I love science metaphors. I try to connect the material I learn in my classes as a science major to everyday life, and sometimes I take things too far. My little sister called me a few weeks ago to discuss a project for her ninth grade English class. She was to give a speech on whether people have a moral obligation to make sacrifices for the greater good. She had chosen to argue yes, and began enumerating a few of the examples she planned to cite. I exclaimed, “You should talk about plasmids!”
I went on to explain that the goal of a bacterium is to become two bacteria, and that it takes a lot of energy and nutrients to do so. However, if a bacterium senses that another bacterium is nearby – even if it’s a member of a different species – it will build a structure called a pilus and transfer a small piece of DNA. This is always a one-way transfer, and the piece of DNA is called a plasmid. Plasmids often encode information that helps bacteria survive in a particular environment, such as genes for antibiotic resistance. From the perspective of the donor bacterium, this can be viewed as an altruistic act: it took energy to make that piece of DNA, energy that it could have instead put toward its goal of dividing into two bacteria, and the donor received nothing in return. And yet bacteria have existed for about 3 billion years, so perhaps herein lies a lesson from evolution.
My sister said that she liked the idea and would incorporate it into her argument, and she left to work on her speech. Left to myself, I continued to ponder the notion of bacterial altruism. If only humans could be like bacteria and share with one another so willingly and selflessly, I thought. But as I examined it further, the idea began to break down.
Evolution selects for traits that allow an organism to better survive and reproduce. So why would evolution select for the tendency to share plasmids when this trait does nothing to help the donor? The only common thread is the plasmid. Plasmids often contain the information necessary for their own reproduction and transfer, so conjugation – one-way plasmid transfer – mainly ensures widespread proliferation of the plasmid. In this view, the plasmid is a parasite!
I couldn’t shake this notion of the plasmid-as-parasite. I thought I had found an example of true altruism, and now it was cruelly wretched away from me; bacteria were only hosts exploited in the process of plasmid proliferation. What if other instances of seeming altruism only masked hidden exploitation? Would exploitation by others be my own fate as well, if I endeavored to do good?
Then I remembered something I’d read in my Cell Biology textbook. One view of primordial eukaryotic cells – the ancestors of modern animal cells – is that they were predators that lived by capturing and eating smaller cells. Plant cells differ from animal cells in that they have chloroplasts (the structures that give plants their green color and allow them to perform photosynthesis) and a thick cell wall. Plant cells can therefore be thought of as primordial eukaryotic cells that transitioned from hunting to farming: they settled down, built cell walls, and were content to farm their chloroplasts.
Carrying this metaphor to its logical extreme, humans farm plants, which in turn farm their chloroplasts! The silliness of this viewpoint cheered me up immensely. So what if plasmids were parasites? I began to feel silly for my distress, for taking the natural course of evolution so personally. I was guilty, not for the first time, of trying to take a metaphor too far.
Strictly speaking, ‘parasite’ wasn’t even the best term. Symbioses – interactions between unlike organisms – are often so complex that it becomes difficult to qualify who contributes more and who benefits more, just as with human relationships. After my reflections, I recognized the futility of trying to understand nature through human moral principles.
But I think I’ll wait to tell my sister until after her speech.
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