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Tag Archives: Biology

Biological classes and ancestries

Taxonomy is the science of classification. Taxonomy applied to biology is a systematic approach to classifying organisms. It can be applied to all organisms at a particular time, throughout time, or within any context. Once a classification is determined, other questions arise such as whether there is an independent reason that organisms are in the same class together.

The basic question in all classifications is whether the objects to be classified fit within a class or belong to another class. The goal of a classification is to minimize the within-class differences and maximize the between-class differences. This is often done by defining a distance metric that quantifies the differences.

Carl Linnaeus is known as the father of modern taxonomy who formalized the binomial nomenclature and called the lowest classes species and genus (no doubt after Aristotle’s method of defining with species and genera). His original expectation was that these biological species were natural kinds that do not change over time. With the discovery that fossils came from dead organisms, it became clear that some of his species had changed over time.

The solution to this problem was to reclassify organisms both living and dead in a new classification system. But this was easier said than done since it took years for fossils to be examined. Meanwhile, people were anxious to know how all the diversity of species arose.

Charles Darwin’s hypothesis was soon adopted: species are temporary population groupings with universal common ancestry. If all species are temporary, there is only one fixed class: the class of all species. Others hypothesize¬† there are classes of species that are fixed and have separate ancestries, which supports design or special creation.

How can this dispute be resolved? Elliott Sober compares these two hypotheses in his book Evidence and Evolution. Sober argues for a likelihood approach to determining the better of two hypotheses. The law of likelihood states that evidence E favors hypothesis H1 over H2 if and only if the probability of E given H1 is greater than the probability of E given H2, or in symbols, P(E | H1) > P(E | H2). Note that this is a comparative approach; it only works when comparing two specific hypotheses.

In this case, the context is all species on the earth over all the history of life on earth. Hypothesis H1 states that there are multiple classes of species that span the history of life on earth, each class with separate ancestry. Hypothesis H2 states that there is only one class of species that span the history of life on earth, all with common ancestry.

Sober notes that Darwin routinely inferred common ancestry if there was some similarity between species. Sober calls this modus Darwin. It is better to have an overall metric of distance between species than rely on a few similarities. However, there is no generally accepted distance metric for species. In its absence, we can still make some inferences.

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Change and stability

Evolution or alteration means change over time. Sameness over time is called permanence or stability. The study of change or the lack of change over time is called history or diachrony.

Change happens. But sameness happens, too. One easily sees that sameness happens in the natural realm much more than change. That is not the result of chance but of law, which is why natural science is able to articulate laws and predict the future. The natural future is like the natural past.

Some changes are unpredictable individually but have predictable distributions or aggregates. Many sciences from statistical mechanics to quantum mechanics to genetics are stochastic in nature.

Similar to the coastline paradox, the amount of change depends on the length of the “ruler” used to measure change. If it is a small ruler, one measures minutiae, and more changes will be found. If it is a large ruler, one measures key features, and fewer changes will be found.

The conceit of evolutionary biology is that very low rates of unpredictable change over very long periods of time can result in all the biological diversity of today. It is an appeal to the imagination more than an appeal to knowledge. Without imagination, the argument becomes an assertion of mere possibility, rather than plausibility, probability, or necessity.

But if very low rates of unpredictable change can determine what happens, how much more can very high rates of predictable stability. One does not need to appeal to the imagination to see that stability is the rule, and exceptions only prove the rule.

Organisms are similar in some respects but not in other respects. If one focuses on minutiae, there are many differences. It is part of the conceit of evolutionary biology to overstate the importance of minor differences such as color and understate the importance of major differences such as body plan.

One might hope that biologists would be working toward finding the optimum characteristics to measure biological change. Alas, they are determined to find the smallest ruler and overstate change as much as possible.

I predict that a more mature biology will seek the optimum measure of change, and will accept that some characteristics are permanent features of a body type.