List of unsolved problems in biology

This article lists currently unsolved problems in biology.

Arthropod head problem, a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other.
Biological aging: there are a number of hypotheses why senescence occurs including those that it is programmed by gene expression changes and that it is the accumulative damage of biological processes.
Extraterrestrial life: might life which does not originate from planet Earth also have developed on other planets?
Paradox of the plankton: the high diversity of phytoplankton seems to violate the competitive exclusion principle.
Cambrian explosion: what is the cause of the apparent rapid diversification of multicellular animal life around the beginning of the Cambrian, resulting in the emergence of almost all modern animal phyla?
In cell theory, what is the exact transport mechanism by which proteins travel through the Golgi apparatus?
Evolution of sex: what selective advantages drove the development of sexual reproduction, and how did it develop?^[1] What causes some organisms to be homosexual?
Butterfly migration: How do the descendants of Monarch butterfly all over Canada and the US eventually, after migrating for several generations, manage to return to a few relatively small overwintering spots?

References

^ Thomas N. Sherratt, David M. Wilkinson. Big questions in ecology and evolution. Oxford University Press US, 2009. ISBN 9780199548613