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Vanessa Couldridge Richard Knight Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/ http://en.wikipedia.org/wiki/Image:Darwin%27s_finches.jpeg Evolution: Selection and Speciation Forms of Selection Different selective pressures will cause a population to respond in different ways The values of the trait under selection will change according to the selective pressure There are three different forms of selection: Stabilizing selection Directional selection Disruptive selection Frequency Trait value Stabilizing Selection Mean values of the trait are favoured It is the most common form of selection Example: In seabirds, individuals with an average body size are less likely to die in a storm than those with large or small body sizes Frequency Trait value Directional Selection One extreme of a trait is favoured over the other extreme Leads to a shift in the mean trait value Example: The long neck of the giraffe evolved as giraffes with longer necks were better able to find food Frequency Trait value Disruptive Selection Both extremes of a trait are favoured Leads to a splitting of the population over time Can produce polymorphic forms Example: The African finch Pyrenestes ostrinus has a bimodal distribution of bill size Frequency Trait value Speciation Speciation is the process whereby a new species arises from an ancestral species It requires reproductive isolation There are three major types of speciation, depending on the degree of geographical separation: Speciation Allopatric speciation Parapatric speciation Sympatric speciation event Allopatric Speciation A new species is formed in complete geographic isolation from the ancestral species Some extrinsic barrier splits a species into two populations and prevents the two groups from mixing and interbreeding with each other The populations continue to evolve and diverge from one another in isolation Eventually they diverge to such an extent that if they were to reencounter one another, they would no longer be able to interbreed Original population Physical barrier New species Allopatric Speciation: Example Formation of the Grand Canyon caused an ancestral population of squirrels to split into two groups that could no longer interbreed Kaibab Squirrel Abert Squirrel Parapatric Speciation Speciation with incomplete geographic isolation Individuals are more likely to mate with their nearest neighbours than with individuals at the opposite end of the geographic range of the population Often results in a cline – gradual change in adjacent populations over the species’ range Varying selective pressures over the geographic range can lead to speciation Original population New niche entered New species Pollution from mines has led to contamination of the soil in part of the range of a species of grass Plants growing on contaminated soil have evolved a tolerance for heavy metals and also flower at a different time to plants growing on uncontaminated soil http://en.wikipedia.org/wiki/Image:AnthoxanthumOdoratum.jpg Parapatric Speciation: Example Ring species Occur when the geographical distribution of a species is circular in shape with clinal variation Each population is slightly different to its neighbouring populations, but can still interbreed with them When the “ring” closes, the two populations at each end of the distribution are sufficiently different that they cannot interbreed Ring species: Example Adjacent populations can interbreed, except for the herring gull and the lesser black-backed gull http://en.wikipedia.org/wiki/Image:Rings_species_example.png Larus gulls have a circular distribution around North Pole http://en.wikipedia.org/wiki/Image:PT05_ubt.jpeg Sympatric Speciation Speciation in the complete absence of any geographical barriers to gene flow A new species arises from within the same geographical area of the ancestral species May occur if there are two or more morphs in the population and assortative mating leads to reproductive isolation between the morphs, eventually resulting in speciation Original population Genetic polymorphism New species Apple maggot flies in the same geographic area feed and mate on either hawthorn (native host) or apple (introduced host) Males and females mate, and females lay their eggs, on the host they grew up on The two races do not interbreed http://www.msu.edu/course/lbs/144/f04/graphics/2_flies.jpg Sympatric Speciation: Example Adaptive Radiation Example: Darwin’s finches of the Galapagos Islands Cactus finch Woodpecker finch Sharp beaked ground finch Cactus eater Seed eater Small tree finch Insect eater Ground finches Occupy different niches Warbler finch Crushing bill Rapid evolution of many new species from a single ancestor Tree finches Fruit eater Medium ground finch Large tree tree Vegetarian tree finch Small ground finch Large Ground finch Hybridization can lead to instant speciation Polyploid hybrids are unable to mate with diploid parental types Example: Tetraploid hybrids of two species of gila monster are reproductively isolated from the parent species http://inst.sfcc.edu/~zoo/images/Gila%20Monster.jpg Hybridization (Polyploidy) Evolution is a gradual process Species continue to adapt at a uniform rate throughout time There is no clear demarcation of the point where one species becomes another species http://www.don-lindsay-archive.org/creation/snails_big.gif Time Phyletic gradualism Character Long periods of stasis interrupted by rapid bursts of change tied to speciation events Most adaptation occurs immediately after speciation Time Punctuated Equilibrium Character Example: A study of fossil Pliocene snails at Lake Turkana showed that species showed no change for long periods followed by bursts of rapid change associated with fluctuating lake levels Prevent different species from interbreeding Prezygotic or premating isolating mechanisms: Ecological isolation Temporal isolation Physiological isolation Mechanical isolation Behavioural isolation (e.g. male calls attract females of the same species in frogs) http://en.wikipedia.org/wiki/Image:Litoria_chloris_calling.jpg Isolating Mechanisms Isolating Mechanisms Postzygotic or postmating isolating mechanisms: Gamete mortality Zygote mortality Hybrid unfitness or breakdown Hybrid infertility or sterility (e.g. mule) http://en.wikipedia.org/wiki/Image:Bg06.jpg