Isogamy is considered to be the most primitive and oogamy the most advanced type of sexual reproduction. The evolution from isogamy to oogamy through unisogamy is seen in different groups of the Chlorophyceae. Since oogamy has arisen quite independently in several groups, sometimes even within the limits of a single genus (e.g., Chlamydymonas), not much phylogenetic significance can be attached to it.