Most flowering plants reproduce through outrcossing. Pollen from one plant is transferred to another where it germinates on the stigma, grows through the style, and sperm from the pollen unites with an egg to form an embryo that develops into a seed.
But a significant minority of flowering plants reproduce through self-fertilization. Much of my early work was focused on understanding why some plants self and others don't.
It's obvious that self-fertilized progeny are generally less fit than outcrossed progeny, which is a significant disadvantage. It's also obvious that self-fertilization is sometimes the only way in which a plant can reproduce, and it's better to have reproduced by selfing, than never to have reproduced at all.
What's less obvious is that a plant that self-fertilizes may have an additional advantage. It leaves offspring not only through self-fertilization but by serving as a pollen parent to the outcrossed offspring of other individuals. That segregation advantage could also favor self-fertilization, but whether it exists depends critically on the details of how pollination happens.
My mathematical models helped to illuminate those processes.