Plants, Polyploidy, and Evolutionary Dead Ends
Sexually reproducing organisms normally have two sets of chromosomes (2N). During normal sexual reproduction, each parent donates half its chromosomes (N) to the offspring. Having received 1 set of chromosomes from each parent, the offspring contains two sets of chromosomes (2N), and is called "diploid." When the offspring matures and can reproduce, the process of sexual reproduction continues.
However, sometimes something goes awry during the process of sexual reproduction, and the offspring receives all of the chromosomes from both parents. Rather than being "diploid" (e.g. 2N, or 2 sets of chromosomes) it has four sets of chromosomes -- 4N, or "tetraploid." Various mistakes that can occur during the biological processes of sexual reproduction can lead to offspring with abnormal numbers of chromosomes (e.g. 3N, 4N, or even more). In general, when offspring end up having more chromosomes than normal, this is called polyploidy.
In animals, these abnormalities are usually lethal. But plant breeders have known for centuries that hybrid crosses between different plant species can sometimes (though certainly not always) result in new varieties. Does this provide a viable mechanism for evolution?
"Speciation" Without Morphological Change
In one example cited by the TalkOrigins FAQ, two highly similar species of flowering plants within the same genus in the mint family (Galeopsis pubescens and Galeopsis speciosa) were crossed in the laboratory to produce a polyploid plant virtually identical to a known species in nature, Galeopsis tetrahit. Members of this species can reproduce with one another allowing this new form to persist. In the entire FAQ, this is the one example that arguably constituted the production of a new species.
The notion that flowering plants can be crossed to produce polyploid hybrids is nothing new. It's long been known that polyploidy occurs commonly in flowering plants.
But duplicating a chromosome doesn't necessarily produce new genetic information, and polyploid plants generally have small small-scale differences from their haploid counterparts. As Jonathan Wells observes regarding this example:
There actually are some confirmed cases of observed speciation in plants -- all of them due to an increase in the number of chromosomes, or 'polyploidy.' In the first decades of the twentieth century, Swedish scientist Arne Müntzing used two plant species to make a hybrid that underwent chromosome doubling to produce hempnettle, a member of the mint family that had already been found in nature. Polyploidy can also be physically or chemically induced without hybridization. Observed cases of speciation by polyploidy, however, are limited to flowering plants. According to evolutionary biologist Douglas J. Futuyma, polyploidy 'does not confer major new morphological characteristics . . . [and] does not cause the evolution of new genera' or higher levels in the biological hierarchy.2Speciation by polyploidy does not produce new morphological characteristics, and in this example, the tetrapoloid daughter species showed only small-scale changes -- the greatest of which is color changes of the kind well known within plants -- from the parent species. These similarities can be seen in the photographs, below, where both the parent species, and the daughter species, have similar leaf shapes and snap-dragon-like flower shapes.
This example thus shows the crossing of two highly similar species without the production of new morphological characteristics.
Is Hybridization or Polyploidy a Viable Mechanism for Most Evolution in Plants?
Speciation by hybridization and polyploidy implies flowering plants may be designed to evolve by forming hybrids, and doesn't necessarily show unguided or unplanned evolution. Jonathan Wells explains why this mechanism -- which entails the joining of two lines -- cannot explain most biological diversity:
Darwinism depends on the splitting of one species into two, which then diverge and split and diverge and split, over and over again. Only this could produce the branching-tree pattern required by Darwinian evolution, in which all species are modified descendants of a common ancestor.3Speciation by hybridization and polyploidy thus cannot be a viable mechanism for the vast majority of evolution even in plants because:
- (1) it occurs only within flowering plants,
- (2) it does not produce new morphological characteristics (as noted by Jonathan Wells, "according to evolutionary biologist Douglas J. Futuyma, polyploidy 'does not confer major new morphological characteristics... [and] does not cause the evolution of new genera' or higher levels in the biological hierarchy").
- (3) polyploid hybrids cannot arise without pre-existing parent species, meaning it entails a collapse -- not gain -- of pre-existing diversity.
The FAQ goes on to suggest that animals may also speciate through hybridization, but the evidence suggests this cannot be a viable mechanism for evolution in animals. One primary paper cited by the FAQ (Vrijenhoek 1994) admits that animal hybrids are "rare organisms."5, and there are good reasons why this is the case.
Darwinian evolution can only operate when there is heritable variation, and selection. Vrijenhoek (1994) discusses hybridogenesis within fish but notes that the genomes of hybrids are not entirely heritable since, "Paternal B genes are expressed in the hybrids but are not heritable. Only the A' 'hemiclonal' genome is transmitted between generations." This is why hybrid fish are often called "clones," because the heritable (female) portion of the genome is simply cloned from generation to generation, and males from the parent "species" are always required to maintain the line.
Such "species" cannot be maintained without the constant presence of the parent species, meaning the hybrids are not a truly independent species. Since hybridization in vertebrates typically involves asexual clonal reproduction, these hybrids cannot persist without both species present. These hybrids always require the parent species to be present in order for them to originate and persist.
The review by Vrijenhoek (1994) explains why such animal hybrids are "evolutionary dead ends":
Asexual species are often considered evolutionary dead ends because of their presumed genetic inflexibility. Among vertebrates and insects. only 0.1% to 0.2% of species arc strictly asexual. This rarity suggests a 'mutation/selection-like' balance. New asexual lineages arise infrequently and go extinct rapidly. Extant asexual 'species' are little more than scattered twigs at the tips of major phylogenetic branches. Except for bdelloid rotifers. asexual lineages have not speciated and diversified into rich asexual clades.6Hybridization is not a viable mechanism for evolution in animals, as Dobzhansky recognizes: "Sudden emergence of new species by allopolyploidy is ... irrelevant to Drosophila and most bisexual animals."7
For additional details, please see my full response to the TalkOrigins Speciation FAQ.
[1.] Arne Müntzing, "Cyto-genetic Investigation on Synthetic Galeopsis tetrahit," Hereditas, Vol. 16:105-154 (1932).
[2.] Jonathan Wells, The Politically Incorrect Guide to Darwinism and Intelligent Design, p. 53 (Regnery, 2006) (emphasis added) (quoting Douglas J. Futuyma, Evolution, p. 398 (Sinauer Associates, 2005).
[3.] Jonathan Wells, The Politically Incorrect Guide to Darwinism and Intelligent Design, p. 53 (Regnery, 2006).
[4.] Theodosius Dobzhansky and Olga Pavlovsky, "Experimentally Created Incipient Species of Drosophila," Nature, Vol. 230:289-292 (April 2, 1971).
[5.] Robert C. Vrijenhoek, "Unisexual Fish: Model Systems For Studying Ecology And Evolution," Annual Review of Ecology and Systematics, Vol. 25:71-96 (1994).
[6.] Robert C. Vrijenhoek, "Unisexual Fish: Model Systems For Studying Ecology And Evolution," Annual Review of Ecology and Systematics, Vol. 25:71-96 (1994).
[7.] Theodosius Dobzhansky, "Species of Drosophila," Science, Vol. 177 (4050):664-669 (August 25, 1972).