Tigriopus californicus is an intertidal copepod species that occurs on the Pacific coast of North America. This species has been the subject of numerous scientific studies on subjects ranging from ecology and evolution to neurobiology.
Found from central Baja California, Mexico to Alaska, USA along the Pacific coast of North America, T. californicus inhabits splash pools in rocky intertidal habitat. T. californicus is limited to pools in the upper end of the intertidal apparently by predation, but it can reach quite high population densities in this habitat. One study found that population densities on Vancouver Island averaged about 800 copepods per liter with some dense pools having as many as 20,000 copepods per liter.
These splash pools are often isolated from the moderating influence of the ocean and therefore the pools can vary dramatically in environmental factors such as salinity and temperature over the course of hours or days. T. californicus has the ability to thrive under these variable environmental conditions (factors that limit predators such as fish to lower pools in the intertidal zone). Temperature in the pools that this copepod inhabits tend to track air temperatures more closely than ocean temperatures and salinities in pools can change as pools evaporate, receive freshwater inputs from rain, or saltwater from wave actions.
The ability of T. californicus to handle extreme high temperatures varies among populations with southern California populations able to handle higher temperatures than those further north. This pattern of higher thermal tolerance in southern populations mirrors the temperature variation seen in copepod pools with southern populations experiencing more extreme high temperatures (over 40 °C or 104 °F on occasion). The genetic basis of this potential thermal adaptation has been studied by looking at genome-wide studies of gene expression and this study showed that differential expression of Hsp70 genes and a number of other genes could contribute to differences in thermal tolerance between these populations.
They have been known to have survived up to six months in laboratory conditions, however their longevity in natural conditions has yet to be determined.
Populations of T. californicus along the Pacific coast of North America show a striking pattern of genetic differentiation among populations. Mitochondrial DNA shows particularly large divergences among populations often exceeding twenty percent total sequence divergence. Genetic divergence of a smaller magnitude extends down to a more local scale and this divergence can be stable for longer than two decades for outcrops that are as little as 500 m (0.31 mi) apart, suggesting that dispersal between outcrops must be relatively rare for this copepod. Surprisingly, genetic divergence is much lower among copepod populations from Washington north to Alaska suggesting that copepods may have recolonized these areas since the end of the last ice age. Crosses of copepods from different populations of T. californicus have been used to study how reproductive isolation accumulates between diverging population to gain insights into the process of speciation. For crosses between many populations a pattern that has been called hybrid breakdown is observed; this means that first generation hybrids have high survival and reproduction (fitness), while the second generation hybrids have lower and more variable fitness. Deleterious interactions between the mitochondrial genome and nuclear genome may play a large role in the reduction in hybrid fitness observed in many of these crosses. Sex determination in T. californicus does not appear to be caused by sex chromosomes and is likely to be polygenic, potentially influenced by environmental conditions. The ratios of males to females produced by females differs among families and in some families seems to be genetically determined largely by the father in a pair. Another interesting feature of the mating system of this species is that the males use their large clasping antennules to clutch females until they are ready to mate. Females will mate only once during their lives but produce multiple clutches of offspring.
This copepod species has also been used as a model system in which to look at some questions in animal physiology including both neurobiology and osmoregulation. In response to increasing or decreasing environmental salinities T. californicus changes the amount of amino acids within its cells to maintain water balance. The amino acid proline is subject to strict regulation in response to changes in salinity and this may be a common mechanism of osmoregulation across crustaceans. For neurobiology, one study looked at the central nervous system of this copepod to get an idea of the organization of the central nervous system of the ancestors to the crustaceans and insects to complement the neurobiological work that has been done in a group of distantly related copepods (the calanoid copepods).
Selfish genetic elements are genetic segments that can enhance their own transmission at the expense of other genes in the genome, even if this has no positive or a net negative effect on organismal fitness. Genomes have traditionally been viewed as cohesive units, with genes acting together to improve the fitness of the organism. However, when genes have some control over their own transmission, the rules can change, and so just like all social groups, genomes are vulnerable to selfish behaviour by their parts.
