Converting migration fraction to number of migrants

[Sofia Hayden Bofill]

Hi,

I now this topic has been covered before, but I am still confused about how to go from the proportion of new migrant m12=M12/(2*Nref) to the number of new migrants per generation.

Until now I have seen two equations:

1) Number of migrants=2*N1*m12 ; which given my low proportion of migrants (2.5e-07) results in <1 new migrants per generation.

2) Number of migrants=2*N1*m12*Nref ; which gives me values on the hundreds of new migrants.

Could somebody please confirm which is the right one here?

Thank you in advance and best wishes,

Sofia

[Ryan Gutenkunst]

Hello Sofia,

First: note that m12 is indicates migration into population 1 from population 2, since that’s a bit confusing.

Second: m12 indicates the proportion of individuals in population 1 that are new migrants each generation.

Third: So to find the number of individuals in population 1 that are new migrants in each generation, you would multiple m12 by the size of population 1. So, depending on the model you’re fitting, that might be something like Nref*nu1*m12.

Fourth: In general in population genetics, a small number of new migrants each generation is enough to dramatically affect genetic variation over moderate timescales. You don’t need hundreds for it to matter.

Does that answer your questions?

Best,

Ryan

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[Sofia Hayden Bofill]

Hi Ryan,

Thank you so much for your quick reply, I really appreciate it.

Amazing, now it is clear. Your reply does answer my questions, thank you again!

Best,

Sofia