The evolutionary transition of multicellular life initially involves growth in groups of undifferentiated cells followed by differentiation into soma and germ-like cells. This is facilitated by trade-offs between traits determining survival and reproduction, favoring the coexistence of cells with extreme trait values and a convex trade-off curve as the multicellular state dominates. However, these transitions remain poorly characterized at the ecological and genetic level. Here, we studied the evolution of cell groups in ten isogenic lines of the unicellular green algae Chlamydomonas reinhardtii with prolonged exposure to a rotifer predator. We confirmed that this trait was heritable and characterized by a convex trade-off curve between reproduction and survival. Identical mutations evolved in all cell group isolates which were linked to survival and reducing associated cell costs. Overall, we show that just 500 generations of predator selection is sufficient to lead to a convex trade-off and incorporate evolved changes into the prey genome. Overall design: We picked one pair of cell group and single cell isolate and inoculated them to nine chemostat in COOL80 medium. After 10 days we either added rotifers (rotifer treatment), switched the resource supply to lower nitrogen concentration (low nitrogen treatment; 40μM nitrogen) or kept the algae as they were (control treatment), each treatment was replicated three times. After five additional days we harvested the algae from the chemostats. We compared the algae from the chemostats by measuring cell cluster sizes (number of cells per colony), percentage of cell clusters, and overall population sizes for the three treatments (Supplementary information Fig. S2). Simultaneously, the harvested algae were prepared for total RNA extraction with RNeasy Plant Mini Kit from Qiagen® for RNASeq analysis. We pooled the RNA extracted by treatment and RNA-sequenced it using Illumina HiSeq (2x150bp).