Home / Genesis / Is Plant Polyploidy a Viable Mechanism for Evolution?

Is Plant Polyploidy a Viable Mechanism for Evolution?

Introduction

Polyploidy is probably a foreign word to many people, but it is a fairly straightforward word to understand. The prefix poly means many or multiple. The word ploid comes from Greek and roughly translates as “fold.”1 Thus, the word polyploidy means many folds. It is a key element of understanding genetics and has strong implications in any creation model.

In practice, polyploidy is used in reference to having extra sets of chromosomes in the genome. This is not the same as trisomy. The normal human complement is 46 chromosomes (23 pairs). Down Syndrome individuals have a single extra chromosome (unpaired). This single chromosome increase is called trisomy instead of polyploidy. Polyploidy is at least one extra copy at every paired chromosome in the genome. This is called a whole genome duplication (WGD). Thus, a theoretical polyploid human would have at least 69 chromosomes (46+23).

Polyploidy arises as part of reproduction, generally as a reproductive error. In sexual reproduction, gametes from both parents are generally haploid, containing just one copy of the chromosomes of the parent. Normally these haploid gametes would combine to produce a diploid offspring, just like the parents. However, on occasion, an errant diploid gamete will be produced and combine with a haploid gamete from the other parent. If this offspring survives, it will be triploid, having three copies of its chromosomes. Available data indicate the creation of new polyploid organisms is exclusively determined by a reproductive error. Maintaining polyploidy relies on the reproductive success of the newly polyploid organisms.

Polyploidy-caused speciation has been known in plants for a very long time.

Polyploidy-caused speciation has been known in plants for a very long time. Ernst Mayr, the chief proponent of the biological species concept, considered polyploidy “one of the important mechanisms of speciation in the plant kingdom.”2 Subsequent authors concurred, writing, “[P]olyploid speciation is instantaneous, sympatric, and may often involve population bottlenecks.”3 The population bottlenecks they mention occur because of the small population sizes available when they arise. Generally, only two or three individuals will be the foundation for a polyploid species. This results in generally a low amount of genetic variability in these species since they are likely only partially heterozygous, provided they reproduce sexually, which is not always the case in plants. Perhaps this is part of the reason that polyploid species go extinct more quickly than their diploid counterparts.4

Polyploid Plants

Plant polyploidy has been well known and documented for decades. Some estimates state that upwards of 70 percent of the plant species are polyploid or had been polyploid.5 While these numbers are undoubtedly high, since they are often based on a universal common ancestor for plants, there is consensus in the literature that plant polyploidy is a common occurrence. One more conservative study estimated that somewhere between 25 and 30 percent of flowering plants, or angiosperms, are polyploid.6 This number is likely to drop as molecular data improves and more information is gained about polyploid plants and how they arise.7 As a side note, while they are not plants, polyploidy also appears to be quite common in fungi.8

Despite its prevalence, Mayr regarded polyploidy as problematic rather than a simple, straightforward speciation event: “Polyploids pose a difficult taxonomic problem. An autopolyploid [defined below] may be virtually indistinguishable, at the time of origin, from the parental diploid. Such a form is often referred to as a ‘polyploid race.’ . . . Yet such a ‘race’ is reproductively isolated from the parental species and is, biologically speaking, a good species.”9 This would seem to imply that polyploid speciation is instantaneous, an idea other more recent authors have also proposed.10 Yet, aside from their genetics, these new species are almost identical to their parent species, making it problematic to delineate their taxonomy.

Plant polyploidy appears in just about every major plant group but is most prominent in angiosperms. In some cases, researchers have suggested that a given angiosperm has undergone multiple rounds of polyploidy.11 Other authors have postulated that every flowering plant species has at least one polyploid generation in the past.12 While this assertion is unprovable and based on the assumption of common ancestry, it is still a remarkable testament to how common polyploidy is among plants.

