Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Lecture 2.3. The Origin of Species. “In the same way, we can account for the separate islands having each their peculiar species, either on the supposition that the same original emigration peopled the whole of the islands with the same species from which differently modified prototypes were created, or that the islands were successively peopled from each other, but that new species have been created in each on the plan of the preexisting ones” (A. R. Wallace (1855), p. 188). 1 Species the Fundamental Units of Evolution. 1. Theory Special Creation as articulated by Lyell allowed that species could vary, but could not “depart indefinitely from the original type.” 2. We now know that species descend the one from the Common descent according other. BUT to Darwin. From The Origin of Species. 3. Species nonetheless possess a permanence and relative stability not observed in subspecies or varieties. 4. Of the several taxonomic categories – species, genera, families, etc. a. Only species can be defined unambiguously, and then only in certain cases. b. The remainder can only be defined subjectively. 2 Biological Species Concept (BSC) “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." [Ernst Mayr (1963), p. 19] 1. Promoted by Ernst Mayr, but the idea actually predates Darwin. 2. With regard to the definition, note the following: a. "Interbreeding" refers both to mating and to the production of fertile / viable offspring. b. "Potentially" refers to the fact that individuals of the same species living in different places would interbreed if given the opportunity. c. “Reproductively isolated" means that mating cannot occur, i.e., for structural or behavioral reasons (pre-zygotic isolating mechanisms), or that offspring, if produced, are sterile and / or doomed to die before reproducing (post-zygotic isolation). 3 d. "Natural" refers to the fact that reproductive isolation can only be assessed with regard to what transpires in nature. e. What goes on in zoos and the breeding pen is immaterial – e.g., ligers and tigons. 3. BSC only applies to sexual species. 4. BSC identifies gene flow as the cohesive force that holds species together. Summer and winter distributions of Baltimore and Bullock’s orioles. Note the regions of hybridization. 5. Gene flow opposes geographic variation in selective pressure. 6. In widely distributed species, the tension between local selection and gene flow produces clines. 4 Previously classified as separate species, Baltimore and Bullock orioles interbreed in the western Great Plains. They are now considered geographic races of a single species. 5 Rose breasted and black-headed grosbeaks also interbreed in the western Great Plains, but manifest assortative mating and are consequently considered “good” species. 6 Diminution of a Melanic Cline in Britain. 1. B. betularia historically evidenced clinal variation. 2. Frequency of principal melanic morph (carbonaria) increased along rural to urban transects. 3. Steepness of one such cline has diminished since 1975 (Saccheri et al., 2008). 4. Consistent with Clinal variation in B. betularia. Top. Environmental transect; a. Less pollution following industrial areas shaded. Botpassage of clean air reg- tom. Carbonaria frequency ulations in the 1960s. along transect. Circles – 19691975; squares – 2002. Both b. Selection against carbo- data sets only intelligible if one considers time, location and naria. selection. From Saachen et al. (2008). 7 Questions. 1. (8 pts) What is the adaptive significance of Allen’s rule illustrated in the picture below? Allen's Rule in North American hares (Lepus spp.). From the northern arctic hare (L. arcticus) through the more southerly desert jackrabbit (L. alleni), members of the genus Lepus show progressively longer extremities (legs and ears) and leaner bodies. In this case, the cline is composed of species as opposed to geographic races. 2. (8 pts) The picture at the right indicates an inverse relationship between temperature (50+ year averages) and body mass in woodrats. Discuss. 8 Other Definitions of Species. 1. Morphological species concept (MSC). a. Based on anatomical differences. b. Look for clustering in character space. c. Differs from “classical” systematics which em- The morphological species phasizes “conserva- concept looks for clusters in tive” characters – e.g., character space. flowers vs. leaves. d. Can be applied to asexual species and fossils. e. Fails if distributions do not segregate cleanly – Lamarck’s problem. f. Can incorrectly split single species into many – think domestic breeds of cattle, dogs, pigeons, etc. 9 2. Phylogenetic species concept (PSC). a. Morphological species concept in a new guise – substitutes “objective” methods of cladistic analysis for the biologist’s judgment. Phylogenetic species concept b. Species are the tips of identifies species with “tip” phylogenetic trees – taxa. i..e., smallest monophyletic groups.1 c. Like MSC, PSC can be applied to asexual species and fossils. d. For most groups, the requisite data and analyses lacking / haven’t been performed. e. Can incorrectly split single species into many, i.e., like morphological concept. 1 As discussed later, a monophyletic group consists of an ancestral group and all of its descendants. 10 Allopatric (geographic) speciation. 