Topic 1 - Key Terms

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Allopatric Speciation - Speciation in which the evolution of reproductive isolating barriers occurs during physical separation of the populations.

Allopolyploid - Individual that exhibits ploidy (more than two chromosome sets) resulting from interspecific hybridisation.

autopolyploid - Individual that exhibits ploidy (having more than 2 chromosome sets) that arises when multiple chromosome sets are derived from a single species. They can also arise by doubling of the genome so that all chromosomes are derived from a single parent.

Biological Species Concept (BSC) - Defines a species as a group of actually or potentially interbreeding populations, which are reproductively isolated from other such groups.

Divergent Natural Selection - The evolution, by natural selection, of phenotypic differences between populations that exploit different environments.

Ecological Speciation - The evolution of barriers to gene flow resulting from ecologically-based divergent selection.

Epistasis - A specific kind of genetic interaction between genes at two or more loci, where the phenotype resulting from variation at each locus in combination differs from that which would be expected if the genes were expressed independently. Such interactions may involve one allele negating or enhancing the effects of other alleles or two or more alleles interacting to produce novel phenotypes.

Fixation - Where an allele increases in frequency in a population to such an extent that all individuals carry only that allele (it reaches a frequency of 1) and other alleles are lost.

Gene Flow - The transfer or incorporation of genes into the gene pool of one population from one or more other populations. Gene flow occurs only if reproduction follows the dispersal of an individual or gamete to a new population.

Gene Pool - The set of all genes or genetic information in a population.

Geographical Isolating Barriers - Physical barrier to interbreeding between individuals of different populations; e.g. mountain ranges, rivers or any other feature of the environment, which are extrinsic to the organism and keep populations apart by physically preventing them from mixing and interbreeding.

Homeobox Genes - A large family of genes that direct the formation of many body structures during early embryonic development of multicellular organisms. Most homeobox genes encode proteins that act as transcription factors. They contain a sequence called a homeobox, which encodes a homeodomain, a part of the protein that binds to DNA and has a role in controlling its transcription and hence gene expression.

Homoploid Hybrid Speciation - Occurs when a stable, fertile and reproductively isolated lineage results from hybridization between two distinct species, without a change in ploidy level. Reproductive isolation between a homoploid hybrid species and its parents is generally attained by chromosome rearrangements, ecological divergence or spatial isolation from the parental species.

Hybrid Breakdown - A postzygotic form of reproductive isolation in which an interspecific hybrid is viable or fertile, but subsequent generations are not.

Hybrid Incompatibility - A postzygotic form of reproductive isolation in which hybrids between species are inviable or sterile.

Hybridisation - The production of offspring following mating between a male and a female from different species.

Hybrid Speciation - Speciation that results from hybridisation between two different species which leads to the formation of a new species that is reproductively isolated from both parent species.

Hybrid Zones - A geographic zone where genetically distinct natural populations come into contact and hybridise. Many hybrid populations exhibit a continuum of variation, spanning the gap between parental populations.

Introgression - The movement of alleles from the gene pool of one species into the gene pool of another by repeated backcrossing of interspecific hybrids with one of the parental species.

Isolating Barriers - Barriers to interbreeding between individuals of different populations; fall into two broad categories, geographical isolating barriers (extrinsic to the organisms and keeping populations apart physically) and reproductive isolating barriers (intrinsic to the organisms themselves, reducing likelihood of interbreeding).

Morphs - Different forms or phenotypes that exist in the same population of a species. Morphs occupy the same habitat at the same time and belong to a random mating population.

Parapatric Speciation - Speciation that occurs in a continuous population (i.e. not separated by an extrinsic barrier to gene flow) but divergence happens because of reduced gene flow between individuals in one part of the range and others in more distant parts of the population's range. (T1P2)

Peripatric Speciation - Speciation by evolution of reproductive isolation in small, geographically isolated (peripatric) populations as a consequence of a combination of genetic drift and natural selection; originally known as founder effect speciation. (T1P2)

Phylogenetic Species Concept (PSC) - Defines a species as a group of organisms that are descended from a common ancestor and that all possess defining characteristics which distinguish them from other such groups. (T1P1)

Phylogeny - Conceptual representation of the history of evolutionary relationships between given organisms, usually depicted as some form of tree-like diagram showing the sequence (and sometimes absolute dates) of branching events between divergent descendants. (T1P1)

Polymorphism - The existence within a population of two or more discrete phenotypes within a population. In the simplest case each phenotype results from different alleles of a single gene. In more complex cases the phenotype results from interactions between many different genes and the environment. (T1P1, EI)

Polyploidy - When cells or organisms contain more than two paired (homologous) sets of chromosomes. (T1P3)

Positive Assortative Mating - Occurs when there is a positive correlation between male and female phenotypes or genotypes across mated pairs. (T1P3)

Quantitative Trait Loci (QTL) - Stretches of DNA that are correlated with variation in a phenotypic trait. The regions contain genes or are linked to genes that underlie a quantitative trait (a trait controlled by the cumulative action of many genes). (T1P3)

Reinforcement - The evolution of reproductive isolating barriers as a result of natural selection against maladaptive hybridisation between two diverging populations that have been geographically isolated from each other for some time and then come back into contact again (also known as secondary contact).

Reproductive Character Displacement - The phenomenon that occurs where differences among similar species whose distributions overlap geographically so that they co-occur is greater compared to when their distributions do not overlap. (T1P3)

Reproductive Isolating Barriers - Intrinsic features of organisms which minimise the chance of interbreeding between individuals of different populations. (T1P1)

Reproductive Isolation - Occurs when reproductive barriers prevent or strongly limit successful reproduction between populations with the result that there is little or no gene exchange between them. (T1P1)

Ring Species - A situation in which two populations that do not interbreed are connected by a geographical ring of populations that can interbreed. (T1P2)

Sensory Drive - The sensory drive hypothesis of speciation refers to the adaptation in sensory and signalling systems to different environments that results in premating reproductive isolation between populations. (T1P2)

Speciation - The evolutionary process whereby new species arise. Speciation causes one evolutionary lineage to split into two or more lineages. (T1P1)

Speciation Genes - Genes that are involved in maintaining reproductive isolation between species or contribute to the cessation of gene flow between populations. (T1P1)

Subspecies - Populations of a species that show geographic variation in morphology and that may differ genetically but that can interbreed. (T1P1)

Sympatric Speciation - Speciation in which the evolution of reproductive isolating mechanisms occurs when populations inhabit the same geographic locality. (T1P1)