Molecular evolution is the process of change in the sequence composition of cellular molecules such as DNA, RNA, and proteins across generations. The field of molecular evolution uses principles of evolutionary biology and population genetics to explain patterns in these changes. Major topics in molecular evolution concern the rates and impacts of single nucleotide changes, neutral evolution vs. natural selection, origins of new genes, the genetic nature of complex traits, the genetic basis of speciation, evolution of development, and ways that evolutionary forces influence genomic and phenotypic changes.
In population genetics, gene flow is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent allele frequencies and therefore can be considered a single effective population. It has been shown that it takes only "one migrant per generation" to prevent populations from diverging due to drift. Populations can diverge due to selection even when they are exchanging alleles, if the selection pressure is strong enough. Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants change the distribution of genetic diversity among populations, by modifying allele frequencies. High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. For this reason, gene flow has been thought to constrain speciation and prevent range expansion by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to differentiation and adaption. In some cases dispersal resulting in gene flow may also result in the addition of novel genetic variants under positive selection to the gene pool of a species or population
Sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap so that they occur together at least in some places. If these organisms are closely related, such a distribution may be the result of sympatric speciation. Etymologically, sympatry is derived from the Greek roots συν ("together") and πατρίς ("homeland"). The term was coined by Edward Bagnall Poulton in 1904, who explains the derivation.
In biology and genealogy, the most recent common ancestor (MRCA), last common ancestor (LCA), or concestor of a set of organisms is the most recent individual from which all the organisms of the set are descended. The term is also used in reference to the ancestry of groups of genes (haplotypes) rather than organisms.
Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introgression is a long-term process, even when artificial; it may take many hybrid generations before significant backcrossing occurs. This process is distinct from most forms of gene flow in that it occurs between two populations of different species, rather than two populations of the same species.
The starlet sea anemone is a species of small sea anemone in the family Edwardsiidae native to the east coast of the United States, with introduced populations along the coast of southeast England and the west coast of the United States. Populations have also been located in Nova Scotia, Canada. This sea anemone is found in the shallow brackish water of coastal lagoons and salt marshes where its slender column is usually buried in the mud and its tentacles exposed. Its genome has been sequenced and it is cultivated in the laboratory as a model organism, but the IUCN has listed it as being a "Vulnerable species" in the wild.
A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form in situ due to the evolution of a new lineage but generally they result from secondary contact of the parental forms after a period of geographic isolation, which allowed their differentiation. Hybrid zones are useful in studying the genetics of speciation as they can provide natural examples of differentiation and (sometimes) gene flow between populations that are at some point between representing a single species and representing multiple species in reproductive isolation.
Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes (mutations) through time, often leading to reproductive isolation and continued mutation even after the populations have become reproductively isolated for some period of time, as there isn’t genetic exchange anymore. In some cases, subpopulations living in ecologically distinct peripheral environments can exhibit genetic divergence from the remainder of a population, especially where the range of a population is very large. The genetic differences among divergent populations can involve silent mutations or give rise to significant morphological and/or physiological changes. Genetic divergence will always accompany reproductive isolation, either due to novel adaptations via selection and/or due to genetic drift, and is the principal mechanism underlying speciation.
In biology, co-adaptation is the process by which two or more species, genes or phenotypic traits undergo adaptation as a pair or group. This occurs when two or more interacting characteristics undergo natural selection together in response to the same selective pressure or when selective pressures alter one characteristic and consecutively alter the interactive characteristic. These interacting characteristics are only beneficial when together, sometimes leading to increased interdependence. Co-adaptation and coevolution, although similar in process, are not the same; co-adaptation refers to the interactions between two units, whereas co-evolution refers to their evolutionary history. Co-adaptation and its examples are often seen as evidence for co-evolution.
The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species.
Human evolutionary genetics studies how one human genome differs from another human genome, the evolutionary past that gave rise to the human genome, and its current effects. Differences between genomes have anthropological, medical, historical and forensic implications and applications. Genetic data can provide important insights into human evolution.
Temperature-dependent sex determination (TSD) is a type of environmental sex determination in which the temperatures experienced during embryonic/larval development determine the sex of the offspring. It is only observed in reptiles and teleost fish. TSD differs from the chromosomal sex-determination systems common among vertebrates. It is the most studied type of environmental sex determination (ESD). Some other conditions, e.g. density, pH, and environmental background color, are also observed to alter sex ratio, which could be classified either as temperature-dependent sex determination or temperature-dependent sex differentiation, depending on the involved mechanisms. As sex-determining mechanisms, TSD and genetic sex determination (GSD) should be considered in an equivalent manner, which can lead to reconsidering the status of fish species that are claimed to have TSD when submitted to extreme temperatures instead of the temperature experienced during development in the wild, since changes in sex ratio with temperature variation are ecologically and evolutionally relevant.
The Bateson–Dobzhansky–Muller model, also known as Dobzhansky–Muller model, is a model of the evolution of genetic incompatibility, important in understanding the evolution of reproductive isolation during speciation and the role of natural selection in bringing it about. The theory was first described by William Bateson in 1909, then independently described by Theodosius Dobzhansky in 1934, and later elaborated in different forms by Herman Muller, H. Allen Orr and Sergey Gavrilets.
Tigriopus brevicornis is a coastal marine copepod. They are a dominant member of shallow supra tidal rock pools along the North Western European coastline. A broad range of studies have been carried out on this species, including: its ecology, physiology, phylogeography, metapopulation genetics, development and reproductive behaviour. T. brevicornis has also recently been used in ecotoxicology studies and has been trialled as a live feed for larvae in several aquaculture-based studies for the past 30 years.
Reinforcement is a process of speciation where natural selection increases the reproductive isolation between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness. The idea was originally developed by Alfred Russel Wallace and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from Theodosius Dobzhansky. He envisioned a species separated allopatrically, where during secondary contact the two populations mate, producing hybrids with lower fitness. Natural selection results from the hybrid's inability to produce viable offspring; thus members of one species who do not mate with members of the other have greater reproductive success. This favors the evolution of greater prezygotic isolation. Reinforcement is one of the few cases in which selection can favor an increase in prezygotic isolation, influencing the process of speciation directly. This aspect has been particularly appealing among evolutionary biologists.
This glossary of evolutionary biology is a list of definitions of terms and concepts used in the study of evolutionary biology, population biology, speciation, and phylogenetics, as well as sub-disciplines and related fields. For additional terms from related glossaries, see Glossary of genetics, Glossary of ecology, and Glossary of biology.
Hybrid incompatibility is a phenomenon in plants and animals, wherein most offspring produced by the mating of two different species are not viable or are unable to reproduce. Examples of hybrids include mules and ligers from the animal world, and subspecies of the Asian rice crop Oryza sativa from the plant world. Multiple models have been developed to explain this phenomenon. Recent research suggests that the source of this incompatibility is largely genetic, as combinations of genes and alleles prove lethal to the hybrid organism. Incompatibility is not solely influenced by genetics, however, and can be affected by environmental factors such as temperature. The genetic underpinnings of hybrid incompatibility may provide insight into factors responsible for evolutionary divergence between species.
Notothenia coriiceps, also known as the black rockcod, Antarctic yellowbelly rockcod, or Antarctic bullhead notothen, is a species of notothen that is widely spread around the Antarctic continent. Like other Antarctic notothenioid fishes, N. coriiceps evolved in the stable, ice-cold environment of the Southern Ocean. It is not currently targeted by commercial fisheries.
Eukaryote hybrid genomes result from interspecific hybridization, where closely related species mate and produce offspring with admixed genomes. The advent of large-scale genomic sequencing has shown that hybridization is common, and that it may represent an important source of novel variation. Although most interspecific hybrids are sterile or less fit than their parents, some may survive and reproduce, enabling the transfer of adaptive variants across the species boundary, and even result in the formation of novel evolutionary lineages. There are two main variants of hybrid species genomes: allopolyploid, which have one full chromosome set from each parent species, and homoploid, which are a mosaic of the parent species genomes with no increase in chromosome number.