Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species, he identified sexual selection as a mechanism, but found it problematic. There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric and sympatric. Speciation may be induced artificially, through animal husbandry, agriculture, or laboratory experiments. Whether genetic drift is a minor or major contributor to speciation is the subject matter of much ongoing discussion. Rapid sympatric speciation can take place through polyploidy, such as by doubling of chromosome number. New species can be created through hybridisation followed, if the hybrid is favoured by natural selection, by reproductive isolation.

In addressing the question of the origin of species, there are two key issues: what are the evolutionary mechanisms of speciation, what accounts for the separateness and individuality of species in the biota? Since Charles Darwin's time, efforts to understand the nature of species have focused on the first aspect, it is now agreed that the critical factor behind the origin of new species is reproductive isolation. Next we focus on the second aspect of the origin of species. In On the Origin of Species, Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species. Chapter 6 of Darwin's book is entitled "Difficulties of the Theory." In discussing these "difficulties" he noted "Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?" This dilemma can be referred to as the rarity of transitional varieties in habitat space.

Another dilemma, related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth." That defined species do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species. It has been argued that the resolution of Darwin's first dilemma lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity; the cost of rarity arises. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a narrow band on that gradient, each species will, of necessity, consist of few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, if any species' population size happens, by chance, to increase, this will make it easier for its members to find sexual partners.

The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, therefore form pairs less than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species driving them to extinction. Only a few species remain, each distinctly different from the other; the cost of rarity not only involves the costs of failure to find a mate, but indirect costs such as the cost of communication in seeking out a partner at low population densities. Rarity brings with it other costs. Rare and unusual features are seldom advantageous. In most instances, they indicate a mutation, certain to be deleterious, it therefore behooves sexual creatures to avoid mates sporting unusual features. Sexual populations therefore shed rare or peripheral phenotypic features, thus canalizing the entire external appearance, as illustrated in the accompanying illustration of the African pygmy kingfisher, Ispidina picta.

This uniformity of all the adult members of a sexual species has stimulated the proliferation of field guides on birds, reptiles and many other taxa, in which a species can be described with a single illustration. Once a population has become as homogeneous in appearance as is typical of most species, its members will avoid mating with members of other populations that look different from themselves. Thus, the avoidance of mates displaying rare and unusual phenotypic features leads to reproductive isolation, one of the hallmarks of speciation. In the contrasting case of organisms that reproduce asexually, there is no cost of rarity. Thus, asexual organisms frequently show the continuous variation in form that Darwin expected evolution to produce, making their classification into "species" difficult. All forms of natural speciation have taken place over the course of evolution.

This is a list of the National Register of Historic Places listings in Northwest Philadelphia. This is intended to be a complete list of the properties and districts on the National Register of Historic Places in Northwest Philadelphia, United States; the locations of National Register properties and districts for which the latitude and longitude coordinates are included below, may be seen in an online map. There are 576 properties and districts listed on the National Register in Philadelphia, including 67 National Historic Landmarks. Northwest Philadelphia includes 73 of these properties and districts, including 6 National Historic Landmarks. One site is split between Northwest Philadelphia and other parts of the city, is thus included on multiple lists. List of National Historic Landmarks in Philadelphia National Register of Historic Places listings in Philadelphia, Pennsylvania

Stellar West is an American indie punk band from Naperville, Illinois formed in 2014. The group consists of Cole Onley, Levi Hansen and JC Kuppinger, their debut release, the EP Songs From the Basement, was released October 22, 2015 and their first album, was released November 13, 2016. Both the EP and LP were engineered by Andy Gerber at Million Yen Studios. Pre-production was done at Sound Summit with Charlie Dresser, their EP Kermit the Fraud was engineered at Sound Summit Studios in Naperville. Stellar West formed in May 2014 after the trio met at a School of Rock in their hometown, not unlike other up and coming young bands, Hippo Campus and Doll Skin who got the same start. Just over a year after their formation, they released their debut EP, Songs From the Basement, digitally and on CD. At the end of 2015, the band was recognized by Forkster Promotions by winning three categories in the website's year-end music awards, including placing 50 out of the top 100 new artists of the year. January 2016 saw the band being invited to perform live on WGN Morning News, where they performed their song Interference and a cover of Nirvana's Smells Like Teen Spirit.

In the summer of 2016, the band won a contest through the music festival Riot Fest and were invited to perform at the event that year. Stellar West's first full-length album, was released at the end of 2016. Following its release, the band was included in Alternative Press's list of the best bands under 21. In April 2017, the band was chosen as a top Chicago finalist to open for Metallica on a five city tour. Songs From the Basement Kermit the Fraud Unfiltered List of alternative rock artists List of punk rock bands, L–Z Official Website YouTube Channel Facebook Profile Twitter Bandcamp Profile Soundcloud