Chapter 17: Speciation

the divergence of biological lineages and the emergence of reproductive isolation between lineages; requires the interruption of gene flow within a species whose members formerly exchanged genes

a state in which two groups of organisms can no longer exchange genes; most important factor in the long term isolation of sexually reproducing lineages from one another

groups of organisms that mate with one another

developed the system of binomial nomenclature

a construct that assumes a species comprises individuals that "look alike," and that individuals that do not look alike belong to a different species; Linnaeus

instances in which two or more species are morphologically indistinguishable but do not interbreed; morphology of little use

proposed by Ernst Mayr; species are of groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups; emphasizes that reproductive isolation is what allows sexual species to evolve independently of one another

species as branches on the tree of life; one species splits into two or more daughter species, which thereafter evolve as distinct lineages; embraces the idea that sexual species are maintained by reproductive isolation

ancestor-descendant series of populations followed over time

is a model of the evolution of genetic incompatibility. It assumes a selective pressure against a combination of alleles in the heterozygous state, hybrids have reduced fertility or viability. The end result of this evolved incompatibility is divergence into separate species.

develops with increasing genetic divergence

speciation that results when a population is divided by a physical barrier; geographic speciation; members cross a barrier and establish a new, isolated population

speciation without physical isolation; occurs with some forms of disruptive selection in which individuals with certain genotypes have a preference for distinct microhabitats where mating takes place

most common means of sympatric speciation; the duplication of sets of chromosomes within individuals; can result in complete reproductive isolation in two generations

chromosome duplication in a single species; originates when two accidentally unreduced diploid gametes combine to form a tetraploid individual; typically sterile

the combining of chromosomes of two different species; produced when individuals of two different species interbreed; often fertile because each of the chromosomes has a nearly identical partner with which to pair during meiosis

selection results in strengthening of isolating mechanisms that prevent hybridization

mechanisms that prevent hybridization from occurring; mechanical isolation, temporal isolation, behavioral isolation; habitat isolation, gametic isolation

mechanisms that reduce the fitness of hybrid offspring; selection against hybridization; low hybrid zygote viability; low hybrid adult viability; hybrid infertility

differences in sizes and shapes of reproductive organs may prevent the union of gametes from different species

many organisms have distinct mating seasons

individuals may reject, or fail to recognize, individuals of another species as potential mating partners; sometimes the mate choice of one species is mediated by the behavior of individuals of other species

when two closely related species evolve preferences for living or mating in different habitats, they may never come into contact during their respective mating period

the sperm of one species may not attach to the eggs of another species because the eggs do not release the appropriate attractive chemicals, or the sperm may be unable to penetrate the egg because the two gametes are chemically incompatible

hybrid zygotes may fail to mature normally, either dying during development or developing phenotypic abnormalities that prevent them from becoming reproductively capable adults

hybrid offspring may have lower survivorship than non-hybrid offspring

hybrids may mature into infertile adults; ex: mules

unless reproductive isolation is complete, closely related species may hybridize in areas where their ranges overlap; contain recombinant individuals resulting from many generations of hybridization