There is a really awful misrepresentation of evolution in a New York Times obituary for Lynn Margulis, the biologist who proposed that the eukaryotic cell originated through a symbiotic merger of two free living bacterial cells. Life on earth is made of up one of two cell types, Prokaryotes or Eukaryotes. Eukaryotes are more complex. They contain their DNA in a nucleus and also possess organelles like mitochondria and chloroplasts which generate energy for cell metabolism.
At one time in the past, perhaps about a billion years ago, one prokaryotic bacterial cell engulfed another prokaryotic bacterial cell type. Instead of the engulfed cell being destroyed, the two evolved a partnership. The engulfed cell was perhaps good at certain tasks like burning food in the presence of oxygen. Over time this cell transferred most of its genes into the genome of the host cell and retained only a few needed for the specialized task of producing energy for cell metabolism. It evolved into the mitochondria. Lynn Margulis was ridiculed for proposing this but over time biologists have accepted this theory of the symbiotic origin of eukaryotes.
Here is a snippet from the obituary by Bruce Weber:
I don't even know where to begin.
If there is any such thing as a neo-Darwinist "belief" then the mechanism of evolution in question is natural selection not random mutation. Mutations generate the variability on which natural selection acts.
And symbiosis doesn't underscore the idea that evolution began at the level of micro-organisms long before it would be visible at the level of species. What does that even mean? Very early life was a world of micro-organisms divided into many species and they evolved through a combination of natural selection and random genetic drift acting on random mutations. If a population of micro-organisms evolve then the species they belong to obviously also evolves! The micro-organisms may be of symbiotic origin or may not. The type of entity i.e. symbiotic or non-symbiotic has nothing to do with the mechanisms of evolution or whether evolution occurs through variations between individuals or variations between species.
Another misconception I have often heard is that symbiosis means that complex cells and complexity in general evolved through a co-operative venture. So evolution is less about competition through natural selection and more about cooperation.
Again, this is a misunderstanding about the nature of evolution.
Sure, at one point in time two cells merged to form one cell...... But then what?
After the merger, there was just one or few of these novel cells in a population of other cell types. The merged cell was more efficient at extracting energy from the environment and reproduced more than other cells. Every round of reproduction threw out varieties of merged cells. Out of these, the more efficient symbiotic cells out-reproduced not just other non-symbiotic cell types but also slightly less efficient symbiotic cells. The symbiotic arrangement became more refined and over time the novel cell type proliferated and became the dominant cell type in that particular ecologic niche. Symbiosis and the cell structure and functionality that is recognized as eukaryote didn't instantly originate through a merger but evolved through natural selection. Eukaryotes became a common cell type by out-competing less efficient versions of itself and other non-symbiotic cell types. Cooperative ventures in life also evolve through competition.
So symbiosis doesn't undermine natural selection. Yes it can create novelty by bringing together different components within one individual, but that novelty if beneficial then evolves. It gets modified and refined and changes through natural selection and genetic drift.
At one time in the past, perhaps about a billion years ago, one prokaryotic bacterial cell engulfed another prokaryotic bacterial cell type. Instead of the engulfed cell being destroyed, the two evolved a partnership. The engulfed cell was perhaps good at certain tasks like burning food in the presence of oxygen. Over time this cell transferred most of its genes into the genome of the host cell and retained only a few needed for the specialized task of producing energy for cell metabolism. It evolved into the mitochondria. Lynn Margulis was ridiculed for proposing this but over time biologists have accepted this theory of the symbiotic origin of eukaryotes.
Here is a snippet from the obituary by Bruce Weber:
The hypothesis was a direct challenge to the prevailing neo-Darwinist
belief that the primary evolutionary mechanism was random mutation.
Rather, Dr. Margulis argued that a more important mechanism was
symbiosis; that is, evolution is a function of organisms that are
mutually beneficial growing together to become one and reproducing. The
theory undermined significant precepts of the study of evolution,
underscoring the idea that evolution began at the level of
micro-organisms long before it would be visible at the level of species.
I don't even know where to begin.
If there is any such thing as a neo-Darwinist "belief" then the mechanism of evolution in question is natural selection not random mutation. Mutations generate the variability on which natural selection acts.
And symbiosis doesn't underscore the idea that evolution began at the level of micro-organisms long before it would be visible at the level of species. What does that even mean? Very early life was a world of micro-organisms divided into many species and they evolved through a combination of natural selection and random genetic drift acting on random mutations. If a population of micro-organisms evolve then the species they belong to obviously also evolves! The micro-organisms may be of symbiotic origin or may not. The type of entity i.e. symbiotic or non-symbiotic has nothing to do with the mechanisms of evolution or whether evolution occurs through variations between individuals or variations between species.
Another misconception I have often heard is that symbiosis means that complex cells and complexity in general evolved through a co-operative venture. So evolution is less about competition through natural selection and more about cooperation.
Again, this is a misunderstanding about the nature of evolution.
Sure, at one point in time two cells merged to form one cell...... But then what?
After the merger, there was just one or few of these novel cells in a population of other cell types. The merged cell was more efficient at extracting energy from the environment and reproduced more than other cells. Every round of reproduction threw out varieties of merged cells. Out of these, the more efficient symbiotic cells out-reproduced not just other non-symbiotic cell types but also slightly less efficient symbiotic cells. The symbiotic arrangement became more refined and over time the novel cell type proliferated and became the dominant cell type in that particular ecologic niche. Symbiosis and the cell structure and functionality that is recognized as eukaryote didn't instantly originate through a merger but evolved through natural selection. Eukaryotes became a common cell type by out-competing less efficient versions of itself and other non-symbiotic cell types. Cooperative ventures in life also evolve through competition.
So symbiosis doesn't undermine natural selection. Yes it can create novelty by bringing together different components within one individual, but that novelty if beneficial then evolves. It gets modified and refined and changes through natural selection and genetic drift.
No comments:
Post a Comment