monophyly, paraphyly and polyphyly
Monophyletic groups (clades)
An important goal of modern systematics is to apply scientific names only to groups of species that aremonophyletic(from Greekmonkeys= one or single, andStammN= species or tribe). A monophyletic group of species has a single common ancestor and also includesatthe descendants of that common ancestor. On a phylogenetic tree, a monophyletic group includes a node and all descendants of that node, represented by both nodes and terminal taxa. Thus a monophyletic group is also a clade (cfSection 2.1).
A phylogenetic tree illustrating the concept of monophyletic groups or clades. Note that monophyletic groups or clades are not mutually exclusive, but are nested within each other. Image by Jonathan R. Hendricks is licensed under aCreative Commons Attribution-ShareAlike 4.0 International License.
As discussed inSection 2.1, monophyletic groups (clades) form nested groups on a tree. Therefore, any given taxon can belong to many monophyletic groups, some more inclusive and some more exclusive, depending on the ancestral node that defines each group. While any node in a phylogenetic tree can be used as an anchor point for defining a monophyletic group, systematists do not necessarily attempt to name every possible group. Rather, systematists generally attempt to define groups by critical features (synapomorphies) that unite them. For example, the class Mammalia is a monophyletic group of animals that share the traits of having hair and mammary glands.
The phylogenetic tree below shows the relationships of seven species of vertebrates, one of which is the extinct dinosaurTyrannosaurus rex. Note that each different colored polygon represents a monophyletic group (clade): a common ancestor and all of its descendants. Each nested monophyletic group represents a different level of classification. For example the crocodileT-Rex, and the bird are all assigned to the monophyletic Archosauria clade. A subclass of Archosauria is the Dinosauria; A subgroup of Dinosauria are Aves, the birds. In other words, birds are descendants of a specific group of dinosaurs - meaning the Dinosauria group is not extinct at all!
Nested vertebrate monophyletic clades, showing how each clade corresponds to a taxon at a different taxonomic rank. Image by Jonathan R. Hendricksis licensed under aCreative Commons Attribution-ShareAlike 4.0 International License.
Each of the clades mentioned in the above cladogram is defined by one or more synapomorphies. For example, Tetrapoda include vertebrates with four legs. All clades nested within the Tetrapoda include organisms with four legs. So four legs is the plesiomorphic statewithinthe clade Tetrapoda, but a synapomorphyin relation tothe broader clade Vertebrata, which includes animals with backbones. Note how the phylogeny corresponds to the classification. The bird is a dinosaur, but it is also an archosaur, a reptile, an amniote, a tetrapod, and a vertebrate. Similarly, you are a mammal, but you are also an amnion, a tetrapod, and a vertebrate.
Paraphyletic Groups
Monophyletic groups can be contrasted with two other types of groups: paraphyletic groups and polyphyletic groups. Aparaphyletic groupcontains a single ancestor and some of its descendants; It resembles a monophyletic group, but some descendants are excluded.
Examples of two paraphyletic groups, one represented by the blue polygon, the other by the yellow polygon. Image by Jonathan R. Hendricks is licensed under aCreative Commons Attribution-ShareAlike 4.0 International License.
In the image directly above, the blue and yellow polygons each enclose a paraphyletic grouping. In the case of the blue polygon, taxon A, taxon B, node 1, and node 2 are included in the group, but taxon C is excluded. The group is paraphyletic because it is not inclusiveatthe descendants of the common ancestor represented by node 2 (i.e. taxon C is absent from the grouping). Can you explain why the yellow polygon also represents a paraphyletic group?
In the traditional Linnaean classification system, reptiles represent a paraphyletic grouping. To understand why, see the table of contents below from Zittel's (1902) Textbook of Paleontology. Note that Pisces (Fish), Amphibians, Reptiles, and Aves are each assigned the Linnaean rank of "class" (e.g. Class Reptiles).
Table of contents from Zittel's (1902) textbook of paleontology.
