Carnival of Evolution #75: A journey

It’s the start of a new month and that means it’s time again for the Carnival of Evolution and this time it’s a special one, the 75th edition! So all aboard as we journey once more through some of the best evolution based blog posts from around the web.

Carnival of Evolution

Our first stop is the History of Evolution so step out and enjoy the view, there are many good things to see here.

Did Darwin and Wallace plagiarise their ideas on Natural selection from an obscure book on Naval timber? George Beccaloni responds with a resounding “NO”!

Why were Darwin’s ideas accepted so quickly in Victorian England? Piers J. Hale thinks one reason is that they endorsed the liberal Whig politics espoused by great thinkers like ” Darwin’s Bulldog” T. H. Huxley.

Christian Wenande tells us about a treasure trove of Darwin’s barnacle specimens recently discovered in Natural History Museum of Denmark.

You may know that Darwin was good friends with Kew botanist Joseph Hooker but did you know that Hooker was the first person Darwin told about his evolutionary ideas which he described in a letter as ” like confessing a murder”.

What would the world be like if Darwin had never existed? Michal Meyer reviews a new book, Darwin Deleted, that asks just that question.

A treasure trove of Darwin's barnacle specimens have recently been discovered in Denmark (photo credit: Natural History Museum of Denmark)
A treasure trove of Darwin’s barnacle specimens have recently been discovered in Denmark (photo credit: Natural History Museum of Denmark)

All aboard! It’s time to move on to our next stop, Evolutionary Ecology.

Greg Laden talks about a new paper published in Science which shows that dinosaurs have been shrinking for at least 50 million years giving rise to those animals we call birds.

Jonathan Weiner discusses the lifes work of legendary ecologists Peter and Rosemary Grant whose 40 years of research on Galapagos finches showed evolution in action in wild animals.

Why are there so many more species in some regions than in others? GrrrlScientist tells us about a fascinating new study which shows that as available niches fill up the rate of speciation slows down, or even stops.

New reasearch shows that two species of Brazilian ants may have evolved without geographic isolation, although not everyone agrees.

Over at The Loom Carl Zimmer talks about an ancient lineage of air-breathing fish called Bichirs which learn to walk better on land if they are raised out of water, even changing the structure of their bones to improve their walking abilities. This may provide clues as to how fish evolved to walk on land.

Travis Park talks about the evolution of penguins and why it is important to calibrate molecular clocks against the fossil record.

Did these two species evolve in sympatry? Mycocepurus goeldii (left) and Mycocepurus castrator (right) are sister species which occupy the same geographic area (image credit: Christian Rabeling, University of Rochester).
Did these two species evolve in sympatry? Mycocepurus goeldii (left) and Mycocepurus castrator (right) are sister species which occupy the same geographic area (image credit: Christian Rabeling, University of Rochester).

And that’s it for evolutionary ecology this time, now our journey must continue as we head into the wonderful world of Evolutionary Theory.

Evolutionary fitness is often misunderstood the mean “healthiest, strongest, biggest, fiercest, and/or fastest”. Fortunately Stephanie Keep is here to tell us what fitness really means.

Charles Goodnight discusses some of his own work showing that within populations similar individuals tend to cluster together to form groups and this may lead to speciation.

Ben Haller talks about the interactions between empirical and theoretical researchers and asks Should theoretical ideas drive new empirical work to look for the patterns and outcomes predicted by theoretical models?  Or should pure “natural history” observations of the real world drive new theoretical work to explain the patterns and outcomes observed?

Our journey is almost at an end but not quite! Hold tight, we have three more posts to go.

Creationist Michael Behe claims the vertebrate glucocorticoid receptor could not have evolved by darwinian means. Thankfully Larry Moran is on hand to dismantle Behe’s arguments which are neither new nor interesting.

Elena Giorgi provides us with a very topical post about her work using ideas based on the evolution of HIV to develop a vaccine for Ebola.

Bradly Alicea shows us some really neat models of breeding networks that could help us better understand how behaviour and physiology lead to structured genetic variation in populations.

And finally, microbes can evolve the ability to form antibiotic resistant biofilms. Luckily for us people like Elyse Hope are working on ways to thwart them so that we still have effective antibiotics.

