John Seager is the President of Population Connection, formerly Zero Population Growth. Since its founding in 1968, Population Connection has been America's largest grassroots organization dedicated to the question of overpopulation. Prior to his work at Population Connection, John worked for the EPA and in congressional politics.
Population has been a regular theme in The Conversation but has not been well developed in previous episodes. John remedies that. He also argues that overpopulation results primarily from gender inequality and a lack of access to affordable contraception—education and affluence matter, but they are secondary to equality. Combatting overpopulation is often thought of in centralized and draconian terms, but John feels that population levels will naturally plateau if individuals are allowed to freely choose the size of their families. Does this make you think of the Constitutional questions discussed in Roberta Francis' episode?
Early in The Conversation, Alexander Rose mentioned his concern that a declining population could threaten our economic system. That question surfaced again, albeit in a slightly different guise, when I spoke to John Fullerton about the challenge of decelerating the economy—though we did not talk about population decline, it's worth asking if our appraisals of corporate value assume a growing population. 
Seager also gives us another perspective on the ideological purity and social pragmatism discussion that Neil and I had at the end of Gary Francione's episode. Like Francione, Seager is a moral realist in certain areas—gender equality being one—but he also embraces incremental change and makes a case for the word "opportunism." Are purity and pragmatism a false binary? Are they equally effective (or ineffective) modes of achieving social goals? Micah, Neil and I will talk about this more at the end of the episode.
One last connection to leave you with: Robert Zubrin. Zubrin claims that overpopulation is a false concept and that, with sufficient freedom and creativity, we can support ever greater populations. Does this make him at odds with Seager? Or does Seager's emphasis on individual freedom and choice make his ideas compatible with Zubrin's? We don't know.