  Particle velocity distributions measured in the weakly collisional solar wind
are frequently found to be non-Maxwellian, but how these non-Maxwellian
distributions impact the physics of plasma turbulence in the solar wind remains
unanswered. Using numerical solutions of the linear dispersion relation for a
collisionless plasma with a bi-Maxwellian proton velocity distribution, we
present a unified framework for the four proton temperature anisotropy
instabilities, identifying the associated stable eigenmodes, highlighting the
unstable region of wavevector space, and presenting the properties of the
growing eigenfunctions. Based on physical intuition gained from this framework,
we address how the proton temperature anisotropy impacts the nonlinear dynamics
of the \Alfvenic fluctuations underlying the dominant cascade of energy from
large to small scales and how the fluctuations driven by proton temperature
anisotropy instabilities interact nonlinearly with each other and with the
fluctuations of the large-scale cascade. We find that the nonlinear dynamics of
the large-scale cascade is insensitive to the proton temperature anisotropy,
and that the instability-driven fluctuations are unlikely to cause significant
nonlinear evolution of either the instability-driven fluctuations or the
turbulent fluctuations of the large-scale cascade.
