welcome to ap daily video topic 7.2 natural selection my name is tanaya hibbler and i work at brophy college preparatory here in phoenix arizona and i'm here to let you know that you can do this what will we learn so why is phenotypic variation important to a population how does natural selection act on phenotypic variation how do changing environments apply selective pressures to populations and what variables increase or decrease fitness of an organism natural selection acts on phenotypic variations in populations genetic variation describes the genotypic and the phenotypic differences between individuals in a population genetic variation can lead to different phenotypes and or adaptations due to natural selection individuals with more favorable traits are more likely to survive and to reproduce genetic variation can increase the probability a population of organisms will survive under changing environmental conditions so if the environment changes one of the phenotypes may be better suited for the changed condition so variation is good environments change and apply selective pressures to populations selective pressure refers to antibiotic or abiotic factors influencing survivability for example disease predation climate and food availability so you can have an organism that has an adaptation and it could be selected for and an organism without that can be selected against and the changing environments introduce different selective pressures on populations and as you see here individuals fitness is relative to the environmental condition so if the environment changes who has the adaptation or what's considered adaptation can change so now after two years the organism that originally had the adaptation is now being selected against and the individual with the other adaptation or the other trait is now being selected for so the phenotype selected for can be selected against when environmental conditions change and vice versa let's look at an example environments change and apply selective pressures to populations in england peppered moths vary in color as you see to the right some are lighter colored and some are darker colored after the industrial revolution soot from coal burning factories and homes covered many of the trees as a result the darker colored moths were well camouflaged and more likely to survive to reproduce over time the dark colored phenotype became most common within this population of moths but after the clean air act was passed and pollution was cleaned up the lighter phenotype type became more common once again some phenotypic variations significantly can increase or decrease fitness of the organism and fitness refers to reproductive success over generations any phenotype that decreases chances of survival and reproduction negatively impacts fitness any phenotype that increases chances of survival and reproduction positively impacts fitness let's look at an example so ddt resistance significantly increases fitness of insect populations ddt was a commonly used insecticide to combat insect-borne diseases but it was later banned in the u.s in 1972 ddt is still used though however in some countries to combat malaria which spreads through mosquitoes due to natural variation in populations some individual insects are resistant to ddt as you can see in the picture above to the right you have the red insect it has a natural resistance there's variation in the population and that individual is resistant resistant individuals are not killed by the ddt and they survive to pass on this resistance to their offspring so over time an entire population of insects can become resistance since those individuals survive to pass on their traits and with ddt this can happen in from two years to up to 20 years let's practice lesson skill focus concept explanation explain biological concepts and or processes go ahead and read the question look at the figure and then choose the best answer pause your video now did you select b so which of the following best explains why the frequency of this trait remains high in some populations but not in others in populations where the sickle cell gene has a high frequency the mutation must serve as an adaptation and confer selective advantage to some individuals what should we take away from today's lesson phenotypic variation increases the probability a population will continue in unstable environments natural selection acts on phenotypic variation by selecting individuals with the most advantageous traits based on certain environmental conditions changes in environmental conditions can change which individuals in the population will be selected for or selected against in any phenotype that increases an organism's chance of survival and reproduction will significantly increase an organism's fitness and ultimately increase the fitness of the entire population thank you so much for joining us for this ap daily video my name is tenea hibler and remember you can do this