Good morning all,
As I noted on Tuesday, we will not meet for lecture today. We have only one textbook chapter remaining, which we will save for our first meeting after the Thanksgiving break.
Instead of lecture today, I'll offer you a reading instead, one that encompasses several of our recent topics. In recent classes, we have considered the degree of cooperation and conflict between reproductive partners, as well as the signaling that occurs to influence each other. When sexual investment is strongly different between the sexes, we expect that sexual selection can drive exaggerated displays, enhance female 'choosiness' of mates, and promote unequal reproductive tactics. But, curiously, sexual displays also are common within pair-bonded species, in which males and females have equal (or nearly equal) roles and should be in cooperative agreement over parental investment, rather than in conflict. An explanation for this paradox has been lacking.
A very recent paper sheds some light on this problem, and present a mathematical model which supports the idea that inter-sexual signaling displays which originate to exploit a sensory bias in the signal receiver can evolve into a cooperative exchange, suggesting that sexual conflict can morph into sexual cooperation. This has significant implications for parental investment and care, as we've noted that the degree of sexual conflict is one of the primary drivers of sexual dimorphism in parental investment.
This paper was published in the Proceedings of the National Academy of Science (PNAS), our national body of 'science experts'. Election to the Academy is reserved for the top thinkers in one's field, and is a prestigious badge of honor. Their Proceedings journal publishes papers submitted by Academy members, as well as those that Academy members recommend for publication.
If you access this link from an IUP campus computer, you can obtain access to the full article and its associated material, through IUP library subscription. If you try to access the article from off-campus, you will be blocked. I've attached the PDF of the article, just in case.
The math of the authors' model is well beyond us. If we accept their model as being sound, it suggests that, instead of females being 'lured' into over-investment in their offspring by male displays, females instead evolve to require (or at least benefit from) the male display in terms of stimulating female condition/motivation to a level of investment which is optimal for the female (but less than that which is maximally optimal for the male). This causes males to remain invested in the pair-bond and their role in parental investment, and reinforces the pair-bond between mating partners. In a sense, the females are now requiring the males to remain present, remain attentive, and to offer displays, in order to ensure that their female partner is providing enough investment of her own.
As do many science journal, PNAS occasionally offers peer commentary on papers which are especially important, or especially difficult (this one is perhaps both). The associated commentary on this paper (link below, PDF attached), describes this result in the context of dove mating pairs, for which male stimulation of female reproductive condition is a well-understood and very necessary component to the reproductive cycle. Interestingly, as the commentary notes, the capitulation of this male-female exchange may ultimately be female self-stimulation of reproductive condition, a result which has been suggested to occur in doves. That may be the current evolutionary end-point to this exchange, but it also has the potential to serve as a type of an "escape clause", which males may now be selected to exploit. It would be interesting to see how much variation exists in this end-point, and whether males can benefit from females which perform more of their own reproductive stimulation.
I hope that you find this article interesting - it represent a nice, theoretical treatment of a difficult (= interesting) problem, and should set the stage for experimental work to come.
I hope that you all have an excellent Thanksgiving break - please be safe, rest, relax, eat, and enjoy. See you early in December for our last chapter.
As I mentioned in lecture on Tuesday, we are caught-up with our lecture material and will not meet for lecture on Thursday. Instead, I am offering a reading (attached) that I had described earlier, along with some explanation of one of the more important points described in this study.
Last weekend, I sent to our class description (below) of a recent publication examining behavioral-genetic associations in domestic dogs. I hadn't yet seen the original research when I wrote to you last weekend, but forwarded a news report about it that came from the home institution of the senior author on the study. I described in my message to you that some of the behavioral-genetic associations the authors reported were as high as 0.7, near to the limit of those ever reported for narrow-sense heritabilities of behavior.
Over the weekend, I requested a copy of the actual research paper from its senior author, and, upon seeing it, wanted to offer some interpretation.
Early in the term, in chapter 03, we discussed trait variation within species, and we noted (using the canine example) that artificial selection has created an abnormally high amount of trait variation within the single species of domestic dog Canis lupus familiaris.