Now look at the group identified in the family tree as members of the class Reptilia (yellow polygon). What is wrong? You can see that the reptiles are paraphyletic in this context because they don't include the birds (class Aves).
Phylogenetic tree showing that excluding birds from the reptiles makes the reptile group paraphyletic. Image by Jonathan R. Hendricks is licensed under aCreative Commons Attribution-ShareAlike 4.0 International License.
If instead we accept that birds (aves) are a subset of reptiles, then we have no problems.
Phylogenetic tree showing that Reptilia is a monophyletic clade when including birds in the grouping. Image by Jonathan R. Hendricks is licensed under aCreative Commons Attribution-ShareAlike 4.0 International License.
Today, ornithologists (scientists who study birds) continue to recognize birds as belonging to the class Aves. However, since Aves is nested within Reptilia, it is logically impossible to continue to recognize Reptilia as a distinct class: just as a state cannot contain another state (e.g. the state of California cannot contain the state of Nevada), a class can not include another class because they both have the same rank. Instead, Reptilia could be placed higher than the class (e.g. superclass or subphylum) or assigned a new name altogether. To avoid confusion, some biologists now recognize Reptilia by nameSauropsid.
Polyphyletic Groups
Apolyphyletic group(from Greekpolys= many andStammN= species or tribe) is a group not defined by a single common ancestor.
In this tree, taxa A, B, E, and H together represent a polyphyletic group. Image by Jonathan R. Hendricks is licensed under aCreative Commons Attribution-ShareAlike 4.0 International License.
Consider a genus with four associated species, taxon A, taxon B, taxon E, and taxon H. Suppose a phylogenetic analysis of these four taxa along with four other taxa yielded the relatedness hypothesis presented above. Such a tree would make this genus polyphyletic since the common ancestor of the four species (node 7) is not part of the clade. In addition, it can be clearly seen that the polyphyletic grouping is widely scattered throughout the tree. Even if the common ancestor were included in the definition, taxa C, D, F, and G would need to be included to make the group monophyletic.
Continued use of paraphyletic and polyphyletic group names
Historically, systematists may have named paraphyletic or polyphyletic groups based on the general similarity in characteristics between a group of organisms and/or a lack of understanding of their relationships. With the advent of new methods in systematics, particularly methods that use DNA sequences to determine relationships between taxa, many previously paraphyletic or polyphyletic groups have been reorganized or disbanded, such that scientific classifications follow the principle that monophyly exists Reason to continue to recognize or use groups that are paraphyletic or polyphyletic?
While systematists may no longer wish to recognize such groups in formal classification schemes, paraphyletic or even polyphyletic groups can still be useful units of study in a structural (anatomical, morphological, and/or developmental), life history, and/or ecological context. For example, in introductory biology or botany courses, the diversity of living terrestrial plants is typically divided into four major groups: mosses (non-vascular plants), ferns and fern-allied plants (also known as seedless vascular plants, free-spored vascular plants, or pteridophytes), gymnosperms (naked plants), and angiosperms ( flowering plants). Two of these groups, the bryophytes and ferns and fern allies, are paraphyletic. (Living gymnosperms are also traditionally considered paraphyletic, although more recent results from molecular systematic studies suggest that living gymnosperms form a monophyletic clade.) Nonetheless, these four categories are convenient entities for comparing and contrasting broad structural and life-history traits and life trends country plants.
The four main groups of land plants are typically used to teach plant diversity at the introductory level. The pictures show examples of plants from each main group. Bryophytes: moss (top) and liverwort (bottom). Ferns and Fern Allies: A fern (top) and lycophytes (bottom). Nudibranch: A pine (above) and a cycad (below). Angiosperms: A passion flower (above) and a tulip poplar blossom (below). Photo credit: All images by E.J. Hermsen. Image by E.J. Hermsenis licensed under aCreative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Polyphyletic groupings of taxa are still partially recognized and used. For example thealgaeare a polyphyletic assemblage of eukaryotic organisms that possess chloroplasts and typically the ability to photosynthesize. (Algae sometimes include the prokaryotic cyanobacteria, or “blue-green algae.”) We now know that several unrelated groups of algae acquired their chloroplasts when their distinct ancestors independently engulfed unicellular, photosynthetic organisms in an evolutionary processEndosymbiose.The three species of algae pictured below are from unrelated groups that obtained their chloroplasts independently. Algae are studied in the field of phycology (from the GreekPhykos= algae).
