Phenetics seeks to express natural relationships among organisms by analyzing large numbers of equally weighted, noncorrelated characters.
Numerous generally means 100-200 characters, thus keeping the sampling error small. In practice, as many as is feasible to collect. Because of the large number of characters, phenetics relies heavily on computers and statistical methods.
Desirable to have characters distributed over various regions, tissues, levels of organization.
Advocate equal weighting to avoid the pitfall of purposely choosing characters that are "thought to be" important by the systematist (subjectivity). But remember - equal weighting is weighting - where all characters have the same weight. Also, by the choice of characters (and lack of others), one will always bias the analysis (partly a function of the observer).
Attempts to use only noncorrelated characters selected after initially studying the organisms. Search for unit characters is made, i.e. ones that cannot be subdivided.
Today, phenetic classifications are generally used at lower taxonomic ranks, such as comparisons among species within a genus.
2. Phenetic classification makes no assumptions about phylogeny, no implications on ancestry, no statements on evolution of the group. But, evolutionary considerations DO enter into phenetic analyses. One must compare homologous features. For example, what purpose would it serve to compare cladodes (flattened, modified stems) in one group with phyllodes (flattened, modified petioles) in another? I will further discuss the topic of homology later in this lecture.