Which taxon is essentially equivalent to the

In contrast, as plants co-evolved with animals, the development of sweet and nutritious metabolites lured animals into providing valuable assistance in dispersing pollen grains, fruit, or seeds. Plants have been enlisting animals to be their helpers in this way for hundreds of millions of years. Land plants, or embryophytes, are classified by the presence or absence of vascular tissue and how they reproduce with or without seeds. The green algae, known as the charophytes, and land plants are grouped together into a subphylum called the Streptophytina and are, therefore, called Streptophytes.

Land plants, which are called embryophytes, are classified into two major groups according to the absence or presence of vascular tissue. Plants that lack vascular tissue, which is formed of specialized cells for the transport of water and nutrients, are referred to as non-vascular plants or bryophytes.

Non-vascular embryophytes probably appeared early in land plant evolution and are all seedless. These plants include liverworts, mosses, and hornworts. Major divisions of land plants : Land plants are categorized by presence or absence of vascular tissue and their reproduction with or without the use of seeds.

In contrast, vascular plants developed a network of cells, called xylem and phloem, that conduct water and solutes throughout the plant. The first vascular plants appeared in the late Ordovician period of the Paleozoic Era approximately million years ago. These early plants were probably most similar to modern day lycophytes, which include club mosses not to be confused with the mosses , and pterophytes, which include ferns, horsetails, and whisk ferns.

Lycophytes and pterophytes are both referred to as seedless vascular plants because they do not produce any seeds. The seed producing plants, or spermatophytes, form the largest group of all existing plants, dominating the landscape.

Angiosperms protect their seeds inside chambers at the center of a flower; the walls of the chamber later develop into a fruit. Privacy Policy. Skip to main content. Seedless Plants. Search for:. Early Plant Life. Early Plant Life A diverse array of seedless plants still populate and thrive in the world today, particularly in moist environments. Learning Objectives Describe the pervasiveness of seedless plants during the history of the kingdom Plantae.

Key Takeaways Key Points Non-vascular seedless plants, or bryophytes, are the group of plants that are the closest extant relative of early terrestrial plants.

The vast majority of terrestrial plants today are seed plants, which tend to be better adapted to the arid land environment. Seedless plants are classified into three main catagories: green algae, seedless non- vascular plants, and seedless vascular plants. Key Terms vascular plant : any plant possessing vascular tissue xylem and phloem , including ferns, conifers, and flowering plants bryophyte : seedless, nonvascular plants that are the closest extant relative of early terrestrial plants.

Evolution of Land Plants The geologic periods of the Paleozoic are marked by changes in the plant life that inhabited the earth. Learning Objectives Summarize the development of adaptations in land plants. Key Takeaways Key Points Land plants first appeared during the Ordovician period, more than million years ago.

The evolution of plants occurred by a stepwise development of physical structures and reproductive mechanisms such as vascular tissue, seed production, and flowering. Paleobotonists trace the evolution of plant morphology through a study of the fossil record in the context of the surrounding geological sediments. Key Terms Paleobotany : the branch of paleontology or paleobiology dealing with the recovery and identification of plant remains from geological contexts mycorrhiza : a symbiotic association between a fungus and the roots of a vascular plant.

Plant Adaptations to Life on Land Plants adapted to the dehydrating land environment through the development of new physical structures and reproductive mechanisms.

Learning Objectives Discuss how lack of water in the terrestrial environment led to significant adaptations in plants. Key Takeaways Key Points While some plants remain dependent on a moist and humid environment, many have adapted to a more arid climate by developing tolerance or resistance to drought conditions. Alternation of generations describes a life cycle in which an organism has both haploid 1n and diploid 2n multicellular stages, although in different species the haploid or diploid stage can be dominant.

The life on land presents significant challenges for plants, including the potential for desiccation, mutagenic radiation from the sun, and a lack of buoyancy from the water. Key Terms desiccation tolerance : the ability of an organism to withstand or endure extreme dryness, or drought-like condition alternation of generation : the life cycle of plants with a multicellular sporophyte, which is diploid, that alternates with a multicellular gametophyte, which is haploid.

Sporophytes and Gametophytes in Seedless Plants Sporophytes 2n undergo meiosis to produce spores that develop into gametophytes 1n which undergo mitosis. Learning Objectives Describe the role of the sporophyte and gametophyte in plant reproduction. Key Takeaways Key Points The diploid stage of a plant 2n , the sporophyte, bears a sporangium, an organ that produces spores during meiosis.

