Evolution of Photsynthetic Sea Slugs

Contributed by Alexandria Albert and Gavon Broomfield

Bright green sea slugs that behave like plants!                                                                 Sea slugs are a diverse family of marine gastropod mollusks characterized by their soft bodies and lack of external shell. Approximately 2,300 species have been documented, all with different physical colorations that allow them to better interact with other organisms and underwater conditions. Sacoglossan sea slugs have mastered the art of kleptoplasty by extracting chloroplasts from various algal food sources and preserving them in digestive tissue, thus creating the kleptoplast. Exploring this symbiotic interaction has provided insight into multiple evolutionary processes. For example horizontal gene transfer, the transfer of genes from one species to another, in this case from the algal nucleus to sea slug cells, has facilitated the long-term use of the kleptoplasts (Cruz et al. 2013). There are still questions about the maintenance of kleptoplasts living in animal tissue, but benefits from this form of energy production have been documented.  

 E.crispate NEWE.viridis NEW

So how is it possible that sea slugs have chloroplasts? Aren’t chloroplasts only in plants?  The key to photosynthetic capable sea slugs is symbiosis. Algal nuclear genes in the sea slug digestive cells encode for chlorophyll synthesis, giving slugs green coloring, and chloroplast proteins, which later become incorporated into the slug’s DNA to get passed onto offspring. For some species of Sacoglossa, these internal or endosymbiotic chloroplasts can be maintained long-term if the slug possesses the nuclear DNA required for photosynthesis. For other species, continual feeding on algae is necessary for long-term, sustained kleptoplastic ability. Cool, right? Since the chloroplast is not native to the sea slug, important behavioral, morphological, and biochemical adaptations have evolved to maintain this symbiosis and kleptoplasty (Schwartz, Curtis, and Pierce 2014, Schmitt, Valerie, et al. 2014). 

E.timida NEW

So how do these photosynthetic sea slugs use these chloroplasts?                               Sea slugs adjust their parapodial lobes, lateral fleshy protrusions on their bodies used for movement, to manage light harvesting. When the parapodial lobes are extended, chloroplasts are exposed to direct sunlight which is then used as an energy source, a process known as phototrophy, as seen in plants. When doing this, sea slugs often resemble leaves. This leaf-like appearance aids in camouflage and avoidance of ocean-floor predators like crab, lobster, and fish (Schmitt and Wägele 2011). This unique behavioral adaptation has evolved to retain endosymbiotic chloroplasts.


Does this really work?                                                                                                   Studies have shown high levels of fitness benefits to kleptoplasty in Sacoglossa when measuring growth efficiency with trade-offs dependent upon algae diet and light exposure (Baumgartner, Pavia, and Toth 2015).  This adaptation has some limitations and may depend upon sunlight exposure and the species of algae. Too much sun exposure could cause photo-oxidative stress on kleptoplasts and decrease the rate of energy production over time (Serôdio, João et al. 2014).

For more information about the photosynthetic qualities of sea slugs, consult these sources:

  1. Baumgartner, Finn A., Henrik Pavia, and Gunilla B. Toth. “Acquired Phototrophy through Retention of Functional Chloroplasts Increases Growth Efficiency of the Sea Slug Elysia Viridis.” Ed. Erik Sotka. PLoS ONE 10.4 (2015): e0120874. PMC. Web. 6 Nov. 2015.
  2. Goodheart, J. A., Bazinet, A. L., Collins, A. G., & Cummings, M. P. (2015). Relationships within Cladobranchia (Gastropoda: Nudibranchia) based on RNA-Seq data: an initial investigation. Royal Society Open Science, 2(9), 150196. http://doi.org/10.1098/rsos.150196
  3. Schmitt, Valerie, and Heike Waegele. “Behavioral adaptations in relation to long-term retention of endosymbiotic chloroplasts in the sea slug Elysia timida (Opisthobranchia, Sacoglossa).” Thalassas 27.2 (2011): 226-238.
  4. Schwartz, Julie A., Nicholas E. Curtis, and Sidney K. Pierce. “FISH labeling reveals a horizontally transferred algal (Vaucheria litorea) nuclear gene on a sea slug (Elysia chlorotica) chromosome.” The Biological Bulletin 227.3 (2014): 300-312.
  5. Schmitt, Valerie, et al. “Chloroplast incorporation and long-term photosynthetic performance through the life cycle in laboratory cultures of Elysia timida (Sacoglossa, Heterobranchia).” Frontiers in zoology 11.1 (2014): 5.  
  6. Serôdio, João et al. “Photophysiology of Kleptoplasts: Photosynthetic Use of Light by Chloroplasts Living in Animal Cells.” Philosophical Transactions of the Royal Society B: Biological Sciences 369.1640 (2014): 20130242. PMC. Web. 6 Nov. 2015.

Helicobacter pylori and you.

Contributed by Thomas Partin and Austin Piccolo

Species do not live in a world separate from each other. Organisms interact with other organisms everyday, and over time adapt to each other accordingly. Often, the evolution of two species can become strongly linked to each other, for better or worse. Heliobacter pylori is a bacteria that thrives in the acidic conditions of the human stomach. It causes stomach ulcers and is strongly correlated with gastric cancer. As recently as the 1980’s, the idea of a bacteria being able to survive in the stomach’s harsh conditions and being responsible for this disease was so controversial that it took one doctor intentionally infecting himself to prove its role in stomach ulcers. That doctor later won the Nobel prize in medicine for his work.

H. pylori did not first start infecting humans in the 80’s though. H. pylori and humans have been living (and battling) together for millennia. The earliest humans also played host to H. pylori. One way this can be shown is a creative use of a phylogenetic tree. Scientists sampled many different strains of H. pylori, and used them to create an ancestral tree of the different strains. They then compared the tree they made to geographic locations of their samples. What they found was that lineages of H. pylori matched perfectly with the migration patterns of ancient humans as they moved out of Africa. Newer strains of H. pylori are found where humans migrated to most recently. The strains were carried and dispersed based on how early humans moved around the globe.

This intimate relation between H. pylori and humans provides a great opportunity to explore coevolution. Humans and H. pylori have been locked in an arms race for thousands of years. H. pylori colonization poses serious health consequences to the host, which creates a selective pressure for humans that can prevent H. pylori infection. Likewise, the human body is an incredibly hostile environment towards foreign invaders like H. pylori, which creates a strong selective environment for H. pylori cells that can overcome human defenses. There is evidence this selective pressure is so strong that H. pylori begins adapting specifically to the host after initial colonization. Although not an innate aspect of human biology, antibiotics are another human defense against H. pylori. Antibiotic use creates a selective pressure for H. pylori that is so strong that resistant strains can develop remarkably quickly after attempted treatment.

Please watch the below video to learn more!


For further information see:

Linz, B., Ballouxm, F., Moodley, Y., Manica, A., Liu, H., Roumagnac, P., Falush, D., Stamer, C., Prugnolle, F., van der Mer, S.W., Yamaoka, Y., Graham, D.Y., Perez-Trallero, E., Wadstrom, T., Suerbaum, S., Achtman, M. 2007. An African origin for the intimate association between humans and Helicobacter pylori. Nature 445: 915-918

Gao, W., Cheng, H., Hu, F., Li, J., Wang, L., Yang, G., Xu, L., Zheng, X. 2010. The Evolution of Helicobacter pylori Antibiotics Resistance Over 10 Years in Beijing, China. Helicobacter. 15: 460-466.

Oh, J.D., Kling-Bäckhed, H., Giannakis, M., Xu, J., Fulton, R.S., Fulton, L.A., Cordum, H.S., Wang, C., Elliott, Glendoria., Edwards, J., Mardis, E.R., Engstrand, L.G., Gordon, J.I. 2006. The complete genome sequence of a chronic atrophic gastritis Helicobacter pylori strain: Evolution during disease progression. PNAS. 103: 9999-10004.

Blecker, U., Landers, S., Keppens, E., Vandenplas, Y. 1994. Evolution of Helicobacter pylori Positivity in Infants Born From Positive Mothers. Journal of Pediatric Gastroenterology and Nutrition. 19: 87-90

Kennemann, L., Didelot, X., Aebischer, T., Khun, S., Drescher, B., Droge, M., Reinhardt, R., Correa, P., Meyer, T.F., Josenhan, C., Falush, D., Suerbaum S. 2011. Helicobacter pylori genome evolution during human infection. PNAS. 108: 5033-5038.

Marshall, B.J., Warren, J.R. 1984. Unidentified curved bacilli in the stomach of patients with gastritis and peptic ulcers. The Lancet. 323: 1311-131.

Avasthi, T.S., Devi, S.H., Taylor, T.D., Kumar, N., Baddam, R., Kondo, S., Suzuki, Y., Lamouliatte, H., Mégraud, F., Ahmed, N. 2011. Genomes of Two Chronological Isolates (Helicobacter pylori 2017 and 2018) of the West African Helicobacter pylori Strain 908 Obtained from a Single Patient. Journal of Bacteriology. 193: 3385-3386.

Sexual Phenomenon as Evidence for the Imperfection in Evolution

Contributed by Sara Allison, Md Saon, Hassan Jassani, Siraj Quad

There is a general misconception that evolution has some sort of directionality associated with it where natural selection pushes organisms only to improve over time towards perfection. The misconception assumes that evolution only produces traits that the organism needs to survive and live long lives. This, however, is not entirely accurate, nor is it the whole story. Natural selection acts on random mutations and selects the ones that best help the organism to survive in a given environment or ones that increase its reproductive fitness. The traits selected for may not be perfect, but are good enough to be maintained through selective pressures. We will focus on a secondary sexual characteristic and sexual cannibalism as evidence for the fact that evolution is not perfect, in that there are not only benefits but also costs for these characteristics.

One example of a trait that is not perfect or optimal for the survival of the organism is the male rhinoceros beetle’s increased horn size. In general, males’ horns tend to be bigger than females’; females prefer males with bigger horns. This trait is not perfect because of the fact that bigger horned rhinoceros beetles experience increased predation, in spite of the trait’s evident advantage when it comes to sexual selection. Thus, the trait is advantageous because it helps males acquire mates and reproduce. However, it is also costly since males with bigger horns have an increased risk of predation. The reason this trait has been favored in spite of its imperfection is because the benefit of big horns being preferred by females outweighs the cost of increased predation through an increase in the number of offspring.


Figure A shows that the big horned male rhinoceros beetle has attracted the female beetle and has a high probability of mating with her—a benefit of big horns. Figure B shows that a smaller horned beetle has failed to attract the same female. Thus, Figure C shows this female fleeing from the small horned beetle without mating with him. Figure D shows that the big horned male receives more predatory attacks than the small horned beetle—a cost to having big horns.

Another example of imperfection in evolution is the evolution of sexual cannibalism, which is an extreme form of sexual conflict that involves the killing and consumption of the mate either before mating or after mating.  It is possible that the occurrence of sexual cannibalism is influenced by low prey availability, which motivates the individual to eat its mate to obtain sustenance—this is known as the foraging hypothesis. Consequent benefits can include an increase in body mass, a healthier body condition, or increased reproductive ability.  A study of the orb-web spider demonstrated that sexual cannibalism increases offspring survival, in which cannibalistic females produced offspring with longer survival times than females that were prevented from consuming their mate.  Although there are benefits, there are also very significant costs. Eliminating the father means a definite loss of any potential paternal care. Additionally, cannibalism can occur before mating and, therefore, cause reproductive failure. In short, the evolution of sexual cannibalism comes with costs and benefits.

The evolution of the big horned male rhinoceros beetle and sexual cannibalism epitomizes the imperfect nature of evolution. Both traits are not perfect in that they come with significant costs. They are maintained because the benefits of each trait outweigh the costs.

For more information, see:

McCullough, Erin L., and Douglas J. Emlen. “Evaluating the costs of a sexually selected weapon: big horns at a small price.” Animal Behaviour 86.5 (2013): 977-985.

Wu, Lingbing, et al. “Factors influencing sexual cannibalism and its benefit to fecundity and offspring survival in the wolf spider Pardosa pseudoannulata (Araneae: Lycosidae).” Behavioral ecology and sociobiology 67.2 (2013):205-212.

Newman, Jonathan A, and Mark AElgar. “Sexual Cannibalism in Orb-Weaving Spiders: An Economic Model.” American Naturalist, The 138.6 (1991):1372-1395.


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Forget Harry Potter… Check Out the Bobtail Squid’s Invisibility Cloak!


Contributed by Kelly Costopoulos, Taylor Werkema, and Nina Zook.

One of the most spectacular biological phenomena is an organism’s ability to luminesce. The light can be produced through chemical reactions in specific cells and even, in more rare cases, through a symbiotic relationship of an organism and unique bacteria. An extraordinary example of this is the cooperative relationship between the Bobtail squid and the bacteria Vibrio fisheri.

The Bobtail squid lives in the coastal waters off the Pacific Ocean and some parts of the Indian Ocean. During the day the squid buries itself in the sand and at night it comes out to hunt. The squid mimics the moonlight by using luminescent V. fischeri and eliminates its shadow on the ocean floor below to avoid predators. It’s nature’s own invisibility cloak!

This incredible act starts every morning when free living bacteria in the water are taken up by special light organs in the squid’s mantle. There, they are nourished by special cells that promote the growth of only V. fischeri, all other competitors are actively selected against. Once the bacteria population reaches a certain density, the bacteria produce light. The squid has the ability to control the intensity of light through specialized filters to match the moon’s light exactly. When the sun comes up in the morning, the squid expels the bacteria in a process known as venting, and the cycle starts over.

The video above gives a graphical representation of how the squid uses its invisibility cloak. As the moon comes out, the squid becomes luminescent and its shadow on the ocean floor disappears. This hides the squid from predators below. 

The bobtail squid is only one of many squid species in the sepiolid family with V. fischeri symbionts. How did such an intricate symbiosis evolve in these squid species? Because both organisms can be easily cultured and raised in the laboratory setting, researchers can do evolutionary studies to shed light on how and why the sepiolid squid-V. fischeri system has evolved.  Researchers have determined the relatedness between similar squid species and the relatedness of their symbiotic bacteria. Interestingly, the bacterial tree lines up with the squid tree, indicating that the squid and bacteria of each symbiotic pair have coevolved. This means that as squid species diverged from one another, so did their native bacteria such that the bacteria became more host-specific.

Evolutionary studies have also helped us understand how bacteria compete to establish symbiosis with the their squid hosts.  Experimental competition, or raising a squid in the presence of two different bacteria, has shown that a squid’s native bacterial symbiont will outcompete nonnative strains.  Further, the more closely related the nonnative strain is to the native bacteria, the more competitive it will be in inhabiting the squid’s light organ.  Thus, the evolutionary history of sepiolid squid species and their unique symbionts is one of fidelity and preference for the native strain with whom the squid has evolved intimately over time.

For more on the squid-vibrio system, check out:

McFall-Ngai, M. 2008. Hawaiian bobtail squid. Current Biology : CB, 18(22), R1043–4.

Nishiguchi, M.K., Ruby, E.G., & McFall-Ngai, M.J. 1998. Competitive dominance among strains of luminous bacteria provides an unusual form of evidence for parallel evolution in sepiolid squid-vibrio symbioses. Applied and Environmental Microbiology 64(9): 3209-3213.

O’Brien, Miles, and Marsha Walton. “Glowing Squid.” National Science Foundation. The National Science Foundation, 22 Nov. 2010. Web. 28 Apr. 2014.

Venom Variation

Contributed by Annelise Bonvillian, Cherisma Patel and Liz Pinkerton

I introduce to you two rattlesnakes. I think they’re generally friendly but they could also be planning to maim me. You just never know with venomous snakes. 

Judging by their appearance, you’d think both rattlesnakes were of the same species. You’d be right, being the clever person you are. They are in fact part of the same subspecies of Southern Pacific Rattlesnake (Crotalus oreganus helleri).

These C. o. helleri rattlesnakes are venomous. Before we dive into their story, let’s explain venom.

Venom Across The Tree of Life

Many animals have evolved venom for protection or to help them capture prey. We, unfortunately have not.

There are also a wide range of structures for delivering venom, such as the cute little fangs in these rattlesnakes.

The interesting thing is, similar types of venom have evolved in completely unrelated species. This is not because all venomous species evolved from one super venomous ancestor or because the venomous critters of the world get together to talk strategy.

Evolution of venom is a solid example of convergent evolution in which organisms that are not very closely related independently evolve similar traits due to similar environmental pressures. The proteins in the toxin end up acting on the same physiological molecule by chance. Most venoms involve disrupting major physiological pathways that are accessible by the bloodstream, especially the hemostatic and neurological systems. Damaging these systems would efficiently cripple any prey or potential predator.

Back to the Rattlesnakes

Getting back to our original C. o. helleri rattlesnakes, you’d think it might be true that they’d have the same type of venom because they’re of the same subspecies, right?

In fact, these two rattlesnakes are very closely related, but the populations that live in the mountains have a distinctly different venom than those that live in the desert.

