The recent discovery of the skeleton of a three-foot tall adult female belonging to a new human-like species, Homo floresiensis, is exciting news to anthropologists. The new species, named after the island on which the skeleton was discovered, appears to be descended from populations of Homo erectus, the closest known relative of modern humans. The skeleton was estimated to be 18,000 years old. This means that populations of Homo floresiensis existed well after modern man appeared approximately 160,000 years ago. Thus, researchers are wondering if the two species interacted. 

The first descendents of Homo floresiensis to reach Flores Island may have been similar in size to Homo erectus. Researchers hypothesize that the small size of Homo floresiensis (only three feet tall) is due to a process known as "island dwarfing." This phenomenon  has been observed in other mammals, where local isolation, absence of predators, and small population sizes, combined with restricted resources, lead to reductions in body size and modifications in brain size. The smaller individuals with reduced energy requirements are favored by natural selection in environments where food is limited and there is no need for defense against predators. In a small population with a limited gene pool, these changes could occur quite rapidly.

The Greater Sundas Islands include Borneo, Java (including the small island of Madura), Sumatra, Sulawesi, and Belitung. The Lesser Sundas (renamed Nusa Tenggara in 1954) are all Indonesian. They include Bali, Lombok, Flores, Sumba, Sumbawa, and Timor. Flores Island, where the new species, Homo floresiensis, was discovered, is located in eastern Indonesia and is heavily wooded, rugged, and mountainous. Homo floresiensis may have evolved from populations of Homo erectus that migrated by boat to Flores island from Java as long as 800,000 years ago.

A number of factors help biologists decide whether an organism belongs to a new species. In the case of Homo floresiensis, the new hominid presented a unique combination of primitive and more recently evolved (derived) features not found in any other taxon. Some of the important characteristics used to differentiate among hominids are: brain size (earlier hominids had brains with volumes around 400-450 cm3, while modern humans have brains averaging 1,300 cm3); jaw shape (during human evolution, jaws have become less elongated, with the development of more pronounced chins); and bipedal posture (whether or not they walked on two legs). Homo floresiensis presents a small brain volume, but has facial and dental features more similar to Homo erectus, the closest known relative to modern humans. In addition, Homo floresiensis appears to have walked on two legs.

The Family Hominidae contains humans, great apes and their extinct relatives (http://tolweb.org/tree?group=Hominidae). Members of this family also are referred to as "hominids." The Tribe Hominini consists of several, related genera (Homo, Ardipithecus Australopithecus and Paranthropus) with bipedal posture, among other shared, derived characteristics. Members of this tribe are called "hominins." Current evidence now points toward three species of the genus Homo: Homo sapiens (modern humans), Homo erectus and Homo floresiensis.

By the early 19th century, scientists had gathered enough evidence to recognize that living creatures had existed on Earth for a long time and that life had changed and diversified since its origin. However, they did not understand the processes or mechanisms that drive biological diversity (variation in life forms), or how physical traits are inherited (passed on) from one generation to the next. One of the prevailing ideas was that of "blending inheritance," which posits that offspring should look like some mixture of the two parents. While this principle had some merit, it did not explain how variation persists in different populations over time. Under the blending inheritance model, all individuals within a given population eventually should end up looking alike. Clearly, this is not seen in nature.

With the publication of "On the Origin of Species" in 1859, Charles Darwin changed the way naturalists and other scientists thought about the diversity seen in nature. Darwin hypothesized that all living things are descendants of one or a few common ancestors and that diversity arises through the process of evolution, which is driven by natural and sexual selection.

Darwin described how natural and sexual selection caused variation to arise in nature, but the genetic mechanisms underlying these processes still were not understood. It was Gregor Mendel, an Augustinian monk who was working around the same time as Darwin, who solved this part of the puzzle. Through his experiments on pea plants, Mendel arrived at a model of "particulate inheritance" that explained how variation can be inherited and maintained over time.

Statistical models developed by G.H. Hardy and Wilhelm Weinberg helped to merge and fill out Darwin's and Mendel's observations into what is often referred to as "The Modern Synthesis" of evolutionary theory. This presentation covers these topics in detail.