Plant polyploids are suggested to be at least situationally more evolutionarily fit to survive than their diploid cousins.13 This proposed fitness varies from being able to adapt more easily, to being able to survive mutations more easily. This is believed to enable the polyploids to gain new habitats. However, there is no consensus on this purported increase of fitness. One recent study found little evidence for such a claim.14 The increase or decrease of plant polyploid fitness remains controversial.

A whole genome duplication has been postulated to be positive in some cases, at least in plants. One explanation for beneficial effects is that the extra chromosomes provide a malleability to the genome, enabling it to survive the duplication. Others have postulated that by duplicating the whole genome, balance is maintained, preventing damage. The two explanations are not mutually exclusive.15 In fact, they could work together; however, neither hypothesis has been demonstrated.

Calling polyploidy beneficial across the board, as some evolutionists have, seems a bit of a stretch based on current knowledge.

One study concluded that polyploidy played a role in the ability of plants to become invasive, though the link was not strong and the exact role not firmly established.16 Another group of authors examined a connection between level of polyploidy and parasite resistance and found limited evidence that polyploidy was beneficial to immune responses in plants.17 Because polyploid plants are believed to self-fertilize more often than their diploid progenitors, it has been speculated that they have a slight reproductive advantage that enables them to establish populations more easily.18 This proposed tendency to self-fertilize is believed to be one of the important factors involved in establishing a polyploid population.19 However, self-fertilization’s prevalence in polyploids has been challenged and is by no means a settled issue.20 Even the suggested benefits from polyploidy are either ambiguous or disputed. Thus, calling polyploidy beneficial across the board, as some evolutionists have, seems a bit of a stretch based on current knowledge.

Polyploid Speciation

Polyploid speciation occurs periodically in the wild. This kind of speciation is enhanced because geographical isolation is unnecessary for reproductive isolation, an important component of speciation. However, polyploidy does not always result in reproductive isolation, as some researchers studying the mustard family discovered in 2008.21

Polyploid species can arise in two ways. The first is a process called autopolyploidy. Autopolyploidy involves a mistake in gamete formation in both parents. Instead of splitting the chromosomes and putting them in separate gametes, the chromosomes remain together and end up creating diploid gametes instead of haploid gametes. When these diploid gametes combine and develop into an offspring, the offspring will be tetraploid (two pairs rather than the normal one). Tetraploid offspring will then produce diploid gametes, resulting in a new, tetraploid species. The second mechanism, which is slightly more complicated, is allopolyploidy. Allopolyploidy is a much more difficult path to a new species. This mechanism requires hybridization between two species and whole genome duplication to occur at the same time. The offspring produced are sterile in most cases, particularly when chromosome numbers in the original were odd. However, if an allopolyploid offspring produces gametes that are diploid rather than haploid, they can breed with each other and produce a tetraploid species. Alternatively, they can produce diploid gametes and breed with one of the parent species to produce a triploid, usually sterile offspring. However, it is possible to take this triploid hybrid and cross it back to one of the parents and occasionally produce viable offspring, which would then create a new species with a chromosome number equal to the sum of chromosomes in the original parents.22

Autopolyploidy

Figure 1.1 Autopolyploidy is the form of polyploidy that arises within one species. A reproductive mistake gives an offspring one or more extra copies of the whole genome. Since the parents in this example are diploid, they can produce a triploid offspring as shown

Allopolyploidy

Figure 1.2 Allopolyploidy is the form of polyploidy that arises when two species hybridize and one of them suffers a reproductive mistake simultaneously. In this example, both species are diploid, resulting in a triploid offspring.

Regardless of the mechanism, speciation by means of polyploidy does occur and often creates a new species within a couple of generations. Evolutionists regularly acknowledge this:“Polyploidy provides a rapid route for species evolution and adaptation.”23 However, just because polyploids can speciate rapidly does not mean that polyploid species will create new species faster than their diploid cousins.24 In other words, that a polyploid population can speciate in one generation does not mean that it will.

It turns out that speciation in quite a few plants is based at least in part on polyploidy. In a genus of cord grasses for example, hybridization between diploid species and polyploids produced numerous varieties, including some tetraploid species. However, in this genus, hybridization is believed to have been the primary mover in speciation.25 In ivies, however, polyploidy played a major role in speciation.26 Yarrows are another group where polyploidy played a major role in forming the existing species and did so rapidly.27 Other groups that have at least some rapid polyploid speciation include the sunflowers, ferns, and mustards. A full list would be extensive. The important point is that polyploid speciation occurs regularly in plants and often does so quickly, in some cases one to two generations.28 Since a generation in plants can be as little as one growing season, some polyploid speciation can take place in one to two years.

Rapid speciation presents solid evidence for biblical creation and the concept of baramin.29 Evolutionists have attempted to explain away the data by suggesting rapid rates of speciation using punctuated equilibrium (rapid bursts of evolution), proposed by Gould and Eldredge.30 In other words, species appear rapidly, then enter a long period of stasis according to this model of evolution. And polyploid speciation often takes just one generation. Rapid speciation like this is what creation scientists expect.31

Polyploidy in the Beginning?

As mentioned above, evolutionists propose that most if not all plants either were polyploid at one time or are polyploid in the present. While the data does not support this, there are still legitimate questions to ask.32 Since plant genomes demonstrate a certain amount of plasticity and appear to tolerate polyploidy in some cases , it is reasonable to ask if at least some plants were created polyploid. Based on the high rates of polyploidy in plants compared to other organisms, this is an appropriate question. Since the original state of creation is unobservable, this will require some deductive reasoning to go from the observable present to the unobserved past.

If polyploidy were created in some of the original created kinds in Genesis 1, it would be expected that a sizeable majority of a given baramin would exist in a polyploid state today. Further, multiple levels of ploidy in the baramin would also be expected. If a kind had been created tetraploid (4 copies), it would produce diploid (2) gametes. However, polyploid levels change due to rare mistakes in the formation of gametes. If, in the post-fall world, a tetraploid kind had a malfunction in gamete formation and produced a triploid (3) and a haploid (1) gamete, the resultant offspring could be hexaploid (6=3+3), tetraploid (4=3+1), or diploid (2=1+1). Whether those offspring could survive or not is debatable, but even if one could, a new polyploid level is created and likely a new species. These predictions can be tested by simply examining known created baramins and seeing how they compare.

Sadly, not many created baramins have been delineated yet, particularly for extinct species. One book by creation scientist Dr. Todd Charles Wood presented 22 baraminology studies for plants.33 While not all of them were conclusive, some may be used to attempt determination of polyploidy in the original created kinds.

One of the baramins delineated was the birthworts, family Aristolochiaceae. One particular genus, which contains the majority of the species, has a vast variety of chromosome numbers.34 However, in this instance, it seems unlikely that polyploidy was the original state of the created kind, as only one genus in the family is heavily “ploidized,” and an evolutionary phylogenetic study indicates only a few polyploidy events in the past.35 While the article is heavily weighed down with evolutionary dogma and used the evolutionarily tainted classification, the authors make a valid point because the ploidy is contained in one genus. This baramin was likely created diploid.

The madder family has been extensively studied in the secular literature and with good reason. It is the fourth largest family of flowering plants and is found throughout the world. However, the most likely reason why this family has been extensively studied is probably sitting on the kitchen counter. The madder family contains coffee. Since coffee is a staple of American life, this family has been subjected to more studies than the average plant family. In the context of polyploidy, the family exhibits a high degree of variability in chromosome numbers, which may have been derived through polyploidy, though part of that may have been through selective breeding.36 They come in tetraploid and diploid variants, and according to evolutionary phylogeny, multiple lineages have resulted in polyploidy.37 Based on this information, it is possible that the madder family was created polyploid, though this cannot be determined definitively. Further genetic studies would be required, particularly focusing on deriving the genotype of the originally created madders.

