IB Biology Option D D2 Species and Speciation All syllabus statements ©IBO 2007 All images CC or public domain or link to original material. “…if I really have as bad an expression, as my photograph gives me, how I can have one single friend is surprising." D.2.1 Define allele frequency and gene pool al·lele fre·quen·cy Allele frequency is the proportion of all copies of a gene that is made up of a particular gene variant (allele). gene pool Noun: The total collection of different alleles in an interbreeding population. Example Say if a recessive allele h made up 2% of the total in a human …then the dominant allele H population… would make up 98%. The frequency for h would be expressed as 0.02 and for H 0.98 Recessive allele frequency + dominant allele frequency = 1 (for characteristics determined by two alleles) http://www.flickr.com/photos/limowreck666/171979083/ D.2.2 State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations New combinations of alleles lead to new phenotypes that can then be selected for or against by the environment. This leads to evolutionary change in the species D.2.3 Discuss the definition of the term species What is a species? D.2.3 Discuss the definition of the term species There are many definitions, here are five! Ecological species A set of organisms adapted to a particular set of resources, called a niche, in the environment. Genetic species Based on similarity of DNA of individuals or populations. Having a common gene pool. Evolutionary species A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, some members may diverge from the main population and evolve into a subspecies. Lots to discuss if you get a Cladistic Species question about this! A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. (This differs from the Evolutionary definition in that the parent species goes extinct when two new species are recognised). Breeding Species Two organisms that are able to reproduce naturally to produce fertile offspring of both sexes. http://en.wikipedia.org/wiki/Species The genetic definition is most widely used and works well for most multicellular organisms The following 4 slides outline the exceptions: …is anything ever simple in Biology? Hors d’oeuvre? It could be (ahem) physically impossible for members of the same species to mate. Therefore they are genetically isolated. http://www.flickr.com/photos/lepetitblonde/2247963815/ 1 : Lesser Black-backed Gull 2 : Siberian population Black-backed gull 3 : Heuglin's gull 4 : Birula's Gull 5 : East Siberian Herring Gull 6 : American Herring Gull 7 : Herring Gull Ring Species: Adjacent populations can interbreed but the populations at the “end of the line” cannot. 1 and 7 cannot produce offspring. So close… …and yet so far :’( http://commons.wikimedia.org/wiki/File:Ring_species_seagull.svg Hybrids are usually infertile and can not produce offspring together, for example the mule (63 chromosomes): a cross between a Male horse (64 chromosomes) and a female donkey (62 chromosomes) The liger is a hybrid cross between a male Panthera leo (lion), and a female Panthera tigris (tiger) and is denoted scientifically as: Panthera tigris × Panthera leo. …conversely… Why are 63 chromosomes a problem when reproducing? The tiglon is a hybrid cross between a female Panthera leo (lion), and a male Panthera tigris (tiger) and is denoted scientifically as: Panthera leo × Panthera tigris. Ligers and tiglons sometimes produce offspring when mated back with a parent species e.g. The hybrid of a male lion and a female tiglon is a li-tiglon! MADNESS!! http://xkcd.com/419/ The genetic definition only applies to sexually reproducing organisms and doesn’t apply to single-celled organisms Rats! http://www.flickr.com/photos/microagua/3721497804/ Additionally: Fossil remains can’t tell us whether species were able to interbreed or produce viable offspring so palaeontologists tend to use the cladistic definition http://www.flickr.com/photos/bruce_mcadam/1393218574/ D.2.4 Describe three examples of barriers between gene pools The circumstances preventing different species from interbreeding are known as reproductive isolating mechanisms Temporal isolation Pinus radiata (Monterey Pine) Pinus attenuata (Knobcone pine) Pollen Production MAX J F M A M J J Month A S O N D Pinus radiata and Pinus attenuata are prevented from hybridising because they have separate pollination times. They can be made to hybridise by pollinating them manually. *Random fact: The Monterey pine is at risk in it’s native range but is one of the most common plantation trees in the world. If you see a pine forest in Australia or NZ, it is probably Pinus radiata http://www.flickr.com/photos/alancleaver/4293345631/ Ecological isolation The two species are in the same area, but live in different habitats I love me some CaCO3 in my soil Blechhh! Acidic soils are more my thing Viola arvensis http://www.flickr.com/photos/annetanne/3035068940/ Viola tricolor http://www.flickr.com/photos/carinemily/644052381/ Behavioural isolation Animals exhibit courting behaviour (song, dance etc.) or release pheremones to attract mates. Individuals are only attracted to, and will only mate with, members of the opposite sex who perform the appropriate ritual or release the correct chemical. Yo! I don’t like your music! Its like, totally