Subject: Increased morphological complexity as the result of natural selection Newsgroups: talk.origins Date: July 11, 2000 Message-ID: slrn8mmj3g.k3s.adhar@elaine21.Stanford.EDU
The evidence that historical evolution has occurred - that organisms have changed radically over the course of hundreds of millions of years since the first life on earth - comes from a combination of examination of the fossil record and studies of the comparative morphology and genetics and the geographical distribution of modern organisms. Most biologists currently find this evidence to be overwhelmingly in favor of not only the historical occurrence of macroevolution but the common descent of all known life. On a smaller scale, including microevolution and low-level macroevolution (e.g. speciation), evolution is observed to continue today.
Scientists have proposed that the mechanisms of historical evolution are most likely to be those that are observable in the present day. Known mechanisms include mutation, selection, and genetic drift.
Steve, who posts with the username "AVE," has asked for examples of observed (present day) evolution which lead to "increases in morphology" by which he presumably means increased morphological complexity. However, the fact that historical evolution has occurred is based on observations about past life (through fossils and comparative analysis of modern organisms) and does not depend upon large-scale changes being observed today. Indeed, given the time scale of historical evolution - in the thousands, millions, and hundreds of millions of years, a few tens or hundreds of years of evolution should be relatively small in effect. The question that remains is if we have correctly identified the mechanisms of evolution. The known mechanisms are capable of producing change on a relatively small scale in the present day. In the absence of other mechanisms, it seems reasonable to extrapolate them to the past. On a much large time scale, these mechanisms should produce much larger changes.
But what if there is some hard limit to the amount or type of change which can accumulate under the known mechanisms of evolution? Steve's request for observation of increased morphology might be seen as a challenge. Can mutation, selection, and drift account for this kind of change?
Many responses to Steve's requests have tried to nail down the definition of morphology. If morphology can be shown to "increase" on even the smallest level, then over larger time periods, there would be no qualitative barrier to larger changes. Thus, while Steve would like to see examples of flagellae evolving in lab populations, would not observed evolution of other transmembrane proteins also qualify as "increased morphology"? Steve has not been willing to allow such changes to be included as morphological thus far. Therefore, debate continues about whether he has set an arbitrary minimum for the level of admissable change in order to exclude cases which should reasonably show that mutation and selection can account for increases in morphological complexity.
Where the goalposts lie aside, I will bring to the newsgroup's attention an experiment in which, under controlled laboratory conditions, an increase in morphological complexity has been observed as the result of selection by predation.
This work was originally discussed on talk.origins some years ago by the last author. I thank Ian Musgrave for tracking down the reference to this paper. The paper is:
Boraas, M.E., Seale, D.B., and Boxhorn, J.E. (1998) Phagotrophy by a flagellate selects for colonial prey: A possible origin of multicellularity. Evolutionary Ecology 12:153-164.
You can view the complete text of this article online by following the pdf link at: <http://www.wkap.nl/oasis.htm/171545>
Predation was a powerful selective force promoting increased morphological complexity in a unicellular prey held in constant environmental conditions. The green alga, Chlorella vulgaris, is a well-studied eukaryote, which has retained its normal unicellular form in cultures in our laboratories for thousands of generations. For the experiments reported here, steady-state unicellular C. vulgaris continuous cultures were inoculated with the predator Ochromonas vallescia, a phagotrophic flagellated protist ("flagellate"). Within less than 100 generations of the prey, a multicellular Chlorella growth form became dominant in the culture (subsequently repeated in other cultures). The prey Chlorella first formed globose clusters of tens to hundreds of cells. After about 10-20 generations in the presence of the phagotroph, eight-celled colonies predominated. These colonies retained the eight-celled form indefinitely in continuous culture and when plated onto agar. These self-replicating, stable colonies were virtually immune to predation by the flagellate, but small enough that each Chlorella cell was exposed directly to the nutrient medium.
Is the change an increase in morphological complexity? The prey alga changes from unicellular to multicellular. This changes the size and shape of the organism. Steve would presumably accept this as an increase in morphological complexity. Chlorella vulgaris is strictly asexual. The colonies that form after about 10-20 generations are composed of clusters of daughter cells that remain within the mother cell's envelope, so they will be as genetically identical to each other as the cells in, say, a human body. Note that this is different than distinct, non-identical cells joining up into clusters. The clusters replicate by division of the internal cells and budding of a small, new, and more predator sensitive cluster which quickly increases in size by concommitant increases in individual cell volumes.
Is the ability to form colonies genetic? The authors discount an alternative hypothesis that colony formation is innate and induced by the presence of the predator. i) Twenty generations are required for colonies to become apparent. This seems too slow to be the result of an environmental trigger. ii) The colonial form persists for more than two years even when the predator is at low density. iii) The colonial form "breeds true" in the absence of predator, in both solid and liquid media.
Is the change the result of mutations? This question also has been raised about the classical example of evolutionary change - the peppered moth. In the Chlorella example, clustering of algal cells was extremely rare prior to the experiment, occurring about two or three times per year over two decades. This is easily explained as the result of rare mutations but very difficult to explain as the persistence of genetic variants from the original wild population, given the large number of generations involved and the extreme rarity of the observation of clustering.
In short, without evidence to the contrary, it is reasonable to conclude that this experiment demonstrates that natural selection acting on rare mutant forms can lead to increased morphological complexity during time scales observable by man. Natural selection and mutation thus remain viable mechanisms for historical, adaptive evolutionary change.
There is also a lesson here. Failure of natural selection and mutation to produce some arbitrary level of change in a short time is not enough to discount these mechanisms' importance during historical evolution, nor does uncertainty about the mechanisms of evolution discount the historical occurrence of evolutionary change. At this point, opponents would do better to propose and test alternative mechanisms or explanations for the existing data.-Adam
-- Opinions expressed are not necessarily those of Stanford University. PGP Fingerprint = C0 65 A2 BD 8A 67 B3 19 F9 8B C1 4C 8E F2 EA 0E
[Return to the 2000 Posts of the Month]
Home Page |
The FAQ | Must-Read Files | Index | Creationism | Evolution | Age of the Earth | Flood Geology | Catastrophism | Debates