* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download SPECIATION Why do species exist? •Altitude symbolises fitness
Survey
Document related concepts
Transcript
SPECIATION •Estimates of the number of species alive today range from 3-100 million •Very few of these have actually been described (approx 1.5 million). Lots of them are insects! •From the fossil record we know that 1. Species can give rise to other species (speciation) 2. Species can go extinct •Why are there different species? •What is a species? Why do species exist? •Altitude symbolises fitness … level of adaptation to environment •Why are there many species? •Because there are many adaptive peaks. •In other words, species exist because there are many possible ways to adapt and survive. •How does speciation happen? What is a species? 1 Western spotted frogs (America) Eastern Blue Jay Cyanocitta cristata Steller’s Jay Cyanocitta stelleri Look different Live mostly in different areas In Colorado overlap and interbreed One species or two? Dog and wolf Once considered a single species Look very similar Don’t interbreed Now classified as two species WHAT IS A SPECIES? •One theoretical definition is that a species is the smallest evolutionarily independent unit – mutation, selection, gene flow and drift operate on different species separately. •However, this is hard to put into practice. •What criteria can we use to identify species? Canis lupus familiaris Canis lupus lupus 2 BIOLOGICAL SPECIES CONCEPT Ernst Mayer, 1942 “A species is a group of actually or potentially interbreeding natural populations that are reproductively isolated from other groups” Main criterion is reproductive isolation - impossible for gene flow to take place between reproductively isolated populations Phylogenetic Species Concept Members of a species must include all genetically closely related organisms Morphospecies Concept Define species on the basis of morphological differences If not done carefully, may make arbitrary species definitions Problems: 1. If the populations do not overlap, it is not easy to tell if they would be able to reproduce together if they did meet 2. It is not possible to apply this definition to the fossil record (extinct species) 3. It is not possible to apply this definition to asexual populations 4. In some plants see hybridisation between strongly divergent populations – what does the biological species concept mean in that case? How does speciation happen? – Mechanisms of Genetic Isolation Classic model: Three steps 1. Geographic isolation of populations 2. Divergence of traits (e.g., mating system or habitat) 3. Reproductive isolation (inability to successfully mate) Recent information indicates that steps 1 and 2 may occur together, and that step 3 may not occur in some cases Physical isolation as a barrier to gene flow When a population is physically isolated from another, it is then free to differentiate through mutation, natural selection and drift. Migration between the populations will be blocked by the physical barrier, and so there will be no homogenising effect of gene flow. This kind of speciation is called allopatric speciation (Allopatric = other country) Describes speciation that happens through geographic isolation Once gene flow has been dramatically reduced or halted, the populations are independent evolutionarily and speciation is under way. 3 Geographic isolation may occur through dispersal to a new remote habitat, or through vicariance (fragmentation of the environment) Examples of vicariance could be the formation of a mountain range, the movement (splitting) of tectonic plates, river formation, formation of isthmus (e.g., Panama Isthmus separates Pacfic and Atlantic Oceans) … etc. etc. 1. Geographic isolation through dispersal and colonisation •Hawai’ian drosopholids (flies) Over 500 named species Huge ecological diversification (habitat, food) Many of the species are found on only one island (endemic) •How did all this speciation come about? Founder hypothesis: small populations of flies, or one pregnant female, disperse to new habitat or island and found a new population. Divergence begins after the founding event. How can we test this hypothesis? It predicts that (1) closely related species should be found on closely positioned islands; and (2) the order of some speciation events should correspond to the order in which the islands were formed. Examine both of these questions by looking at DNA sequence similarity. The order of speciation events is seen in the order of branches in the tree. 4 Observe: 1. The newest species are on the newest islands. 2. Some of the species branching events correspond to island formation events 3. Hawai’I (Big Island) flies are more closely related to Maui flies than to flies from other islands (closer genetic relationships of flies from geographically closer islands) These observations are in line with the predictions of the founder hypothesis. What about speciation in the absence of geographic isolation? Can this happen? Called sympatric speciation Sympatric = same country Examples of sympatric speciation come from polyploidy (more about this in your plant evolution lectures of this module). Polyploidy is a type of mutation event and is the instantaneous doubling (or tripling, … etc.) of the genome of an organism. For example, humans have 23x2 chromosomes, two of each kind. A polyploid human (if he arose and could survive) would have more than two of each kind of chromosome, e.g., a tetraploid would have 23x4 chromosomes. 1. Geographic isolation through vicariance •A species’ geographic distribution is split between two isolated regions. •Gene flow between the groups is impeded or prevented by geographic separation. => populations start to evolve independently. •Example: rise of the land bridge between North and South America (Panama Isthmus) isolated the marine populations in the Atlantic from those in the Pacific. •Did this lead to speciation? •Yes – in an experiment where male and female shrimp from either side of the isthmus were placed together in a tank, those with a greater genetic distance (and therefore a longer isolation time) were less interested in courting each other. •A polyploidy individual is genetically isolated from nonpolyploid relations – even if they are crossed, the offspring will not survive (or will have a very low survival rate) or will be infertile because of the unbalanced number of chromosomes. •It also has been observed in plants that plants with different ploidy levels (different numbers of copies of the chromosomes) have different flowering times. This further decreases the likelihood of gene flow. •Polyploidy produces instant genetic isolation in the absence of geographic separation. 5