                                                                                                                  

And now we come to the end of the line. We hope you had a pleasant journey and will travel with us again soon. The next Carnival will be held in October at Eco-Evolutionary Dynamics, if you would like to submit anything you can submit it on the Facebook page. You can keep up with the Carnival of Evolution via Facebook and Twitter.

Advertisements

Cooperative sperm, killer sperm and the competition for reproductive success

ResearchBlogging.org

In the closing paragraph of on the origin of species Darwin famously said that nature was a war in which individuals struggle against each other and the environment for survival. However, while survival may be important from an individuals point of view, from an evolutionary perspective mere survival is not enough. Reproduction is what matters and success or failure at producing offspring is what determines an individual’s evolutionary success. Of course, survival is important too, but only when it leads to reproduction.

In most species the reproductive success of females is limited by the rate at which they can produce offspring. When a female is pregnant or carrying eggs she has no choice but to wait until she has given birth or laid her eggs before she can reproduce again, and this can take a long time. Males have no such constraints to their reproductive success and can potentially mate with hundreds of females over their lifetime and raise an enormous number of offspring. The only thing stopping them is that there just aren’t enough females to go around. This shortage of females coupled with the need to reproduce leads to intense, and often aggressive, competition among males for limited mating opportunities.

Male red deer (Cervus elephus) compete for females by fighting
Male red deer (Cervus elephus) compete for mating opportunities by fighting

Male red deer (Cervus elephus) fight for their chance to mate by using their huge antlers to batter their rivals into submission, while male northern elephant seals (Mirounga angustirostris) grow to enormous sizes allowing them to dominate harems of many females and guard them against the advances of smaller, weaker males. Not all species are so aggressive in their tactics. Males of many bird species such as peacocks (Pavo cristatus) and birds of paradise produce fantastic and colourful displays with which they attempt to attract females, as do a large number of insects and fish. In these species, rather than fighting with each other, males try to out-perform and out-class each other in the hope that females will choose them while their rivals are left unwanted on the sidelines. This may seem a more peaceful strategy but make no mistake, although these males don’t actively fight each other the competition between them is every bit as intense as among more aggressive species.

Male peacock (Parvus cristatus) aim to attract females by out-perform their rivals displays.
Male peacock (Pavo cristatus) aim to attract females by out-performing the displays of rival males.

So fighting or displaying are two ways in which males can improve their reproductive chances, but what happens in species in which each female mates with lots of different males in quick succession? How is a male to improve his odds of being the true genetic father of the offspring? Well, as is often the case evolution has found a way and that way is called sperm competition (yes, really).

In species in which females mate promiscuously males compete not just for mating opportunities but also for direct access to eggs. In these cases competition between males happens after mating has occurred as the sperm of multiple males compete with each other within the females reproductive tract as they race towards the eggs. In species in which sperm competition is known to exist an incredible variety of different sperm adaptations have been found, all of which serve to improve the sperms chances of reaching the eggs first.

For individuals of many species adaptation to sperm competition simply means producing more sperm so as to swamp the sperm of their rivals and increase the odds that some of their sperm will make it to the eggs before anyone elses. For other species adaptation to sperm competition is more complex. For example, the wood mouse, Apodemus sylvaticus, has evolved sperm that have a hook-like structure on the head which allows them to intertwine with one another to form long sperm ‘trains’ which are much faster at swimming than individual sperm.

The sperm of the wood mouse (Apodemus sylvaticus). (a) Image of the sperm head with the hook clearly visible. (b) 50 sperm hooked together. (c) A clip from video footage of the sperm train. (d) Another view of the sperm train with an arrow and asterisk marking the position of hooks. (e) One sperm latching onto another. (f) Another view of a sperm hook.
The sperm of the wood mouse (Apodemus sylvaticus). (a) Image of the sperm head with the hook clearly visible. (b) 50 sperm hooked together. (c) A clip from video footage of the sperm train. (d) Another view of the sperm train with an arrow and asterisk marking the position of hooks. (e) One sperm latching onto another. (f) Another view of a sperm hook. Image from Moore et al. (2002).

