Biology 1B Evolution Lecture 6 March 8 2010 selection co evolution sex evolutionarily 1 Selection Darwin thought that evolution by natural selection was a very slow process however it can also be very fast only taking a few generations Scientists can generate experimental evidence using organisms that have a short life cycle such as bacteria yeast and flies Drosophila For example consider guppies Females are more attracted to male guppies with many large brightly colored spots than to male guppies with fewer smaller or duller spots However bright colors attract predators So the brightly colored guppies survive less but reproduce more while duller fish survive more but reproduce less Endler and Reznick transplanted fish from a predator filled pool at the bottom of a waterfall into the predator and guppy free waters at the top of the waterfall Therefore a population of fish with drab colors was moved into an environment with no predators There within generations the male fish quickly came to have more spots that were larger and more brightly colored This field experiment demonstrates the response of heritable phenotypes to a change in selection for a trait Sexual Selection Why are females attracted to males with brighter colored spots There are two primary hypotheses 1 Good Genes Hypotheis The bright coloration of males is a trait linked to other traits that allow for increased selection and survival Therefore females can use it as a proxy to determine their approximate fitness a Don t anthromorphize this however this doesn t mean the fish are consciously considering mate choice it is simply an adapted preference much as humans enjoy the taste of sugar and fat 2 Sensory Bias The food that guppies consume is brightly colored so the female is biased to be attracted to bright objects since they may be food Therefore the female preference for brightly colored males is simply that same existing color preference being exploited by the males to increase their mating potential Figure 1 Guppy Experiment Campbell 8th Edition Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings pg 460 Moritz Lecture 6 Notes page 1 Biology 1B Evolution Lecture 6 March 8 2010 selection co evolution sex evolutionarily 2 Genomic Signatures are another way to see whether natural selection is operating in a population It has become increasingly popular as the technology to do so has become more advanced and the cost of such tests has decreased markedly A candidate gene approach targets genes consider the pocket mice example MC1r agouti known to cause such effects Contrarily a genome scan sequences an entire genome and searches for regions of very low variation Why would we look for regions of low variation A gene that is not under selection will accumulate variations through both mutation and the opposing effect of genetic drift However if a gene is under strong and recent directional selection then there will be much less variety as one variant of the gene will be more conducive to survival for the lava flow mice the darker phenotype Figure 2 genetic variation for traits under selection and under no selection Copyright J Stovz 2005 For example consider small and large breeds of dogs All of these dogs are descended from a common ancestor that has been subject to intense artificial selection A genome scan of small dogs revealed that all small dogs that were sequenced shared a single genetic location that exhibited very low variation This location contained a known gene for a growth factor when sequenced larger dogs did not have the same variant as small dogs This implies that recent selection on this growth factor is what makes small dogs small As for what makes small dogs so yappy that s presumably still being researched Directional Selection is when the ideal phenotype shifts from that of the original population to a phenotype more adapted to the environment For example light colored mice living on a lava floe would shift to a darker coat color Moritz Lecture 6 Notes page 2 Biology 1B Evolution Lecture 6 March 8 2010 selection co evolution sex evolutionarily 3 Figure3 change in phenotypes with strong directional selection 2010 Pavia Coevolution Coevolution or reciprocal selection is when each species affects the fitnesses of phenotypes in the other species Mutualistic coevolution is when both species receive a benefit from the coevolutionary relationship This can become an obligate required trait For example attine ants also known as leaf cutter ants harvest leaves and bring them back to the ant nest where they are digested by fungi The fungi receives its nourishment from digesting the leaves but the ants receive their nourishment from consuming the product produced by the fungi digesting the leaves Antagonistic coevolution is when species interact negatively Consider a host and a pathogen coevolving the pathogen would develop surface proteins that the host cannot detect The host s immune system would then develop sensors that could detect that pathogen This sequence of evolutionary events has often been called an evolutionary arms race or a Red Queen situation to remain in one place survive without being vulnerable to pathogens you have to keep evolving new defenses In this case there is frequency dependent selection where the common phenotype has a reduced fitness pathogens will easily infect individuals who all have the same genes and the rarer phenotype has an increased fitness This works to maintain genetic variation Neat note The MHC or major histocompatibility complex is one of the most variable genomic regions in mammals and is responsible for our immune systems It s very complex and has a lot of different interacting genes Another example would be a predator and its prey predators which are more able to catch the prey will be favored while prey that can escape from predators more often will be favored Alternatively a prey species such as the Pacific newt might be very poisonous However its coevolved predator the gartner snake has the ability to detoxify the poison and prevent itself from being harmed by the poison This coevolutionary process might continue with the newt evolving more toxic poisons while the gartner snake would become more poison resistant Why have sex Evolutionarily there are several strong disadvantages to sex 1 Requires having two separate genders and expending resources on creating an entire gender males that waste energy by not directly producing offspring 2
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