Natural Selection of Large Mammals, not anymore.

When teaching evolution by natural selection it is very easy just to think about what the examiners need to read in the students’ responses. I know that I constantly hammerhome the sequence:

Within a population there is genetic variation due to random mutations. Some of these mutations may result in different phenotypes. Following environmental change, those with the beneficial characteristic survive and breed. Over many generations the frequency of the beneficial allele increases until all of the population possesses it.

But there is a lot to unpick in that sequence, but the concept always seems to be taught towards the end of the GCSE and A level courses, and the perceived time pressure creeps in preventing me from really digging deeper with my students. However following the Linnean Society Christmas Lecture by Prof Sam Turvey (https://youtu.be/o_lG5RJ5aBs) I am finding myself questioning what we are calling natural selection. 

Again @CMooreAnderson has a lot to answer for,  this time with one of those requests for examples of natural selection. I am sure that I am not alone in thinking of examples of the outcomes rather than the process itself. We think of organisms like the star-nosed mole with a particular set of adaptations, specialists for their environment.

What if what we consider to be specialists were actually on the edge of their existence? Getting squeezed out of their desired habitats to those less than favourable conditions until they are gone forever. 

Prof Turvey gave a fantastic talk that pulled the historic data from spoken and written records in South East Asia to plot range changes since the last ice age to the present day of many large mammals. The range of some species, like the hog badger, is more or less unchanged over the last 11 000 years. However most show a contraction in distribution, but for many it is not a contraction from the edges, but instead the data showed species being pushed to the edges. This is particularly evident with the Sumatran and Javan rhinoceroses. His team showed that the distribution of both overlapped significantly, but are now found in the tiniest discrete pockets in Malaysia and Indonesia. 

It’s not just the rhinos, the patterns of the squeeze are seen in other large mammals like the giant muntjac, Hainan gibbon, giant panda, sika deer, Asian elephant to name a few. So a couple of questions remain, what was the environmental change that they were trying to outrun, and why hasn’t natural selection made them more resilient?

Prof Turvey’s research on the matter points the finger squarely at humans. Our success in growing our population has directly impacted on the megafauna through hunting, food chain disruption, habitat destruction and introduction of non-native species. The rate of change over the last 100 years is simply too fast for what have been relatively stable species for the last 11 000 years. 

From the lecture and the reading, natural selection is not a success story of the specialists but rather a tragedy happening before our eyes. It was this that my Year 11s really engaged with, contemplating the loss of the species that once co-inhabited the planet with us as demonstrated with the ghosts of the megafaunal mammals that once covered the globe. Undoubtedly the loss of species is important for natural selection in the theories we teach, but how can we continue to teach it as a success of the species that are currently still here, when many of these iconic species will be gone within the lifetime of our students.

So perhaps it is time to consider the mundane species, those ubiquitous in our local habitats. Be it bracken, oatgrass or a holly, could you give a convincing story of their evolution through natural selection? Would your students engage with the same enthusiasm when dealing with the weeds outside against the “Attenborough” species from far flung lands? I guess that’s a challenge for me over the next term or so.

References:

Turvey et al (2016) Holocene range collapse of giant muntjacs and pseudo-endemism in the Annamite large mammal fauna. Journal of Biogeography 43: 2250–2260.

Turvey (2018) Mammal extinction risk and conservation in Zachos & Asher Mammalian Evolution, Diversity and Systematics.

Turvey et al (2017) Long-term archives reveal shifting extinction selectivity in China’s

postglacial mammal fauna. Proc. R. Soc. B 284: 20171979.

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