In our next lecture (Chapter 04), we discussed behavioral genetics and narrow-sense estimates of heritability, describing the upper limit of such associations as around 0.7. We saw in that same chapter (as well as in later chapters, including Chapter 10) some estimates of narrow-sense behavioral-genetic heritability estimates that all were < 0.3, which is typical.
In this new report, the authors report behavioral-genetic associations that are much higher than those typically reported. How can this be? It stems from the artificial (and unusually large) degree of trait variation within this domestic species.
Typically, when one examines associations between traits within a natural (e.g., not artificially-selected) species, we expect some small, defined range of trait values, with correlation (association) between traits of some relatively low magnitude. Here in my Figure 1, I show the trait values and the within-species bivariate trait association for two (hypothetical) different species, such as a fox and a wolf. Within either species, there is some defined range of values for trait X (such as body length) and some defined range of values for trait Y (such as body mass). In my hypothetical example, these traits are correlated somewhat weakly within species A, and uncorrelated within species B. Notice that the two species do not overlap in trait values - a small wolf is always larger than than a large fox.
If domestic dogs were a natural species, chances are good that their trait values would fall somewhere in between these two species, perhaps closer to the 'wolf' end of the spectrum. Nonetheless, they would be expected to occupy only a small potion of the overall trait ranges.
Now, consider what the authors have done in their analysis. They have considered all domestic dog breeds to be of the same species, a fact that is technically true but which ignores the other fact that their range of trait values is anything but normal. They have analyzed behavioral-genetic associations across breeds within this single species, but here the individual breeds represent much more trait variation than natural species might, as size variation across domestic dog breeds is much greater than size variation across canine species in the wild. When associations are evaluated across multiple species (such as in my hypothetical Figure 2), the associations are often of higher magnitude. In the current study, analysis across the very artificially-distributed dog breeds behaves in the same way, resulting in behavioral-genetic associations much higher that those reported within single, natural species. 13/14 of their within-breed estimates (their Figure 1) are <0.3, just as one would expect.
This study is quite interesting, and represent the application of some very modern techniques (canine SNP chip, anyone?) to this interesting question of the heritability of behaviors. It also serves as a very useful reminder of
- the power of artificial selection - modern dog breeds are estimated to have been developed only over the last 300-500 years. For a natural species to evolve as much trait variation in this short time is unheard of.
- the danger of reliance upon secondary news sources - the original news story that I sent to you accurately describes the gist of this research study, and highlights the very strong associations found. But, it also leaves out enough detail that it is not possible to immediately assess why the associations are of such magnitude.
- the importance of proper modeling of evolutionary constraint - as shown in my hypothetical example Figure 2, trait associations across species can be artificially inflated if simple, linear techniques are used instead of methods that account for shared evolutionary history, such as independent contrasts analysis or nested ANOVA. The authors do have a phylogenetic model for their dog breeds; I am not schooled well-enough in the jargon of their analytical models to know if they have fully controlled for relatedness. Whether they have, or have not, these types of broad comparisons should always be examined with an eye for that type of concern.
- the imperfection of any one study - this is a research report describing one body of work on this topic, and I'm certain we could find other, similar/related studies. Is this study perfect? Certainly not. Is it still interesting, and useful? Absolutely. Any one research study can only advance our understanding incrementally. It's too easy, and too common, to dismiss work outright for containing flaws - it's more important to ask, given such flaws, is there anything that we can learn? The latter approach is more fruitful, and provides a much better return on one's investment of time and effort. Here, the traits with the highest across-breed heritabilities are trainability, aggression, and attachment - exactly those traits we might expect to have been key in the artificial selection/shaping of the human-dog relationship. It's a nice confirmation that these are strongly heritable, in ways that have translated into very powerful differences among breeds.
I've spent perhaps too much time dissecting some of these points, but I do so because they put some of our lecture material into sharp relief. Textbook examples are often too carefully culled to represent cutting-edge investigation; it's both fun as well as useful to see where current researchers in these areas actually are working.
