Convergence , or convergent evolution, is the development of a similar anatomical feature in distinct species lines after divergence from a common ancestor that did not have the initial trait that led to it. The common ancestor is usually more distant in time than is the case with parallelism. The similar appearance and predatory behavior of North American wolves and Tasmanian wolves thylacines is an example. The former is a placental mammal like humans and the latter is an Australian marsupial like kangaroos.
Their common ancestor lived during the age of the dinosaurs million years ago and was very different from these descendants today. There are, in fact, a number of other Australian marsupials that are striking examples of convergent evolution with placental mammals elsewhere. Australian Tasmanian wolf or tiger now extinct North American wolf Last Tasmanian Tiger, Thylacine, silent film : To return here, you must click the "back" button on your browser program.
To return here, you must click the "back" button on your browser program. Both parallelism and convergence are thought to be due primarily to separate species lines experiencing the same kinds of natural selection pressures over long periods of time. Analogies are anatomical features that have the same form or function in different species that have no known common ancestor. For instance, the wings of a bird and a butterfly are analogous structures because they are superficially similar in shape and function.
Both of these very distinct species lines solved the problem of getting off of the ground in essentially the same way. However, their wings are quite different on the inside. Bird wings have an internal framework consisting of bones, while butterfly wings do not have any bones at all and are kept rigid mostly through fluid pressure.
Analogies may be due to homologies or homoplasies, but the common ancestor, if any, is unknown. Problems in Classifying Organisms. Listing characteristics that distinguish one species from another has the effect of making it appear that the species and their distinctive attributes are fixed and eternal. We must always keep in mind that they were brought about by evolutionary processes that operated not merely at some time in the distant past, but which continue to operate in the present and can be expected to give rise to new forms in the future.
Species are always changing. As a consequence, they are essentially only a somewhat arbitrarily defined point along an evolutionary line. It is also important to realize that most species are physically and genetically diverse. Many are far more varied than humans.
When you think of an animal, such as the jaguar shown on the right , and describe it in terms of its specific traits fur color patterns, body shape, etc. To do so, however, is to ignore the reality of diversity in nature.
Archaea are more similar to Eukarya than to Bacteria. The domain Eukarya includes all organisms that have DNA contained within a nucleus. Organisms with similar characteristics are grouped within these broad kingdoms. Organisms are usually grouped together based on their unique characteristics. The classification of an organism often provides useful information about its evolutionary history and which other organisms are related to it.
At each level of hierarchy listed in Table 1. All organisms in Eukarya often referred to as eukaryotes have DNA contained in a nucleus rather than in the cytoplasm like the domains Prokarya and Archaea. Next is the kingdom Animalia box. Everything in this box must consume other organisms to survive. Other kingdoms within Eukarya, like the kingdom Plantae, have organisms that can make their own food.
Within the kingdom Animalia box, there are several other boxes, each labeled as a different phylum. One is the phylum Chordata box. This box contains everything that has a notochord, gill slits, and a dorsal nerve cord.
The phylum Chordata box contains many classes, one of which is the class Aves. Aves are the birds, with feathers and hollow bones. The class Aves box includes the box labeled order Anseriformes, the waterfowl that are grouped together due to their webbed front toes. The order Anseriformes box contains two family boxes. One of these is the family Anatidae—the swans, ducks, and geese that have a broad bill, a keeled sternum, and other unique features. The family Anatida box contains the genus Branta.
Geese in the genus Branta are noted for bold plumage and legs and bills that are black in color. The genus box Branta holds the species sandvicensis. By examining each level of classification, it becomes clear that Branta sandvicensis is a Hawaiian goose with a black broad bill, legs, webbed toes, feathers, hollow bones, and a notochord.
It must also eat other things. However, no other organism on earth is given the genus Branta and the species sandvicensis. The classification system tells something about the evolutionary relationships among species.
Moving down through each level of classification, the number of species in the group decreases Table 1. Two species within the same genus likely share a recent common ancestor in their evolutionary history.
These two species would be more closely related to each other than two species classified into different families. The levels of classification might also provide information on the evolutionary history of a species or other taxonomic group. Such is the case with the coelocanths Latimera spp. West Indian ocean coelacanth Latimeria chalumnae ; Fig. They are also the only living members of their family Latimeriidae and of their order Coelacanthiformes.
All other species belonging to these levels of classification are now extinct. Coelacanths are also some of the very few surviving fish species within the class Sarcopterygii, a group known as the lobe-finned fishes. All four-limbed vertebrate animals—amphibians, reptiles, birds, and mammals—also belong to class Sarcopterygii.
The coelacanths, and the six species of lung fish, are more closely related to each other and to the four-limed vertebrates than to other fishes. For this reason, the coelacanth offers a rare glimpse into the evolutionary history of vertebrate animals and their limb-development.
Classification systems are used in many ways. Compare the classifications shown in Fig. Most people know something about water vehicles, so it is not necessary to say that a speedboat has a motor. In the same way, there is general knowledge that a tuna is classified as a fish. So, a tuna can be described without needing to say that it is a fish because. Thus, if we make the statement that a skipjack tuna is caught while fishing in a speedboat, many details can be left out of the description because there is general, underlying knowledge of the classification of boats and tuna.
