The Descent Inference

The Descent Inference:How Transitional Fossil Sequences are Evidence of Common Descent

Creationists and intelligent design advocates often argue that transitional sequences are not evidence of common descent but instead are merely assumed to be such. They frequently bring up the arguments of paleontologist Henry Gee, who has said we cannot know for sure if a fossil group is directly ancestral to another. Gee, however, is convinced that transitional sequences are evidence of common descent. So, how can this be? This essay will show that common descent is not an assumption, but an inference drawn from observed phenomena in extant organisms as well as fossil data, and will lay out the reasoning behind the inference. It will also, where possible, point out aspects of the reasoning that are understood intuitively by most people. Those confronted by students or other evolution critics can use this to effectively rebut the 'common descent is an assumption' charge.

I. Relatedness is directly proportional to genes shared in common

This is actually something most people understand intuitively. Common expressions like 'blood relative' reflect this understanding. Empirical data confirm this intuitive concept. It is the basis for DNA paternity testing, for example.

II. All organisms share at least some genes in common.

This has been confirmed by protein sequence studies. Cytochrome c is a well-known example.

III. Morphological characteristics are primarily under genic or polygenic control.

This is also something most people understand intuitively. Plant and animal breeders certainly use this concept to guide their efforts. Geneticists have also confirmed this, even to the point of identifying genes that control basic body plans (homeotic genes).

IV. Morphological similarity between organisms is directly proportional to genes shared in common.

Again, this is understood on an intuitive level, when we expect siblings to resemble each other more than they resemble distant cousins. It also follows from (I), (II) , and (III). Comparative molecular studies are consistent with this as well. For example, humans and chimpanzees, which share almost all of the genes in common, are much more alike morphologically than humans and fish, who share relatively less.

V. Populations that exchange genes are more alike genetically than those that do not.

This is confirmed via the observation that when reproductive barriers occur within a population, creating subpopulations, those isolated subpopulations will independently accumulate different mutations and will undergo different allelic frequency changes over time.

VI. Barriers to reproduction between populations lead to new species.

All observed instances of speciation involve barriers to gene exchange leading to genetic divergence. These barriers can include geographic, ecological , ethological and genetic factors, but all lead to genetic divergence. In addition, no species have ever been observed to arise spontaneously: all are descendants from pre-existing, closely-related populations.

VII. Genetic divergence between isolated populations grows over time.

Once the barriers to gene exchange are in place, divergence only grows, due to independent accumulation of more and more mutations and combinations of alleles.

VIII. Degree of ancestry is a function of time since divergence.

From V, VI and VII, it follows that recent speciation events result in descendant populations that are more genetically alike than in populations that diverged much earlier. It also follows from all of the above that:

IX. Degree of ancestry is proportional to genetic similarity.

X. Degree of ancestry is also proportional to morphological similarity.

This follows directly from IV.

Once we have reached this point, the question of fossil evidence can be addressed:

1. Fossils are the remains of ancient living organisms.

Many fossils are not completely mineralized, and show traces of organic material.

2. Fossil organisms, therefore, had ancestors and left descendants, just like extant organisms.

There is no observed evidence that fossil organisms lived and reproduced any differently than extant ones now do.

3. Common Ancestry can be inferred from morphological similarity

This follows from I-X, and 1-2. 4. Transitional morphological 'sequences' indicate chains in ancestry.

While Henry Gee's point about a transitional organism not necessarily being the direct ancestor of the next organism in the sequence is valid, from the above it can be inferred that the organism came from a population very closely related (and therefore morphologically similar) to the population that was the direct ancestor.