|Athro, Limited Biology Evolution|
|Evolution: Fact, Theory, and Rhetoric|
Sponges from Virgin Gorda,
British Virgin Islands.
(Photographed while snorkeling
on our Honeymoon...)
Contemplate an organism. Say a snail, or a flower, or a strand of seaweed, or a sponge:
The more closely we look at even the simplest organisms, the more complexity we see.
Why are organisms complex? A biologist will answer that this complexity is the result of evolution. But what is evolution? Evolution is an observational fact of nature. All life on earth is genealogically related in a great tree.
Actually when biologists talk about evolution they may mean any of three different things: Evolution is an observational fact of nature. We also have a theory, natural selection, to explain how evolution occurs. But much of what has been said about evolution includes lots of added rhetorical statements about what it all means. In particular, evolution has been associated with progress; progressive improvement of organisms in nature, leading to humans as the pinnacle of creation. Conversely, one can find rhetorical statements about how nature is random and contingent, and that humans are nothing special in the universe.
|The Fact of Evolution|
We do not, after all, expect to encounter a page-one story with the headline "New Experiment Proves Earth Goes Around Sun, Not Vice Versa. Galileo Vindicated." The fact of evolution has been equally well documented for more than a century.
We have two reasons to say evolution is fact: First, all the patterns we observe in nature point towards a nearly four billion year long history of evolution of life on earth. Putting a fancy word on it, this is Consilience. Multiple independent lines of evidence (from classification, development, fossil record, molecular biology etc.) all point to a branched evolutionary tree of life. Second, the anatomy and biochemistry of organisms is filled with quirky features, things that only make sense as artifacts of history. Again, putting a fancy term on it this is Dissonance.
The canonical example of quirky artifacts of history is thePanda's thumb. All biological systems are full of similar examples. My favorite is in the asymmetries of snail anatomy. Even Lamarck understood the concept of contingent history.
Biological systems often appear well designed. For example, eyes are very precise effective optical systems. Unlike quirky artifacts of history, examples of good design in nature cannot be used to support the existence of evolution (though they can be explained by it). Perfect design wipes clean the slate of history.
|Theories to explain Evolution|
Whatever the cause may be of each slight difference in the offspring from their parents - and a cause for each must exist - it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive.
We have a theory to explain how evolution happens - Charles Darwin's theory of evolution by natural selection. Core idea is widely accepted by biologists, how species evolve is area of active research,, multiple ideas, and disagreements (signs of active science).
Darwin's concept of evolution by natural selection may be expressed as a very simple set of statements:
Any time we look around we see that organisms are all slightly different from each other - in a classroom, just about everyone looks a little bit different from everyone else. All organisms are capable of producing more offspring than there are parents. Given time, this automatically leads to there being more offspring of any organism than can possibly survive. Therefore, some of these offspring must not survive. Those that survive will be those who are best suited to their own local environment. This is natural selection. We also observe that children tend to look like their parents, puppies like their parents, and so forth. There is some mechanism for inheritance of appearance. Since organisms vary, some of this variablity is inherited, and those organisms best suited to their local environments are the ones to survive to produce offspring, traits that improve fit to local environmental conditions will tend to be passed on to future generations. Conversely, those traits that are locally disadvantageous will tend to be lost. This is evolution by natural selection.
Natural selection is easy to study - it occurs over fairly short periods of time.
The classical textbook example of natural selection at work is observation of changes in the wing color of English moths as soot from industrial polution darkened the trees they lived on. The classical story goes: Widespread use of coal in England during the 1800s led to accumulations of soot on the bark of trees. Moths that spent their days hiding on the bark of trees had wings that were camouflaged, colored to match the color of the bark of the trees. As soot darkened the trees, moths that happened to have slightly darker wings tended to be less likely to be picked off by birds, since their darker wings stood out less agaist the newly dark background. This variablity happend to be heritable, thus through natural selection, the typical wing color of these moths shifted over time. Conversly, later in the 1900s, as polution from soot was reduced in England, tree bark became lighter, and natural selection shifted moth populations back to lighter wing colors.
One of the most powerful observations of natural selection at work comes from recent work on wild flowers in the Canadian arctic.
However, Evolution by Natural Selection doesn't quite get us all the way - we still need to explain how this can lead to the formation of new species.
|Rhetoric and the uses of Evolution|
The meaning of human life and the destiny of man cannot be separable from the meaning and destiny of life in general.
This quote is the first line of the Paleontologist George Gaylord Simpson's book The Meaning of Evolution. Evolution is often given a meaning beyond simply stating that organisms adapt to local conditions and that different species share common ancestry. Consider Ernst Haeckel's Tree of Life.
|Pages in this section|
|Fact||Evolution of the Panda's thumb|
|Snail soft parts|
|Eyes and the argument from design|
|Theory||The Elephant Problem|
|Inheritance of human eye color|
|Inheritance of Coat Color in Shetland Sheepdogs|
|Selection in Canadian Arctic Wildflowers|
|Rhetoric||Evolution in Haeckel's tree|