In a similar and recently discovered case, a team led by Morgan Pearcy of the Université libre de Bruxelles looked for evidence of sperm competition in the desert ant, Cataglyphis savignyiThe queen ants of this species mate with up to 14 males in rapid succession and store their sperm jointly in a special storage organ called the spermatheca. Only those sperm which make it to this storage organ have any chance of fertilising an egg and so competition for access to the spermatheca is intense. In response to this pressure C. savignyi males have evolved highly cooperative sperm that team up into bundles of 50-100 cells which can swim much faster than they could alone and so are better able to outcompete their rivals.

Sperm from the desert ant Cataglyphis savignyi work together to increase their swimming speed. Image from Pearcy et al, (2014).
Sperm from the desert ant Cataglyphis savignyi work together to increase their swimming speed. Image from Pearcy et al, (2014).

It is not just the way sperm behave that can change due to sperm competition, the shape and function of sperm can change too. For example, Philip Byrne and his colleagues from the University of Western Australia found that in a group of Australian frogs those species under the most intense sperm competition produced sperm with the longest tails, possibly to improve their swimming speed. Other species known to have oddly shaped sperm include the water beetle Dytiscus marginalis which has sperm that fuse at the head into pairs with two tails, and the tiny fruit fly Drosophila bifurca which at 6cm long produces the longest sperm on earth.

Some species have taken a more sinister approach to sperm competition and have evolved infertile “parasperm” which contain enzymes capable of breaking down the sperm of rivals. A similar and fantastically named kamikaze sperm hypothesis has even been proposed for humans in which some sperm are adapted to kill the sperm of rivals rather than fertilise eggs. The evidence for this hypothesis is equivocal at best but given the adaptations that have been discovered in other species it is not entirely unbelievable. In fact, given the adaptations that have been discovered so far, almost nothing is completely unbelievable.


References

Sperm competition by producing large quantities of sperm
Moller, A. (1989). Ejaculate Quality, Testes Size and Sperm Production in Mammals Functional Ecology, 3 (1), 91-96 DOI: 10.2307/2389679

Sperm trains in the wood mouse
Moore H, Dvoráková K, Jenkins N, & Breed W (2002). Exceptional sperm cooperation in the wood mouse. Nature, 418 (6894), 174-7 PMID: 12110888

Cooperative sperm in the desert ant
Pearcy M, Delescaille N, Lybaert P, & Aron S (2014). Team swimming in ant spermatozoa. Biology letters, 10 (6) PMID: 24919705

Sperm competition in Australian frogs
Byrne PG, Simmons LW, & Roberts JD (2003). Sperm competition and the evolution of gamete morphology in frogs. Proceedings of the Royal Society B: Biological Sciences, 270 (1528), 2079-86 PMID: 14561298

The two tailed sperm of the water beetle
Mackie JB, & Walker MH (1974). A study of the conjugate sperm of the dytiscid water beetles Dytiscus marginalis and Colymbetes fuscus. Cell and tissue research, 148 (4), 505-19 PMID: 4836644

The world’s largest sperm in drosophila
Bjork A, Dallai R, & Pitnick S (2007). Adaptive modulation of sperm production rate in Drosophila bifurca, a species with giant sperm. Biology letters, 3 (5), 517-9 PMID: 17594959

Killer ‘parasperm’
Buckland-Nicks, J. (1998). Prosobranch parasperm: Sterile germ cells that promote paternity? Micron, 29 (4), 267-280 DOI: 10.1016/S0968-4328(97)00064-4

Kamikaze sperm
Baker, R., & Bellis, M. (1989). Elaboration of the Kamikaze Sperm Hypothesis: a reply to Harcourt Animal Behaviour, 37, 865-867 DOI: 10.1016/0003-3472(89)90074-2

Criticism of the kamikaze sperm hypothesis
Moore, H., Martin, M., & Birkhead, T. (1999). No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro. Proceedings of the Royal Society B: Biological Sciences, 266 (1436), 2343-2350 DOI: 10.1098/rspb.1999.0929

Coelacanths are not living fossils

ResearchBlogging.org

The term ‘living fossil’ is often misleadingly used in the popular press to describe species which have, supposedly, stopped evolving. Commonly cited examples include horseshoe crabs, Ginkgo trees, hagfish and, perhaps the most famous of all, the coelacanths, a group of lobe finned fish with a very long evolutionary history of which two species still survive in the deep waters of the West Indian Ocean.