This use of italics is part of the rules that the scientific community has developed for the naming of organisms. There are three main codes that govern the naming of organisms. Scientific names are useful outside of science. Common names vary from place to place, and the scientific nomenclature system helps eliminate confusion.
This example also brings up another problem with common names. Notice that one of the common names for this fish uses the word dolphin, which is also the common name of a marine mammal. Scientific names are also valuable in navigating the classification system. The classification system provides great deal of information about the characteristics of organisms. Using scientific names can therefore act as a shorthand method for describing a plant or animal.
For example, following a whale stranding along the Maui coastline, an observer might record this information:. This is all information needed to identify the organism and avoid mixing it up with other similar organisms. Of course, when reporting the mammal stranding to her supervisor, the observer will report stranding of a Megaptera novaeangliae , which is the species name that describes the humpback whale.
It is the lowest and most strict level of classification of living things. The main criteria for an organism to be placed in a particular species is the ability to breed with other organisms of that same species. The species of an organism determines the second part of its two-part name. The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands.
Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden. This website uses cookies to ensure you get the best experience on our website. Learn more. Skip to main content Skip to acknowledgement of country Skip to footer On this page Toggle Table of Contents Nav What is classification? Levels of classification. Toggle Caption Blue Butterflies are Morpho spp. The orange and yellow butterflies are in the family Pieridae whites, yellows and sulphurs , and come from Brazil, Peru, Malaysia and Indonesia.
Figure 3. In this pedigree analysis for alkaptonuria, individuals with the disorder are indicated in blue and have the genotype aa.
Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. For example, if neither parent has the disorder but their child does, they must be heterozygous.
Two individuals on the pedigree have an unaffected phenotype but unknown genotype. In the sections to follow, we consider some of the extensions of Mendelism. Figure 4. These pink flowers of a heterozygote snapdragon result from incomplete dominance.
However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents. Note that different genotypic abbreviations are used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness. This pattern of inheritance is described as incomplete dominance , denoting the expression of two contrasting alleles such that the individual displays an intermediate phenotype.
The allele for red flowers is incompletely dominant over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses.
A variation on incomplete dominance is codominance , in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance is the MN blood groups of humans.
The M and N alleles are expressed in the form of an M or N antigen present on the surface of red blood cells. In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct. However, the genotypic ratio characteristic of a Mendelian monohybrid cross still applies.
Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans and all diploid organisms can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed.
All other phenotypes or genotypes are considered variants of this standard, meaning that they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele. An example of multiple alleles is coat color in rabbits Figure 5. Here, four alleles exist for the c gene.
The chinchilla phenotype, c ch c ch , is expressed as black-tipped white fur. The Himalayan phenotype, c h c h , has black fur on the extremities and white fur elsewhere.
In cases of multiple alleles, dominance hierarchies can exist. In this case, the wild-type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino. This hierarchy, or allelic series, was revealed by observing the phenotypes of each possible heterozygote offspring. Figure 5. Four different alleles exist for the rabbit coat color C gene. Figure 6. As seen in comparing the wild-type Drosophila left and the Antennapedia mutant right , the Antennapedia mutant has legs on its head in place of antennae.
For the allelic series in rabbits, the wild-type allele may supply a given dosage of fur pigment, whereas the mutants supply a lesser dosage or none at all. Alternatively, one mutant allele can be dominant over all other phenotypes, including the wild type.
This may occur when the mutant allele somehow interferes with the genetic message so that even a heterozygote with one wild-type allele copy expresses the mutant phenotype. One way in which the mutant allele can interfere is by enhancing the function of the wild-type gene product or changing its distribution in the body.
One example of this is the Antennapedia mutation in Drosophila Figure 6. In this case, the mutant allele expands the distribution of the gene product, and as a result, the Antennapedia heterozygote develops legs on its head where its antennae should be. Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae Figure 7a , and is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia.
Plasmodium falciparum and P. When promptly and correctly treated, P. However, in some parts of the world, the parasite has evolved resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by geographic region.
Figure 7. The a Anopheles gambiae, or African malaria mosquito, acts as a vector in the transmission to humans of the malaria-causing parasite b Plasmodium falciparum, here visualized using false-color transmission electron microscopy. Varying degrees of sulfadoxine resistance are associated with each of these alleles. Being haploid, P. In Southeast Asia, different sulfadoxine-resistant alleles of the dhps gene are localized to different geographic regions.
This is a common evolutionary phenomenon that occurs because drug-resistant mutants arise in a population and interbreed with other P. Sulfadoxine-resistant parasites cause considerable human hardship in regions where this drug is widely used as an over-the-counter malaria remedy. As is common with pathogens that multiply to large numbers within an infection cycle, P.
For this reason, scientists must constantly work to develop new drugs or drug combinations to combat the worldwide malaria burden. In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes. The sex chromosomes are one pair of non-homologous chromosomes.
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