mutual! http://www.flickr.com/photos/nrk-p3/2333221093/ http://www.flickr.com/photos/rowelbg/2895578034/ Mechanical isolation Animal example: Different species of bush baby (Galago) have particular shapes for their genitalia and they are physically incapable of copulation*. It is like a lock and key. In plants, mechanical isolation occurs when different species have different pollinators that are not able to service the flowers of other species She says “We’re not a good fit”. What is that supposed to mean? *Take care when Googling “Bush baby genitalia”! http://www.flickr.com/photos/joachim_s_mueller/4113758487/ Hybrid Inviability Remember: Male Horse + Female Donkey = Mule Horse 2n = 64 ∴ sperm n = 32 Donkey 2n = 62 ∴ ovum n = 31 Sperm + Ovum = Mule zygote 32 + 31 = 63 Mule 2n = 63 ∴ gamete n = ? Note: Sad eyes http://www.flickr.com/photos/pirateparrot/301798478/ D.2.5 Explain how polyploidy can contribute to speciation So far you’ve learnt that cells contain two homologous sets of chromosomes. Well….. that isn’t always the case. It goes on: Pentaploid Hexaploid Septaploid Octaploid Etc. up to: 84-ploid and 1260 chromosomes Ophioglossum reticulatum A small fern. The incredible thing is that this plant is able to carry out meiosis accurately with 1260 chromosomes to divvy up http://commons.wikimedia.org/wiki/File:Haploid,_diploid_,triploid_and_tetraploid.svg How it happens: Remember: When non-disjunction occurs during meiosis in humans, an individual can end up with an extra chromosome or missing chromosomes. E.g. An extra chromosome 21 means Downs syndrome (see 4.2.4) Self fertilisation Total non-disjunction, is when one of the two cells produced during Meiosis I gets all of the chromosomes. The other cell is not viable and is reabsorbed. This results in two (2n) daughter cells from meiosis instead of the usual four (n) daughter cells. See animation http://commons.wikimedia.org/wiki/File:Polyploidization.svg Few polyploid organisms exist in the animal kingdom. Can you think of the reasons why not? Animal polyploid species include salamanders, goldfish and salmon. However, polyploidy is a great source of speciation amongst plants. Polyploidy often leads to increased size, resistance to disease and overall vigour. Many plants used by humans are polyploid. Including cereal crops like wheat. Polyploid crops generally have bigger fruits, seeds and storage organs Two versions of Polyploidy: • Autopolyploidy* • Allopolyploidy *This is not autopolyploidy http://www.flickr.com/photos/leapkye/3224058317/ Autopolyploidy (Auto = “self”) • Autopolyploids are polyploids with multiple chromosome sets derived from a single species as described a couple of slides ago. • Autopolyploids form following fusion of 2n gametes • Autopolyploidy can be induced in plants using colchicine, a chemical extracted from the autumn crocus. • Autopolyploids with odd ploidys eg triploid or pentaploid have trouble reproducing sexually WHY? • That does not stop them from being good crops if they can be propagated asexually Allopolyploidy (Allo = “different”) Allopolyploids come about when a sterile F1 hybrid doubles all of its chromosomes and becomes fertile. + = Wheat Rye Triticale For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines soughtafter characteristics of the parents, but the initial hybrids were sterile until doubling of the number of chromosomes occurred Remember the poor sterile mule with 63 chromosomes? Imagine if we could somehow induce sperm and ova with 126 chromosomes Voila! The mule born would be fertile. Of course, it would need to be done a couple of times to get a few mules to breed together http://en.wikipedia.org/wiki/File:Wheat,_rye,_triticale_montage.jpg http://jonathanturley.org/2009/03/01/a-happy-mule/ D.2.6 Compare allopatric and sympatric speciation Allopatric speciation (Allo = “different”, patric = “fatherland”) This arises when a species is subject to geographic isolation. This can occur when a population is split by: • A river • A mountain range Gene flow is cut off between the two split • A desert populations and they can evolve in different • A road directions (See animations below) • The sea etc. Remember Darwin’s finches? http://commons.wikimedia.org/wiki/File:Darwin%27s_finches_by_Gould.jpg Once the populations have been separated into two gene pools they can diverge through natural selection or through random genetic drift http://upload.wikimedia.org/wikipedia/commons/b/b6/Random_sampling_genetic_drift.gif Allopatric speciation of Drosophila in the lab Even when the “geographic barrier” is removed, the populations are still genetically isolated Sympatric speciation (Sym = “same”, patric = “fatherland”) The formation of two or more descendant species from a single ancestral species all occupying the same geographic location. Whether it actually happens is still contested. Find a pair of species that are thought to have diverged by sympatric speciation http://etc.usf.edu/clipart/2200/2288/salamander_1.htm Diagrammatic comparison http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Speciation_modes.svg/500px-Speciation_modes.svg.png D.2.7 Outline the process of adaptive radiation Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits with which they can exploit a range of divergent environments. Wikipedia Think Darwin’s finches (AGAIN!) They originated from a population of an ancestral species that flew or were blown to the Galapagos islands from mainland South America. They colonised the islands and (while geographically isolated) evolved via natural selection to have beaks that suited the types of food available on their islands. Their beaks are homologous structures in that they have evolved from a common structure to have different functions. D.2.8 Compare convergent and divergent evolution What do humans, octopi and box jellyfish have in common? http://www.flickr.com/photos/jlambus/2303592201/ We all have complex camera* eyes. They evolved independently in organisms only very distantly related. They are an example of convergent evolution Complex eyes have evolved 50 to 100 times! *Camera means ‘room’ Convergent evolution describes the acquisition of the same biological trait in unrelated lineages. Other (random!) examples include: - Penguins in the southern hemisphere and Auks in the northern hemisphere both use wings as flippers - Echolocation in bats, toothed whales and shrews to capture prey. It even evolved independently twice amongst the bats - Super strong jaws on different genuses of ants (Trapjaw ) - Flight/gliding in birds, pterosaurs, bats, insects and flying fish! Little Auk http://commons.wikimedia.org/wiki/File:AlleA Little Penguin http://commons.wikimedia.org/wiki/File:Little_penguin_Eudyptula_minor.jpg I’m including this image because I mentioned bats twice on the last slide and bats are awesome! * *Whatever he’s saying, its ultrasonic http://www.flickr.com/photos/furryscalyman/673915993/ Features that come about by convergent evolution are known as analogous structures http://www.flickr.com/photos/sniffette/6705872/ http://www.flickr.com/photos/volk/1038089969/ http://www.flickr.com/photos/jaybock/4006029348/ http://www.flickr.com/photos/martynr/76538849/sizes/o/in/photostream/ Divergent Evolution is another way of saying adaptive radiation (D.2.7). As natural selection acts on two or more species that have arisen from a common ancestor, they become phenotypically different. It gives rise to homologous structures, features that now look different or have a different purpose for each species that has evolved http://commons.wikimedia.org/wiki/File:Evolution_pl.png Convergent evolution Time Divergent evolution Parent species (common ancestor) Parent species Parent species D.2.9 Discuss ideas on the pace of evolution including gradualism and punctuated evolution Phyletic Gradualism, as the name suggests, is the idea that evolution occurs at a slow-butsteady pace. Punctuated Equilibrium is the idea that, for most of the time, species are stable. But every now and then there is a disruptive event that prompts rapid change. The slope of the line indicates rate of change. • Vertical lines = little/no change • Horizontal lines = very rapid change Gradualism is the older idea. Darwin is one of the originators of the concept, borrowing from his friend Charles Lyell. Darwin recognised however that not all species evolve at the same rate all of the time "I think case must be that one generation should have as many living as now. To do this and to have as many species in same genus (as is) requires extinction . Thus between A + B the immense gap of relation. C + B the finest gradation. B+D rather greater distinction. Thus genera would be formed. Bearing relation" (next page begins) "to ancient types with several extinct forms" http://commons.wikimedia.org/wiki/File:Darwin_tree.png Punctuated equilibrium was first proposed by palaeontologists Niles Eldredge and Stephen Jay Gould in 1972. They were the first to suggest that species did not change for long periods of time but were in stasis until events punctuated (disrupted) the equilibrium (balance) Richard Dawkins is a prominent critic of the theory TOK: Find out more: • What evidence are the two theories based on? • Gould (deceased) and Dawkins have both become popular writers. How does this affect the weight of their opinion: • In the scientific community? • In the wider community? http://www.flickr.com/photos/ideonexus/4022727065/ http://www.flickr.com/photos/mrccos/288136783/sizes/m/in/photostream/ Revisiting the tree for punctuated equilibrium it should be noted that the “sudden” speciation events are only sudden in terms of geological time. They would still take many generations and possibly thousands of years. The periods of stasis may be explained by stabilising selection The punctuation could be explained by directional selection or disruptive selection You should be able to understand and interpret these diagrams. Practise sketching them. The downward facing arrows indicate selection pressure against individuals with that morphology Directional Disruptive After Before Stabilising All images CC Andrew Colvin Darwin’s Finches (again!) are an example of disruptive selection A B Short-beaked birds (A) and long-beaked birds (B) were able to exploit different food sources and this selection pressure led to the evolution of two species http://www.flickr.com/photos/kookr/2917861361/ Lake Turkana (Kenya, Ethiopia) contains several species of snails that have a fossil record showing long periods with little change followed by sudden change (punctuated equilibrium) The periods of change coincide with times when