What series of evolutionary events would lead to isogamous (not isogynous) animals?

Isogamy: "sexual reproduction by the fusion of similar gametes." Not to be confused with Isogyny, "marriage between people of similar status or age."

All such species on Earth lack tissue-level organization, such as algae and fungi. What hurdles would an isogamous species need to overcome to achieve metabolism and organ systems comparable to animals without becoming anisogamous in the process?

EDIT: I may be wrong, but to me it seems the most obvious hurdle is how they decide who lays eggs or gives birth. Fungi conceive using mating bridges that produce fruiting bodies that release spores which grow into a mycelium. Obviously this is infeasible for non-sessile organisms.

posted over 8 years ago

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