The question of the spread of an aposematic or of an otherwise visually defended plant type within a non-aposematic or a visually non-defended population is a long-standing enigma that has received considerable theoretical attention. However, the spreading of aposematic or otherwise visually defended plant genotypes within a non-aposematic or a non-visually-defended population has never, as far as is known, been shown or studied in nature in wild plant populations. This study investigates the loss of the various simultaneous types of defensive coloration in the spiny thistle Silybum marianum by a mutation that occurred independently and found in 13 wild populations in Israel. Mutant plants have plain leaves rather than leaves of the zebra-like wild-type, which has a white network of stripes on the upper leaf surface. The mutants never spread beyond several dozen meters and usually only over several meters. The mutation has a simple developmental origin, since the white variegation is the result of small air spaces formed between the epidermis and the photosynthetic parenchyma (causing no loss of photosynthetic capacity in white areas), and the mutants have no such subepidermal air spaces. In order to examine the possibility of establishing a pure population lacking this type of defensive coloration, seeds of mutant plants were collected from two wild populations where they grew mixed with the wild-type and a pure mutant population was selected and maintained for 4 years. Thus, 13 cases of very restricted spread of the visually defenseless mutant demonstrate the probable contribution of the variegation to plant fitness, supporting the hypothesis that conspicuous leaf variegation functions as defensive coloration.