Resolving an ancient radiation: gingers, phylogenies, and fossils.
– with Selena Y. Smith, University of Michigan
The purpose of this research is to examine genetic, morphological, and ecological evolution of Zingiberales on broad spatial and temporal scales by combining multiple lines of evidence from extinct and extant taxa, and to develop methods of integrative analysis of fossil and modern data that will be applicable to other organisms with a diverse fossil record.
An ancient rapid radiation can be understood as rapid and successive divergence of lineages within a relatively short period of evolutionary (geologic) time, resulting in ambiguous molecular and morphological changes for use in phylogenetic analysis. However challenging to resolve, rapid evolutionary radiations are thought to be a common theme across the tree of life and are thought to explain poorly-resolved phylogenies in many groups including insects, birds, bees, turtles, mammals and angiosperms. It has been argued that while ancient rapid radiations render phylogenies with low support for basal relationships, characters that evolved rapidly during divergence but have subsequently slowed may help to resolve branching patterns of basal lineages.
Understanding the specific drivers and responses of biotic (e.g., dispersal, speciation, extinction) and ecologic (e.g., community assembly, functional niches) changes is an important component of understanding the evolutionary history of a group, and may ultimately help resolve patterns of ancient diversification. Earth’s environment and climate has fluctuated significantly over geologic time; structure and composition of ecosystems has consequently changed to respond and adapt to new climates and environments, a process that is intimately linked with biotic evolution and may be reflected in the genetics and morphology of extant taxa. The fossil record provides us with the data necessary to reconstruct ancient environments and past biotic responses.
Resolving rapid radiations and their evolutionary implications requires the integration of morphological, molecular, ecological, and fossil data, or a “consilience approach” that integrates and interprets comparative data in an appropriate framework. Integrating temporal and character data is important to phylogenetic inference, and to fully understand the dynamics of evolution and diversity, we need to integrate molecular phylogenies with the fossil record. A well-supported and resolved phylogeny should be consistent with stratigraphy, and morphology (e.g., leaf, floral, or seed structure) should reflect viable evolutionary patterns and processes. Coupled with detailed phylogenetic studies, environmental niche modeling and reconstruction of ancestral habitats can provide important data for understanding ecological evolution and for predicting how climate may be linked to changes in morphology and diversity. If physiological uniformitarianism7 is demonstrated through hindcasting, quantification of fundamental niche space for modern taxa could help to reconstruct climate variables such as mean annual temperature and precipitation in deep-time, providing testable constraints for climate modelers.
The plant order Zingiberales is a model group to apply a consilience approach for reconstructing evolutionary history. Zingiberales are a diverse group of tropical monocots, including edible plants (e.g., ginger, cardamom, bananas) and ornamentals (e.g., cannas, bird-of-paradise, prayer plants). Eight families are recognized with ca. 2500 species. Fossil zingibers are known since the Cretaceous on the basis of fruits, seeds, leaves, rhizomes, and phytoliths. Zingiberales are thought to have diverged from the sister order Commelinales9 approximately 120MY ago, but relationships among the basal lineages are not confidently resolved using multi-gene phylogenies, likely due to an early rapid radiation. This project seeks to reconstruct the evolutionary history of Zingiberales through integration of morphological, molecular, ecological, and fossil data to determine when, where, and how these tropical monocots evolved.
Research questions
The specific objectives will be accomplished as three integrated aims:
Aim 1: Use morphology to understand patterns of evolution across Zingiberales.
We will investigate current and deep-time biodiversity by critically reviewing and updating the taxonomy of fossils, and build a morphological data matrix from fossil and extant taxa in order to address the following questions:
- What is the range of morphological diversity in flowers, fruits, seeds, and leaves?
- Which morphological characters are taxonomically significant for fossil identification?
- What are the most reliable fossils of Zingiberales?
Aim 2: Produce a well-supported, resolved phylogeny of extant and fossil taxa.
We will utilize DNA sequence data, plastid genomes, morphological characters, and stratigraphy to answer the following questions:
- Did Zingiberales undergo an early, rapid radiation?
- What are the relationships between extinct and extant taxa?
- Can fossil and stratigraphic data help resolve deep phylogenetic relationships?
Aim 3: Characterize the spatial evolution of Zingiberales through time.
We will utilize biogeography, environmental niche modeling, and hindcasting to address:
- When/where did different lineages originate? What is the paleobiogeography?
- What are the environmental niches of key groups? Can these data be used to understand ancestral niches and predict responses to environmental change?
- Can fossil zingibers be used for quantitative paleoenvironmental reconstruction?