They are likely referring to Lysenkoism. This was the USSR's alternative to natural selection, partially because they believed natural selection was not compatible with Marxism. (There were other justifications, but this is the one relevant to the parent comment)
"Some Marxists, however, perceived a fissure between Marxism and Darwinism. Specifically, the issue is that while the "struggle for survival" in Marxism applies to a social class as a whole (the class struggle), the struggle for survival in Darwinism is decided by individual random mutations. This was deemed a liberal doctrine, against the Marxist framework of "immutable laws of history" and the spirit of collectivism."
"Some Marxists, however, perceived a fissure between Marxism and Darwinism. Specifically, the issue is that while the "struggle for survival" in Marxism applies to a social class as a whole (the class struggle), the struggle for survival in Darwinism is decided by individual random mutations. This was deemed a liberal doctrine, against the Marxist framework of "immutable laws of history" and the spirit of collectivism."
Link: https://en.wikipedia.org/wiki/Lysenkoism