Sentence:  Another type of theory emphasises the sometimes accidental nature of evolution. A spandrel is a structure arising as a by-product of some other architectural configuration. In evolutionary terms, a <WSD>spandrel</WSD> is some trait of an organism which emerges not through direct adaptionist or exaptationist means, but rather as a side effect of some other evolutionary development. 
Meaning of the word marked with <WSD> tags:  A phenotypic characteristic that evolved as a side effect of an adaptation in response to evolutionary pressure.