Distribution of Crotalus oreganus

Distribution of Crotalus oreganus

A research team from the University of Queensland studied the variation between rattlesnakes in the mountainous area of Idyllwild, California and those in the desert of Phelan, California. The extent of venom variation is surprising for two populations because the two areas are relatively close together geographically.

The venom of the desert-dwelling rattlesnake contains proteins that break down blood vessels and prevent clotting.

This venom is slow acting. It is described as haemotoxic and would lead to uncontrollable bleeding. Neat!

Its close relative in the mountains is equally as morbid. The mountain populations have proteins in their venom that stop nerves from sending signals to the muscles.

This venom is fast acting. It is described as neurotoxic and results in paralysis. Sweet!

The venoms are completely different even though these two populations are from the same subspecies.

In fact, venom can vary between species, subspecies, individuals, sexes, and ages. Another research team from the Biomedical Institute of Valencia that loves poisonous reptiles just as much as the the previous one studied venom variability in Mojave rattlesnakes in Arizona. They found that you were 10 times more likely to die from a bite in one county than another. Here, the severity, not the type, of venom was what varied.

So, getting back to our C. o. helleri rattlesnakes (don’t worry, we love you best), here’s another question: which would be better to have in the snake’s arsenal? The fast acting neurotoxin venom or the slow acting haemotoxin?

Evolution, the lovely process that developed these fine creatures is NOT about having the best of a trait. The fitness of a certain trait is relative. What is good for one population might not necessarily be the best for another. Based on where they live, different venom types might be more beneficial in capturing prey or warding off predators.

So why would it be more beneficial for the mountain rattlesnake to have a faster acting venom?

Researchers suspect that environmental differences between these two populations of rattlesnake are likely to have promoted the huge variation in venom between the two.

If the mountain rattlesnake’s venom was slow acting the, prey could hide before the venom had properly incapacitated it.

And venom isn’t cheap. Creating venom costs a lot of energy and wasting it time and time again would be a shame. Natural selection would thus lead to a venom composition that would reduce metabolic cost. Natural selection is the process by which organisms that have higher fitness and are more adapted to their environment tend to produce more offspring and survive.

Now, in the desert, rattlesnakes don’t have to worry as much about their prey hiding before they can get to them. They might not have evolved the quicker neurotoxic venom because there was no selective pressure to have their venom be fast acting.

As with most things in this incomprehensible universe, we are not sure if the desert dwelling Southern Pacific rattlesnake maintains its haemotoxic venom due to lack of selection pressure. Or perhaps somehow the snakes have co-evolved with their prey and the haemotoxic venom is just the most efficient one for the prey type.

All in all, it’s important to realize that evolution isn’t a race where the end goal is to have the best of something. Traits change randomly due to mutations, and factors such as the environment or prey type select for variations that are most beneficial for survival.

In related news, using the vast powers of human intelligence to learn from the natural world, snake venom is being adapted to heal instead of hurt. Venom works in the same way as many medicines, and the enzymes in venom are being modified to affect disease processes.

One specific example includes the using ACE inhibitors in Brazilian pit viper venom to prevent hypertension.

However, with the decline of snake diversity due to environmental degradation, the diversity of venom and its medical potential is decreasing.The fact that venom from various snakes can be used to target certain  diseases is a very important implication for evolutionary medicine. Population divergence in snakes increases the potential for variation in venom type, which can ultimately increase the antidotes possible for fighting certain diseases.

So why research things like rattlesnake venom? Not only is the subject wildly fascinating, but unraveling the complexities of snake venom can help humans better counter its life-threatening effects and can also promote the development of new medicines. In conclusion,  though we may be terrified of you, dear rattlesnakes and other venomous denizens of this world, we’d also like to say thanks. May you continue to amaze us.

Check out these sites for more information:

Caswell, Nocholas R., Wolfgang Wuster, Freek J. Vink, Robert A. Harrison, and Bryan G. Fry. 2012. Complex cocktails: the evolutionary novelty of venoms Trends in Ecology and Evolution.

