A portal of entry<span> is the site through which micro-organisms enter the susceptible host and cause disease/infection. Infectious agents enter the body through various </span>portals<span>, including the mucous membranes, the skin, the respiratory and the gastrointestinal tracts.</span>
Answer
Environmental pressures
Basically every animal adapts to pressures from their environment in different ways, causing them to have different shapes and sizes
