Pulsed plant evolution in mountain ecosystems: identifying rate changes of range shift and morphology

Understanding the origin of plant diversity remains a central and relevant scientific goal. This statement is reflected by a survey conducted by Grierson et al. (2011), where the one hundred most important and urgent questions in plant science were collected. Here, the listed questions were grouped into five sections, and “plant diversity” is prominently represented as one of these sections. Remarkably, these questions - even though some of them might be as old as Darwin’s work – are very urgent to be answered, especially in a world where society is confronted with the consequences of climate change. For instance, understanding the rapid and relatively recent diversification of angiosperm species can provide valuable insight into the evolution of genomes and the underlying evolutionary processes (Grierson et al., 2011). Therefore, studying angiosperm plant diversification is crucial, yet addressing such a broad question presents a considerable challenge.
Phylogenetic trees serve as a central tool for studying patterns and processes that underlie angiosperm diversification. Charles Darwin (1809 - 1882) was one of the most famous scientists who promoted this “tree thinking”. In “Origin of Species” (Darwin, 1859), he introduced one of the first phylogenetic diagrams. The “Tree of Life”, the sole illustration in Darwin’s book, represents the relationships among species, both living and extinct, across evolutionary lineages. The diagram’s y-axis symbolizes time, where the top represents the present, and below the past with extinct lineages. Identical to family trees the lines (called branches) are used to show the relationship between species and branches below a joint (called internal nodes) represent an ancestral species. These structural representations were coined as phylogenies by Ernst Haeckel in 1866 (McLennan, 2010) and are still used in modern biology. Nowadays specific programs exist to compute phylogenetic trees allowing them to be inferred from morphological, genetic, or both data types (e.g., Höhna et al., 2016). However, thinking of related species as trees is not sufficient to explain the numerous angiosperm species.
Critically, closely related species introduce statistical nonindependence, posing a challenge, and more complex analytical tools are required to study rapid diversification accurately. Felsenstein (1985) addressed this issue using a fictive phylogeny of 40 species, which displays two groups of 20 closely related species. Next, a two-dimensional data set (from these 40 species) is presented, showing a positive correlation between the two characters. Felsenstein then confronts the reader with the same data but reveals what point belongs to which of the closely related groups. The points within each group show no significant correlation, yet the overall significant pattern is illusory and caused by the position of the two “data clouds”. Ultimately, Felsenstein suggests overcoming this problem by correcting for the nonindependence caused by the taxa’s shared ancestry. Therefore, he uses the structure of the phylogeny and treats evolutionary lineages as replicates. This pioneering work created a new world of powerful and diverse methodological tools called “phylogenetic comparative methods” (Cornwell & Nakagawa, 2017).
These analytical tools enable me to investigate alpine plant diversity and tackle the overarching question: How do rates of evolution vary among lineages and what factors account for potential differences in rates?