Field of Science

Time to update my taxonomy and YOURS: part 2

A reader has asked the Phytophactor “why seed plants evolved into monocots and dicots?” This question actually can’t be answered because science is not very good at answering WHY questions. This question is even more difficult to answer because flowering plants can no longer be simply divided into monocots and dicots, and it would be wrong to say they evolved into monocots and dicots. This is because phylogenetic studies tell us that monocots (Liliopsida) evolved from a common ancestry with dicots, and as a result, they share many characters with basal angiosperms. However, dicots no longer form a single lineage, the requirement for being recognized as a taxonomic group. The rest of the groups shown in this simplified phylogenetic diagram are dicots, except for the outgroup, gymnosperms, but even some gymnosperms share the character of having two cotyledons. So dicots, a lineage of plants with two cotyledons, does not.
None of this tells us why such groups evolved. Flowering plants presumably were more successful in many habitats than other seed plants. This may be attributed to many things, not the least being their interactions with animals involved with pollen and seed dispersal, although cycads use both as well. Why monocots evolved cannot be answered other than to say a set of modifications in the ancestors of this lineage were successful and plants with these characteristics proliferated.
The second part of the answer is that the taxonomy of flowering plants is far more complicated than monocots and dicots. Even this diagram does not reflect many of the newest findings or even show all the lineages (it was just handy), but it shows the general organization and gives examples of familiar plants. The ANITA grade is a series of basal lineages now just called ANA as some new relationships have combined what were separate lineages. The label refers to their names (e.g., A is for Amborella, N is for waterlilies, A is for Austrobaileyales), and the AA lineages have two cotyledons while the waterlilies only have one. The eudicots (eu- means true) all share a particular type of pollen. So, although this is not reflected in formal taxonomies because of the difficulty of translating such knowledge into the traditional taxonomic framework, flowering plants consist of the following groups/lineages: three lineages forming a basal ANA grade, monocots, the magnolids, and the eudicots. Even this is simplified, but next spring the Phactor will be teaching plant taxonomy.

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