What Evolution Is
In my continuing pursuit to publish the entire contents of Dobzhansky and Pavlovsky's "Indeterminate Outcome of Certain Experiments on Drosophila Populations" on my blog, I present you with some more gems from one of the foremost researchers of the modern synthesis.
First, I give you an excerpt from the introduction, in which Dobzhansky describes how biologists study evolution:
"Repeatability of observations and experiments is taken for granted in science. If experiments on similar conditions fail to yield similar results, one suspects that some variables in the materials or in techniques have escaped detection. Yet, some natural processes happen only once; history, either on the biological or on the human level, does not repeat itself. There is, of course, nothing mysterious in this uniqueness. Evolutionary processes in nature are influenced by usually very numerous internal and environmental variables. It is unlikely that the whole constellation of variables can remain constant for long, or that it can recur in the future. The elementary components of the evolutionary process, the mutational and selectional steps, are both repeatable and reversible; evolution is however unrepeatable and irreversible. Geneticists and other experimentalists concentrate their attention chiefly on the elementary components, and are consequently reproached by those who study evolution by other means for not being able to observe evolutionary changes of any consequence. This reproach fails to take into account that historical events and trends become comprehensible only through understanding of the unspectacular everyday events which bring them about." (The emphases are my own.)
How about that!? It's a bit wordy, but a nice refutation to the anti-evolution movement of today -- and it was written in 1953. No, we cannot recreate the major evolutionary changes that resulted in the diversity of life on earth that we observe today. This is not because evolution did not happen, but because so many damn parameters go into each step in evolution to try to understand every detail would be akin to assembling an adult human in a laboratory cell by cell. You wouldn't argue that because we cannot describe the process of human development taking the trajectory of each and every cell into account that biochemistry and molecular and cellular biology should not be taught. Well, the same goes for evolutionary biology. We can, however, try to understand the processes that underlie evolution (mutation, natural selection, genetic drift, gene-environment interactions, etc) in order to get a clearer picture of how it occurred.
The paper from which this quote is taken deals with laboratory experiments in which changes in allele frequencies over time are repeatable and can be described using simple mathematical functions. In essence, the authors show that certain rules govern evolutionary change (i.e., the repeatable/reversible processes) and they contributed to the wealth of knowledge developing at the time.
The article concludes with a distinction between micro- and macro-evolution. While I have a particular distaste for those terms, Dobzhansky makes an important point:
"Even though these new adaptive genotypes were compounded from pre-existing genetic elements, the changes observed in populations of mixed origin surpass the scale of the changes observed in populations of geographically uniform origin. In the former, new adaptive genotypes arose and became established, while in the latter the alterations involved chiefly the relative frequencies of certain previously available genotypes. If the latter changes are labeled microevolution, the former may perhaps be referred to as mesoevolution. The words macroevolution and megaevolution would, then, be reserved to designate emergence of new types of body structure and function, requiring time in geological scale, and usually leading to formation of new genera, families, and still higher categories. Needless to say, these four words refer only to the dimensions of the changes which they describe, while most evolutionists regard it as probable that the essential genetic mechanisms are similar throughout." (The emphases are my own.)
When a biologist describes an evolutionary change as microevolutionary or macroevolutionary he/she is only referring to the scale of the change itself and not to the processes underlying that change. We can gain an understanding of the large scale irreversible/unrepeatable evolutionary events through studying the small scale repeatable/reversible events. Both processes are governed by the same mechanisms such as meiosis, natural selection, population level phenomena, and environmental effects.
In fact, where we draw the line between micro and macro is somewhat arbitrary. If we say that speciation is a macroevolutionary process, then recent results indicate that population level phenomena are responsible for macro level changes.