This post covers chapters 12 & 13 from Futuyma and Kirkpatrick’s book on Evolution (2018). The author of this post is Jacqueline Hoppenreijs.
In blog 4, we briefly touched upon one of the advantages of dispersal: it decreases competition for resources with your nearest and dearest. Residency can have multiple outcomes that vary from extremely cooperative, via neutral, to extremely violent.
Cooperation and conflict
When two individuals coexist, one can help the other (“cooperate”) through altruism or mutualism, or do the opposite (“conflict”). Altruism is probably the most renowned example of these interactions, and indicates that one individual sacrifices something to another’s benefit. In the case of mutualism, there are only winners: everyone benefits. Conflict can consist of direct harm, but also indirect actions such as cheating. That’s not to say that a plant and a bee are sitting down for a game of Monopoly, but it might look like pollinators that are not doing their job by bypassing a plant’s anthers and stealing the nectar (Figure 1).
Figure 1. Pollinators robbing plants of their nectar instead of taking the front door and take some pollen with them.
Now, it is tempting to think that those that cheat will win from the others, because they get more resources with less effort, and might thus be able to withstand selection pressure better. But wouldn’t populations come to consist exclusively of cheaters and then collapse? Not quite, so there must be another mechanism at work. That mechanism is not group selection, which was a popular hypothesis until the 1960s and assumed that selection pressure didn’t work on the individual but on the level above. There is, however, a bunch of other mechanisms through which evolution has resulted in cooperation rather than conflict.
A well-known example of non-family cooperation is to find safety or success in numbers. This behaviour can directly increase the chance of not-being-eaten, or increase the chance of catching a prey. Other types of cooperation take more time to pay off, or only work as such because the individuals involved interact repeatedly. The latter means that actions are reciprocal, as opposed to one-off selfish behaviour. This might look like individuals copying each other’s behaviour or copying their own, previous behaviour until that’s not rewarding anymore. A series of cooperative interactions can also change if a helping individual decides to punish a cheater for being selfish. Or even worse, when one individual damages another at a cost for itself! All of this changes when family enters the game. Now, there’s not just profit for yourself and increased direct fitness to be gained, but you can gain by helping others survive. Why? Because they’re likely to be carrying the genes you carry, which means there’s an indirect fitness advantage to be gained! Kin selection like this is advantageous if the relatedness and benefit for the recipient outweigh the costs of the helping individual.
Conflict within family, on the other hand, can take some really extreme forms. That starts at the parental level: we’ve heard about male-male competition in blog five, but males can go as far as to harm the female they’re mating with (called sexually antagonistic competition) in order to increase their own reproductive success. The female praying mantis’ sexual cannibalism barely makes up for all the suffering her sisters go through. A less deadly form of parental conflict is the distribution of parental care to the offspring, which takes time and energy from either or both parents.
Moving onto the next generation, we see that there are parents that kill other individual’s offspring or even their own, which is called infanticide. The former makes sense in that they are making space for their own genes in the population by removing another individual’s genes, and the latter might make sense if there’s resource limitations. The same reasoning might apply when siblings compete for the same resource, which is called siblicide. This touches upon the conflict that exists between parents and their offspring. While one or few offspring are usually selfish as they profit most when they themselves survive, it’s in the interest of the parent to produce as many successful offspring as possible.
After so much dysfunctional family dynamics, we’re moving to animals that have taken family relationships to a whole other level: eusocial animals, such as many bees and termites. Here, we see the positive side of kin selection, in that infertile offspring all increase their indirect fitness by taking care of their mother, the nest, or helping rear their (half) siblings. There’s a, for lack of better words, negative side, too. To keep the relatedness with reproductively successful offspring high, eusocial animals don’t hesitate to disadvantage some family members that are not as related to them as others.
We’ve seen that conflict can exist between individuals because they want to increase their direct or indirect fitness. It also exists within individuals, for example when alleles kill off other alleles during meiosis, leading to segregation distortion, or in the form of transposons, bits of DNA that keep replicating themselves within a genome. Conflicts between mitochondrial and nuclear DNA, which are spread via the maternal line and both parents respectively, also occur widely. It’s in the mitochondrion’s interest to increase female production also when it comes at a cost to male production, so the nuclear DNA has to jump into an arms race to try and undo any changes that might disadvantage male reproduction. The previously mentioned concept of group selection shouldn’t get dismissed entirely, as there are examples of traits evolving in groups that are not necessarily advantageous for its individuals. Groups of individuals with such benevolent traits survive better than groups without them. Benevolence doesn’t always mean pure altruism, by the way: lower production of toxins by plants (Wilson, 1987) and “less-deadliness” of pathogens are well-described examples, too. Especially pathogens want to be careful: they’ll want to spread as much as they can, but there’s little use in killing off the entire host population since that means they’re doomed themselves. Units that depend on vertical transmission, that is reproduction as opposed to horizontal transmission/infection, profit from making their host as successful as possible. Not so surprising then, that they are the driving forces behind some of the major transitions that our planet has seen. The adoption of mitochondria and cyanobacteria have led to successful single-cell organisms. Some of these have, through mechanisms similar to kin and group selection, been able to transform to what we know as multicellular organisms today.
