Well, I had to write something about evolution on Darwin's 200th birth anniversary coming up February 12. I am going to be writing two posts on evolution and evolutionary patterns that may occur at a biological level of organization below and above that of an individual. Darwin didn't address evolution at these levels of biological organization. I am not taking pot shots at any perceived inadequacies in Darwin's thinking but rather trying to highlight how successful his theory really is.Darwin's contribution is huge because his theory of natural selection is a general explanation that is scalable to all levels of life. It provides us with the intellectual tools to expand the theory beyond what he used it for and explain life's patterns at the level of cells, individuals and species. You have to say that is a pretty powerful idea. As Richard Dawkins put it, his theory has an extraordinarily high explanatory bang for the buck!
Alright so part 1 is about evolution below the level of the individual.
Darwin didn't know anything about genes or even what causes variation. So his theory of evolution didn't address evolution at the level of molecules or cells. His great obsession was to explain how diversity arises and how organisms acquire adaptations or rather evolve a state of adaptedness to their environment over time. To this effect he argued that natural selection acts at the level of the individual. Birds may evolve smaller or larger beaks over time depending of the food available, butterflies may evolve patterns mimicking a poisonous relative, peppered moths may evolve a dark color that acts as a camouflage. Individuals vary and natural selection filters this variation, retaining traits that help individuals survive and reproduce and eliminating traits that are harmful.
But natural selection doesn't only act at the level of the organism. It can act on any entities which shows certain properties. If entities vary in certain traits, if these traits are heritable and if these traits affect "fitness" i.e. they enable one variant to reproduce more than the other then natural selection is off and running. In the natural world these conditions are most familiarly met by whole organisms but in principal they can be met by cells or genes within cells.
Our cells contains two copies of each gene. During cell reproduction a fair system would ensure that each gene has a 50% chance of being passed down to the next generation. But there is scope of this system being subverted. If a mutant gene acquires the ability to increase its chance of being passed down to more than 50% by copying itself and proliferating inside the cell or by killing its sister gene, natural selection will favor it. Genes might then engage in a war among themselves to increase their own reproduction at the expense of the body. In fact complex bodies will not evolve if such genomic conflict is the norm. Mark Ridley has written an engrossing book on how evolution has come up with ingenious solutions to minimize this conflict enabling complex life to emerge.
Still evolution has not done a perfect job of prevention. Whole organisms - and I am talking about multicellular entities- are made up of populations of cells. During development these cells divide and establish cell lineages that perform different functions in the body. Each cell division comes with a probability of a copying error as 4 billion bases get read and a copy of the genome is assembled. Over time as somatic cells divide differences might arise between cells and natural selection might then act on those differences.
The most well know example of this is cancer, where a mutant somatic cell arises and increases in frequency relative to the "normal" cell type. This happens even though the change is harmful to the body. The birth and death rate of cells is a faster process than the birth and death rates of individuals and so natural selection will favor any mutant cell that concentrates on its own reproduction even if it is to the detriment of the body.
Evolution of cells within an organism that confer benefits to the individual also occurs and a recent paper- Evolution of Highly Polymorphic T Cell Populations in Siblings with the Wiskott-Aldrich Syndrome - in PLoS One describes this. The paper reports on a case of two brothers who have both inherited a disease causing allele. But the researchers found that over time in this cell population there have been multiple corrective somatic mutations. These corrective mutations confer an advantage to the host cell and consequently it occurs at higher frequency in that cell population than the diseased cell type. The researchers found that corrective mutants have been positively selected for and the diseased cells selected against.
The development, function and life cycle of somatic cells is part of a great co-operative venture that make bodies work. But population level evolutionary processes can occur in these cell lines within an individual. This is evolution, though not in a form we are familiar with.
By familiar evolution I mean a form where one type of individual is favored over another. That is what Darwin tried to explain. Darwin didn't know about the genes part. He understood something was passed from parent to child and his theory of natural selection worked just as well with this notion and explained the origin of adaptations. We know now that those adaptations develop and change over time through the flow of genes from the passage of germ line cells.
Somatic cell genes don't flow across generational times of whole organisms. The evolutionary histories of somatic cell lineages are short lived and are terminated as these cell lines go extinct with the death of the organism.
But just when you start getting comfortable with the idea of a well established evolutionary pattern, nature or rather evolution itself finds a way to get around it. A facial tumor that has spread and nearly decimated the Tasmanian devil - a marsupial carnivore - shows that somatic cells can sometimes develop an evolutionary history that can extend beyond the lifetime of an individual. Olivia Judson wrote a nice essay about this tumor and its effect on the Tasmanian Devils. The cancerous cell initially arose through a mutation. Devils are aggressive creatures and they often bite each other especially during mating. The cancer cells graft themselves on facial tissue of the other individual and grow and spread.The tumor gene thus spread from animal to animal by bypassing the normal channels of reproduction.
The mutant gene that produced this cancer is a good example of a selfish gene. This is a concept made famous by Richard Dawkins and represents a way of thinking about natural selection. A selfish gene is a mutant gene that enhances its own reproduction relative to the other copy of the gene or other genes in the cell often to the detriment of the body. So it is not just any old mutation. Most copying accidents or mutations harm the gene as well as the body. But "selfish genes" are mutations that natural selection favors at the level of the gene or the cell and opposes at the level of the individual.
So, selection is operating at two levels here. At a lower level cells which contain the mutant gene are fitter than cells that don't contain this gene. But at a higher level individuals that don't contain this mutant gene are fitter than individuals that do. The gene spreads even though it is harmful to the body since cellular reproduction is faster than the generation times of individuals.
The cancer is spreading rapidly in the Tasmanian Devils. Their populations have crashed in some areas by nearly 90% since the inception of this infectious cancer. It is possible that the devils may become extinct.
Natural selection operates on the immediate advantage. It has no long terms plans.
Our bodies are collections of tightly integrated cells. This fantastic example of co-operation has evolved through natural selection over hundreds of millions of years. We don't think of somatic cells as a separate life form. The cells that make up our bodies are us. But occasionally as in the case of the facial cancer in the Tasmanian devils the us can morph into the other. A cell which was part of the Tasmanian devil is evolving into a parasite.
Natural selection is an unsentimental process. It doesn't care for a billion years of co-operative evolution. If the opportunity arises, cells will just as easily rebel against our elaborate body politic and develop lives of their own.
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Tomorrow's post will be on the effect of species selection or species sorting on biotic recovery during and after mass extinctions.
Excellent post! I'll have to share it with my Evolution students.
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