DOES SIMILARITY MEAN DESCENT?
New Light on the Concept of Homology
Wayne Frair

INTRODUCTION

Legs of animals (as birds, frogs, turtles) and humans

Before Darwin: forms of life as modifications of great themes

For Darwin: similarity as evidence of relationship

HOMOLOGY DEFINED & ILLUSTRATED

Owen (1840s): "essential similarity" in
- relative position and connections
- adult structure
- embryonic development

Current popular definition: similarity based on descent from common ancestors

Homology illustrated

Homology contrasted with Analogy
- similarity in use, with no prerequisites re/ structure

HOMOLOGY SINCE 1859

Thought of as powerful evidence for organic evolution

"The central task of evolutionary biology is the separation of homologous from analogous likeness." (Gould, 1988, p 26)

For Christians: evidence of design by one Creator

THE BIG QUESTION:
Does homology mean ancestry or not?

Review of basic embryology
- cleavage
- blastula
- gastrula
- differentiation
- organogenesis

The trend of thought
- Darwin, 1872
- Ballard, 1976
- Denton, 1985
- "Homologous organs are arrived at by different routes." (p 146)
- Roth, 1988
- changed to view of Denton


Pioneering work of Sir Gavin de Beer, 1971
"Homology of phenotypes does not imply similarity of genotypes." (p 15)

Some examples supporting the concept that different genes lead to diverse developmental pathways yet similar end products
- Vertebrate limbs
- Spinal cord

Plants
"in general the homology of structure such as organs or molecules cannot be ascribed to inheritance of homologous genes or sets of genes. (Sattler, 1984, p 386)


BIOCHEMICAL HOMOLOGIES?

Sequence similarities, yes; homologies, no

Ancestry unknown

Suggestions for use of the term homology" in the future
- Morphology, yes
- Common ancestry, no
- Discontinue use in biochemistry

Conclusions
"Evolutionary considerations are considered as secondary. What matters are the processes that realize a body plan, and not whether a shared plan implies the same ancestry. Under this perspective, perhaps a return to the pre-Darwinian concept of homology will be more useful to biologists ... [homology] restricted to morphology and not in molecular biology." (Aboitiz, 1988, p 28)

NONRANDOM MUTATIONS

PRIMARY CAUSATION
- naturalism vs. supernaturalism

SECONDARY CAUSATION
- macroevolution vs abrupt appearance

I will praise you because I am fearfully and wonderfully made; your works are wonderful, I know that full well. Psalm 139:14


BIBLIOGRAPHY

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Acta Biotheoretica 37:27-29.

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Quarterly Review of Biology 18:228-241.

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Frair, W., 1985. 'Biochemical evidence for the origin and dispersion of turtles.'
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