Note: in lay terminology, the word "theory" often is used as a synonym for a hunch or guess. Consequently, people sometimes misinterpret the phrase "evolutionary theory" to mean some kind of guess that lacks critical support. In scientific terminology, however, a theory is a well-developed integration of observations, experiments, and interpretations. Scientists use the word "hypothesis" to refer to a "possible explanation" that remains to be tested.

Darwin spent many years collecting evidence from different sources to support his theory that evolution occurs through the process of natural selection. He carefully studied specimens that he and others had gathered from around the world, including several different species of finches from the Galapagos Islands. Darwin recognized, for example, that the different types of beaks he observed among the finches were related to different food sources and foraging patterns. Finches that fed on large seeds, for example, had thicker, stockier beaks, which contrasted with the more pointed beaks of finches that fed on cactus.

Darwin proposed that natural selection could explain how diversity-such as the diverse forms of beaks in the Galapagos finches-arises in nature. He reasoned that when environmental conditions change (e.g., alterations in temperature or sources of food), some individuals will have characteristics that allow them to continue to survive in the changed environment. These "successful" individuals will be more likely to produce more offspring than other less successful, and perhaps less well adapted individuals. Over time, the useful adaptive traits would become more common in the population, and the detrimental traits would become increasingly rare. In the example of the finches, birds with thick, stocky beaks would have a foraging advantage if the most abundant food source consisted of large, hard-shelled seeds. Individual finches whose beaks were most suited to seed eating would, theoretically, be able to consume more food. Therefore, birds with thick stocky beaks generally would be healthier and produce more offspring than individuals with less effective beaks. Over time, the population would come to be predominated by the stocky beak type.

It is important to recognize that evolution by natural selection, which many people think of as "survival of the fittest," is not strictly based on physical attributes, but rather, on differential reproductive success of individulas within a population. In a biological sense, "fitness" is equivalent to success in producing offspring that also survive and reproduce.

Darwin had little understanding of the underlying genetic mechanisms that drove natural selection. He knew, however, that for this system to work, the offspring must inherit the parent's physical characteristics. Thus, the basic elements of natural selection are that: (1) variation is present; (2) variation is heritable; (3) individuals within a population have different reproductive successes; and (4) individuals with higher reproductive success leave disproportionately more offspring.

Darwin used examples of artificial selection to help explain the process of natural selection. For thousands of years, humans have applied artificial selection to obtain domesticated plants and animals with desirable combinations of traits. Darwin noted how humans developed hundreds of dog breeds from one common ancestor (now known to be the wolf). Some dog breeds were developed for a particular purpose. For example, many of the characteristics of the dachshund breed of dogs-such as short legs, slender bodies, and courageous dispositions-were selected to develop a breed well suited to maneuvering through narrow holes while hunting badgers. Most plant and animal products we eat have been similarly modified through careful selection and breeding of individuals with desirable characteristics. For example, broccoli, brussels sprouts, cabbage and cauliflower all have been derived from the same common wild ancestor, a single species of wild mustard.

Darwin expanded his principle of natural selection to explain sexually dimorphic traits (features that differ between the sexes), including why, for example, males of many species tend to have more showy traits, while females often are comparatively drab. Darwin proposed that competition for mating opportunities drives the process of sexual selection as long as the fitness benefits conferred by this selection outweigh the costs imposed by natural selection. For instance, elaborate male traits, such as the spectacular train (tail coverts) of the male peacock, are clearly important to attracting females during courtship. However, the large elongated train increases susceptibility to predators by reducing the males' flight capability. In contrast, peahens (female peacocks), for whom the males are competing, are relatively drab, and blend more successfully with their environments. Females are less vulnerable to  predators, particularly during nesting season. Darwin noted that since females and non-breeding males lack exaggerated colors or displays, these features of breeding males probably are disadvantageous. 
There are two basic forms of sexual selection: intrasexual and intersexual. Intrasexual selection is driven by direct competition among members of one sex. This can involve contests between males of a species to gain mating opportunities with females. Such male-male combat is found among deer, for example. Competition between males also may take place during reproduction. In some species, sperm from more than one male may compete to fertilize the female's eggs. In contrast, intersexual selection is driven by abilities of one sex to attract the attention of the opposite sex and be chosen as a mate. Females generally drive intersexual selection, as they choose males with which to mate based on "attractive" features, such as the showy courtship display of the peacock, or other features, such as size or vocalizations, or dominance of other males.