The family Cupressaceae contains the junipers, redwoods, and several other conifers. Polyploidy in conifers in general is very rare.38 While polyploidy can be induced in this family for decorative purposes,39 most of the family is not polyploid, with the notable exception of the coast redwood, which is hexaploid.40 Given the low incidence of polyploidy in this family, it is very unlikely that it was polyploid in the beginning.

Based on this admittedly small sample, there is potential that polyploidy was not the created form in the majority of thus-far delineated baramins. The madder family is the only one mentioned here that could potentially have been created polyploid.

Diploid Cross

Figure 2.1 This Punnett square demonstrates the possible combinations of multiple alleles from a pair of heterozygous diploid parents. There are four possible outcomes

Figure 2.2.

Figure 2.2 2 This Punnett square demonstrates the possible combinations of multiple alleles from a pair of heterozygous tetraploid parents who produce diploid gametes. Notice that there are far more possible combinations than there are for diploid parents

Created polyploid is significant because of heterozygosity. Heterozygosity in the genome means having two different alleles that code for the same trait. Alleles are the genetic information that codes for a given trait. In this article, allele will be used to represent a sequence of nucleotides coding for a given trait. Many alleles are expressed in the observable variation of the organism, depending on how the alleles are combined. Sometimes the alleles work together to produce the outward appearance, while other times a dominant allele overrides the other and is the only visible allele. The more available alleles in a population for a given trait, the more variability that is available to that population for that trait. Thus a more heterozygous population will appear more diverse.

The madder family is quite large, while birthwort and cypress families are quite small for plant baramins, leading to the obvious assumption that the madder family is more diverse. The madder family does display great diversity, containing trees, shrubs, and herbs. To diversify to this point from one originally created kind, a massive amount of genetic information would have been required. Adding an extra two copies of the genome doubles the potential alleles in the genome. However, assuming normal Mendelian genetics are in play, a pair of diploid creatures that are heterozygous for a given trait can create four different combinations, though two are the same and thus potentially three different phenotypes. However, if those same two individuals are heterozygous tetraploids, instead of four possible alleles, there are eight. Each parent passes two alleles to their offspring, leading to at least thirty-six separate combinations of alleles, though this may be a conservative figure.41 Doing this for several generations might produce significant variation within one created kind. An example of this comes from strawberries. Some species of strawberries are polyploid; others are not. The differences in the ploidy level affect numerous traits from flavor and size of the fruit to the appearance of the plant.42 While it is not as simple as this due to rates of recombination and linkage between alleles, it gives a general idea of how much variation could be produced by a tetraploid original kind. Since a change in ploidy level generally results in a near-instant speciation event, having this increased amount of information built into the created kind would have resulted in a vast variety of species as ploidy levels changed. Traits would have split and changed ploidy level, resulting in an increased number of phenotypes and species, some separated by ploidy level, some separated by conventional variation.

Conclusions

Polyploidy is a vast topic, with a significant array of polyploid plants. Whether it is generally beneficial or not remains to be determined since there is some evidence pointing to both benefit and detriment. It is therefore unwise to draw a broad-brush conclusion about polyploidy. It is worth considering that some of the more diverse plant kinds could have been created at some level of polyploidy, leading to the potential for much greater genetic diversity in their offspring. The creation of polyploid organisms in the present requires a reproductive mistake that adds extra copies of the same information pre-existent in the genome. Polyploidy is thus not an evidence for evolution or an increase in actual information. Two of the same dictionaries do not increase your vocabulary any more than one would. In fact, quick polyploidy speciation conflicts with the slow process evolutionists have traditionally expected. Instead, rapid speciation, such as afforded via polyploidy, better fits the biblical model of diversity within created kinds held by creation scientists.

Let’s block ads! (Why?)

Answers in Genesis: Daily Articles

About admin

Check Also

Is Capital Punishment for Today?

When passages from the Old Testament are used to justify or condemn a modern practice, …

Leave a Reply

Your email address will not be published. Required fields are marked *