Penguin link spam system
Webspam algorithm targeting manipulative link building and over-optimised anchor text; launched April 2012 and later made real-time inside core ranking.
What changed
Penguin algorithmically demoted sites whose backlink profiles showed paid links, link networks, article-directory spam or heavily repeated commercial anchor text. Google said the first release affected around 3% of English queries and framed it as an anti-webspam measure rather than a quality measure. Recovery required refreshes of the algorithm, which made the early versions notoriously slow to escape. From Penguin 4.0 in September 2016 it ran continuously as part of core ranking and devalued bad links rather than demoting whole sites.
Timing
Rollout began 24 April 2012. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.