Speciation Ch 22 Speciation • Darwin titled his great book, On the Origin of Species. • In Minnesota (2009), about ¼ of high school biology teachers see creationism as a viable explanation for the origins of life on Earth. • A 2009 Gallup poll reported that about 60% of American believe that humans are not the result of evolution. The same poll found that 16% of high school biology teachers believe that God created humans in their present form sometime during the last 10,000 years • The Creation Museum, opened in 2007 in Petersburg, KY, presents the Earth as 6,000 years old, with humans and dinosaurs co-existing. • Florida’s State Dept of Education adopted NEW standards in 2008 for teaching science, using the word “evolution” for the FIRST TIME. The adoption passed by only 4-3 and is already under pressure to allow “alternative” explanations. • Speciation is highly controversial!! SPECIATION - Definition • Folk concept (morphological): All societies give groups names like dogs, cats, humans, flowers, fish. Dogs beget dogs, fish beget fish, and people beget people. • BIOLOGICAL SPECIES CONCEPT (BSC): a community of populations (isolated from each other) that occupies a specific niche in nature and that can potentially reproduce among themselves. • CHALLENGES: not always testable (16,000 lakes in Wisconsin, each with bluegill sunfish. Are they separate species or the same? Limited to sexual reproduction. Bacteria exchange genes among themselves and among unrelated organisms. • The question, What is a species? is not always answerable. Other Concepts • PHENETIC: smallest groups consistently distinct and distinguishable by ordinary means, mostly morphological. If it looks like a duck…Cryptic species, however, often look very much alike. • PHYLOGENETIC: smallest group that possesses at least one diagnostic character fixed in reproductive units, i.e. ability to do photosynthesis or swim. But may not look alike. • CONCLUSION? Different definitions work for different organisms. species are not fixed and are not extensions of human behavior or feelings. Intermixing genetic material is far more fluid and common that human-centered thinking allows. Post-mating Pre-mating Isolation allows for microevolution to lead to divergence. Reproductive isolation leads to speciation Classical isolating mechanisms must eventually be fixed through genetic means, or the organisms can remain different populations of the same species. Mechanical Barriers — Reproductive • Female tract twist • • Penis twist • • • • Duck reproduction was studied primarily by Patricia Brennan at Yale in 2007. She asked, “Why do duck males have such large phalluses?” (i.e. 40 cm long when uncoiled) Waterfowl (ducks, loons, swans, etc) often resort to forced copulation, sometimes involving 2 males and 1 female where males bite and hold the neck of females during copulation. Only the most “fit” male ducks can successfully mate with females, an example of intra-male competition, as aspect of sexual selection. Or an example of female choice since females can relax their oviducts (analogous to vagina) if they choose to. Other species cannot successfully mate with female water birds. Considered by Brennan as a malefemale “competitive race” to control reproduction. Examples • POSTMATING • PREMATING • Darwin’s finches, • Drosophila melanogaster mate Camarhynchus with D. simulans, psittacula females but the male hybrids won’t choose are sterile. ONE mates with beaks GENE is as large as those responsible (nuclear of C. pauper, a pore gene) closely related species. Mechanisms of Speciation • ALLOPATRIC – geographic reproductive isolation • PARAPATRIC adjacent with limited gene flow until complete reproductive isolation • SYMPATRIC speciation within a panmictic population (i.e. “interbreeding”) Ultimately, genetic incompatibility is required for speciation. Genetic incompatibility can be at the level of the gene or with chromosomal structure. In the Dobzhansky-Muller Model, simple mutations in key genes can result in one population of an ancestral species become reproductively incompatible with another population of the same ancestral species. If that happens, the two populations cannot interbreed successfully (to lead to viable offspring). Gene flow blocked Any additional mutations will accumulate in one population but not the other, leading to microevolution divergence. Why is reproductive isolation important? Without reproductive isolation, gene flow occurs and ALL offspring have combinations of the same alleles and remain the same species Modern genetics and the theory of evolution by natural selection explains the existence of “hybrids”, something that’s quite mysterious otherwise. When reproductive isolation is incomplete — incomplete divergence and speciation — then limited gene flow occurs, but selection (fitness for survival) is often against the hybrids • Cattleya mossiae (left) and Laelia purpurata (rt) can interbreed • But they produce infertile offspring • They’re closely related species undergoing “speciation”, diverging from a common ancestor. • Hybrids signify gene flow between 2 species Non hybridizing species • Lions and tigers can interbreed, but more distantly related species cannot