Geography/Human sciences personal statementWatch
Well genetics and evolution do influence ecology and biodiversity, and relating specifically to humans, in terms of human behavioural ecology (considering how human behaviours and culture evolved over long term as a response to their environments). So you can discuss more broadly both how humans affect their environment and how the environment affects humans in the long and short runs, perhaps.
I'm not sure about Exeter but at the HumSci talks at both UCL and Oxford at their Open Days both said that they don't consider personal statements as much as other courses because it's such a niche subject, and they understand that people will be applying to different subjects. I personally wouldn't worry about trying to balance your personal statement by trying to shoehorn in genetics/biology (which wouldn't look as good to Geography admissions tutors), and instead would focus on writing something really good for Geography. Maybe you could focus on the bits of the HumSci syllabus that you really like which overlap with Geography?