Marco Biagi plans to go up against former SNP deputy leader Angus Robertson for the key Holyrood seat.
Carlaw steps down less than a year before the next Holyrood election.
Polling expert Professor John Curtice said 'never before' had public support for the union 'looked so weak'.
The Highlands politician, who has served as a councillor, MP and MSP, will step down aged 65.