A modern day coelacanth (Latimeria chalumnae)
A modern-day coelacanth (Latimeria chalumnae)

Coelacanths have long been known from the fossil record with the oldest specimen dating back to the Devonian period, some 400 million years ago. They were however thought to have gone extinct, along with many other animals, in the end Cretaceous mass extinction event. That all changed one day in 1938 when a South African museum curator named Marjorie Courtenay-Latimer discovered a coelacanth amongst the catch of a local fisherman. The discovery was a sensation, a fish that had been thought to have been extinct had been rediscovered 65 million years later, it was not extinct! It was alive! It was amazing!

That’s how the story goes at least, and ever since it’s discovery journalists have talked about the fish that has been “left behind by evolution”. But is this really true? Can a species really exist for a span of time so great that it will have seen ice ages come and go, mountain ranges form and the great super-continent of Gondwana break apart, and through all this not change at all? Over recent years a mountain of evidence has been steadily growing showing that this is in fact not the case, coelacanths, like any other species, are constantly evolving to adapt to changing conditions.

A comparison of the living coelacanth (Latimeria) with some of it's extinct relative. The morphological differences are striking
A comparison of the living coelacanths (genus Latimeria) with some of its extinct relatives. The morphological differences are striking. Image from Casane and Laurenti.

It is sometimes claimed that there is a low rate of change in coelacanth DNA and that this leads slow evolution. However, this idea is now being challenged by systematic studies of the coelacanth genome which do not detect slow rates of genetic change. In one study forty-four genes were analysed and no dramatic decrease in the rate of change compared to other species was detected. Furthermore, there is no known reason why coelacanths should have slowly evolving genomes. Their environment in the deep ocean, while relatively stable, is not particularly unusual and is inhabited by other species which are not considered living fossils. Another factor that may lead to a slow rate of evolution is a slow generation time, however, the reproductive rates of coelacanths are not thought to be particularly long. Finally, coelacanth populations are small, and small population size is known to increase the rate of genetic change within a species. We might therefore expect these species to be evolving rapidly, not standing still.

Probably the most widely held belief about coelacanths is that, even if they are genetically different, they look exactly the same now as they did millions of years ago. This belief is mistaken. No fossils are known for either species of surviving coelacanth or even for members of its genus, Latimeria. This suggests that the scientists responsible for classifying the fossil and living species consider the morphological differences so great the they should be placed in widely separated groups. In fact, there are significant differences in the body shape and structure of modern and extinct coelacanth species. These include changes in the number of vertebral arches and substantial differences in skull morphology. The swim bladder of coelacanths has also changed from being filled with oil in the extinct genus Macropoma, to being ossified in modern species, suggesting that the two groups lived in very different environments. Lastly, there are substantial differences in size, with modern coelacanths being three and a half times larger than their closest extinct relative (one and a half vs half a metre).

Comparison of the skeleton of modern and extinct coelacanths. A) Latimeria chalumnae (a modern species), B) Macropoma lewesiensis (extinct), C) L. chalumnae skull D) M. lewesiensis skull, E) Pectoral fins of L. chalumnae (above) and Shoshonia actopteryx (another extinct relative) (below). Image from Casane and Laurenti.
Comparison of the skeleton of modern and extinct coelacanths. A) Latimeria chalumnae (a modern species), B) Macropoma lewesiensis (extinct), C) L. chalumnae skull D) M. lewesiensis skull, E) Pectoral fins of L. chalumnae (above) and Shoshonia actopteryx (another extinct relative) (below). Image from Casane and Laurenti.

The view that coelacanths are ancient prehistoric fish which have stopped evolving has been around for a very long time. However, the evidence is now in and it shows that it is time to put this mistaken idea to bed.