Holland, Jennifer S. 2013. Venom: The Bite That Heals. National Geographic: The New Age of Exploration. http://ngm.nationalgeographic.com/2013/02/125-venom/holland-text

Kartik Sunagara, Eivind A.B. Undheimc, Holger Scheibd, Eric C.K. Grene, Chip Cochrane, Carl E. Persone, Ivan Koludarovc, Wayne Kellne, William K. Hayese, Glenn F. Kingd, Agosthino Antunesa, Bryan Grieg Fry. Intraspecific venom variation in the medically significant Southern Pacific Rattlesnake (Crotalus oreganus helleri): Biodiscovery, clinical and evolutionary implications. 2014. Journal of Proteomics. http://www.sciencedirect.com/science/article/pii/S1874391914000256

Massey DJ, Calvete JJ, Sánchez EE, Sanz L, Richards K, Curtis R, Boesen K. 2012. Venom variability and envenoming severity outcomes of the Crotalus scutulatus scutulatus (Mojave rattlesnake) from Southern Arizona. Journal of Proteomics. http://www.ncbi.nlm.nih.gov/pubmed/22446891

Yong, Ed. 2014. Rattlesnakes Two Hours Apart Pack Totally Different Venoms. National Geographic: Phenomena. Online. http://phenomena.nationalgeographic.com/2014/01/27/rattlesnakes-two-hours-apart-pack-totally-different-venoms/

Zimmer, Carl. 2013. On the Origin of Venom. National Geographic: Phenomena. Online. http://phenomena.nationalgeographic.com/2013/01/09/on-the-origin-of-venom/

Evolution of Eusociality

by Lingshan Chen

EusocialView Original Graphic

Eusociality is a sociobiological phenomenon in which adult members are divided into reproductive and non-reproductive castes and have overlapping generations of parent and offspring. The reproductive caste contains only one or a few members of the entire colony and is responsible for producing all the offspring. Conversely, the non-reproductive caste is composed of the majority of the colony. They cooperatively raise the young and otherwise provide and protect the colony. This extreme form of altruism and social life has long perplexed scientists as it contradicts the intrinsic selfishness displayed by organisms.

Although some mammals are eusocial, the majority of eusocial species belong to  the phylum Arthropoda and order Hymenoptera, most commonly seen in bees, wasps, and ants.  There are several advantages of the organized structure of eusociality. Resources such as food, territory, and protection are maximized in comparison to solitary individuals.

For example, the leaf-cutter bee, Megachile rotunda, is a solitary species. These bees reproduce, forage, and raise eggs individually. Each female leaf-cutter bee adult must cut leaves to build nests for each egg. Inside each nest, the female must provide pollen and nectar to feed the larvae. When the bee leaves to forage for food, the nests are left unprotected. In contrast, honey bees have a queen that lays many eggs each day to populate the colony. Worker bees are sterile and provide food and protection for juvenile siblings. Though many honey bees are not reproducing, the productiveness and safety of the colony as a whole has increased.

If eusociality is advantageous, why is it so rare? To investigate this question, we can look at the origin of eusociality. Evolutionary theories propose that at first, solitary organisms group together for mutual benefits. In Hymenoptera, eusociality may have arisen because relatedness between individuals is maximized because of their reproduction method. In this system, fitness benefits from related individuals are a lot greater than the cost to the individual. An intermediate step occurs when workers develop the choice to stay and help with the colony or start their own colony. Other theories also suggest that eusocial evolution follows a series of stages that start with the formation of groups between related or unrelated individuals that must persist. For the group to remain cohesive, the acquisition of pre-adaptive traits such as nest building are necessary. Following this stage, eusocial genes emerge through mutation or recombination. As a result of multiple driving forces, primitive eusocial colonies reach a transition stage termed the  “point of no return”, during which different castes develop and maintain morphological differences, and evolve into advanced eusociality.

For more information please see the following papers:

Bang, A., & R. Gadagkar. 2012. Reproductive queue without overt conflict in the primitive eusocial wasp Ropalidia marginata. PNAS 109:14494-14499.

Dolezal, A.G., Flores, K.B., Traynor, K.S., & G.V. Amdam. 2013. “The evolution and development of eusocial insect behavior.” Advances in Evolutionary Developmental Biology (2013): 37-57

Grüter, C., Menezes, C., Imperatriz-Fonseca, V.L., & F. L. W. Ratnieks. 2012. A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee. PNAS 109 (4) 1182-1186.

Nowak, M.A., Tarnita, C.E., & E.O. Wilson. 2010. The evolution of eusociality. Nature 466(26) 1057-1062.

