UVA BIOL 2200 Lessons 6-7 Textbook Notes

L6&7 Textbook Notes: The Origin of Species the biological species concept • states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring —but do not do so with members of other groups reproductive isolation • formation of new species hinges on reproductive isolation • the existence of biological barriers that impede members of two species from interbreeding and producing viable fertile offspring • limit the formations of hybrids, or offspring that result from an interspecific mating kinds of reproductive isolation • habitat isolation: two species occupy different habitats within the same area • temporal isolation: species breed during different times of the day, year, or lifespan • behavioral isolation: courtship rituals that attract mates and other behaviors serve as a barrier • mechanical isolation: morphological differences prevent mating • gametic isolation: the sperm of one species cannot fertilize the eggs of another results of interspecific mating reduced hybrid viability: the genes of different parents species • may interact in ways that impair the hybrid’s survival/development reduce hybrid fertility: hybrids can be vigorous but sterile • if the parent species differ in chromosome number or • structure, meiosis in the hybrids may fail to produce normal gametes hybrid breakdown: some first-generation hybrids are viable and • fertile, but when they mate with one another or either parent species, offspring of the next generation are feeble or sterile barriers • barriers can be classified according to whether they contribute to reproductive isolation before or after fertilization • • prezygotic barriers: block fertilization from occurring; act by: • impedes members of different species from attempting to mate • preventing an attempted mating from being completed successfully • hinders fertilization is mating is completed successfully postzygotic barries: contribute to reproductive isolation after the hybrid zyogte is formed • developmental errors may reduce survival among embryos • problems after birth may cause hybrids to be infertile or decrease their chance of living long enough to reproduce limitations of the the biological species concept • there is no way to evaluate the reproductive isolation of fossils • does not apply to organisms that reproduce asexually • in this concept, species are designated by the absence of gene flow, but there are species that are distinct yet experience gene flow other definitions of species • morphological species concept: distinguishes a species by body shape and other structural features • ecological species concept: defines species in terms of its ecological niche, the sum of how members interact with parts of their environment allopatric speciation • gene flow is interrupted when a population is divided into geographically isolated subpopulations • once separated gene pools diverge, different mutations arise, and natural selection and genetic drift may alter allele frequencies of each • reproductive isolation can evolve due to the genetic divergence • a behavioral/reproductive barrier to reproduction is established when there is a preference for one phenotype over another • the divergence of species occurs when geological barriers are formed • sister species at shallow levels of the ocean diverged more recently than those at deeper levels, as the barrier reached that point later on • regions that are isolated or subdivided by barriers typically have more species than do other regions that lack such features • while geographic isolation prevents interbreeding, physical separation is not a biological barrier to reproduction sympatric speciation • occurs in populations that live in the same geographic area • less common than allopatric speciation • can occur if gene flow is reduced by factors such as polyploidy, sexual selection, and habitat differentiation polyploidy • a species may originate from an accident during cell division that results in extra sets of chromosomes • more common in plants • includes autopolyploidy and allopolyploidy autopolyploidy • an individual that has more than two chromosome sets that are all derived from a single species • a failure of chromosome separation during meiosis (nondisjunction) could double a cell’s chromosome number from the original number (2n) to a tetraploid number (4n) • can produce fertile tetraploid offspring by self-pollinating or mating with other tetraploids • reproductively isolated from 2n plants of the original population; their resulting triploid (3n) offspring would have reduced fertility allopolyploidy • when two different species overcome reproductive barriers, interbreed, and produce hybrid offspring • most hybrids are sterile because the set of chromosomes from one species cannot pair with those from the other • the infertile hybrid might be able to propagate asexually • various mechanisms can make this sterile hybrid into a fertile polyploid, which is an allopolyploid • if the hybrid’s chromosomes spontaneously increase before meiosis, the homologous chromosomes will be able to pair and form viable gametes • fertile when mating when each other but not with either parent species; thus, they are a new biological species • the new species has a diploid chromosome number equal to the sum of the diploid chromosome pairs of the two parent species sexual selection • typically, females select males based on their appearance, acting as a reproductive barrier • some species can still interbreed when this prezygotic barrier is breached in the laboratory habitat differentiation • can occur when a subpopulation exploits a habitat or resource not used by the parent population • habitat isolation acts as a prezygotic barrier between populations • can result in temporal isolation, in which populations mate at different times, and natural selection toward one or both populations hybrid zones • a region in which members of different species meet and mate, producing at least some offspring of mixed ancestry • happens when reproductive barriers are incomplete patterns within hybrid zones • some are narrow bands, which are more extensive in length than width • the frequency of alleles decreases moving from that species’ region (100%), to the hybrid zone (50%), to the other species’ region (0%) there is a limit to the hybrid zone because of • obstacles to gene flow; otherwise, alleles from each parent species would be common in the gene pool of each other • ex. hybrid mortality or natural selection in different environments hybrid zones are usually located • wherever the habitats of interbreeding species meet hybrid zones and environmental change • hybrid zones can move to a new location or new ones may form when environmental conditions alter the habitats • ranges of two species can start overlapping and begin hybridization • a hybrid zone can improve the ability of one or both parent species to cope with changing environmental conditions • an allele found only in one parent species transfers to the hybrid and then to the other parent species when hybrids breed with the second hybrid zones over time • can cause polyploid speciation, in which hybrids become reproductively isolated from their parents and form a new species • interspecific hybrids may not become reproductively isolated, causing reinforcement of barriers, fusion of species, or stability reinforcement: strengthening reproductive barriers • because hybrids are often less fit, natural selection can reinforce prezygotic barriers • reproduction barriers between species end up being stronger for sympatric populations than for allopatric populations fusion: weakening reproductive barriers • gene flow may become so frequent that reproductive barriers weaken • the gene pools of the two species become increasingly alike and can cause them to fuse into a single species stability: continued formation of hybrid individuals • hybrids can continue to be produced, mostly because they survive or reproduce better than either parent species • some stable hybrids can be selected against, but hybridization can continue due to high narrowness; migration from outside to inside occurs patterns in the fossil record • new species can suddenly appear in a geologic stratum, persist unchanged throughout several strata, then disappear • punctuated equilibria: periods of apparent stasis punctuated by sudden change • some species do not show a punctuated equilibria and instead change gradually over long periods of time • if a species took tens of thousands of years to undergo morphological changes, it would not be distinguished in fossil strata, making it seem as if they appeared suddenly • a punctuated pattern indicates that speciation occurred relatively rapidly • through gradual change, it is difficult to tell when a new biological species formed speciation rates • it is suggested that once the process of speciation/divergence begins, it can be completed relatively rapidly • natural selection can produce extensive genetic changes in hybrid populations over short periods of time • rise in fertility can occur as selection eliminates regions of dna from the parent species that were not compatible with one another • time between speciation events ranges, being 6.5 million years on average and rarely less than 500,000 years • speciation only begins after gene flow is interrupted, such as through environmental change or unpredictable events studying the genetics of speciation • in some cases, reproductive isolation can result from one gene, which can influence reproductive preferences and traits • in other organisms, speciation is influenced by a larger number of genes and gene interactions from speciation to macroevolution • speciation begins with small differences, but as it repeats, differences can accumulate and become more pronounced • this can lead to the formation of new groups of organisms that differ greatly from their ancestors • as one group of organisms can increase in size, another might shrink and become extinct