Sunday, July 11, 2010
One of the more interesting problems in evolution is how we went from single celled organisms to multicellular organisms.
It's an interesting dilemma. After all, one celled organisms are perfectly fine. They evolve well. They manage to be the largest biomass on the planet. Why did some individuals decide to give up their autonomy to join together?
Remember, evolution is always working. Therefore, there had to be an advantageous reason for these organisms to band together. The problem is, of course, that the non-reproductive cells in an organism give up their reproductive rights-- which seems counter-intuitive for a single celled organism to do.
Sergey Gavrilets has published a mathematical model in PLoS of how such a differentiation can occur. He used as his model the algae Volvox, a primitive colonial algae. He concentrated on two genes, one for viability and one for fertility. Viability (v) was considered to be contribution towards survival of the colony and fertility (f) was defined to be the ability to found a new colony.
Since single cells found colonies all the time and still retain their individuality, we can use this as a starting point. Both v and f contribute to the proliferation of colonies but neither can be maximized at the same time. In his model individual selection favors larger values of f and colony level selection favors larger values of v. Mutation can affect the value of f or v. For individuals with mutations favoring f in the individual cell, the viability of the colony is decreased. Similarly, individuals with mutations favoring v cause a loss of colony fertility.
Volvox has a division of labor between two cell types: germ and soma, i.e., reproduction and the colony operation. The differentiation between the two is regulated by three types of genes and has an environmental factor. Gavrilets posulated similar genetic processes in his model such that the f and v values could be suppressed or initiated with respect to one another. He was able to control the regulation processes and see the results.
The coupling of the f and v regulation resulted fairly quickly (about 1 million generations) in a germ/soma differentiation very similar to Volvox. That is, the germ cells were the only ones that reproduced and the soma cells were exclusively producing material for colony consumption. Depending on the degree of f and v coupling he could create populations of varying differentiation. The degree of differentiation reflected a range of plasticity, the ability of a germ or soma cell to do the other type's job. Plasticity directly reflect the fitness cost. If that cost was high, germ/soma differentiation proceeded to full differentiation. If that cost was low, the differentiation remained limited or perhaps non-existent.
What's interesting here is that these are particularly simple numerical models. What is necessary is the coupling of only two traits (f and v) and selection towards a particular outcome. The question of where multicullular organisms came from gets turned on its head: why aren't all organisms multicellular.
Hazarding a guess, I would think that there are two areas where single celled organisms might not evolve into multicellular organisms. One is where there is no real coupling between fertility and viability. Bacteria organize quite well into colonies. However, the viability of the colony doesn't have to have a negative effect on the ability of individuals to colonize.
The other is the cost of plasticity. If there's a high cost to retaining plasticity (possibly as a result of the coupling of f and v) then, like anything else in evolution, plasticity is jettisoned. However, if there is a positive attribute to plasticity than it would be retained. One can see that plasticity is highly advantageous in many single celled animals. Thus, multicellular systems can evolve easily when the circumstances are right-- which they can often be-- but won't evolve from organisms which don't have the right repertoire.
Original article here.