Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

While most modern definitions of species recognize only a monophyly as a species, i.e. a related line of the population derived from a single original parental population, it is almost certain that some biological species are formed polyphyletically, i.e. their members evolve several times independently within various populations.  A typical example consists in botanical species formed by polyploidization of some existing diploid species (see XXI.5.2).Polyploid plants exhibit a different phenotype from their diploid ancestors and can also generally not cross with the original species because of differences in the number of chromosomes (however, some species of plants are capable of this across several ploidy levels (Petit, Bretagnolle, & Felber 1999)).However, they are usually capable of productively crossing with independently formed plants with the same ploidity level, i.e. with plants that have the same number of chromosomes.Tetraploids formed by duplication of the genome of a diploidal inter-species cross have relatively the greatest chance of full renewal of fertility.Their chromosomes will most probably not form an aberrant chromosomal arrangement containing tetrades during meiosis, but rather regular chromosomal arrangements containing twice the number of chromosomal pairs compared to the original diploidal species.It is apparent that some polyploids can be formed repeatedly within a species, and can successful reproduce together because of having the same number of chromosomes.As a consequence, they comply with the requirements of the definition of a biological species although, for example, they do not comply with the requirements of the definition of a phylogenetic species (see below).