Interactions among species
Neither single-cell nor multicellular organisms exist in a vacuum or only interact with organisms of their own kind. Species interactions are variable over time, in how much species depend on each other and in its effect on the respective participants of the interaction. Examples include a specific insect and plant needing each other a few weeks per year for food and pollination respectively, to a parasite that spends its entire life in its host and depends on it for all its food and the dispersal of its offspring. Close associations are called symbiosis and species that are very attuned to each other, such as corals and zooxanthellae (Stanley & Swart, 1995), are said to be co-evolved. This means that they have undergone reciprocal genetic change because they’ve exercised selection on each other.
Not all of these interactions are positive; becoming the host to a parasite or a prey to a predator usually doesn’t bode well for an individual or a population. Both the enemy and the victim in these relationships need to evolve fast enough and in such a way that they can outperform the other; this can turn into an evolutionary arms race if both are under selection for the same trait. Co-evolution doesn’t have to be unidirectional, as enemy and victim can follow each other to increase and decrease trait values over time.
There are multiple strategies for escaping or outperforming enemies and becoming successful in this arms race. Many are visual-based: by displaying warning signals of being dangerous (aposematism) or mimicking a dangerous species (mimicry). If you’re not a dangerous species yourself (Batesian mimicry), you disadvantage the actually dangerous species by teaching enemies that risks are low. Being dangerous yourself and mimicking a dangerous colleague (Müllerian mimicry), on the other hand, reinforces the defence mechanism. Other strategies can be olfactory or taste-based, such as in plants that produce chemical compounds to protect themselves from herbivory. Throughout history, such successful adjustments have often been seen to lead to quick radiation and many new species with that adjustment, both on the enemy and the victim side. Whether a parasite can successfully infect its victim, can depend on whether the victim can recognise and resist (gene-for-gene model), or whether the parasite matches the victim’s genetic profile (matching allele model). Parasites and pathogens have been shown to adapt relatively quickly to their victims, which means that the above-mentioned shift towards benevolence doesn’t always take place. This depends, amongst others, on the genetic diversity on both the enemy- and victim-side and the way in which the parasite or pathogen is transmitted and received. The latter can be as simple as the difference between vertical and horizontal transmission, but can also have to do with the life cycle of host, pathogen and potential intermediate hosts. In the case of malaria, for example, humans and mosquitoes are hosts of most Plasmodium species, which is complicated as a life cycle in itself. There seems to be at least one Plasmodium species, however, that has found another intermediate host in macaque monkeys, complicating the transmission process further (Centers for Disease Control and Prevention, 2020).
Moving on to the sunnier side of life, there are also a lot of species interactions from which one or both species profit. This mutual exploitation, can, just as with the cheating family or population members that we saw earlier, go awry when one of the participants starts cheating the other. That can lead to immediate sanctions being taken (e.g. lower rewards for the cheater) but can also continue to exist because cheating happens relatively little or there are non-cheaters to compensate potential losses. Even here, one could speak of an arms race where both participants exploit the other as much as they can, and increasingly over time.
Not all species interactions are one-on-one, or as close as some of the previously-mentioned examples. Very often, species interact simply because they use the same space or other resources. If there are limited resources, however, this may lead to competition. One consequence of competition might be that one or more species is driven to extinction. This principle, and the related “Diversity Paradox”, have been the topic of many studies (Simha et al., 2022). A second consequence is the selection-by-lack-of-resources that can cause divergence of resource use and may lead to the evolution of new species. If species that overlap in their use of resources co-occur, they might each display more or different resource-use compared to when they occur alone. This is called ecological release, but shifts to interference competition when one of the species starts to affect its competitor to keep it from using the resource in question. If this is done by a species not native to the area, one can speak of invasive alien species (Figure 2).
Figure 2. Competition for light and space in a Swedish riparian zone by the American Skunk-cabbage (Lysichiton americanus). Photo by Owe Nilsson.
Whether friends or foes, newcomers and the species that already were in a certain place together form an ecological community. The local environment and its inhabitants impose a “filter” on the species that occur in a larger area to decide which of them manage to live in a certain spot. The mechanisms of competition can lead to evolutionary divergence, whereas the environmental filtering can lead to evolutionary convergence. That’s why we often see functionally similar, but taxonomically different species communities in ecologically similar, but geographically different areas. Uniting community ecology and evolutionary biology helps us discern these patterns and the eco-evolutionary past and present.
References
Centers for Disease Control and Prevention. (2020). Malaria. (DPDx – Laboratory Identification of Parasites of Public Health Concern). https://www.cdc.gov/dpdx/malaria/index.html
Futuyma, D., & Kirkpatrick, M. (2018). Evolution (4th ed.). Oxford University Press.
Simha, A., Pardo-De la Hoz, C. J., & Carley, L. N. (2022). Moving beyond the “Diversity Paradox”: The Limitations of Competition-Based Frameworks in Understanding Species Diversity. American Naturalist, 200(1), 89–100. https://doi.org/10.1086/720002
Stanley, G. D., & Swart, P. K. (1995). Evolution of the Coral-Zooxanthenllae Symbiosis During the Triassic: A Geochemical Approach. In Paleobiology (Vol. 21, Issue 2, pp. 179–199).
Wilson, J. B. (1987). Group selection in plant populations. Theoretical and Applied Genetics, 74(4), 493–502. https://doi.org/10.1007/BF00289828