"Seeing is believing." Many evolutionary biologists lean on this old adage to argue for the validity of the evolutionary paradigm. They claim that evolution must be a fact, because we can observe it happening. We can see evolution in action. In fact, Oxford biologist Richard Dawkins makes this very case in his recent book, The Greatest Show on Earth, which presents what Dawkins believes to be the best evidence for biological evolution.
One example of evolution happening in real time is the Long-Term Evolution Experiment (LTEE) conducted by Richard Lenski's group at Michigan State University.
Long Term Evolution Experiment
This study, inaugurated in 1988, has been designed to monitor evolutionary changes in Escherichia coli.The LTEE began with a single cell of E. coli that was used to generate twelve genetically identical lines of cells.
The twelve clones of the parent E. coli cell were separately inoculated into a minimal growth medium that contained low levels of glucose as the only carbon source. After growing overnight, an aliquot of each of the twelve cultures was transferred into fresh growth media. This process has been repeated everyday for about twenty years. Throughout the experiment, aliquots of cells have been frozen every 500 generations. These frozen cells represent a "fossil record" of sorts that can be thawed out and compared to current and other past generations of cells.
The forces of natural selection have been carefully controlled in this experiment. The temperature, pH, nutrients, and oxygen exposure have been constant for the last twenty years. Starvation is the primary challenge facing these cells.
Lenski and coworkers have noted evolutionary changes in the cells, some which have occurred in parallel. For example, all of the populations evolved to increase cell size, grow more efficiently on glucose, and grow more rapidly when transferred to fresh media. These changes make sense given the near starvation conditions of the cells.
Are Evolving Bacteria Evidence for Evolution?
But does the evolution of E. coli in the LTEE validate the evolutionary paradigm? Not necessarily. Just because evolution is routinely observed doesn't mean that evolutionary processes can adequately account for life's origin and history, and the full range of biodiversity.
I like to think of evolutionary changes as falling into one of five categories.
Microevolution refers to changes happening within a species. A textbook example would be the change in wing color of the peppered moth in response to changes in pollution levels in the UK.
Speciation occurs when one species gives rise to a closely related sister species. Take for example the evolution of the finches on the Galapagos Islands from an ancestral finch species that came to this archipelago from South America. Upon arrival this ancestral finch evolved into a variety of species that vary primarily in body size and in beak size and shape. Both microevolution and speciation have been repeatedly observed in nature and, in my opinion, are noncontroversial.
Macroevolution refers to putative changes that require that evolutionary processes have genuine creative potential. Examples include humans evolving from a primate ancestor, whales evolving from a terrestrial wolf-like mammal, and birds evolving from theropods. Whether or not macroevolution has occurred defines the creation/intelligent design/evolution controversy. I am skeptical that macroevolution is a real process that shaped life's history. (Go here, here, and here to read a couple of representative articles that help explain my skepticism.)
Chemical evolution is another type of evolutionary process I'm skeptical about. This term refers to the processes that presumably generated the initial life-forms. According to this model, chemical selection transformed a complex chemical mixture of simple compounds into protocellular entities that further evolved to yield the first true cells. (I would refer readers to the book I coauthored with Hugh Ross, Origins of Life, for a detailed rationale for my skepticism about chemical evolution.)
Microbial evolution helps make sense of the evolutionary changes associated with the LTEE, which don't really fit in any of the previous four categories. These types of transformations involve changes in viruses, bacteria, archaea, and single-celled eukaryotes—changes like the acquisition of antibiotic resistance in bacteria, the ability of viruses to hop from one host to another (such as SARS and HIV), and the emergence of drug-resistant strains of the malaria parasites. Microbial evolution would also include horizontal gene transfer between microbes, which accounts for the evolution of pathogenic bacteria from non-pathogenic strains (like E. coli O157:H7). Again, I don't find microbial evolution particularly controversial. A preponderance of evidence exists for it, including the LTEE.
In a sense, it is not surprising that single-celled microbes and viruses can evolve given their extremely large population sizes and capacity to take up large pieces of DNA from their surroundings and incorporate it into their genomes.
Just because scientists have observed microevolution, speciation, and microbial evolution doesn't mean that macroevolution is necessarily valid. The scale of the biological changes that take place in microevolution and speciation are radically different than those that presumably take place in macroevolution. As is true in other areas of science, processes happening at one level can't automatically be extrapolated to other levels without proper validation. In my opinion, this validation doesn't exist for macroevolutionary changes. As for microbial changes, it is hard to maintain that what is true for viruses and single-celled prokaryotic organisms is valid for complex, multicellular eukaryotes. In fact, many biologists don't even think that the concept of a species applies to bacteria and archaea in the same way it applies to complex, multicellular organisms, if it applies at all.
The LTEE is a remarkable scientific study that has and will continue to reveal important understanding about microbial evolution. But I don't think that it can be counted as overarching evidence for the evolutionary paradigm. It is evidence that microbes can evolve and nothing more. Next week I will discuss recent work associated with the LTEE that actually raises broader questions about molecular evolution and, consequently, the evolutionary framework.
|Part 1 | Part 2|