Selection pressures can reduce or eliminate heritable variability and thus many traits that have undergone strong selection in the past no longer show heritable variability (e.g., all genetically normal human beings have two legs, an inherited but nonvariable characteristic).
(Selection pressures; can reduce; ) (Selection pressures; eliminate; heritable variability) (thus many traits; have undergone; strong selection; T:in the past) (thus many traits that have undergone strong selection in the past; no longer show; heritable variability)
Selection pressures Selection pressures thus many traits thus many traits that have undergone strong selection in the past
can reduce eliminate have undergone no longer show
heritable variability
strong selection T:in the past
heritable variability