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Regents Living Environment 16-2: Evolution as Genetic Change Oswego High School Looking at evolution from a genetic standpoint offers a new way to look at key evolutionary concepts. Every time organisms reproduce, they pass on their genes to their offspring. We can look at evolutionary fitness as how successful an organism is in passing on its genes. We can also view adaptations as any genetically controlled trait that increases an organism’s chances of survival. Natural selection rarely acts on genes alone because it is an entire organism that survives to reproduce or dies without reproducing. Therefore, natural selection can only affect which individuals survive and reproduce and those who don’t. If an individual dies without reproducing, they do not contribute their genes to the population. If an individual produces many offspring, its alleles will stay in the gene pool and may increase in frequency. Recall that evolution is the change in relative frequency of alleles in a population. This means that evolution can only occur in populations and not individual organisms Natural Selection on Single-Gene Traits Natural selection on single gene traits can lead to changes in allele frequencies and thus can lead to evolution. For Example: There is a population of brown lizards. A mutation occurs that turns some of the lizards black and red. The red lizards become visible to predators and their alleles will be less likely to be passed on. The black color may allow the lizards to absorb sunlight and cause them to warm up faster on cold days. If high body temperature means that the lizard will escape predators, the black lizards may produce more offspring than the brown lizards. The black allele may increase in frequency over time. If a change in color does not affect fitness, the allele that produces the color change will not be under pressure from natural selection. When traits are controlled by more than one gene, the effects of natural selection are more complex. As we said before, the action of multiple alleles on traits can produce a wide range of phenotypes that fit a bell curve. The fitness of those close to each other on the curve will not be much different. However, fitness can vary from one end of the curve to another. Natural selection acts where fitness varies. Natural selection affects allele distributions in one of three different ways. Stabilizing Selection Stabilizing selection occurs when individuals in the Middle of the curve have a higher fitness than those at the ends. Stabilizing selection selects for the intermediate and against the extremes. Example: The mass of human babies is under the influence of stabilizing selection. Babies of below average mass are more susceptible to illness and are less likely to survive. Babies born of above average mass will be more difficult to pass through the birth canal. These babies will be less fit than those who are born of average mass. Directional Selection Directional selection occurs when individuals on one end of the curve have a higher fitness than those in the middle and the other end of the curve. Directional selection selects for one extreme and against the middle and the other extreme. Example: Finches with larger beak sizes will be able to feed on harder and thicker seeds. A food shortage causes the number of small and medium sized seeds to decrease. Finches with larger beaks will have better access to food than those with small and medium sized beaks. This results in the large beaked finches having a higher fitness than those with small and medium sized beaks and the average beak size of the population would increase. Disruptive Selection Disruptive selection occurs when individuals at both ends of the curve have a higher fitness than those in the middle. Disruptive selection selects for both extremes and against the middle If natural selection is strong enough and lasts long enough, two distinct phenotypes can be created. Example: Finches live in an area where medium sized seeds are less common, while small and large seeds become more common. This means that birds with ______________ and ______________ beaks will have a higher fitness than those with ________________ sized beaks. Genetic Drift Natural selection is not the only source of evolution. Sometimes, in small populations, evolution can occur by alleles becoming more or less common by chance. This random change in the allele frequency is called genetic drift In small populations individuals that have a certain allele may produce more offspring that others strictly by chance. Founder Effect One way genetic drift can occur is when a small group of individuals from a population colonize a new area. It is possible that these individuals have a different allele frequency than the larger population. As a result of this, the new population will be genetically different than the original population. The situation where a small number of individuals from a larger population colonize a new habitat, creating a new gene pool, is called the Founder Effect Bottleneck Effect Genetic drift can also occur as a result of a random event. These events may include floods, earthquakes, hurricanes or forest fires As a result of these chance events, the majority of the population is wiped out, leaving few survivors. These survivors have a much smaller gene pool than the original population. These individuals will mate with each other, causing the offspring to be more related to each other than they would have in the original population. Changes in allele frequency as a result of chance events is called Bottleneck Effect Evolution vs. Genetic Equilibrium Scientists often find it helpful to determine what happens when no change takes place in order to clarify how evolutionary change takes place. The Hardy-Weinberg Principle states that allele frequencies in a population will remain constant unless one or more factors cause them to change. The situation in which allele frequencies remain constant is called Genetic Equilibrium There are five factors that are required to maintain genetic equilibrium. 1. Random Mating Ensures that all individuals in a population have an __________________ chance of reproducing and passing their alleles on to the next generation. In natural populations, mating is almost __________________ random. Many species select mates based on certain __________________ traits like size and strength. This is called __________________ _____________________. If there is non-random mating, genes for certain traits are not in equilibrium and are under ______________ __________________. 2. Large Population Large populations help maintain genetic equilibrium because genetic drift has _____________ effect on larger populations than on small populations. 3. No movement into or out of the population If new individuals enter a population, they bring ___________ alleles into the population. In order to stay in equilibrium, a population’s gene pool must be kept _______________ and __________________ from the gene pools of other populations. 4. No Mutations Mutations cause __________ alleles to be introduced into a population, causing a change in ________________ ____________________. 5. No Natural Selection All genotypes need to have an ______________ chance of survival in the population. No phenotype can have a selective _____________________ over others. All of these conditions must be met in order for genetic equilibrium to take place. If any or all of these are disrupted, evolution will occur.