Children often outgrow their parents by their teens. TripAdvisor Inc. TRIP -1.63% did it in less than 20 months.

That milestone was crossed after Expedia Inc. EXPE -0.33% executives spent months warning analysts and investors that aggressive marketing tactics from old rivals like Priceline.com Inc. PCLN -0.83% could hurt profits this year.

But a more painful and unexpected sting came last month from an old ally: TripAdvisor, the former subsidiary that still feeds Expedia many of its customers.

A year and a half after being spun off by Expedia and with a newly independent board, TripAdvisor redesigned its website to show shoppers more hotel options right there among its traveler reviews, rather than referring them to sites like Expedia's via pop-up windows, as it had in the past.

Expedia reacted late to the shift, and its sales suffered this spring partly as a result. TripAdvisor, meanwhile, booked its fourth-straight quarter of better-than-expected earnings.

The former subsidiary's success shows how swiftly currents can shift online, where travelers are increasingly planning their trips on sites that aggregate offerings from a number of different online sources.

The shift is evident in the two companies' stock market value. At $6.9 billion, Expedia is now worth about 40% less than TripAdvisor, which is valued at more than $11 billion.

It is also evident in data from Google GOOG -0.66%