

Annelids

Rotifers

Tardigrades

Arachnids

Myriapods

Crustaceans

Insects

Cartilaginous fishes

Ray-finned fishes

Lissamphibians

Reptiles

Mammals

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New Species: March 1 to 10

by Piter Kehoma Boll

Here is a list of species described from March 1 to March 10. It certainly does not include all described species. Most information comes from the journals Mycokeys, Phytokeys, Zookeys, Phytotaxa, Zootaxa, International Journal of Systematic and Evolutionary Microbiology, and Systematic and Applied Microbiology, as well as journals restricted to certain taxa.

Pristimantis_attenboroughi

Pristimantis attenboroughi is a new frog species described in the past 10 days and named in honor of Sir David Attenborough.

SARs

Plants

Fungi

Sponges

Entoprocts

Annelids

Kinorhynchs

Nematomorphs

Nematodes

Arachnids

Myriapods

Crustaceans

Hexapods

Ray-finned fishes

Lissamphibians

Reptiles

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Filed under Systematics, taxonomy

New Species: February 21 to 28

by Piter Kehoma Boll

Here is a list of species described from February 21 to February 28. It certainly does not include all described species. Most information comes from the journals Mycokeys, Phytokeys, Zookeys, Phytotaxa, Zootaxa, International Journal of Systematic and Evolutionary Microbiology, and Systematic and Applied Microbiology, as well as journals restricted to certain taxa.

nyctibatrachus_manalari

Nyctibatrachus manalari is a new tiny frog described in the past 8 daysdescribed in the past 8 days.

Bacteria

Plants

Fungi

Sponges

Annelids

Nematods

Arachnids

Crustaceans

Hexapods

Cartilaginous fishes

Ray-finned fishes

Lissamphibians

Mammals

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New Species: February 11 to 20, 2017

by Piter Kehoma Boll

Here is a list of species described from February 11 to February 20. It certainly does not include all described species. Most information comes from the journals Mycokeys, Phytokeys, Zookeys, Phytotaxa, Zootaxa, International Journal of Systematic and Evolutionary Microbiology, and Systematic and Applied Microbiology, as well as journals restricted to certain taxa.

pseudomacrochenus_wusuae

Pseudomacrochenus wusuae is a new longhorn beetle described in the past 10 days.

SARs

Plants

Fungi

Nematodes

Arachnids

Myriapods

Crustaceans

Hexapods

Tunicates

Ray-finned  fishes

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New Species: February 1 to 10, 2017

by Piter Kehoma Boll

Here is a list of species described from February 1 to February 10. It certainly does not include all described species. Most information comes from the journals Mycokeys, Phytokeys, Zookeys, Phytotaxa, Zootaxa, International Journal of Systematic and Evolutionary Microbiology, and Systematic and Applied Microbiology, as well as journals restricted to certain taxa.

heliconia_berguidoi

Heliconia berguidoi is a new plant species from Panama. Photos by R. Flores and C. Black, seen in the lower picture beside one specimen. (License CC BY 4.0)

Archaeans

Bacteria

SARs

Plants

Fungi

Cnidarians

Flatworms

Annelids

Nematodes

Arachnids

Myriapoda

Crustaceans

Insects

Ray-finned fishes

Reptiles

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Filed under Systematics, taxonomy