Examples of algae from three unrelated groups, each inheriting its chloroplast from a specific ancestor. Left: A green alga. This alga inherited its chloroplast from an ancestor that devoured a cyanobacterium. Center: A seaweed. This alga inherited its chloroplast from an ancestor that devoured a single-celled red alga. Right: A eugenoid. This alga inherited its chloroplast from an ancestor that devoured a unicellular green alga. Picture credits: Links: Gabriele Kothe-Heinrich (Wikimedia Commons,CC BY-SA 3.0license;shortcut). Mittens: J. R. Hendricks. Credits: Deuterostome (Wikimedia Commons,CC BY-SA 3.0license;shortcut). Image by E.J. Hermsen is licensed under aCreative Commons Attribution-ShareAlike 4.0 International License.
FAQs
How can you know for sure that individuals II 3 and II 4 are heterozygous? ›
2. How can you know for sure that individuals II-3 and II-4 are heterozygous? Because their offspring have the disease so they are both carriers of it.
How do you fill out a pedigree chart in biology? ›Draw any siblings in birth order from left (oldest) to right (youngest). Siblings are connected by a horizontal line above the symbols, with vertical lines connecting the symbols to the horizontal line. Leave space to add any partners and children. Add aunts, uncles, grandparents in the same manner.
How can you tell a couple is married on a pedigree? ›A male is represented by a square or the symbol ♂, a female by a circle or the symbol ♀. Mating is shown by a horizontal line (marriage line) connecting a male symbol and a female symbol; offspring symbols are connected in a row (sibship line) beneath the mated pair.
What is pedigree short answer? ›(PEH-dih-gree) A diagram of family history that uses standardized symbols. A pedigree shows relationships between family members and indicates which individuals have certain genetic pathogenic variants, traits, and diseases within a family as well as vital status.
What is pedigree analysis with answer? ›Answer: Pedigree analysis is the study of a particular trait that is inherited from one generation to another. It helps to know the trait of inheritance for a particular trait, and also know whether the trait is actually getting inherited or not.
What is the chance in percentage (%) that the child being heterozygous for the trait? ›The Punnett square below makes it clear that at each birth, there will be a 25% chance of you having a normal homozygous (AA) child, a 50% chance of a healthy heterozygous (Aa) carrier child like you and your mate, and a 25% chance of a homozygous recessive (aa) child who probably will eventually die from this ...
How many heterozygous genotypes are possible with 4 alleles? ›Number of genotypes for a given number of alleles Given n alleles at a locus, the number genotypes possible is the sum of the integers between 1 and n: With 2 alleles, the number of genotypes is 1 + 2 = 3. 3 alleles there are 1 + 2 + 3 = 6 genotypes. 4 alleles there are 1 + 2 + 3 + 4 = 10 genotypes.
How do you tell if a gene is heterozygous or homozygous dominant? ›To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined.
How do you read a pedigree chart worksheet? ›The Components of a Pedigree:
Squares are used to indicate males in a family. Circles are used to indicate females. If the individual is “affected" by the trait (dominant or recessive) we darken the shape. A line between a male and a female indicates a marriage or union.
Pedigrees are normally used to represent simple dominant and recessive traits. For example, having a widow's peak hairline is dominant. If an individual has that trait, their symbol on the pedigree will be shaded in.
How do you calculate pedigree probability? ›
In general, if one parent is not a carrier, the probability that a child will be carrier is: ½ times (the probability the other parent is a carrier). That is, we multiply the probability of passing a disease allele, ½, times the probability that the parent does, in fact, carry the disease allele.