Evolution is a change in allele frequency in a population over time. It involves the process of adaptation through mutation allowing more desirable characteristics to appear in the next generation. Evolution occurs in populations not individuals. Darwin wrote On the Origin of Species by Means of Natural Selection after studying the Galapagos finches. Over time, characteristics beneficial to survival are retained through selective pressure. For living organisms to adapt and change to environmental pressures, genetic variation must be present. With genetic variation, individuals have differences in form and function that allow some to survive certain conditions better than others. This ability to survive and reproduce is referred to as genetic fitness. Organisms pass favorable adaptations to their offspring. Eventually, environments change, and what was once a desirable, advantageous trait may become an undesirable trait. This exposes populations to variations in selective pressure.

Darwin observed variety in finch beaks. He hypothesized that ancestral beaks had adapted over time for different food sources.

Evolution may be convergent with similar traits evolving in multiple species or divergent with diverse traits evolving in multiple species that came from a common ancestor. Unrelated animals, such as the arctic fox and grouse, living in the arctic region have been selected for seasonal white phenotypes during winter to blend with the snow and ice. These similarities occur not because of common ancestry, but because of selection pressures—the benefits of not being seen by predators. The fox and grouse are both related to populations in other geographic areas with differing coloration.

Humans can also apply selective pressure by artificial selection, or selective breeding. In artificial selection, a human chooses desired features, then allows only the individuals that best express those qualities to reproduce. This accounts for breeds of dogs, domesticated animals and specific farm crops.

A heritable trait that helps an organism survive and reproduce in its present environment is called an adaptation. The bones in the appendages of a human, dog, bird, and whale all share the same overall construction resulting from their origin in the appendages of a common ancestor. These populations have maintained the same overall appendage layout, despite changes in the shapes and sizes of specific bones in different species.

The similar construction of these appendages indicates that these organisms share a common ancestor.

These are not to be confused with structure that appear similar – like the wings of bats and bees. These structures are referred to as analogous structures, because each function for flight but the structures are not similar. Bats are mammals, and bees are insects. Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales.

Evidence of shared ancestors is found in DNA code, the fossil record, and existence of homologous and vestigial structures. Like anatomical structures, DNA reflects descent with modification. Ancestry is reflected in DNA as the genetic material across species, the genetic code, the machinery of DNA replication and protein construction. Fundamental divisions between the three domains are reflected in major structural differences in structures like ribosomes and membrane structures.

Natural selection leads to beak changes in medium-ground finch populations in the Galápagos. This does not mean individual finch beaks are changing. If one measures the average beak size among all individuals in the population at one time and then measures the average beak size in the population several years later, this average value will be different. Although some individuals may survive from the first measurement to the second, they will still have the same beak size. However, there will be many new individuals contributing to average beak size.

Natural selection depends on the variety of alleles already in a population and does not arise in response to an environmental change. The total set of gene copies for all genes in a population is referred to as its gene pool. For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that resist antibiotics. The resistance, caused by a gene, did not arise by mutation because of the application of the antibiotic. The gene for resistance was already present in the gene pool of the bacteria, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects individuals that are resistant, since these would be the only ones that survived and divided. Experiments demonstrate mutations for antibiotic resistance do not arise as a result of antibiotic use. Instead, using antibiotics selects for mutants.

In a larger sense, evolution is not goal directed. Species do not become “better” over time. The changing environment maximizes reproduction in a particular environment at a particular time based on existing adaptations. Evolution has no goal of making faster, bigger, more complex, or even smarter species. A’s, C’s, T’s and G’s do not think or feel. Characteristics in any species are a function of the variation present and the environment, both of which are constantly changing in a non-directional way. A beneficial trait in one environment at one time may well be fatal at some point in the future. This is true for any species from bacteria to human.

Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that occur within a shared habitat (sympatric speciation). Both pathways isolate a population reproductively in some form. Mechanisms of reproductive isolation act as barriers between closely related species, enabling them to diverge and exist as genetically independent species. Prezygotic barriers block reproduction prior to formation of a zygote, whereas postzygotic barriers block reproduction after fertilization occurs. For a new species to develop, something must cause a breach in the reproductive barriers. Sympatric speciation can occur through errors in meiosis that form gametes with extra chromosomes (polyploidy).

Speciation can occur when two populations occupy different habitats. The habitats need not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, and the cricket (b) Gryllus firmus prefers loamy soil. The two species can live in close proximity, but because of their different soil preferences, they became genetically isolated.

Organisms reproduce with other similar organisms. The fitness of hybrid offspring have led scientists to propose two models for the rate of speciation. One model illustrates how a species can change slowly over time, and the other model demonstrates how change can occur quickly from a parent generation to a new species. Both models continue to follow the patterns of natural selection.