Karyotypic Megaevolution by Any Other Name: A Response to Marks

Marks (1983) objected to our (Baker and Bickham, 1980; Haiduk and Baker, 1982) use of the term "karyotypic megaevolu- tion" as a name for the occurrence of a major repatterning of the euchromatic G-band sequences in a karyotype. As an alternative to "karyotypic megaevolu- tion," Marks (1983) proposed the name "chromosomal tachytely. " We believe chromosomal tachytely is not an appro-priate name for the phenomenon and pro- vide information as to why we chose Simpson's (1944, 1953) abandoned term. Simpson (1944) proposed two terms to describe relative rates of evolution, "bra- dytely" which refers to a slower than av- erage rate and "tachytely" to refer to a faster rate. Chromosomal tachytely as pro- posed by Marks (1983) for an alternative to "karyotypic megaevolution" would perhaps be acceptable, if karyotypic mega- evolution were simply a fast rate of chro- mosomal evolution. As explained below, this is not true. Relatively rapid rates of chromosomal evolution have been dem- onstrated in a wide variety of mammals, including mice of the genera Mus (Capan-na, 1982) and Onychomys (Baker et al., 1979) and in the examples cited by us in describ- ing "karyotypic megaevolution," as well as examples cited by Marks (1983). All of these different patterns could be included under Marks' usage of the term "chromo- somal tachytely. " For example, in Onych-omys (Baker et al., 1979) there has been the addition of many heterochromatic short arms (possibly as many as 32) without al- tering a single euchromatic linkage group. In Mus (Capanna et al., 1977; Capanna, 1982), numerous centric fusions (at least 17) have occurred, yet the internal struc- ture of the euchromatic arms was unal-tered. On the contrary, in all examples of "karyotypic megaevolution" (Baker and Bickham, 1980) many different types of euchromatic rearrangements have become established and most if not all major link- age groups have been altered. Factors affecting chromosomal evolu-tion were outlined in detail by Lande (1979) who documented that, from a the- oretical probability standpoint, certain types of rearrangements (such as hetero- chromatic additions and centric fusions) are more likely to become established in a species than are other types of rearrange- ments (such as telomere-centromere translocations and reciprocal transloca-tion~). From a theoretical and practical cy- togenetic standpoint, it is much easier to explain the evolution of the observed variation in Mus and Onychomys than it is to explain the observed change in exam- ples of karyotypic megaevolution (Baker and Bickham, 1980). The types of rearrangements found in Mus and Onychomys could occur in the heterozygous condition without causing severe problems in fertility. Without se-vere constraints on fertility, these rear-rangements are more likelyto become es- tablished. On the other hand, many of the rearrangements found in examples of "karyotypic megaevolution" are thought to cause significant meiotic problems and, therefore, should rarely evolve in natural populations (Lande, 1979). Yet, in exam- ples of karyotypic megaevolution, many such rearrangements (minimally 15 to 20) have become established in species since they separated from their nearest relative. Herein lies the significance of karyotypic megaevolution. Based on the assumptions outlined in Lande (1979), karyotypic megaevolution cannot occur in natural populations during a relatively short pe-