The meaning of population

the meaning of population

The passage below is from ‘the Blank Slate’ by Steven Pinker.

http://evolbiol.ru/blankslate/blankslate.htm

But even then the evidence was there: schizophrenia is highly concordant within pairs of identical twins, who share all their DNA and most of their environment, but far less concordant within pairs of fraternal twins, who share only half their DNA (of the DNA that varies in the population) and most of their environment.

In this passage I’m not sure what the underlined ‘population’ means.

http://www.thefreedictionary.com/population
1. a. All of the people inhabiting a specified area.
1. b. The total number of such people.
2. The total number of inhabitants constituting a particular race, class, or group in a specified area.
3. The act or process of furnishing with inhabitants.
4. [Ecology] All the organisms that constitute a specific group or occur in a specified habitat.
5. [Statistics] The set of individuals, items, or data from which a statistical sample is taken.

I am hovering between #2 and #5 but cannot decide which is the right one in this context.

Hope for your reply.

Regards.