With the 1859 publication of On the Origin of Species, Charles Darwin put forth the idea that all species have descended, with modification, from a common population of ancestors. All life on earth is therefore united by evolutionary history. This idea, which also is credited to Alfred Wallace, revolutionized scientific thought and is the cornerstone of modern evolutionary theory. However, what Darwin didn't fully understand, what he called the "mystery of mysteries," is how new species arise. We now know that there are many modes, mechanisms, and evolutionary forces that play a role in the formation of new species.
To understand speciation (the formation of new species) we must first have an idea of what species are. Many different definitions have been proposed; none of them are universally applicable and all of them are useful in particular contexts. At present, one of the most widely used definitions of species is based on the biological species concept. This definition is appropriate for our discussion of speciation because it not only defines species, but it also emphasizes the process by which species arise. The biological species concept states that "species are groups of interbreeding natural populations that are reproductively isolated from other such groups." Reproductive isolation refers to biological differences between two populations that impede or greatly reduce gene exchange between them. Most biologists recognize that some gene flow occurs between populations and do not require complete reproductive isolation in order to recognize a population as a distinct species. The study of speciation can therefore be thought of as the study of how a population evolves genetic distinctiveness and becomes reproductively separate from other populations.

Genetic distinctiveness arises through processes that introduce new alleles and genes into a population (mutation and migration) and processes that alter the frequencies of genes that already exist within a population (natural selection, sexual selection, and genetic drift).

A mutation is a random change in an organism's genetic material. Heritable mutations (those that are carried by the gametes) can introduce new alleles and genes into a population and, therefore, provide raw material for the evolutionary process.

Migration, or the movement of organisms from one place to another, can also cause changes in the gene pools of different populations. Through migration, new alleles can be introduced or taken away from a population, or the frequencies of alleles and genotypes in a population can be altered.

Through natural selection, heritable traits (and the alleles that confer those traits) that are beneficial to reproductive success become more common in a population while those that are disadvantageous become increasingly rare.

Sexual selection is a special form of selection that leads to the development of sexual dimorphic traits (traits that differ between the sexes). Through sexual selection, alleles that confer an advantage in the ability to obtain mates become more common in a population.

Genetic drift refers to changes in a population's allele frequencies that occur due to chance. In general, the smaller the population, the greater the impact of genetic drift. 
These processes, which drive evolution and speciation, can be reviewed in the BioEd Online presentation entitled "Biological Evolution." In addition, information about different species concepts, as well as mechanisms of reproductive isolation, can be found in the presentation "Species Concepts."

In On the Origin of Species, Darwin wrote that all species, whether living or extinct, form a great "Tree of Life," now commonly referred to as a phylogenetic tree. A phylogenetic tree graphically represents the evolutionary history (phylogeny) of species, groups of related species, or other taxonomic groups. A branch point of the tree represents divergence of one or more new populations from a parent population. In some phylogenetic representations, the branch length is proportional to the predicted evolutionary time between branching events. These branches and nodes represent two different patterns of evolution: cladogenesis and anagenesis.  

Cladogenesis, also known as branching evolution, is the evolutionary pattern by which a new group splits from a parent group. Because this results in an increase in the total number of existing taxa, cladogenesis is responsible for the diversity of organisms seen in nature. In contrast, anagenesis, also known as phyletic evolution, results in the linear transformation of a population. There is no branching of the phylogenetic tree and no increase in the total number of species. Instead, anagenesis causes two species that split from the same parent species diverge more and more from one another over time.