The basic principle of all molecular genetic methods is to employ inherited, discrete and stable markers to identify genotypes that characterize individuals, populations or species. Such genetic data can provide information ori the levels and distribution of genetic variability in relation to mating patterns, life history, population size, migration and environment. Although molecular tools have long been employed to address various questions in fisheries biology and management, their contributions to the field are sometimes unclear, and often controversial. Much of the initial impetus for the deployment of molecular markers arose from the desire to assess fish stock structure based on various interpretations of the stock concept. Although such studies have met with varying success, they continue to provide an impetus for the development of increasingly sensitive population discriminators, yielding information that can be valuable for both sustainable exploitation and the conservation of fish populations. In the last major synthesis of the subject, Ryman and Utter (1987) summarized progress and applications, though this was prior to the wide-scale adoption of DNA methodology. New sources of genetic markers and protocols are now available, in particular those that exploit the widely distributed and highly variable repeat sequences of DNA, and the amplification technique of the polymerase chain reaction.

Although Gregor Mendel is considered the father of genetics, he has never taken the credit for his principles on heredity. Mendel's treatises, though they were part of the collection of the largest European libraries in the 19th century, were only rediscovered in 1900, 16 years after his death. Mendel's revolutionary ideas would have given greater strength to the formulation of Charles Darwin's ideas about common descent and gradual evolution through natural selection presented in 1859 in "The Origin of Species." However, Darwin was not totally ignorant of the possibility of genetic heredity. He even described “invisible characters” emerging in atavistic situations and named his hypothetical particle of heredity as “gemmules.” It is remarkable that the “invisible characters” and “gemmules” referred by Darwin are what we now know as genes – a term coined in 1909 by Wilhelm Johannsen that was widely accepted. During the 1930s and 40s, the findings of great proponents of genetics and evolution such as Mendel, Darwin, Wallace, Fisher, Haldane, Wright, Dobzhansky, Mayr, and several others were brought together to form the neo-Darwinian synthesis. In addition, in the 40s, genetics started its molecular revolution, which in the late 70s, driven by sequencing technology, gave rise to the genomics era. It took approximately 100 years to formulate the theoretical foundations of genetics to understand how information is transmitted to the next generations. Now, less than 45 years after the beginning of the genomic era, science is fully capable of identifying complete genomes. Among animals, fishes are one of the most relevant groups in genetic studies. Although fish studies were important in applying and corroborating Mendel's findings in the first decades of the 19th century, these studies contributed little to the development of classical genetics. However, fish have been of great importance for the development of molecular genetics. Several species such as Carassius auratus, Oryzias latipes, and Danio rerio (among several others of productive interest such as Salmo salar, Oreochromis niloticus, and Cyprinus carpio) have been used around the world as biological models. These models can be used for the study of genes and genomes, epigenetics, and genetic expression. Genetic studies using fish, in addition to increasing genetic knowledge about the species, also serve for a better general understanding of the physiology of metabolic pathways, diseases, evolution, systematics, dispersion, creation, and selection of individuals and lineages. Considering this, this Research Topic aims to bring together studies that present applications of fish as targets in genetic studies.

The aim of this book is to systematize and discuss population genetic studies of freshwater fish in a region that harbors the greatest diversity of species among all inland water ecosystems. This volume explores the genetic evaluation for a number of orders, families and species of Neotropical fishes, and provides an overview on genetic resources and diversity and their relationships with fish domestication, breeding, and food production.

Fisheries genetics researchers will find invaluable the thirty-eight peer-reviewed contributions in this book, presented at the 20th Lowell Wakefield Fisheries Symposium "Genetics of Subpolar Fish and Invertebrates," held in May 2002 in Juneau, Alaska. Looming over concerns of lost fisheries stocks and persistent erosion of genetic variability are predictions of global warming, which may further tax genetic resources. One consequence is an increased reliance on genetic applications to many aspects of fisheries management, aquaculture, and conservation. The contributions in this book are important to modern fisheries science and genetics, and illustrate the evolution of the field over the past decade. The improved technology provides tools to address increasingly complicated problems in traditional applications and ecological and behavioral studies. The union between molecular and quantitative genetics, where many of the major questions about population structure and evolution remain unanswered, will also benefit from the new technologies.