Homosporous plants produce one type of spore which develops into a gametophyte 1n with both male and female organs. Heterosporous plants produce separate male and female gametophytes, which produce sperm and eggs, respectively.

In seedless plants, male gametangia antheridium release sperm, which can then swim to and fertilize an egg at the female gametangia archegonia ; this mode of reproduction is replaced by pollen production in seed plants.

Key Terms gametophyte : a plant or the haploid phase in its life cycle that produces gametes by mitosis in order to produce a zygote gametangium : an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants sporopollenin : a combination of biopolymers observed in the tough outer layer of the spore and pollen wall syngamy : the fusion of two gametes to form a zygote sporophyte : a plant or the diploid phase in its life cycle that produces spores by meiosis in order to produce gametophytes.

Structural Adaptations for Land in Seedless Plants Plants developed a series of organs and structures to facilitate life on dry land independent from a constant source of water. Learning Objectives Discuss the primary structural adaptations made by plants to living on land. Key Takeaways Key Points Many plants developed a vascular system: to distribute water from the roots via the xylem and sugars from the shoots via the phloem throughout the entire plant.

An apical meristem enables elongation of the shoots and roots, allowing a plant to access additional space and resources. Because of the waxy cuticle covering leaves to prevent water loss, plants evolved stomata, or pores on the leaves, which open and close to regulate traffic of gases and water vapor.

The taxonomic-nomenclatural system is a device for communicating about the complexly interrelated products of evolution. Generally it works well, even though many aspects of it are arbitrary. For example, whether Dendroica is distinct enough to be recognized as a full genus, or should be merged with Vermivora and Parula is not self-evident, and ornithological taxonomists disagree on it.

Some taxonomists are "lumpers" and would like to combine the three; others are "splitters" and wish to keep them separate. Furthermore, as new studies of the relationships of various higher categories are published, scientists must modify the taxonomic system, and as a result names of taxonomic groups may change, as may the organisms included in them. For example, recent DNA-DNA hybridization studies have led some scientists to conclude that the Emberizidae should be considered a subfamily Emberizinae of the family Fringillidae, the wood warblers a tribe Parulini of that subfamily, and both the orioles and blackbirds combined in yet another emberizine tribe, Icterini, with the tribal name Agelaiini disappearing.

Changes in latinized specific names are inevitable as knowledge about birds increases, and most should simply be accepted as the price of progress. Common names, at least within North America, show more stability and facilitate regional communication.

Attempts to reconcile both functional and taxic views under a single umbrella concept are doomed. It would continually be unclear whether the term was being used to refer to taxa understood in various ways or functional entities of various kinds.

One way to defend pluralism is to suggest that different kinds of species should have different monikers, for example, biospecies, ecospecies, and phylospecies Ereshefsky ; Baum However, although such terms may help achieve more nuanced communication in evolutionary and ecological theory, it is hard to see how these could help in taxonomy.

It seems unrealistic to plan on developing multiple parallel taxonomies of life, one for each kind of species—we have a hard enough time maintaining one taxonomic system without trying to juggle 2, 3, or more. So, in the context of taxonomy we should aspire to monism. At their most basic, species are taxa assigned the rank of species.

As a practical reality, the discovery of a new species involves 2 steps, first deciding that a group of organisms constitutes a distinct taxon and, second, deciding that that group is a species rather than a more or less inclusive taxon subspecies, genus, etc. And even after their initial discovery, taxa ranked as species may be later recognized at another rank or vice versa. Thus, the practice of taxonomy implies that species is a rank of taxon, thereby supporting the species-as-taxon approach.

Equating species with taxa can also be defended for theoretical reasons. Most modern systematists would agree that taxa are groups of organisms that have a unique common history i. In that case, provided species are also taxa, it becomes valid to refer to the position of a species on a phylogeny or the evolutionary relationships of one species to others. In contrast, if species are functional entities of any sort, there is no reason to assume that they will show historical unity, in which case species could not be said to occupy a single position on the tree of life.

In order to develop a coherent species-as-taxa concept, we need a clear understanding of what taxa are. Then, we can ask: what makes some taxa, but not others, species? In this paper, I first clarify the concept of a taxon in light of recent advances in analyzing genealogical discordance.

I then argue that the assignment of taxa to the rank of species cannot be fully objective without undermining the demand that species be taxa. The recognition that there is nothing distinctive about the species rank aligns with Darwin's views Mallet ; Ereshefsky The concept I present is an update to the genealogical species concept Baum and Shaw except with regard to ranking, for which my position is closer to the ideas of Mishler and coworkers Mishler and Donoghue ; Mishler and Brandon ; Mishler ; Fisher et al.

