What are the processes and conditions that generate biological diversity?


Humans have always been fascinated by the diverse array of species on our planet. Understanding the origins of this diversity is no easy task, because most of the details of evolutionary history have been lost to the sands of time. Without a time machine, our primary means for understanding evolutionary change is to study organisms that are in the process splitting into two new species. I study examples of speciation in action using cutting edge tools, mainly genomics, to understand the processes, mechanisms and conditions cause their origin. The following gives a brief description of some of the more detailed research topics and detailed questions that interest me.

Some examples of 'speciation in action' I have been fortunate enough to study. Top left: Most recently, snapdragons (Antirrhinum) in the Spanish Pyranees.  Top middle:  Heliconius butterflies. Photo taken by Dr. Emma Curran. Top right: Rhagada snails, which I studied during my PhD. Bottom left: The red and yellow ecotypes of the bush monkeyflower (Mimulus aurantiacus) which I studied during a postdoc with Prof. Matt Streisfeld. Bottom Middle: Joshua Trees (Yukka) in the Mojave desert which I studied with Anne Royer and Chris Smith. Bottom right: Littorina snail, which I studied during a postdoc with Roger Butlin (Photo credit Zuzanna Zagrodzka). 

Evolution of barriers to gene flow

Barriers to gene flow are critical to the maintenance of biodiversity because they allow divergent populations to coexist while remaining distinct. Without barriers to gene flow, interbreeding would cause different populations to completely mix with one another, meaning that they would not be able to adapt to different ecological niches. Many traits have the potential to act as barriers to gene flow. In a flowering plant, potential barriers might include differences in flowering time, different levels of attractiveness to alternative pollinators, poor adaptation of hybrid offspring and reduced fertility of mixed offspring. I am interested in understanding importance of different types of barriers, the order in which they evolve, and their genetic basis.

Left: a conceptual diagram showing how different barriers work together in a chain to contribute to an overall barrier to gene flow. In this hypothetical example, a strong reduction in gene flow between two populations of flowering plants results from differences in their flowering time, differences in their attractiveness to different pollinators, reduced survival of hybrid offspring, and reduced fertility of hybrid offspring. These barriers work in a chain to reduce the amount of gene flow (m) to a much lower rate (me) that if there was no barrier. 

Interpreting the genomic 'landscape'

When we look across genomes, we find that patterns of genetic variation are not the same all the way along them; some regions have more genetic diversity than others, or show greater differences in the genetic code between populations or species. These bumpy patterns have been describe as 'landscapes', because they look like they are made up of peaks and valleys. Like real landscapes, the features of the genomic landscape are created by forces of nature - for example, natural selection and gene flow. By interpreting the patterns, we can work out what processes have been important in species evolution, and we may even able to pick out landscape features that tell us about why species look, function, or behave differently, and how they stay distinct.

Right: Monument valley in Colorado, USA is known for its rugged topography made up of sandstone peaks that tower above the valley floor. Patterns of genetic variation can also have rugged topographies, as seen here  in the along 3 chromosomes of the bush monkeyflower genome.

Analysis of hybrid zones

Hybrid zones are locations where divergent populations come into contact and mate to produce mixed individuals, commonly referred to as hybrids. It is hard to overstate the contribution that hybrid zones have made to our general understanding of the speciation process. Unlike experimental crosses made in the lab, hybrid zones allow us to understand how barriers to gene flow play out in the real-world, over time scales that are more relevant to the long-term persistence of species boundaries. Coupled with a rich body of theoretical work and powerful methods for their analysis, it not surprising that hybrid zones have been described as ‘natural laboratories and ‘windows’ on the evolutionary process. I am fortunate to have been able to work on hybrid zones in a variety of organisms, including snails (Rhagada and Littorina), Monkeyflowers (Mimulus), butterflies (Heliconius), Joshua trees (Yukka), and beeltes (Acalles).

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A hybrid zone between tall- and flat-shelled snails which are locally adapted to grassy plain environments and rocky hilltop environments, respectively. interbreeding (Hybridization) between the tall and flat shelled forms has caused a cline of intermediate shell shapes to form at the boundary between the habitats. The hybrid zone is kept narrow because the intermediate shell shapes are poorly adapted to life in either habitat.

Drivers of adaptive radiation

Rather than multiplying at a constant rate, speciation often happens in rapid bursts that are more commonly referred to as radiations. But how and why do radiations occur? I seek to understand the drivers of adaptive radiations, so far focusing on the radiations of Mimulus monkyeflowers and Rhagada snails. 

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Left: The bush monkeyflower radiation in California has seen a burst of speciation occur in a  short period of time. The difference taxa show many morphological difference that are thought to be due to the divergent pollinator preferences across their geographic distribution.  

Concepts in speciation research

The language and concepts of speciation have always been points of furious debate. We now know that there is no single set of definitions that all speciation researchers adhere to. If we cannot agree on fundamental concepts, how can we advance as a field? Myself and Mark Ravinet are currently addressing this problem using a social science approach, based on the results of an online survey. By gauging and synthesising the thoughts of other speciation researchers, we hope that we can help each other understand why we see species and speciation so differenty. 

The speciation survey, conducted in 2019, received more than 400 responses from 30 countries. The results will be shared via publication and in an interactive online explorer later this year.

Natural history, taxonomy and conservation genetics

A strong understanding of natural history and evolutionary relationships sets the stage for detailed evolutionary studies, and can inform conservation and management plans. While it is not a primary goal of my research, I always endeavor to develop and share more ‘basic’ information about the organisms I study. In addition to describing several new species of land snail, I have collaborated with biodiversity management groups and museum staff to address specific conservation and taxonomic issues. 

Shell of Rhagada ngurrana, a new, geographically restricted species of land snail from the Burrup peninusla (Murujuga). The name is derived from ngurra which means land, country or home in the language of the Ngarluma people. The name was offered by the Murujuga Circle of Elders as an appropriate name for this species from the Murujuga area, the traditional home of the Jaburara tribe.