All organisms live in environments that vary. Environments vary in very many ways, such as how much food there is available and the temperature. Some of this variation may be predictable, such as temperature because of seasons. Other aspects are less predictable, such as food availability which may depend on how many other individuals are eating it. Organisms have therefore evolved ways to maximise their survival and reproduction in such changing environments. For one soil-dwelling nematode worm (called Caenorhabditis elegans) we have investigated how it arrests its development as an immature larva and how it alters its reproduction when the environment changes. Studying laboratory strains of this species, we have found that worms differ in how they alter their larval arrest and reproduction when the environment changes. We think that these differences between worms from different parts of the world occur because they have evolved in different environments and that these different responses are best for these different environments. These responses to changes in the environment are controlled by genes and we have discovered the 10 regions of the genome that control this. In summary, this previous work has shown how larval arrest and adult reproduction can be affected by environmental change; that these changes differ between different worm strains and that these responses to environmental change are controlled by relatively few regions of the genome. We now want to test these findings further. In particular, we want to see whether strains of Caenorhabditis elegans recently taken from the wild, respond in the same way to environmental change and whether these responses can be altered by artificial selection. We also will investigate further the 10 regions of the genome to try and identify the particular genes that control these environmental responses. The reason for doing this is to work out if our ideas for how and why we think Caenorhabditis elegans responds to environmental change in these ways are correct. This work is therefore seeking to deeply understand how one well studied species responds to environmental change and the genes that control this. This work is important because environmental change is a common feature of the life of all organisms. Understanding this relationship in great detail for one species will help us to develop our understanding of the general principles that apply to other organisms. Understanding this has some urgency because the environment of many organisms is changing, largely due to changes brought about by human activity.