Manhattan(P199): the dash is a flexible punctation mark that the GMAT occasionally employs. You can use a dash as an emphatic comma, semicolon, or colon.
In short, you cannot really go wrong with a dash!
----at some time ... our ancestoers suffered an event that greatly reduced their numbers and thus our genetic variation: ... 遇到一些事降低了他们的数量,并以这种方式(降低)了基因变异
thus有一种用法:in this manner/in this way,翻译成中文:像这样,以这种方式
我觉得这里应该是这个意思