Systematics and Biogeography of the Laurales, a case study: Atherospermataceae

Lecture © Stefan Little
BIOL 606 Session, University of Alberta, February 14, 2001

Systematics encompasses three main areas, taxonomy, the study of phylogeny, and the process of evolution (Steussy, 1990). Therefore, in order to understand the systematics of a group of organisms one must first know the identification and classification of individuals in the group. Then one must know how the members of the group are related to each other (phylogeny), as well as know some processes by which these organisms may have diverged from their common ancestor. Biogeography, the science that seeks to explain the distribution of species and higher taxa on the earth (Ridley, 1996), is often connected to the understanding of phylogeny and divergence events in groups of organisms. In particular, the biogeographic concept of vicariance is used as an alternative hypothesis to the concept of dispersal when the same taxon has a disjunct distribution over several continents that were continuous landmasses in the geological past.

The order Laurales has been often implicated in the origin of angiosperms due to its early fossil record and the primitive characters that members of this order possess. Bessey's Dicta (1915) specifies his ideas of plesiomorphic characters for flowering plants. Bessey places what is now considered Laurales is his concept of the 'Ranales', a 'primitive' flowering plant group. Ranales also contained angiosperm groups that are now considered to be more derived groups, such as Ranuculaceae (the buttercup family). Cronquist (1981) places Laurales in the Magnoliidae, his modified version of Ranales. Important support for these concepts of plesiomorphic characters is found in recent molecular studies that place Amborella and Trimenia, historically members of Monimiaceae (Laurales), in the basal most grade of the angiosperm clade (Renner, 1999).

Renner (1999) circumscribed the Laurales using both molecular and morphological data, recognizing the families Calycanthaceae, Lauraceae, Monimiaceae, Hernandiaceae, Siparunaceae, Gomortegaceae, and Atherospermataceae. Schodde recognized Atherospermataceae as a family in his unpublished Ph.D. dissertation (1969). The Atherospermataceae have a distribution over Australia, South America, New Guinea, New Caledonia, and New Zealand. This type of transantarctic disjuction is common in Laurales (Berry 1935), and whether this distribution is due to vicariance or dispersal has been a commonly asked question. Schodde suspected Atherosperma (in Australia) and Laureliopsis (in Chile) to be closely related, and that due to their large fruitlets, they were not likely distributed via dispersal.

Renner (2000) used this relatively small family to ask two questions. 1) What are the relationships in Atherospermataceae? 2) Is the distribution of this family due to dispersal or vicariance? Cladistic analysis from DNA sequence characters found that Laureliopsis and Atherosperma are not sister to each other. Therefore the important disjunctions appear to be between Atherosperma and Nemuaron (in New Caledonia), within Laurelia (in South America and New Zealand), and between Laureliopsis and Laurelia. This topology is relatively well supported and therefore this phylogenetic tree was used to test the hypothesis that vicariance is the reason for the distribution of the taxa in the family.

Molecular distances were calculated and four of the tree nodes were separately calibrated with the occurrence of Coniacian atherospermous pollen, the separation of New Caledonia from Australia, the separation of New Zealand from Austral-Antarctica, and the occurrence of Paleocene Laurelia/Laureliopsis-like leaves. Each node calibration either places the age of the family into the Jurassic (well before any angiosperms appear in the fossil record), or the calibration places the divergence of disjunct sister lineages to well after the separation of their landmasses. Along with the very late occurrence of Gomortegaceae, the sister family to Atherospermataceae, the paper concluded that the present distribution of atherospermous taxa was not likely due to vicariance. Final statements indicated that the fruitlet of this family are likely more able to disperse via wind and/or animal mediated dispersal.

Concluding class discussion focused on the problems of conflicting molecular data in this kind of analysis, whole plant biology concepts, stem group versus crown group sampling in the fossil record in relation to character evolution. Points were brought up on the low support for certain clades in the paper, indicating that there is potentially a shorter molecular tree that may better illustrate the hypothesis of vicariance for the family. Further discussion on molecular clock estimates focussed on the more common trend towards rejecting molecular clock estimates due in part to the inconsistent apparent mutation rates between lineages in a tree. It was noted that the majority of the Atherospermataceae fossils were Tertiary, supporting the idea that the family likely diversified perhaps later in geological history, even though the family appears to be at least 80 mya (Coniacian pollen record).

The problem that perhaps some of these fossils do not represent members of a living lineage, therefore a stem group, was mentioned. If this is the case, then these estimates of age may be overestimating the age of the [crown-group] family, depending on whether the single feature (pollen, leaf type etc.) only occurred in this family in geological history. For example, if a particular fossil is used, and it represents a stem group that shared the pollen type with [crown] Atherospermataceae, but was not in fact a member of the family, then the estimate of age of the node calibrated by this fossil is overestimated. This would not be the case if the pollen type is in fact unique to this family. This final idea brought up the point that characters evolve separately from each other in some cases, and therefore in order to be more confident with the results of a study of this kind, it would be preferable to use whole plant reconstructions as node calibration points.

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Bessey, C. E. 1915. The Phylogenetic Taxonomy of Flowering Plants. Annals of the Missouri Botanical Gardens. 2:108-164

Berry, E. W. 1935. The Monimiaceae and a new Laurelia. Botanical Gazette. 96(4): 751-754

Cronquist, A. 1981. An integrated system of classification of flowering plants. Columbia University Press, New York, NY.

Renner, S. S. 1999. Circumscription and phylogeny of the Laurales: evidence from molecular and morphological data. American Journal of Botany. 86(9): 1301-1325

Renner, S. S., D. B. Foreman, and D. Murray. 2000. Timing transantarctic disjunctions in the Atherospermataceae (Laurales): evidence from coding and noncoding chloroplast sequences. Systematic Biology. 49(3): 579-591

Ridley, M. 1996. Evolution second edition. Blackwell Science, Inc. Cambridge Massachusetts.

Schodde, R. 1969. A monograph of the family Atherospermataceae R. Br. Ph.D. dissertation, University of Adelaide, Adelaide, Australia.

Stuessy, T. F. 1990. Plant Taxonomy: The Systematic Evaluation of Comparative Data. Columbia University Press, New York, NY.