Evolution, Natural Selection & Selective Pressures
Page 1: Concept Map Evolution! Definition: Natural Selection Definition: Genetic Variation: Selective pressure: Many Generations:
Page 2: Categories of Natural Selection Directional selection: What is it? Example graph with the change Stabilizing selection: What is it? Example graph with the change Disruptive selection: What is it? Example graph with the change
What is Evolution? Evolution = Change in allele frequency in a population over time (generations) Ex: If any inherited trait is increasing (or decreasing) in frequency, the population is evolving
How does Evolution Occur? Evolution occurs through a process called natural selection Natural selection occurs when certain individuals are better able to survive and reproduce due to their phenotypes (traits)
Theory of Evolution by Natural Selection Charles Darwin was the first person to propose the idea of evolution via natural selection in 1859 Many people before him had hypothesized about evolution, but he was the first person to propose a valid mechanism for how it works
What is Needed for Nat. Selection to Occur? 1.A population containing genetic variation 2.Selective pressures in the environment that cause certain individuals to have greater fitness (survival and reproduction) 3.MANY reproductive generations (time)
What is a Selective Pressure? Selective pressures are any factors in the environment that affect the survival &/or reproduction of individuals in a population Can be biotic (living) or abiotic (non-living)
What is a Selective Pressure? Examples of biotic selective pressures: Predators, competitors (organisms that compete for the same resources), pathogens. Predation is a biotic selective pressure
What is a Selective Pressure? Examples of abiotic selective pressures: Temperature, sunlight, humidity, rainfall, snow, wind, soil minerals, salinity, fire Fire is an abiotic selective pressure
Monogenic Traits Some traits are monogenic coded for by only one gene Ex: Widow s peak, blood type, earlobe attachment, many genetic disorders Monogenic traits appear as distinct categories, e.g. blood type A, B, AB or O
Monogenic Traits Phenotype frequencies for monogenic traits are shown on simple bar graphs If evolution is occurring, the phenotype frequencies will change over generations 3 phenotypes: brown OR black OR red
Initial P opulation G eneration 10 Frequency of phenotype G eneration 20 G eneration 30 What do you notice in this population of lizards with three skin color phenotypes?
Initial Population Generation 10 Frequency of phenotype Evolution is occurring! Generation 20 Generation 30 What selective pressure could cause this selection for black lizards over red and brown?
Polygenic Traits Most traits are polygenic coded for by more than one gene Polygenic traits show a wide range of phenotypes Polygenic traits are continuous,and NOT separate categories Ex: Height, eye color, skin color
Polygenic Traits Frequency The fact that there is 3 5 3 0 such a wide range of 2 5 lengths indicates this trait is polygenic 2 0 1 5 1 0 5 0 2 0 m m 3 0 m m 4 0 m m Length Polygenic traits generally have frequencies that follow a bell-shaped curve
S e a G u ll P o p u la t io n - G e n e r a t io n 1 2 0 What do you notice about the wingspan phenotypes after 250 generations of sea gulls? F r e q u e n c y 1 5 1 0 5 0 8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5 W i n g s p a n ( c m )
S e a G u ll P o p u la t io n - G e n e r a t io n 1 There has been selection for longer wingspans F r e q u e n c y 2 0 1 5 1 0 5 0 8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5 W i n g s p a n ( c m ) Sea Gull Population - Generation 250 What selective pressure could cause this? Frequency 30 25 20 15 10 5 0 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 Wingspan (cm)
Three Categories Of Natural Selection Directional (common) Stabilizing (common) Disruptive (uncommon)
Directional Selection Hawk vision ratings in population before selection Vision ratings after many generations selection for better vision has occurred
Stabilizing Selection When phenotypes in the middle give the organism an advantage Ex: Human birth weight Low birth weight = infant mortality High birth weight = delivery complications
Stabilizing Selection Peak weight does NOT change over time since weights at the ends are selected against
Disruptive Selection Selection in favor of phenotypes on both ends of the curve Ex: Short and long beaks when two sources of food are present Short-beaked birds can eat the seeds more easily, long beaked birds can eat the insects more easily
Disruptive Selection Over many generations, birds with short and long beaks become more common, while those in the middle die out