                                                                         

For a comprehensive review of the evidence showing that coelacanths are not living fossils see: –

Casane D, & Laurenti P (2013). Why coelacanths are not ‘living fossils’: a review of molecular and morphological data. BioEssays : news and reviews in molecular, cellular and developmental biology, 35 (4), 332-8 PMID: 23382020

For the study analysing forty-four coelacanth genes see: –

Takezaki N, Figueroa F, Zaleska-Rutczynska Z, Takahata N, & Klein J (2004). The phylogenetic relationship of tetrapod, coelacanth, and lungfish revealed by the sequences of forty-four nuclear genes. Molecular biology and evolution, 21 (8), 1512-24 PMID: 15128875

For a contrasting study claiming slow molecular evolution in these species see: –

Amemiya CT, Powers TP, Prohaska SJ, Grimwood J, Schmutz J, Dickson M, Miyake T, Schoenborn MA, Myers RM, Ruddle FH, & Stadler PF (2010). Complete HOX cluster characterization of the coelacanth provides further evidence for slow evolution of its genome. Proceedings of the National Academy of Sciences of the United States of America, 107 (8), 3622-7 PMID: 20139301

Five books for evolutionary biologists

Whenever I visit someone’s house, particularly the house of a fellow scientist, I like to browse their bookshelves, partly because I’m curious about what other people are reading and partly because I’m always on the lookout for new books to buy for my own collection. Another way I find out about new books is through other people’s blogs and websites, in fact some of my most visited blog posts have been book lists which I return to periodically when searching for new reading material. With that in mind I thought it was about time this blog had a book list, so here it is, the Ecologica top five books on evolution and behaviour. 1. On the Origin Of Species by Charles Darwin (Amazon). No list of books on biology would be complete without including Darwin’s seminal work outlining his theory of evolution by natural selection. Although great advances have been made in evolutionary theory since The Origin was first published in 1859, this book is still hugely important and influential and a must read for biologists of any stripe. If you want to fully understand evolutionary theory, or at least how the theory has developed since its formation, you must read this book.

Origin_of_Species_title_page[1]2. The Selfish Gene by Richard Dawkins (Amazon). This book is a classic and a truly great work of science writing. Dawkins makes the case for selection acting on genes rather than individuals or groups. This idea may have been more controversial when this book was first published in 1976 than it is now, although with a recent resurgence in interest in group selection this book is still highly relevant.

The_Selfish_Gene3[1]3. The beak of the Finch by Jonathan Weiner (Amazon). This book documents the work of Peter and Rosemary Grant, two biologists whose research in the Galapagos islands has shown that, for the finches that live there, evolution is happening so rapidly that we can actually see changes occurring year by year. A fascinating book and one that I really enjoyed reading.

125286[1]4. Wonderful Life by Stephen Jay Gould (Amazon). The late Stephen Jay Gould was a fantastic writer and populariser of science and this is one of his best known books. Gould discusses the pre-Cambrian fossils of the Burgess shale and their importance for our understanding of evolution. Some of his ideas have proven controversial but this is still a great book and well worth a read. For a response to Gould’s ideas you might also read The Crucible of creation by Simon Conway Morris (Amazon).

Wonderful life5. Why Evolution is True by Jerry Coyne (Amazon). Coyne presents a compelling argument that evolution is not just a theory but a fact. If you don’t accept that evolution is a reality then read this book and change your mind. You might also read The Greatest Show on Earth by Richard Dawkins (Amazon).

dn16347-1_300[1]

So that’s my list, what do you think? And what would you add to it?

The enigmatic fossa and the evolution of sociality

Madagascar is a land filled with wonderful creatures, from charismatic lemurs to chameleons so small they can stand on the tip of a pencil. It is also home to the fossa, Cryptoprocta ferox, a carnivorous cat-like mammal endemic to the island and thought to be closely related to mongooses (although this is contested). Unusually for a mammal, male fossas sometimes form long-term social groups of two, or occasionally three, individuals that work as a team when hunting allowing them to take down big prey such as some of the larger lemurs. Sociality clearly provides animals with major benefits, not only when hunting but also in terms of decreased predation risk and an improved ability to defend resources such as territory, food supplies or mates. However, there are also significant costs such as increased competition for food when supplies are low or for mates when mating opportunities are limited.  In most male mammals it seems that the costs of sociality outweigh the benefits. Although it is known from a few other species such as cheetahs and kinkajous, for male mammals to form long-lasting social bonds is extremely unusual, especially when the females are solitary, as is the case with the fossa.

