The evolution of plants

In my diversity of plants post, I discussed the breadth of extant plant life in the natural world. However I wish now to discuss this diversity in an evolutionary context aiming to address questions such as how are different plant groups related to one another and how they have evolved.

The plants I discussed in this previous post are illustrated in figure below. This basic phylogeny demonstrates the relative relatedness of plants on a broad evolutionary scale, navigating us through the major transitions in the ancestral history of all plants. In this last post, I highlighted all groups from the bryophytes to the angiosperms and mentioned any major distinguishing transitions or features. Collectively these plants are referred to as the Embryophyta or land plants.

Evolution of plants

The two groups in this evolutionary tree or phylogeny that have not been mentioned are the Chlorophyta and Charophyta. The ancestors of these plants are thought to have led to the emergence of the very first land plants. Chlorophyta, a group of both freshwater and marine photosynthetic algae, have approximately 4,500 species, with both unicellular and multicellular forms. Examples of chlorophytes include Caulerpa lentillifera (Sea grapes) and Ulva spp. (Sea lettuce) [1]. Ernst Haeckel, a german biologist and artist born in the 19th century, was famed for depictions of marine organisms. Below is a scientific illustration studying chlorophyte algae from his 1904 publication, Kunstformen der Natur.

Charophyta are exclusively freshwater algae which commonly occur in shallow or transient water. There are six constituent lineages of Charophyta which include Mesostigma, Charophyceae and Zygnematophyceae which represent approximately 3,500 species [1]. The ancestors of these plants are thought to have transitioned from water to land. In the context of my PhD project, this is really interesting as these ancient plants are hypothesised to have some capacity for drought tolerance. Understanding the genes that aid modern day Charophyta may provide some inference into the genetic potential of the ancestors of all land plants.

The dating of the emergence of different plant groups is a fairly controversial area of evolutionary plant biology. It is both technically complex and sometimes conflicts with fossil evidence. However throughout this article, I will provide rough approximations of the timing of evolutionary divergences. Around 450 – 500 million years ago (mya), the first land plants which would have been the ancestors of modern bryophytes evolved. The major lineages of the bryophytes (mosses, liverworts and hornworts) remain fairly unresolved. However a recent highly cited paper supports a clade of (Hornwort (Liverwort, Moss)) [2]. Their position relative to the other major plant groups does not change throughout analyses.

bryophytes.png

Lycophyta have been dated to the Silurian period, approximately 420 mya. They include most notably club mosses, spikemosses and quillworts. Monilophytes are ferns and fern-like plants which includes horsetails and whisk ferns. The ancestors of ferns diverged approximately 380 mya. Gymnosperms evolved around 275 – 320 million years ago and have over 1000 extant species. Along with angiosperms, they are referred to as the spermatophytes or seed-bearing plants [3].

Finally we come to the angiosperms, distinguished by, amongst other evolutionary innovations, flowers. The flowering plants are by far the largest group of extant plants having diverge approximately 150 – 200 mya. As such, I plan to examine and explain the diversity within the angiosperms in a future post. Understanding how the major transitions in the evolutionary history of plants have influenced adaptations will have exciting consequences for the function specific gene identification. Gene diversification in the context of species evolution can also be further understood. From today’s post, I hope I have explained how a diverse range of plants are related and why this is important to my PhD.

If you want to learn more: 

  1. Guiry, M. D. 2018. Retrieved from http://www.algaebase.org/.
  2. Puttick, M. N., Morris, J. N., Williams, T. A., Cox, C. J., Edwards, D., Kenrick, P., Pressel, S., Wellman, C. H., Schneider, H., Pisani, D. & Donoghue, P. C. J. 2018. The Interrelationships of Land Plants and the Nature of the Ancestral Embryophyte. Current Biology, 28(5): 733–745.
  3. Willis, K. & McElwain, J. 2014. The evolution of plants (2nd Edition). Oxford: OUP.

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