Have a great rest of the week - see you on Tuesday for Chapter 11.
Good morning all,
At several points this term, we have discussed the genetics of behavior, including both the ability of single genes to influence behavior, as well as the heritability of individual behaviors and how traits can potentially be mapped onto phylogenetic histories. In the recent behavioral news is a report of a study that used large databases on dog behavior and genetics to look for behavioral traits that were associated with consistent genetic features. The researchers found >100 potential sites in the genome that were strongly associated with dog breed characteristics, including train-ability, aggression, excitability, and others.
One of the strengths of the method used here was that the researchers restricted themselves to a subset of the data pertaining to purebred dogs. This has the advantage of eliminating cross-breed variation which could dilute the strength of the genetic signals they were trying to detect. Dogs also are an advantageous species for a study like this, because they are popular, have long been bred in relatively pure lines, and have been artificially selected for a range of behavioral characteristics.
Some of the associations reported are quite strong, with heritability estimates as high as 60-70%. Those are very high values, near the limit reported for animal behavior-genetic comparisons. It's also surprising, in that, while this study has several strengths in its design, it also has one specific weakness: the researchers did not have genetic and behavioral information from the same individual animals, but instead were relying on databases (and breed averages) assessed across different individuals. That suggests that some of the associations, if tested within individual subjects, could be even stronger.
The human-dog relationship is a long one, and our artificial selection of dogs has been enormously powerful - when you think about all of the different dog breeds in the world, from Danes to dachshunds, Newfoundlands to chihuahuas, they all are the same species. That is testament to an enormous phenotypic plasticity (reaction norm) within their development. I'm going to request a copy of the original research article that this news report references, if anyone would like to see it - I'll bet it is interesting reading. Perhaps it will shed some light on my dog's (a rescue Rottweiler) behavior...
Have a great rest of the weekend - see you on Tuesday.
We've considered recently the concept of aposematism, the display of warning coloration to indicate to potential predators that one is unpalatable or otherwise unsuitable as a prey item. As we have seen, there are many implications to this type of signaling, including the costs involved, the degree to which it is effective, and its potential to be mimicked (and thus rendered potentially less effective) by palatable species.
The issue of aposematic costs is one that has been considered for some time, particularly the metabolic costs of producing warning coloration as well as the predation cost of being conspicuous. In addition to these are the metabolic costs of actually being unpalatable, and in no system has this been better explored than in monarch butterflies, conspicuous in both larval and adult forms, as well as highly unpalatable in each for the glycosidic compounds they acquire and sequester from milkweed plants (their near-exclusive forage). These compounds are highly toxic disruptors of Na+ channels, and being able to ingest and store them has required some evolutionary tinkering.
In the recent science news is consideration of this phenomenon, with some genetic work that explains the evolution of caterpillar resistance to these glycosides. The plant defenses have evolved to deter caterpillar feeding, but the caterpillars were able to evolve resistance with as few as three genetic mutations. These researchers were able to induce these same mutations in fruit flies, rendering them resistant to the glycosides as well - a very powerful experimental demonstration. The researchers also demonstrate some of the costs associated with the evolution of resistance to glycosides, including reduced ability to withstand physical shock. No evolutionary benefit is free, and beneficial changes to genes often are paired with deleterious side-effects. Here, the benefit (unpalatability) appears to outweigh the costs (reduced ability to withstand physical rotation).
Many of the plants and animals around us are conspicuous, while many others are cryptic. Those that are colorful and eye-catching may be silently playing potentially-deadly games of chemical warfare. Nature has been described as 'red in tooth and claw' (William Congreve); we might expand that to '... tooth, and claw, and toxin', for many toxins (including these glycosides) are quite deadly. What is remarkable to me is the role of simple sugars in glycosides, forming one side of the glycosidic bond. This is why some dangerous chemicals (such as automotive antifreeze, ethylene glycol) taste sweet and thus are dangerously attractive to the uninitiated. It makes me wonder whether glycosides have ever been used in nature as deadly bait, to lure, and then poison, potential prey. I'm willing to bet that it has...