Fossa

As a general rule female social groups occur in response to the distribution of food in the environment, while male social groups form in response to the distribution of females. When food sources are clumped females tend to aggregate at feeding areas while males form groups around them. In contrast, when food is evenly distributed both sexes are expected to lead mostly solitary lifestyles coming together only to reproduce. The fossa then is a bit of an oddball. It is the largest carnivore in Madagascar preying on small to medium-sized animals, its food supply is evenly distributed and, as theory predicts, the females live solitary lives. Why then do some males go against theory and form social groups? It may be that more sociable males benefit not only from cooperative hunting but also from increased mating success.

The fossa’s mating system is unique among mammals. During the breeding season large numbers of males gather at traditional mating trees which are occupied by small groups of up to three females. This leads to fierce contest competition among the males for access to mates and reportedly intense sperm competition. Those males that are able to monopolise access to females and gain the longest copulations are likely to reap the benefits of increased reproductive success. It may be that those males that form social groups share access to females and so gain a reproductive advantage over their solitary counterparts. However, intense sperm competition and typically small litter sizes may also impose high fitness costs to individuals belonging to groups of unrelated males thereby constraining the evolution of sociality.

This image from Arkive shows the fossa mating tree.
This image from Arkive shows the fossa mating tree.

The evolution of group-living from solitary ancestors has long been a hot topic in behavioural ecology. The unusual social behaviour of male fossas makes them an ideal study species for testing predictions about why sociality arose in the first place. Answering the question of why some male fossas go it alone while others form associations is the subject of recently published research by Mia-Lana Lührs and her colleagues.

Lührs and her fellow researchers were able to track and gather data on 22 wild male fossas between 2007 and 2010. They began by capturing individuals using live-traps and taking DNA, tissue and hair samples together with data on body mass and length, testis volume, and canine width. Some of these males were also fitted with GPS-accelerometer collars before release.

Since social groups of males were expected to move and hunt in synchrony, accelerometers were used to record when, and how vigorously, individuals were moving. This allowed the researchers to determine when their tracked males were moving and hunting and if they were hunting as a group or alone. In order to determine male mating success seven females were observed during the breeding seasons between 2007 and 2010 and the number and duration of copulations by known males recorded.

The results of this study are fascinating. Male fossas were found to exhibit two distinct phenotypes which differ according to social organisation. Those males that formed associations were found to be 38% heavier and 13% larger than solitary males, most likely due to differences in diet and an increased foraging efficiency. Dietary analyses revealed that the diet of social males contains larger and more agile bodied prey than solitary males and this has been linked to cooperative hunting. Data from associated males fitted with accelerometer tags shows that their movements, including movements typically associated with hunting, were highly synchronised, supporting the view that associated males hunt cooperatively. These results suggest that the benefits of hunting in a group may more than compensate for the costs of increased competition for scarce food resources. The physical superiority of social males may also grant them reproductive advantages as they are able to out-compete solitary males for access to females.

This boxplot shows the proportions of major mammal species in the diet of solitary (grey bars) and social (black bars) males. Prey species are sorted by size from left to right, largest to smallest. You can clearly see that the diets of the two phenotypes differ with social males consuming far more large bodied prey than solitary individuals.
These boxplots shows the proportions of major mammal species in the diet of solitary (grey bars) and social (black bars) males. Prey species are sorted by size from left to right, largest to smallest. You can clearly see that the diets of the two phenotypes differ with social males consuming far more large bodied prey than solitary individuals.

Since males appear to benefit from sociality it is worth considering why females of this species do not also form associations. Lührs and her colleagues propose that the energetic costs of competition for food when communally raising offspring are too high, for female fossas it just does not pay to be social. This may also explain why males usually only associate in pairs and, when groups of three have been observed, one member is usually in very poor condition. There is a fine balance between the benefits of cooperation and the costs of increased competition, two’s company but three’s a crowd.

