A primer on symbiosis
4.9 (6 ratings)
Course Ratings are calculated from individual students’ ratings and a variety of other signals, like age of rating and reliability, to ensure that they reflect course quality fairly and accurately.
31 students enrolled

A primer on symbiosis

Learn what symbiosis is (and isn't), how it works and how it evolves.
4.9 (6 ratings)
Course Ratings are calculated from individual students’ ratings and a variety of other signals, like age of rating and reliability, to ensure that they reflect course quality fairly and accurately.
31 students enrolled
Created by Scott Turner
Last updated 5/2019
English [Auto]
Current price: $13.99 Original price: $19.99 Discount: 30% off
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This course includes
  • 1.5 hours on-demand video
  • 9 downloadable resources
  • Full lifetime access
  • Access on mobile and TV
  • Certificate of Completion
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What you'll learn
  • What symbiosis is
  • The categories of symbiosis
  • How the lichen symbiosis actually works
  • The physiological underpinnings of symbiosis
  • What coral bleaching is and why not to worry about it much
  • The selective forces operating to enforce or disrupt symbiosis
  • How mutualism and parasitism evolves
  • A basic knowledge of biology (e.g. an introductory biology course)

  This course provides a brief introduction to the phenomenon of symbiosis, multiple organisms living together in an intimate relationship. Symbiosis can include symbiotic organisms, like lichens, but also including complex ecological association, as in microbial mat communities, and the extraordinarily diverse associations between individuals and various microbial communities, as in the intestine, skin and genitals. An understanding of the broad phenomenon of symbiosis also opens the door to radically new ways to think about life and its evolution. 

  The course is divided into six short lectures: Introduction to symbiosis; The lichen symbiosis; Physiology of symbiosis; Dynamics of symbiosis; The evolution of mutualism; and Symbiogenesis. It will take you about 90 minutes to watch them all. Please take a look at the promo video for a fuller description. 

  This course is aimed at anyone who has an interest in this phenomenon, from people with little biological training, to undergraduate college students who want an entry point into this fascinating subject, to professionals and graduate students looking for a quick brush-up on the subject. 

  When you’ve completed the course, my goal is for you to have learned something about symbiosis, and that you will want to go out and learn more! 

Who this course is for:
  • Undergraduate students looking for an introduction to symbiosis
  • Graduate students and professionals looking for a brush-up on symbiosis
  • Anyone who is curious to know more!
Course content
Expand all 8 lectures 01:22:16
+ Course intro and organization
2 lectures 04:41

This document is the syllabus for the course. It outlines titles, lengths and descriptions for the videos, as well as helpful keywords you can use as search tokens to find out more.

Preview 02:03

This video outlines the course topics and tells you a little about the course organization. You can also get the course syllabus and reading list here.

Preview 02:38
+ A primer on symbiosis
6 lectures 01:17:35

In this lecture, I will introduce you to the “classical” concept of symbiosis, established by the 19th century scientist Anton de Bary. This scheme categorizes symbiosis into three broad categories: mutualism, where both partners benefit, parastitism, where one partner benefits at the expense of the other, and commensalism, where neither partner benefits or is harmed. Despite this scheme being very popular and widespread, it is, in many ways, a distorted view of the phenomenon of symbiosis.


Preview 07:18

Here, we will delve into the details of a “classical” symbiotic association: the mutualism between fungi and photosynthetic micro-organisms that we know as the lichen. The classical story of the lichen is that it is a mutualism. The photosynthetic partners provide carbohydrates to the fungi and the fungi provide a moist shelter for the photosynthesizers. A detailed look at how lichen function reveals that the supposed mutualism looks more like a parasitism. This underscores the difficulty of trying to fit symbiosis into neat categories like mutulism or parasitism.

The lichen symbiosis

Symbiosis is better defined by the physiological interaction between the symbiotic partners than by trying to jam an association into a neat category. Symbiosis is ultimately a manipulation of matter and energy flows through their partners, and this leads us to a definition of the “symbiotic organism”: an association of different organisms that is like an organism physiologically. Symbiosis is sustained by “open-loop” flows of matter and “closed loop” flows. “Open loop” flow is illustrated by the cycling of nitrogen through microbial mat communities. “Closed loop” flow is illustrated by the cycling of nitrogen between coral polyps and their photosynthetic symbionts, their zooxanthellae.

Physiology of symbiosis

A physiological conception of symbiosis allows us to characterize symbiosis by the effect of the partnership on metabolism of the respective partners. This makes symbiosis not a matter of categories (i.e. mutualism, parasitism, commensalism), but a balance of physiological costs and benefits. Because these can change depending upon circumstance, this means that the nature of a symbiotic association is dynamic, capable of shifting from one state to another. This is illustrated by the phenomenon of coral bleaching, where the coral polyps expel their photosynthetic partners en masse, and try to re-establish a new suite of symbiotic partnerships.

Dynamics of symbiosis

Symbiosis is not just a physiological association, it is an evolutionary one. As such, the formation, dynamics, permanence and function of a symbiosis has been subject to natural selection. The “classical” conception of symbiosis imagined that symbiotic associations followed a predictable arc from parasitism to mutualism or commensalism as they evolved, the argument being that it was in the selective interests of all partners not to kill or disadvantage the other. Parasites would, therefore, always evolve toward “accommodation” with their hosts. This neat story not only misunderstands the nature of Darwinian natural selection, it obscures the complex selective games that play out in a symbiotic association. This is illustrated by the symbiosis between fig trees and fig wasps, which, depending upon circumstance and strain, can evolve either toward mutualism or parasitism.

Evolution of mutualism

Symbiosis has long been presented as a kind of “biological curiosity”, worthy of attention because they are supposedly rare and wonderful to behold. However, symbiosis is emerging as a fundamental property of life, with symbiosis playing a vital role in all aspects of life. This undermines not only our standard conception of the organism, it also undermines many of the presuppositions and tenets of modern Darwinism. This has led to the emergence of a radically new theory of evolution known as symbiogenesis, in which evolution is driven not by mutation and selection of genes, but by evolving associations of entire genomes. Symbiogenesis not only accounts for the ubiquity of symbiosis in the biosphere, it also provides a superior explanation for the major evolutionary events of life on Earth. I illustrate this with Lynn Margulis’ conception of the symbiogenic evolution of the eukaryotic cell.