Tuesday, 27 September 2011

Tim Tyler: Seven steps to understanding evolution

Hi! I'm Tim Tyler and this is a video about some of the domains within biology to which evolutionary theory can be applied.

Evolution is a powerful theory with a broad range of applications to biological systems. For some reason, the application of evolutionary theory to many of these domains is widely unappreciated. So, this video contains a brief primer, to help interested parties get up to speed with some of the basic applications of evolutionary theory.

Evolutionary theory applies most directly to systems which involve copying. Essentially that means biology, since biology is the science of life, and life is that which persists via copying. Biological evolution is the domain of evolutionary theory which people tend to be most familar with. It typically deals with the evolution of plants, animals, fungi and microorganisms over millions of years. This is an umbrella category, which includes all the other processes discussed here. However, within biology, there are many sub-processes which have their own evolutionary dynamics - and those are what we will be looking at here. The approach taken in this video may be reminiscent to readers of the material in Gary Cziko's book "Without Miracles - Universal Selection Theory and the Second Darwinian Revolution". Seven of the main areas within biology which can be usefully be modelled by evolutionary theory are:

  • Immunology
  • Ontogeny
  • Neurobiology
  • Psychology
  • Memetics
  • Cybernetics
We will briefly cover each of these in turn - starting with the simplest area to understand:

  • Immunological evolution - In order to deal with rapidly evolving parasites, vertebrates employ an adaptive immune system which works using the principles of evolutionary theory. This uses a process known as somatic hypermutation to rapidly generate adaptive variants. It employs genetic recombination to allow a small number of genes to generate a large number of different antigen receptors which are expressed on the surfece of lymphocytes. Lymphocytes are then tested and successful ones are bred from. Lymphocytes can also produce memory cells that mediate long-term immunity. The result is a minature evolutionary process within the organism that helps it to adapt in response to rapidly-changing parasites. To most people, this is similar to the type of evolution they are familiar with - except that mutations are performed deliberately, and since there are mechanisms for actively making them, they are more likely to be somewhat biased towards being adaptive.
  • Ontogenetic evolution - In multicellular creatures, development is an adaptive evolutionary process. Tree roots grow around rocks, their trunks grow around obstacles and their branches seek out holes in the canopy. If you have pulled plants out of pots, you will see that there is often an adaptive fit between the roots and their containers. However the evolutionary dynamics underlying this kind of adaptation during development often remain unappreciated. It is obvious that development involves copying on a cellular level. However, evolution requires variation - and since all the cells are clones of each other what is not always clear is where variation comes in. However, if you look at the cell phenotypes, it is obvious that some kind of variation is coming from somewhere - since liver cells look totally different from muscle spindle cells, which look totally different from neurons. The variation arises not in the DNA of the cells involved, but rather from their environment and it persists via a form of environmental inheritance. This inheritance includes location, chemical concentartions, cell neighbours and other local feaures. To give an example of how such processes work, consider the evolution of plant root tips. Plant roots have identifiable tips. The tips divide, which represents a form of reproduction for them. The offspring tips inherit their physical location from their parent - and this location varies from tip to tip. Location represents the main form of evolving heritable information in this system. The plant allocates more resources to root tips in locations with moisture and yielding soil, and withdraws resources from those tips in locations that are too dry or rocky. Those resources are then used to fuel root tip reproduction - resulting in differental reproductive success of root tips, and root tip evolution. This generates an adaptive fit to the environment in which the roots find themselves.
  • Neurobiological evolution - Brains exhibit several interesting forms of evolution as they are used. Copying processes are ubiquitous in the brain. Since axons branch liberally any signal sent down them is copied many times - there is also variation and differential reproductive success of these signals. This results in neural spike train evolution. Also both axons and dentrites have a dynamic branching tree-like structure. The argument I gave explaining why plant roots evolve also applies to growing neurite tips. Neurons and glial cells also reproduce and undergo selection - as is documented in a book called "Neural Darwinism". There are other forms of low-level competition in the mind as well: axon tips compete for attachment points, synapses compete for neurotransmitters, and so on.
  • Psychological evolution At a higher level in the mind a virtual world is implemented - containing thoughts, ideas and memories. In this virtual world more copying, variation and differental reproductive success takes place. Copying takes place when memories are recalled - since this adds a memory of recalling the original memory. Copying takes place when actions are repeated or when they are rehearsed. Ideas and action plans are also copied. Lastly the mind contains a model of the world which is used to make predictions about the future. The elements that make up this model also evolve. Psychological evolution, can feature directed mutations and intelligent design. The psychologist, B. F. Skinner was among those how appreciated the evolutionary side of mental development. He wedded his theory of learning to Darwin's theory of evolution, and talked about the "extinction" of learned behaviours.
  • Memetic evolution - Memes also evolve over time in social spaces - resulting in social and cultural evolution. Like psychological evolution, memetic evolution features directed mutations and intelligent design. I discuss memetic evolution in a lot more depth in my book on Memetics - which is now available. Memetic evolution has snowballed, producing accumulating innovation and technological progress - which is in the process of leading towards...
  • Cybernetic evolution - Machines evolve too. Today machines mostly coevolve relatively slowly with humans - but the evolution of computer viruses, genetic algorithms and memetic algorithms gives us a taste of what future cybernetic evolution is likely to hold, once humans get eliminated from the innovation loop.

These, then, are seven different domains within biology to which evolutionary theory applies. Together they illustrate some of the breadth of the systems to which evolutionary theory can usefully be applied.


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