Plowes, N. 2010. An Introduction to Eusociality. Nature Education Knowledge 3(10): 7

Richards, M. H., Wettberg, E.J., & A. C. Rutgers. 2003. A novel social polymorphism in a primitively eusocial bee. PNAS 100 (12) :7175-7180.

Rueffler, C., Hermisson, J. & G.P. Wagner. 2012. Evolution of functional specialization and division of labor. PNAS 109(6) E326-E335.

Strassmann, J.E., Queller, D.C., Avise, J.C., & F. J.Ayala. 2011.  In the Light of Evolution V: Cooperation and Conflict Sackler Colloquium – Introduction. PNAS 109 10787-10791.

Wilson, E.O. & B. Hölldobler. 2005. Eusociolity: Origin and consequences. PNAS 102(38) 13367-13371.

The Evolution of Helping in Nature

You would share a cookie, but why would a meerkat?

Contributed by Jessica Etienne, Kavya Reddy, Madison Malone, Okeoghene Ogaga

Evolution, the inherited change in a group of organisms over time, is often misunderstood. One major misconception is that evolution leads to immoral behavior. However, many animals, insects and plants have evolved altruistic behavior, an interaction between two individuals in which one individual reduces its own fitness (the ability of an individual to survive and reproduce) for the benefit of another. In other words, the individual is doing something good for someone else. Altruism is important evolutionarily because it interacts with natural selection by promoting cooperation of a group as a method of competition against others. Altruism can come in two forms: either an organism performs an action without anything in return or an organism becomes less competitive with other individuals without receiving any benefits. It seems counter intuitive for organisms in nature to perform a selfless action, however a chimpanzee will help another without receiving anything in return. One idea as to why this happens is that it has evolved due to indirect fitness. The total fitness of an individual includes indirect and direct fitness; indirect fitness is benefit for oneself gained through the fitness of others, while direct fitness is the benefit gained directly from one’s own actions. Contrary to what many believe, the fittest individuals are the ones that manage to spread their genes the most because fitness incorporates more than an individual’s physical capabilities.

Typically within a population, organisms tend to help those that are related to them rather than strangers; for example, plants will compete less for resources when they are next to their relatives than when they are next to strangers. This is most likely because individuals that are related to one another share a percentage of DNA. A unit for expressing this percentage is called coefficient of relatedness (r). One can model the likelihood of altruistic behavior using relatedness and Hamilton’s Rule, which states that the coefficient of relatedness, multiplied by the benefit (b) that the related individual receives, should be greater than the cost (c) that the individual performing the action experiences:  rb-c>0  or rb>c

Despite the accuracy of Hamilton’s Rule in predicting the evolution of altruistic behavior, it is important to remember that it a model that describes why altruism occurs, and is still being investigated. For more on Hamilton’s rule, see the video below.

In order for organisms to show altruism preferentially towards related individuals, they need to be able to know who they are related to; organisms do this two different ways, either using kin recognition (they can recognize who their brothers are) or kin fidelity (families stay together).

In addition, altruistic behavior does not imply that an individual will sacrifice itself for the “good of the species,” but rather Individuals are sacrificing themselves to increase the fitness of their relatives, and therefore increase their own fitness indirectly.


For more information, go to:

Abbot, P., Abe, J., Alcock, J., Alizon, S., Alpedrinha, J. A. C., Andersson, M., Andre, J.-B., et al. (2011). Inclusive fitness theory and eusociality. Nature, 471(7339), E1–E4. doi:10.1038/nature09831

Curry, O., Roberts, S. G. B. and Dunbar, R. I. M. (2013), Altruism in social networks: Evidence for a ‘kinship premium’. British Journal of Psychology, 104: 283–295. doi: 10.1111/j.2044-8295.2012.02119.x

Dudley, S., Murphy, G., and File, A. 2013. Kin recognition and competition in plants. Functional Ecology 27(4) 898-906. DOI: 10.1111/1365-2435.12121

Ratnieks, F. and Wenseleers, T. 2008. Altruism in insect societies and beyond: voluntary or enforced? Trends in Ecology and Evolution 23(1) 45-52. doi:10.1016/j.tree.2007.09.013

Yamamoto, S., Humle, T., and Tanaka, M. 2012. Chimpanzees’ flexible targeted helping based on an understanding of conspecifics’ goals PNAS 109 (9) 3588-3592. doi:10.1073/pnas.1108517109