Maybe its just me, but I've never researched any of the body-modification techs that make up the vast majority of the Biology tech tree. After researching all the Infantry Upgrades, I assign all but my 2 most capable biologists to other fields since they'll only ever be researching terraforming upgrades from then on.
That is a shame since I think biology could be good for a number of upgrades that would vastly improve your empire overall, like the construction/production tree. Here is a list of suggestions:
1. Triage Technology: Each level increases the amount of casualties returned to their units after battle and increases the chances of officers and crew escaping from ships alive. Also reduces the chances of officers being killed in accidents.
2. Healthcare Technology: Increases the lifespan of your leaders and reduces the chances they develop health issues.
3. Medical Technology: Increases the population growth rate.
4. Viral Agents: An alternative to using missiles or beam weapons to clear out an enemy planet. Viral Agents are launched from specialized bomb-bays requiring the ship to be in orbit of the target, causing no damage on their own but introducing a viral agent that will slowly (relative to bombardment) clear out the planet population and garrison without damaging the planet or infrastructure. Viral Agents are designed in the component designer and each agent needs to be custom made for each species you encounter, with the option to do so presenting after you conduct an autopsy. Using viral agents have a serious effect on you relations with any empires in contact with you or the target. Viral Agents would have two components; Viral Agent Potency, and Viral Agent Target (both researched in Biology).
5. More options to upgrade infantry genetic enchantments. It could be split into three different upgrades; Health, Stamina, and Reflexes. Health increases health, much like the existing enchantments do now. Stamina increases the shots they can take per turn (they don't have to rest as often). Reflexes increases the damage done per shot to reflect better accuracy.
I think adding at least the first three of these will greatly flesh out the biology tree and make it a more useful field worthy of investing in. What do you think of these suggestions and is there anything you'd like to add?