INTRODUCTION TO BIOCHEMICAL SYSTEMATICS 



43 



phological character represents convergent evolution or phylogenetic 

 affinity. Should it come as a surprise or disillusionment to find that 

 the same problem may confront one who is attempting to evaluate a 

 biochemical character? Hansel (1956) has discussed some of the 

 problems raised in this paragraph. He illustrates clearly the point that 

 the same basic problems are involved in the phyletic interpretation of 

 biochemical as well as morphological data. The "percentage of 

 frequency rule," illustrated with an example from indole alkaloid 

 distribution (Fig. 4-1) is often useful in the interpretation of the 

 systematic significance of members of a related series of substances. 



At the present time there is no phylogenetic system based on 

 the distribution of biochemical constituents, nor is there likely to be 

 one, at least one derived out of the first biochemical level referred to 

 previously. What comparative biochemistry has to offer is supple- 

 mentary evidence which, when added to other systematic knowledge, 

 may clarify or help to clarify a given situation. If comparative 

 biochemistry seriously contradicted any part of the major structure 

 of plant systematics, it would be equally as disturbing to the pro- 

 ponents of comparative biochemistry as to other biosystematists. 



The matter of weighing equitably biochemical data, of evalu- 

 ating it, and comparing it with a given unit of morphological, cytolog- 

 ical, physiological, or anatomical data is so important that a separate 

 chapter will be devoted to this topic. In the final analysis one would 

 hke to translate all differences into gene differences. It is difficult to 

 do this in the case of most biochemical or morphological data, and 

 unless hybridization is successful, it is impossible to analyze directly 

 the genetic basis for a particular difference. 



There is reason to believe that in special situations bio- 

 chemical characters provide advantages if one is considering the 

 question of the genetic basis for a particular difference. In work to be 

 more fully described in Chapter 15, Turner and Alston (1959) have 

 demonstrated recombination of species-specific characters in individ- 

 uals from natural hybrid swarms of Baptisia species. Most species- 

 specific substances of the parents were present together in the hy- 

 brids though often in reduced amounts. In order to translate these 

 species-specific chemical characters into genetic units of differences 

 one must produce an F2 generation. Theoretically, if a particular sub- 

 stance required only one gene from one parent not present in the 

 other parent, three-fourths of the F2 generation should produce it, 

 and if n genes were required to form the substance, (|)« of the F2 

 generation should contain that compound. Therefore, a moderately 

 large F2 generation should suffice to translate units of biochemical 

 data into unlinked gene differences, assuming that pairing relation- 



