Floral Evolution

The angiosperms burst upon the fossil record long ago (in human terms) with tremendous diversity already in place. Some of that fact rests with their site of evolution: places with lots of turnover such as tropical rain forests (where no dead plant parts remain unrecycled to become fossilized) and/or places with no sedimentation possible for fossilization (mountain slopes, etc.). Regardless of how they got here, controversies rage between what constitutes plesiomorphic and apomorphic among the angiosperms. Here we will take the Cronquist perspective.

Floral Characters and Character States in Angiosperms

Plesiomorphic StateCharacterApomorphic State
elongatefloral axisshort
spiralarrangement of flower partswhorled
radialfloral symmetrybilateral
manynumber of flower partsreduced
freefusion of flower partsfused (adnate and/or connate)
largeperianth sizeperianth missing (incomplete)
beetlefloral pollinatorwind or bees
bisexual (perfect)floral genderunisexual (imperfect)
hypogynousperianth positionepigynous
superior ovaryovary positioninferior ovary
single flowersgrouping of flowersinflorescences
all flowers alikeinflorescence organizationdivision of labor

A small sample of plant families

Below is a diagram depicting a few of the many plant families. Please remember a family is the grouping of organisms just above the binomial level. In mammals there are the cat, dog, and bear families. At this same level of classification we have plant families. Thus the families depicted below are just as different as cats and dogs are. Please further note that, of the 250,000 species of angiosperms on the planet, only a small number are shown here...not even half. And the fact that I have chosen to show the larger families, means that I am including far less than half of the plant families on the planet!

In looking at this diagram, please observe the trends from plesiomorphic to apomorphic from the table above. The flower formulae given here show you these trends! The diagram below shows an arrangement of families that indicates patterns from more plesiomorphic (at bottom) to more apomorphic (at top). The goal of the diagram is to help you understand the table above...please do not memorize floral formulae...focus on the trends among the family lineages.

Finally, and this is important, please remember that, while a family may posess many plesiomorphic character states (and therefore be near the bottom of the diagram), it is not the ancestor of a family that is nearer the top of the diagram. Families near the top of this diagram merely have a greater preponderance of apomorphic characters...that's all! This is NOT a cladogram! In human terms, we did not evolve from modern chimpanzees...rather, chimps and humans share a common ancestor. Keep that thought carefully in your mind.

In class, various examples of flowers are shown from each family to point out some basic characteristics of plant families. The goal of that was to expose you to some of the diversity in angiosperms and to reinforce the trends from primitive to advanced. Again, please do NOT get confused about this: The Apiaceae did not evolve from the existing Ranunculaceae...they probably share a common ancestor...both have diverged from that common ancestor.

The most plesiomorphic flower was something approaching a Magnolia flower. The common ancestor for all flowering plants probably had an elongate floral axis with many parts in each of what became the four whorls. The parts were probably large and more leaf-like than those of modern flowers. The filaments were probably more blade-like. The ovary was likely superior and the flower hypogynous. The flower was probably pollinated by beetles and provided much excess pollen and nectar to satisfy that animal. From some common ancestor like this evolved all modern floral types.

Key elements to take home with you from this portion of the course include that plants appear to be quite diverse in terms of floral structure. This makes floral structure an important area for studying meaningful character state changes. Second, this study shows four major lines of evolution among flowering plants. Of these two are monocot and two are dicot...

One line of dicot evolution culminates in the Lamiaceae (the mint family) and the other terminates in the Asteraceae (the daisy family). These two lines appear to have gone through a reduction in flower size, a reorientation through zygomorphy, and a division of labor in inflorescences to minimize display costs.

One line of monocot evolution has evolved in a parallel manner, floral reduction into a tight inflorescence, and display costs have gone to zero by moving to wind pollination (the Poaceae). The other line of monocot evolution has moved into zygomorphy in a big way and very specialized pollination mechanisms with bilateral symmetry, but it has not resulted in diminution of the individual flower (Orchidaceae).