5 Feb - Trends in floral evolution; paleoherbs and basal eudicots

In 1753 Linnaeus made the first widely accepted, comprehensive classification of plant species. After Darwin proposed his theory of descent with modification, many botanists began to work toward a classification that was based on evolutionary relationships. The authors of these classification systems tried to figure out which plant groups were ancestral and which were derived, often based on perceived simplicity of morphological structures. During the 20th century, many plant classifications were published, each based on slightly different ideas about evolutionary trends in plants. Some examples:

1899 Adolf Engler & Karl Prantl published the 20 volume Die natürlichen Pflanzenfamilien which covered algae through angiosperms. It is still the basis for the arrangement of many modern floras and herbaria.

1910 Charles Bessey became the first American botanist to publish a natural classification of plant families. His system was based on a series of 28 rules or "dicta" (e.g., woodiness is primitive, hypogyny is primitive, separate floral parts are primitive). His system can be summarized with "Bessey's Cactus", which shows a basal split between monocots and dicots, and the families with fused floral parts and inferior ovaries considered most "advanced". Bessey recognized that despite a general trend toward complexity, degradation or reversal can occur in evolution. Bessey's ideas were extremely influential and elements of his classification were incorporated in many of the treatments that followed: Stebbins 1974, Cronquist 1981, Takhtajan 1987, Dahlgren 1989, Thorne 1992.

These classifications recognize some basic trends in floral evolution:

Ancestral characters: spiral arrangement of floral parts, many floral parts, separate parts, radial symmetry, undifferentiated perianths (tepals), laminar (leaf-like) stamens, superior ovary, bisexual flowers, monosulcate pollen (has 1 slit or groove).

Derived characters: floral parts in distinct whorls, reduction in number of floral parts, fusion of parts, bilateral symmetry, distinct petals and sepals, distinct filament and anther, inferior ovary, unisexual flowers, tricolpate pollen (3 grooves).

The terms "ancestral" and "derived" describe characters, NOT taxa, and they do not necessarily mean "primitive" and "advanced".

An ancestral or plesiomorphic character is one that has persisted through evolutionary time.

A derived or apomorphic character differs from the ancestral condition and has arisen more recently in time. Determining whether a character is derived requires knowledge of the ancestral condition. This is typically done by examining the character in question in a closely related, or sister, taxon. More about this in a few weeks.

Most classifications (and therefore most keys and floras that you will use to identify plants) recognize a basic split between the monocots and dicots, even though we now know that this is somewhat inaccurate. The monocots are a monophyletic group, but the traditional dicots are paraphyletic and includes all the basal angiosperm lineages plus the monophyletic true or eudicots which are united by triaperturate pollen. Still, there are some distinct structural differences between monocots and dicots that you will find useful when you start to make identifications:

Monocots: one cotyledon in embryo, parallel venation in leaves, adventitious roots only, secondary stem growth in scattered bundles, floral parts in 3's, pollen with one opening.

Dicots: two cotyledons in embryo, net venation in leaves, embryo radicle forms primary root and a variety of root types found in mature plants, secondary stem growth in ring around perimeter, floral parts in 4's or 5's, pollen with 3 openings (eudicots only).

The classifications above have been based on morphological, anatomical and chemical characters. These can be subject to convergence. In the last two decades, systematists have begun to explore molecular characters (usually DNA sequence in some form), because these are likely independent of morphology, resulting in a classification that more accurately reflects evolution. Most molecular analyses to date have been based on the sequences of 1) the large subunit of RUBISCO, an important photosynthetic protein encoded in the chloroplast, 2) atp synthase, another chloroplast-encoded protein important in to plant cell's energy budget, and 3) the nuclear encoded gene for ribosomal 18S RNA, critical for the translation of the genetic code.

1) monocots monophyletic, but "dicots" NOT monophyletic.
2) monocots vs. dicots NOT the most basal split in flowering plants, the monocots have diverged more recently than previously thought.
3) the earliest angiosperms may not have been woody with many parted flowers - we still don't know whether the first angiosperms were woody or herbaceous.
4) phylogenetic relationships among the non-monocot paleoherb lineages are poorly understood. See lab handout for 02/06/03 for more information on paleoherb family Nymphaeaceae (water lilies).
5) an important split occurs between the one and three grooved pollen types. Triaperturate pollen is more recently evolved than previously thought and plants with this type of pollen are referred to as eudicots.
6) within the eudicots are 3 large monophyletic groups and and several smaller lineages:

Several basal eudicot lineages include the poppy and buttercup familes. See lab handout for 02/06/03 (Ranunculaceae, Papaveraceae) for more details and terminology.

Caryophylliid group (monophyletic) includes pinks, cacti, buckwheats, sorrels. Polypetalous flowers, betalain pigments, special seed nutritive tissue called perisperm.

Rosid group (monophyletic) very large group includes roses, legumes, oaks, mustards. Polypetalous flowers.

Asterid group (monophyletic) very large group includes mints, nightshades, carrot family, composites. Mostly sympetalous, single integument, iridoid compounds.

In addition to the three major monophyletic angiosperm groups, there are several lineages that are likely related to, but can't be assigned to these groups. For example: blueberry family, mistletoe, geraniums.