one thing you must look at is, diploids only have half the chromosomes therefore less of the bad traits to be represented.
Don,
I have learned that I was premature in asserting that autopolyploids have no selective advantages in the wild. Sometimes they do ⌠though usually not in areas where the species is comfortable. The advantages will more often be found at the limits of a speciesâ range, particularly where pushing northwards or upwards (in mountains). Tetraploid races of diploid species are often slower growing (one way to avoid late frosts); and sometimes (e.g. tomatoes) higher in vitamin C. Ascotbic acid and ascorbates are antioxidants that protect plants against frost damage.
The Evolutionary Significance of Autopolyploidy
Arne MĂźntzing
http://booksc.org/book/14857264
So, where a diploid species is comfortable, tetraploid forms gain no great advantage and are soon lost. But where two diploid species are present and both are fairly comfortable, hybrids between the two may give rise to happy tetraploids â if the chromosomes of the two species are sufficiently differentiated.
Then, if a species is migrating upwards or northwards, autotetraploids can gain physiological advantages over the diploid parent species.
And then, if two diploid species are migrating in the same direction, and each pushes forward by doubling their chromosomes, the autotetraploids can then cross to form new allotetraploid species.
Or, an autotetraploid race of one species entering a new (to the species) habitat may encounter a different diploid species that is already native to the region. Triploid hybrids result, which may then produce allotetraploid progeny. Or the diploid may generously provide an unreduced gamete, giving an allotetraploid hybrid immediately.
Allotetraploids (and higher polyploids) gain a greater advantage over autotetraploids through increased phenotypic flexibility by modulation of multiple gene-sets.
Now, how does a higher polyploid break down and produce lower levels of ploidy? They must, or they will be trapped in the far north or at high altitudes.
New Phytol. 37: 72-81. 1938
PHYLOGENY AND POLYPLOIDY IN ROSA
By EILEEN W. ERLANSON, D.Sc.
A plant of octoploid > R. acicularis > from Alaska was transferred to Ann Arbor, Michigan, where it leafed out soon after the first April thaw and was frequently badly damaged by frost in May. In some seasons all the floral primordial tissue was destroyed and no flowers were produced. The alternating thaws and frosts which are a feature of continental climates between 40 and 50° of latitude North, may militate against the spread southwards of octoploid races in Rosa. The Alaskan octoploid thrived at Pasadena (Erlanson, 1934) in southern California, and came into flower early in February, 3 1/2 weeks before the hexaploid type.
http://bulbnrose.x10.mx/Roses/breeding/Erlanson/ErlansonPhylogeny1938.html
Obviously, an Alaskan octoploid will not migrate to Pasadena without passing through areas where it is not likely to survive long enough to reproduce. To gain the ability to survive at lower latitudes (or lower altitudes) it must lose some of its chromosomes. This might be achieved by crossing with different species with lower chromosome numbers (if the species are available) or by being pollinated by plants of other genera.
Or, the higher polyploids may become mitotically unstable.
Journal of Heredity 48: 11-20 (1957)
MITOTIC INSTABILITY IN RUBUS
DONALD M. BRITTON AND J. W. HULL
In one population of 66 seedlings of Eldorado (4x), (2n=28) selfed, five of the young plants were determined to be 2n=42 by means of axillary leaf-bud squashes. These 6x plants presumably resulted from fertilization of unreduced eggs by reduced pollen grainsâan event not uncommon in Rubus. In contrast to the regular-appearing leaves of 4x Eldorado selfed seedlings (Figure 5), the 6x seedlings exhibited, in addition to their symptoms of higher ploidy a noticeable degree of âcheckingâ or mosaicism in their leaves (Figure 5B) Cytological examination of their root-tips indicated that many cells had 42 chromosomes and others had fewerâsome markedly fewer (Table II). No evidence of mitotic instability has been found in 4x Eldorado plants or 4x selfed seedlings of Eldorado.
http://bulbnrose.x10.mx/Heredity/Britton_Instability/Britton_Instability.html
The point, here, is that higher polyploids are not evolutionary dead-ends forever trapped in the far north or at high altitudes. They may give rise to lower polyploids and even diploids. These derived diploids will not be identical to the original diploid parents, but will often possess a âfamily resemblanceâ. Likewise for tetraploids derived from hexaploids or octoploids.
I gave the wrong link above:
The Evolutionary Significance of Autopolyploidy
Arne MĂźntzing
http://booksc.org/book/14680575
In view of a recent post, I thought I might just bring this old thread back to the surface.