Individuality, Evolution, and Dancing
What is the unit of evolution, the level of life upon which natural selection acts? A geneticist would say the gene; Charles Darwin saw it in the unique populations on the Galapagos. On Friday, Leticia Aviles, associate professor of zoology at the University of British Columbia, singled out the individual as dividing the cellular from the group level. ?But what an individual is depends on one's frame of reference,?
What is the unit of evolution, the level of life upon which natural selection acts? A geneticist would say the gene; Charles Darwin saw it in the unique populations on the Galapagos. On Friday, Leticia Aviles, associate professor of zoology at the University of British Columbia, singled out the individual as dividing the cellular from the group level. ?But what an individual is depends on one's frame of reference,? she said, and the level at which natural selection acts remains an unresolved issue.
Added Sean Rice, an associate professor of ecology and evolutionary and
biology at Yale University, in his commentary, ?Levels at which selection acts is not a philosophical question, but is a property of the biological system. You can't correctly represent evolution if you don't recognize the right level of selection.? Aviles offered an example of selection at the population level in social spiders: when the sex ratio veers from 1:1, selection begins to favor the rarer sex until the ratio is restored.
But defining the individual may not be straightforward. For unicellular life, clearly cells are individuals. But is a eukaryotic cell a part of a larger system, an individual entity, or a community amassed from endosymbionts acquired long ago? To consider these questions, the agenda returned to the idea of individuality to close the conference. Richard Michod, head of ecology and evolutionary biology at the University of Arizona, who studies unicellular and multicellular algae such as Chlamydomonas and Volvox, began by defining the term. ?The Latin root of individual is indivisible. You can't chop an individual organism into parts and still maintain the function of the individual.?
How, then, do we classify a eukaryotic cell, whose heritage has been to incorporate simpler cells and earn their dependency by borrowing some of their genes, eventually forming a host cell-organelle relationship? Lynn Margulies, distinguished professor of geosciences at the University of Massachusetts, Amherst, presented a panorama of endosymbiosis with films and animations depicting various cells swallowing others, which she contends is fairly common and a clear and powerful route to evolutionary change other than natural selection.
According to the theory, aerobic bacteria begat mitochondria, cyanobacteria begat chloroplasts, and various protein assemblies evolved into the motile appendages of some eukaryotic cells, such as cilia and sperm tails. ?Modern nucleated cells began as different kinds of bacteria. The cell is an integrated bacterial community that happened before the Cambrian-Precambrian boundary and before the Varangerian Ice age of 600 million years ago,? Margulies explained.
Margulies and others have discovered many ?symbiogenic? associations that may have led to the amalgamation that is a eukaryotic cell, and are seeking the founding eukaryote. She distinguished symbiosis, which is an ecological phenomenon and the long term physical association of members of different species, from symbiogenesis, which is the evolutionary consequence of symbiosis. Symbiogenesis forms a new type of organism that has acquired a novel trait from the symbiosis.
In her commentary, Colleen Cavanaugh, the Edward C. Jeffrey Professor of Biology at Harvard University, described animal-bacterial symbioses in the deep-sea thermal vents that she studies. ?Symbiosis clearly played a role in the radiation of these animals,? she said. The phenomenon seems universal. ?I don't want to get into personal body flora, but we are not alone,? she added. If symbiosis is indeed the precursor to symbiogenesis, then evolution certainly has a great deal of raw material for this route.
I was thinking about the biological meaning of individuality as I walked to
the farewell party given by the mayor and residents of San Cristobal after the last talk. On the town square, ringed by the 150 attendees, schoolchildren danced to the captivating music of local bands. Then each child drew a scientist into the center of the circle, and soon nearly everyone was dancing. I watched the various individuals and couples, trying to match faces to talks, then joined in. And a remarkable thing happened. We all made eye contact, and quite suddenly everyone joined hands and we formed a snake-like, undulating being. Like single cells aggregating into a colony, so too did we lucky attendees at this first World Summit on Evolution become one. It was the perfect metaphor to end a perfect meeting.
The experience has been unique in many ways, this Woodstock of scientific meetings. But it won't take a generation to have another - the second World Summit on Evolution is already being planned. The focus: communicating what we know about evolution to the public.
Ricki Lewis, embedded with the biologists on San Cristobal