interbreed. • Wolves, coyotes, dogs (and several bear species) can (and do) interbreed in nature as well as in captivity • As divergence continues for longer periods of time, speciation becomes more obvious to everyone Chromosomal fusions often occur. In this case, two bat species differ from their ancestral species because of two different chromosomal fusions. Whole chromosomal arm translocations are the least disruptive because the breakage occurs at the centromere (called centric fusions). The impact on fertilization and meiosis of a hybrid, however, is deadly. We’ve talked about how chromosomal differences between human and chimp DNA is not in the gene sequences (about 1.5% difference), but in chromosomal arrangement. Human chromosome #2 has split during chimpanzee evolution into 2 different chromosomes. It isn’t known when this happened during the 6 x 106 years since the human and chimp lineage began diverging, but the earlier it happened, the earlier reproductive isolation would have occurred. The human-chimp line of primate evolution shows some frequency in centric fusions in certain chromosomes because of the presence of highly repetitive rRNA genes, possibly leading to breaks because of the possibility of pairing due to homology and then crossing over. In 2005, Stephen Scherer published a paper on regions of chimpanzee-human DNA “flipping” or inversions, another potential source of early reproductive incompatibility (after which mutations and microevolution continued to the point where humans and chimps today have DNA that differs by < 2%). Scherer’s group identified 1576 presumed inversions between the human and chimp species, 33 of which are larger than 100,000 base pairs (human genes average about 60,000 base pairs in length). Comparing chimp DNA with the gorilla genome (phylogenetic analysis) demonstrated that at least half the chimp inversions occurred after the chimp-human divergence (during human evolution). Some of the human inversions are polymorphic (humanhuman differences), suggesting ongoing human evolution. One human inversion is associated with human colorectal cancer. Human Evolution I • Where controversy lies • Even among people who agree that evolution is the over-arching theory for understanding organisms (here at Hunter, for example) • Human behavior “feels” different than behavior of About other animals 4 million• Genetic evidence is that years human evolution has ago speeded up since mutation in FoxP2 gene and evolution of speech New Hominid Species reported April 8, 2010 • • Remains of a 4-ft boy ≃ 1.8 – 1.95 million yrs old • Unclear whether it’s genus is Homo or Australopithecus • Changes that are decidedly Homo related start occurring 2.6 million years ago, including shaping stone tools • Homo-like face (small protrusion and teeth) but brain size and limbs like Australopithecus http://video.nytimes.com/video/2010/04/08/science/1247467554351/australopithecus-sediba.html Primate family tree Given the network nature of organismal evolution and divergence, should there be a MISSING LINK?? Pan paniscus & Pan troglodytes Homo sapiens & Homo neanderthalensis Newly reported species of Australopithecus sediba • Doonesbury 2006 Darwin’s finches relate evolution to developmental biology and gene expression (and facial development in particular). During embryonic development, all vertebrate faces look remarkably similar. In 2005 Tabin et al. evaluated two species, the large ground finch and the cactus finch. In situ hybridization analysis — using miRNA linked to fluorescent probes — indicated a different level of expression of Bmp4. The authors wanted to test whether a spatial or temporal change in Bmp4 could account for the relative size & shape beak differences. Bmp4 expression levels control beak depth and height. To test this, they misexpressed Bmp4 throughout the chick frontonasal prominence and converted the narrower, short chick beak into a much broader bigger beak. Bmp4 promotes chondrogenesis and beak growth. Natural selection of individuals with different levels of Bmp4 expression leads to beak evolution and speciation Helms J A et al. Development 2005;132:851-861 • From 25 October 2013 [Science, 342: 440] • Study identified enhances that “fine tune expression of genes during craniofacial development in mice.” • The authors conclude that variation in enhancers may contribute to “the spectrum of facial variation in human populations.” • Craniofacial enhancers are located in the human genome regions associated with craniofacial birth defects such as cleft lips and cleft palates. • Noncoding regions may also contribute to changes in expression of the Bmp4 gene. 1. Plot the number of species of Darwin’s finches and the number of islands in the Galàpagos archipelago as a function of time. 2. Are the data consistent with the hypothesis that isolation of populations on newly formed islands is related to speciation in this group of birds? Why or why not? 3. If no more islands form in the Galàpagos archipelago , do you think that speciation of geographic isolation will continue to occur among Darwin’s finches? Why or why not? What additional data could you collect to test your hypothesis (without waiting to see if speciation actually occurs)?