Understanding Anagenesis and Cladogenesis in Island Biogeography

Anagenesis: This refers to the evolutionary process in which a single species transforms into a different species without branching. It typically entails gradual changes in the characteristics of an organism over time, which may lead to the extinction of the original species. Anagenesis can occur through various mechanisms such as mutation, natural selection, and genetic drift, often influenced by environmental changes.
Cladogenesis: Unlike anagenesis, cladogenesis is the evolution that results in the splitting of a lineage into two or more separate species. This process often creates a bush-like structure in evolutionary trees, showcasing biodiversity. Cladogenesis is significant in understanding how new species arise and how species adapt to different ecological niches.

Emerson and Patiño argue that these terms can occasionally confuse patterns of speciation—specifically, the differences between the ways new species arise (cladogenesis) and how existing species may evolve in a linear fashion (anagenesis)—with the evolutionary mechanisms driving these processes.
Despite the potential for confusion, the authors assert that anagenesis and cladogenesis are still valuable concepts, particularly in studying phenomena such as island biogeography, where unique evolutionary paths can be observed due to isolation.

Colonization Assumptions: Emerson and Patiño challenge the notion that anagenesis on an island implies the source population remains static in its evolutionary journey. They clarify that both insular populations (species that evolve on islands) and mainland counterparts can exhibit ongoing evolutionary changes after a population has colonized a new area, leading to adaptations that enhance survival in specialized environments.
Critique of the Argument: The authors criticize the oversimplified approach to defining cladogenetic or anagenetic events strictly based on observable phenotypical changes. They highlight that significant evolutionary alterations may occur at a genetic level without any apparent changes in external traits, emphasizing that a deeper understanding of genetic and ecological contexts is necessary.

Cryptic Cladogenesis: This term designates scenarios where speciation occurs, and the resulting island and mainland species show no observable phenotypic differences, yet possess distinct genetic identities. Such instances indicate the importance of genetic studies in revealing hidden diversity within species that appear morphologically identical.
Peripatric Speciation: Commonly occurring in island settings, this form of speciation involves populations that become geographically isolated from their mainland relatives. An example is the North American woodrat, Neotoma lepida, where paraphyletic relationships allow the source population to evolve continually while fostering cladogenesis in isolated populations. This continuous evolution enhances understanding of how species adaptive traits can diverge.

Within-Island Speciation: Populations that achieve a particular evolutionary status post-colonization exemplify cladogenesis; however, they may also undergo anagenetic evolution within their insular confines. Evidence from fossil records, such as those found in Myotragus, provides illustrative examples of anagenetic evolution, indicating significant changes within species that thrive in isolated environments.
Terms of Speciation: The authors call for prudent usage of terminology regarding island endemics and diversification. While maintaining clarity in definitions is crucial, they advocate against discarding established terms like anagenesis and cladogenesis, given their relevance in evolutionary biology.

Value of the Terms: The importance of retaining the concepts of anagenesis and cladogenesis is emphasized, as they are not intrinsically linked to observable trait changes or direct selection processes. Maintaining these terms within the lexicon of evolutionary biology is vital for effective communication among researchers, aiding in the overall understanding of complex evolutionary dynamics, particularly in the distinctive contexts of island biogeography.

Emerson, B.C. and Patiño, J. (2018) Anagenesis, cladogenesis, and speciation on islands. Trends Ecol. Evol.
Thomas, G.H. et al. (2009) Body size diversification in Anolis: novel environment and island effects. Evolution.
Gordon, K.R. (1986) Insular evolutionary body size trends in Ursus. J. Mammal.
Millien, V. and Damuth, J. (2004) Climate change and size evolution in an island rodent species: new perspectives on the island rule. Evolution.
Patton, J.L. et al. (2007) In The Evolutionary History and a Systematic Revision of Woodrats of the Neotoma lepida Group.
Collen, B. et al. (2004) Biological correlates of description date in carnivores and primates. Global Ecol. Biogeogr.
Meiri, S. (2016) Small, rare and trendy: traits and biogeography of lizards described in the 21st century. J. Zool.
Nicholson, K.E. et al. (2005) Mainland colonization by island lizards. J. Biogeogr.
Raia, P. and Meiri, S. (2011) The tempo and mode of evolution: body sizes of island mammals. Evolution.