Have a great weekend-
Good morning everyone,
In the recent science news are articles related to several of the topics we have considered recently - this is a nice confirmation that our course topics are 'up-to-date'!
Early in the term we considered the behavior of parasitic wasps, that stun prey and then oviposit eggs within them so that their larvae have a ready food supply during early growth. In the news this week is description of a different kind of parasitic wasp, one which parasitizes other wasps.
Here, the form of parasitism is less direct, in that the parasite deposits its eggs into the same plant gall that its host occupies. The parasite larvae then can attack the host, and in doing so, they accomplish a form of behavioral and physiological 'hypermanipulation'. Not only do they use the host tissues for their own nourishment, but they actually trigger a malformed version of the hosts normal escape behavior, which ensures that the host itself doesn't escape the gall but which provides the parasite an escape route.
The degree to which parasites manipulate their hosts can be extraordinary. We are used to thinking that parasites can make use of host tissues, but examples like this reveal more complicated interactions, with some parasites hijacking host behavior as well. There are plenty of examples, such as these:
All are good reminders that host behavior, as well as host tissues, can be exploited by parasites.
Even more recently, I sent you some information about humans who have developed some ability to perform echolocation. Just this week came a report on this topic, suggesting real, functional remapping of the brain's visual cortex to support this new capability:
At some level, neural plasticity is responsible for all that we can learn, but to have whole-scale re-functioning of a part of the brain from one sense to another is very impressive.
Have a good weekend -
Good morning all,
As our term comes to a close, I'll use my last news message to send along the latest news from two ongoing news stories in genetics:
The first bit of news is about a newly reported fossil find, from a branch of ancestral hominins known as the Denisovans. While scientists and anthropologists have been studying our Neanderthal relatives for decades, Denisovans are only recently discovered. They are thought to have represented a 3rd lineage of ancestral hominin, that co-existed with and likely inter-bred with both Neanderthals as well as early humans.
Until very recently, all information on Denisovans came from fossils collected from a single location, the Denisova cave in modern Siberia (Russia). This new report describes a Denisova fossil from much farther south, in modern Tibet, which suggests that Denisovans were more broadly distributed, expanding the ranges of times and locations over which they may have interacted with modern humans. We know so little about Denisovans that this new information has been described as 'game changing'. If you recall the patterns of early human migration we considered, the first humans may well have had opportunity to interact with the last Denisovans. We all likely have some 'Neanderthal DNA' in us; we may come to realize that we all have a little 'Denisovan DNA', too.
The second news story I will send here relates to the promise, and difficulty, of genome editing. We've discussed a number of times the concept of genes and alleles, and we've considered both gene therapy as well as some of the news related to human genome editing. Recently, a group of prominent scientists has argued that, given our current state of knowledge, the use of gene editing to produce 'designer babies' is more fiction than fact. Even apart from the difficulty of successfully edited the human genome, they suggest that the likelihood of finding individual genes with pronounced effects is very, very low. If you remember, genome-wide association studies (GWAS) can be used to identify genes associated with particular aspects of our physiology and health, but the strength of these associations normally is very low (e.g., often <1%). As such, we may not yet have good, individual targets for gene manipulation.
That said, it is very likely that both our gene-editing as well as our genome evaluation skills are going to improve over time, so perhaps the current limitations on the likelihood of 'designer genome editing' are just that: current, but not permanent. It seems impossible that this topic, or interest in it, is going away any time soon.
I'm signing off for the term now. I hope that these weekly news messages have been useful to you. This is the first semester that I have used them to this extent, and it has been a learning experience for me. In particular,
In the end, though, I remain very optimistic. Science is "mankind's organized quest for knowledge" (Floyd Bloom), and we already know that "knowledge is power" (Francis Bacon). It is science that offers us the best hope to deeper understanding, new therapies and treatments, new cures, and new adventures. We will encounter many speed-bumps along the way, to be sure. I hope that our course has inspired you to be a part of this quest, and to make the best use of the knowledge that you gain while on it.
Have a great weekend, and best of luck with all of your exams next week.