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.

Biological Evolution is a basic overview of evolutionary theory.

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.

Hardy-Weinberg calculations identify the allelic and genotypic frequencies expected from generation to generation, when a population is in Hardy-Weinberg equilibrium. For a population to reach and maintain this equilibrium, it must meet criteria (e.g. random mating patterns, no mutation, no selection) that are essentially unattainable outside of a laboratory setting. Thus, evolution occurs when natural populations deviate from Hardy-Weinberg equilibrium, causing shifts from the expected allelic and genotypic distributions. The major mechanisms that drive these shifts are selection, mutation, migration, genetic drift, and non-random mating.

Selection is one way that genetic shifts in populations (evolution) occur. Through natural selection, traits (and the alleles that confer those traits) that are beneficial to reproductive success become more common in a population, while those that are detrimental become increasingly rare. There are three main modes of natural selection: directional, diversifying, and stabilizing.

In directional selection, individuals with one extreme phenotype exhibit an advantage in fitness (reproductive success) over the others. In time, the mean (average) of the population shifts toward that extreme phenotype (see figure "a"). For example, if dark fur helped mice absorb heat from the sunshine on a cold winter day, and thereby gave them a survival advantage, a disproportionate number of their dark fur alleles would be passed on from generation to generation. The mean coat color of the population would shift to darker and darker values over time, thereby increasing the proportion of individuals with good warming features.

Under diversifying selection, individuals with extreme phenotypes at either end of the spectrum (the lightest and darkest coats in this example) have higher fitness than those with the average phenotype, and thereby pass on a larger number of alleles to descendent generations. Through time, the distribution of the phenotype within the population changes such that most individuals exhibit one of the extreme phenotypes. Figure "b" illustrates this process. Mice with the darkest or lightest coats have higher fitness than those with medium coat colors, and therefore, become more common in the population.

In stabilizing selection, individuals with the average phenotype have higher fitness than those with the extreme phenotypes. In this scenario, the range of phenotypes decreases over time (see Figure "c"). Mice with average coat color have higher reproductive success, perhaps because their plain fur helps them hide them from predators. Their fitness results in a disproportionate number of alleles for the average coat color being passed on to future generations. In contrast, mice carrying alleles for extreme coat colors eventually would be weeded out because, in this example, they are easier targets for predators and thus, less likely to reproduce.

Populations under selection pressures will have changes in allelic frequencies and will, therefore, deviate from Hardy-Weinberg expectations.

Migration, or the geographical movement of organisms, can cause deviations from Hardy-Weinberg expectations (and therefore can provide raw material for evolution) by changing 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.

Suppose that population 1 has only alleles "a" and "b," while population 2 has only alleles "x" and "y." If individuals from population 2 move into the same area as population 1 and breed with individuals from population 1, they will introduce the "x" and "y" alleles.

Now suppose that all individuals from population 1 carrying the "a" allele migrate into the area containing population 2 and interbreed with that population. Population 1 (individuals that did not migrate into the area of population 2) will undergo a decrease in genetic variation, because it is left only with individuals carrying the "b" allele. Meanwhile, population 2 will experience an increase in variation, because it has gained the "a" allele.

A more subtle way for migration to alter Hardy-Weinberg expectations is by shifting the relative frequencies of alleles, even when the number of alleles remains unchanged. Suppose population 3 also has only alleles "a" and "b." And suppose the "a" allele is common in population 1, but rare in population 3. If population 1 migrates and interbreeds with population 3, allelic frequencies will change and the differences between the two populations will be reduced.