The increased competition that comes with sociality not only applies to food but also to mating opportunities. This raises the question of how associated males share these limited opportunities to reproduce. A likely answer is that the costs of competition are accounted for by inclusive fitness benefits. This does appear to be the case. In four out of six associations the males were found to be likely litter mates, presumably brothers. Furthermore, the only unrelated male associates in this study were found to share mating opportunities. This suggests that forming associations provides males with significant benefits whether they are related or not.

This raises one final question, if forming associated pairs is so beneficial to males then why are there still any solitary males at all? Why hasn’t natural selection removed solitary behaviour from the population? One possibility is that the solitary males that were observed did not have any litter mates and so are constrained in their opportunities to associate with a familiar male. Put simply, they are solitary because they had no choice. No male littermates were found for any of the solitary males observed in this study, supporting this conclusion. Another possibility is that solitary males represent an alternative life strategy maintained by frequency dependent selection. Lührs and her colleagues suggest that the small size of solitary males may allow them to extract small lemurs from tree-holes or rodents from burrows. This seems unlikely since, when it comes to competition for mates, solitary males are at a significant disadvantage. Interestingly, Lührs and her colleagues note that in some species inferior males invest relatively more in sperm competition by producing more sperm. In fossas however this is not the case and this suggests that, for fossas, going solo is not an alternative strategy. At present the view that solitary males occur due to a lack of littermates with which to associate seems the most likely.

                                                                                                                        

claimtoken-51352dc0d52f9Luhrs M.L., Dammhahn M. & Kappeler P. (2012). Strength in numbers: males in a carnivore grow bigger when they associate and hunt cooperatively, Behavioral Ecology, 24 (1) 21-28. DOI:
(pdf)

A frog with teeth and the violation of Dollo’s law

In 1893 a Belgian palaeontologist by the name of Louis Dollo formulated his law of irreversibility which stated that evolution is not reversible. According to Dollo, structures that have been lost over evolutionary time (such as gills in mammals) cannot re-evolve. To put it in his own words: –

“an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors”

However, if there’s one thing you should know about biology it’s that rules almost always have exceptions and often it is these exceptions that are the most interesting areas of research. A paper published last year in the journal Evolution and written by John J. Wiens documents one such case, an apparent violation of Dollo’s law, the loss and re-evolution of teeth in the lower jaw of frogs.

The class amphibia is divided into three major groups; the little known caecilians, the salamanders and the frogs and toads. Of these groups both the caecilians and salamanders have teeth in their lower jaws, or mandibles. This is the ancestral condition that has been retained ever since the first amphibians pulled themselves from the water during the Devonian period. However, in the lineage leading to frogs and toads, the order Anura, mandibular teeth were lost at some point after it split from its sister group, the salamanders, between 225 and 338 million years ago. This is shown in the phylogenetic tree below.

Phylogenetic analysis of the amphibians showing the presence (red lines) and absence (blue lines) of mandibular teeth. This trait was lost in the Anuran clade at some point after its split from the salamanders 338mya. Taken from Wiens (2011). Click to enlarge.

This is where it gets interesting. If, as we know they were, mandibular teeth were lost in the frog lineage hundreds of millions of years ago then, according to Dollo’s law, they should be found in no species of frog alive today. But this is not the case. Gastrotheca guentheri, or Guenther’s marsupial frog, does possess teeth in its lower jaw, in fact it is the only living species to have this trait.

I couldn't find an image of this species showing the mandibular teeth but this image shows what Gastrotheca guentheri looks like.
I couldn’t find an image of this species showing the mandibular teeth but this image shows what Gastrotheca guentheri looks like.

If you look closely at the phylogenetic tree at the top of this post you can see G. guentheri is buried deep within the Anuran clade indicated by an arrow (re-evolution of mandibular teeth). No other species within the genus Gastrotheca, or indeed any other species of frog, possesses these teeth which means that they must have evolved independently in G. guentheri relatively recently. What we have here is an anomaly, G. guentheri is a species that violates Dollo’s law.

Before we consider why teeth have re-evolved after such a long period of absence it is worth considering why they were lost in the first place. Although teeth play a crucial role in prey capture for most carnivorous animals, many frogs instead catch their prey with their tongues which they flip out and downwards over the lower jaw. Mandibular teeth are therefore not required for most frogs and may actually be a hinderance when it comes to catching their prey, this may explain why they were lost so long ago.