However, I believe that my formulation adds clarity to previous work by refining the concept of a taxon, and hence species, based on genealogical exclusivity and by enumerating a set of semisubjective ranking criteria.

I begin by providing a brief summary of my species concept and then revisit and expand on some important details. I end by suggesting that treating species as ranked taxa will help to revolutionize taxonomy so that it can serve modern needs as a repository of biodiversity information. We can assume that every homologous nucleotide position shared by a group of contemporaneous organisms has a single, true tree-like history. This tree has some reasonably high probability of being identical to that of the neighboring nucleotide positions.

However, as one increases the spatial distance among nucleotide positions, it becomes increasingly likely that different positions will have tracked different histories. This problem, genealogical discordance, poses a significant conceptual challenge.

Thus, if our objective were to base taxonomic decisions on the assignment of organisms to clades, genealogical discordance would seem to undermine our endeavor. How can we articulate an ontology of taxa that is meaningful even when genealogical discordance applies? A set of organisms either forms a clade or does not form a clade on a particular nucleotide position's true tree.

Therefore, there is some actual if unknown proportion of the homologous nucleotide positions for which a particular subset of organisms forms a clade. The concordance factor refers to the probability of drawing a single homologous site at random from each organism and having those organisms form a clade see Baum , for more discussion. If there is no systematic difference in the length of loci as a function of their true genealogy, then the proportion of loci having a clade should equal the proportion of nucleotide positions having the clade.

This fact predicts that full genomic approaches which have yet to be developed and multilocus approaches will yield similar concordance factor estimates in practice. Thus, an exclusive group is a set of organisms whose concordance factor is higher than that of any set of organisms that includes at least one organism from outside the group and some, but not all, organisms from within the group. Therefore, this concept of exclusivity, although similar in spirit to that proposed by Baum and Shaw , is much more liberal in that it allows one to recognize divergent tree-like structures when concordance factors fall below 1.

I propose that taxa should be defined as exclusive groups of organisms. Thus, assigning an organism to a taxon represents a hypothesis that it forms a clade with all other members of that taxon for more of the genome than any overlapping set of organisms.

The nature of this taxon concept ensures that taxa, so defined, will always be hierarchically nested. However, because exclusivity can apply even when a set of organisms forms a clade for only a small proportion of the genome, hierarchical structure and thus taxa may exist well below the level that is typically associated with the species rank.

The revised genealogical species concept views species as those taxa among a nested series that are designated as being at the species rank. There is no available fully objective ranking criterion for species. The closest would be time since common ancestry Hennig ; Avise and Mitchell , but different parts of the genome can have different histories and hence different times since common ancestry. Furthermore, a strict application of a temporal ranking criterion would likely lead to the recognition of species within species and would also likely rank as species some taxa that do not warrant such a designation for other reasons.

Given the lack of a single, objective ranking criterion, the best we can do is to recognize that there is some ambiguity in the ranking of taxa, but that nonetheless there are certain features that we expect of those taxa i. These features should each refer to real biological attributes, giving them some measure of objectivity. However, because there are multiple criteria that can be used to rank species Table 1 , and these will sometimes conflict with one another, the ranking of taxa as species is best viewed as semisubjective.

These ranking criteria fall into 5 general categories: biological significance, utility, predictive power, robustness, and precedent, as outlined below. Species should correspond as closely as possible to units that are perceived to have evolutionary or ecological importance. For example, when 2 or more nonoverlapping taxa occur in sympatry and are not in the process of merging due to interbreeding, they should be ranked as separate species. The species rank should apply if at all possible to taxa that are internally homogeneous, can readily be distinguished from other nonnested taxa, and have biological properties that give us a reason to talk about them.

It is also helpful if the degree of phenotypic distinction between species has some degree of constancy within a larger group. The species rank would generally be applied to taxa about which generalizations can be made.

One important facet of the ability to generalize about all the organisms within a species is the expectation that they have a common genealogical history for more than a trivial proportion of their genome.

Thus, it would be preferable to recognize as species only those taxa that have a reasonably high concordance factor. This is likely to align with being able to make predictions about the biological properties morphology, physiology, ecology, distribution, etc.

Because of the importance of maintaining the stability of species names, taxa assigned the rank of species should ideally be those whose status as exclusive groups is confidently determined. What matters is not our confidence that the group forms a clade on a given gene tree, but that it forms a clade on more gene trees than any overlapping set of organisms. And it does not necessarily matter how high the concordance factor is.