1. Formerly isolated populations remerge or speciate. 2. If populations remerge, gene flow follows – introgression. 3. Hybrid infertility / inviability (post-zygotic reproductive isolation) selects for pre-zygotic isolation and ecological divergence. 11 Hybridization. 1. Can facilitate expansion of species into new areas. 2. Coyotes an example. a. Have recently extended their range eastward. b. Northern expansion has proceeded more rapidly than in the south. Expansion of coyotes in the northeastern United States. Postulated wolf-coyote in the north based on analysis of mitochondrial DNA analysis of eastern coyotes and source populations. c. In the north, but not in south, wolf genes incorporated into coyote genome. d. Reflected by larger body size that allows them to hunt deer. 3. Now recognized that hybridization can produce new species. a. Recall the 19th century debate pre-Darwin and Mendel’s interest in the stability of hybrids. b. See HW question on Darwin’s finch below. 12 Parapatric and Peripatric Speciation. 1. Parapatric. New species from peripheral populations. 2. Peripatric. New species from disjunct colonies. 13 Sympatric Speciation. 1. Polyploidy – “instant” speciation results from chromosomal duplication and resultant hybrid sterility. 2. Other mechanisms, e.g., behavioral changes / mating preferences, producing reproductive isolation The simplest form of polyploidy, autopolyploidy, involves the production of diploid gametes and results in a doubling of chromosome number. Self-fertilization is most often required to perpetuate the new species. If outcrossing is obligate, perpetuation requires simultaneous formation of multiple individuals. More complex mechanisms can produce polyploid species resulting from hybrids. In these cases, one speaks of allopolyploidy. 14 Question. (On Daphne Island in the Galapagos Islands, Peter and Rosemary Grant have identified the formation of what appears a new species of Darwin’s finch. The birds are descended from a single individual – probably a hybrid from another island. They differ from the locals morphologically and with regard to song. Importantly, they mate solely amongst themselves. For additional details, go to http://www.nature.com/news/2009 /091116/full/news.2009.1089.html A new Darwin’s finch. The bird’s ancestor, # 5110 (almost all the birds are tagged) had a larger bill than the indigenous medium ground finches and a different song. Drought reduced its descendants to a single brothersister pair whose offspring 3. (8 pts) Discuss this event have since mated only with with regard to the modes of each other. speciation given above and also with regard to pre- and post-zygotic isolation. Note: (1) young birds learn their songs (how to sing and what to respond to) from Dad; (2) both song and bill morphology are important cues for recognizing conspecifics and hence, potential mates. 15 Adaptive Radiations. 1. Differentiation of a single lineage into many. 2. Often observed on oceanic islands. 3. In the fossil record, adaptive radiations follow major extinction events, but often delayed. 4. In both instances, multiple speciation events a response vacant ecological niches. 5. Island specialists often less well adapted to new ways of life than mainland counterparts. May reflect lack of a. Genetic diversity; b. Competition from other species. 16 Top. Tarweeds and silverswords. Bottom. Hawaiian silversword. Examples of Adaptive Radiations Exemplifying Ecological Divergence. 1. Within the genus Penstemon, variation in flower shape / color allows for utilization of different pollinators. 2. Cichlid fishes of Lake Nyasa. Variation in skull / jaw morphology facilitates specialization on different food sources. Above. Variation in flower color and morphology within the genus Penstemon correlates with pollinators. Right. Variation in skull morphology in cichlid fishes correlates with diet. 17 3. Early Radiation of Mammals. Two-thirds of mammalian evolution occurred in the shadow of nonavian dinosaurs. Shown here are Jurassic mammals, their evolutionary relationships and ecological niches. From Lee and Beck (2015), 18 Questions. 4. (10 pts) Most hummingbird-pollinated flowers are red and have long tubes. How have evolutionary biologists accounted for this? (Requires outside reading.) 5. (8 pts) Where / what is Lake Nyasa (Malawi)? Why would the cichlid fishes of this lake have undergone Satellite view of extensive adaptive radiation? Lake Nyasa. 19 Extinction. 1. The flip side of speciation. Over 99% of all species have gone extinct. 2. Background level of extinction. Result of a. Biotic interactions (ecological / evolutionary), b. Gradually changing climates / landscapes. c. Chance extinction small populations. of Extinction rates for invertebrates since the Cambrian. Peaks call attention to times at which extinction rates rose above background levels (dots). The end Cretaceous event (KT) resulted entirely or in part from the Chicxulub impact in what is now the Yucatan peninsula. After Raup and Sepkoski (1982). 3. Mass extinctions. Result of unique events that occurred over geologically short time periods – recall Cuvier’s revolutionary upheavals of the globe. 4. Regarding background extinction, ecologists have suggested an approximate balance between speciation and extinction – recall Lyell. 20