Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Node colour indicates country of origin. Size of each node is proportional to the number of direct offspring. Colour coding: pink (Australia), orange (Canada), light green (Germany), bright green (France), turquoise (UK), light blue (Italy), purple (Mexico), fuschia (the Netherlands), yellow (Sweden), red (US), dark blue (other countries). For a higher resolution image, see S1 Fig.