The tongue method of hunting is however, only effective for those species that feed on relatively small prey items such as insects. Some species of frogs tackle much larger prey (such as other frogs) and have evolved stiffer mandibles and large fang-like teeth in the upper jaw to facilitate this. A few of these species have even evolved tooth-like structures on the lower jaw known as odontoids or serrations, these are not actual teeth but serve a similar function. As Wiens points out, it is clear that for some species of frogs mandibular teeth would be a significant advntage when hunting. Why they have re-evolved only in G. guentheri and not in other species is not known but may be due to developmental constraints that limit the re-evolution of mandibular teeth in most species of frogs.

Wiens study used genetic data to estimate the divergence times of the major groups of frogs and to pinpoint the times at which mandibular teeth were lost and when they later re-evolved in the lineage leading to G. guentheri. His results show that teeth were lost between 338 and 225 million years ago didn’t reappear again until only 20 million years ago, thus they were absent for 200 million years at least and most probably much longer. Further research is required to reveal why it is that mandibular teeth have only re-evolved once and what constraints may be preventing their re-evolution in other species.

If there’s one thing we can take away from this study it is that Dollo’s law is not really a law at all but a rule. This means that while it is true in most situations there are exceptions as we have seen. Dollo’s rule then is really just a statement about the statistical improbability of complex traits evolving in exactly the same way in the same lineage twice. It’s still important but it is not a law.

                                                 

Wiens J.J. (2011). Re-evolution of lost mandibular teeth in frogs after more than 200 million years, and re-evaluating Dollo’s law, Evolution, 65 (5) 1283-1296. DOI:

Help prevent creationism spreading in the UK

You may have seen or heard in the news over the past year or so that three groups intending to teach creationism have gained government approval to open Free Schools, which are independent but fully funded by the state, in 2013 and 2014. These are: –

  • Grindon Hall Christian School in Sunderland, due to open this September, which has a “Creation Policy” stating that they “affirm that to believe in God’s creation of the world is an entirely respectable position scientifically and rationally’, and say that ‘We will teach creation as a scientific theory”
  • Exemplar – Newark Business Academy which due to open in September 2013. The plans have been put forward by the same people who proposed the Everyday Champions Academy which was rejected by the Government specifically because of fears it would teach creationism as science.
  • Sevenoaks Christian School in Kent which is also due to open in September 2013. They have stated that they will not teach creationism in science classes but will teach it in RE instead.

If you care at all about the quality of science education in the UK you should be concerned by these proposals. Even if these schools do not teach creationism in science classes it is difficult to see how teachers who explicitly reject the theory of evolution, which is the very core of biology, and who may also reject an old age for the earth, the big bang and other established scientific principles could possibly teach science effectively or accurately.

The British Humanist Association explains the problem (their bold):

Both Exemplar and Sevenoaks intend to teach creationism, but in RE, not science lessons. Grindon Hall have as a private school taught creationism in science, but now also intend to teach creationism in RE and assemblies. It is quite common for primary schools to teach about the Christian, Jewish or Muslim creation narrative, as it is a prominent story in the Bible. But it is extremely rare for a secondary school to do likewise, and this raises concerns that these schools are intending to promote creationist views to their students as scientific theories.

The proposed schools have now been ‘pre-approved’ by the Government, who will work in support of the plans between now and when they are due to open. However, we can still stop the plans. The proposals for Exemplar and Sevenoaks will not gain final approval until next year. The proposals for Grindon Hall do not appear to yet have final approval, but if they haven’t already, they are due to any day now. This means we must urgently contact our MPs and Michael Gove to voice our opposition.

You can help stop these plans by sending an email to your MP using this form. It only takes two minutes and could help ensure that thousands of children get a decent education free from indoctrination.

I am lucky to have an MP who took my concerns seriously and wrote to the education secretary on my behalf. I received this response in the post last week:

It sounds promising but it is vitally important everyone who cares about science in the UK makes their feelings known and remains vigilant. Write to your MP Here.