For example, a group with an estimated concordance factor of 0. Unless other considerations weigh strongly, taxa previously assigned species rank should continue to be recognized. By maintaining continuity of usage, species-ranked taxa become better units for communicating biological information. There is a fundamental difference between species grouping, which is tied to the objective concept of exclusivity, and species ranking, which is tied to a set of semisubjective ranking criteria.

Either it is a taxon or it is not. We might be uncertain as to the correct answer, and even when we are confident in our answer, we could be mistaken, but there is a true answer. Consequently, scientific data can be used to arbitrate a dispute as to whether a group of organisms is a taxon and, thus, potentially a species. Although each ranking criterion relates to a more or less objective property of taxa, the species rank is not defined based on any one criterion, but rather is determined by the judicious balancing of multiple potentially conflicting considerations.

Different criteria will often be at odds with one another; for example, a clade may be geographically, and hence reproductively, isolated but lack phenetic or ecological distinctiveness from related taxa. Furthermore, ranking decisions are not made by looking at taxa one by one, but by considering a larger taxon and evaluating the best way to divide it into species such that all organisms are in species.

Consequently, although data can be helpful in resolving a dispute about the species rank, there is no underlying ontological claim that can be rigorously tested. The species rank is not a hypothesis, but a judgment. The art of monography is to work within the rigid constraints of genealogical relatedness to find a balance of conflicting ranking criteria that gain the support of other specialists and serve the needs of the user community. Having now introduced the basics of my genealogical species and taxon concept, I will now expand on some issues that I have glossed over.

In the process, I will discuss and counter some possible objections. The use of gene genealogical exclusivity rather than organismic exclusivity as the core of the taxon and hence species concept deserves explanation. Gene genealogical exclusivity refers to genetic ancestry and specifically to groups that form clades for a plurality of the genome.

As used here, organismic exclusivity refers to descent from common ancestral organisms. There are several ways one might formally define organismic exclusivity, but I have developed with E.

Sober and J. Velasco, University of Wisconsin—Madison a relatively stringent definition: A set of contemporaneous organisms, M, shows organismic exclusivity if there is at least one organism, A, that is an ancestor of all individuals in M, such that A is also a descendant of all the common ancestors shared by any individual in M and any contemporaneous individual outside M. Given this definition, 4 sets of contemporaneous organisms in Figure 1 constitute organismic exclusive groups. Because genetic ancestry is constrained by the paths of parent—offspring descent, organismic and gene genealogical exclusive groups will usually have identical content.

So the bracketed groups of organisms in Figure 1 would probably also show gene genealogical exclusivity. Therefore, a substitution of organismic for gene genealogical exclusivity in the taxon concept would not have major consequences in terms of the groups that would actually satisfy the concept. In both cases, there will be a hierarchically nested series of groups satisfying the criterion, thus requiring the application of semisubjective ranking criteria.

A hypothetical pedigree to illustrate the concept of organismic exclusivity. Circles represent individual organisms, each connected by lines to 2 parents in the preceding lower generation. For each of the 4 organismically exclusive groups composed only of organisms in the most recent generation , one ancestral organism M is marked. This organism has the property both of being an ancestor of all living members of the exclusive group and of being a descendant of all ancestors shared by members of the group and any contemporaneous organisms outside the group.

Despite the similarity of the 2 concepts of exclusivity, they are not identical and will not always identify the same sets of organisms. Of particular importance, directional selection acting on loci distributed across the genome can result in gene genealogical exclusivity even in a group that does not show organismic exclusivity.

Conversely, some groups that show organismic exclusivity may, nonetheless, fail to show gene genealogical exclusivity if patterns of genetic segregation have deviated from Mendelian expectations, as could happen by chance or due to selection. Given the potential for an occasional discrepancy between the 2 kinds of exclusivity, an unambiguous taxon concept should specify which has priority.

Although I can see arguments in both directions, I favor giving primacy to the gene genealogical criterion. Organismic ancestry constrains how genetic ancestry should be structured under Mendelian inheritance and in the absence of selection. When the realized patterns of genetic relatedness deviate from expectation, I think we should recognize taxa based on what actually happened rather than worry about what should have happened.

Pterophyta d. Bryophyta e. Problem Details Which taxon is essentially equivalent to the "embryophytes"? Learn this topic by watching Land Plants Concept Videos.

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