the water level of the lake dropped and it became a series of smaller lakes. What happens then? http://en.wikipedia.org/wiki/File:Lake_turkana_satellite.jpg That’s right: geographic isolation Smaller gene pools are more susceptible to directional selection So evolution of the isolated populations may be faster than when they were one big happy gene pool By the time lake levels recovered and the populations were united, isolating mechanisms were in place that prevented hybridisation After each extinction event, the number of genera has bounced back Phanerozoic_Biodiversity.svg The K/T extinction event (250 MA at the Cretaceous-Tertiary boundary) wiped out over half the genera, including most of the dinosaurs. A layer of iridium has been found in sediments laid down at that time all over the globe. Iridium is in higher concentrations in meteorites than on Earth generally. Therefore it is postulated that a large meteor or comet hit the Earth and caused the extinction. Individuals in the species that survived could move into the empty ecological niches and directional selection led to rapid evolution http://www.flickr.com/photos/53402955@N08/4928503884/in/photostream/ D.2.10 Describe one example of transient polymorphism Darwin's finches…. Have little to do with this point (for a change!). Instead, the peppered moths (Biston betularia) are the best known example Polymorphism is the existence of two or more different forms of a species Poly = “many” morphism = “shapes” Prior to 1840 peppered moths in Britain were light grey with dark spots to blend in with the grey lichen that grew on the trees in their habitat http://www.flickr.com/photos/wildhastings/4720082589/ The first dark variant was reported in 1848 and by 1895 most of them were black. The term industrial melanism was coined. Soot and acid rain from the burning of coal changed the colour or the trees that the moths rested on. Directional selection did the rest. http://www.flickr.com/photos/naturalhistoryman/817332984/ Before long the majority were dark. This situation reversed after 1956 when Britain instituted the clean air act. Less coal was burnt and most trees returned to their original colour. Now in polluted areas most moths are dark and in rural areas most moths are light. They are not distinct species because they still interbreed. The theory that natural selection due to predation was the cause of these changes has been confirmed experimentally by Dr HBD Kettlewell D.2.10 Describe sickle cell anaemia as an example of balanced polymorphism Sickle cell anaemia occurs when a single-base mutation in the gene that codes for haemoglobin causes the amino acid valine to be produced in a particular spot rather than glutamic acid. Valine is non-polar, unlike glutamic acid, and this causes the mutant variety of haemoglobin (haemoglobin S) to crystallise at low concentrations of oxygen. This in turn pulls the red blood cell into a sickle shape. It is less able to carry oxygen and can get stuck in small capillaries, causing blockages, pain and damage. Homozygous individuals (HbS HbS) are subject to a debilitating condition and have a shortened life expectancy On the brighter side, while individuals who are heterozygous (HbA HbS) will have some mutant haemoglobin. They can lead normal lives. As a benefit, they are resistant to malaria as the plasmodium parasite that causes it is not able to use sickle cells to reproduce. Individuals that are homozygous normal (HbA HbA) have no sickle cells and no resistance to malaria. Historical distribution of malaria Distribution of the sickle cell trait Heterozygous: Sickle cell trait Heterozygous: Sickle cell trait A S A A A S A S A S S S HbA HbA Haemoglobin: Normal RBCs: Normal O2 Capacity: Normal Malaria resistance: None HbA HbS Haemoglobin: 50% normal, 50% mutant RBCs: Usually normal, sickle when [O2] low O2 Capacity: Mild anaemia Malaria resistance: Moderate HbS HbS Haemoglobin: mutant RBCs: Sickle O2 Capacity: Severe anaemia Malaria Resistance: High Homozygous: ‘Normal’ 25% chance Heterozygous: Sickle cell trait 50% chance Homozygous: Sickle Anaemia 25% chance http://en.wikipedia.org/wiki/File:Autorecessive.svg This is an example of balancing selection and balanced polymorphism People who are homozygous for sickle cell are severely anaemic and have less chance of surviving to reproduce. Likewise individuals homozygous for normal haemoglobin are likely to contract malaria and are less likely to survive. Heterozygous individuals have what is termed heterozygote advantage. They are the most likely to survive and reproduce. Therefore both alleles are maintained in the population http://en.wikipedia.org/wiki/File:Plasmodium.jpg http://en.wikipedia.org/wiki/File:Sicklecells.jpg http://commons.wikimedia.org/wiki/File:Simple_balance_scales-01.jpg Hypotheticals: France has the greatest number of sickle cell sufferers in Europe because of immigration from its former African and Caribbean colonies. What do you expect will happen to the sickle cell allele in France over time given: 1. no more immigration, 2. modern medicine, 3. and the absence of malaria? What do you expect will happen to the sickle cell allele in West Africa over time if: • We eradicate malaria? Or • We develop medication that helps all sickle cell sufferers live normally Further information: