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Evolution and Natural Selection (Chapter 2)
Key definitions
Population: A group of organisms of the same species living in the same geographic region. Gene: Section of DNA instructing how to produce a trait Allele: One variant of a gene (several may exist) Evolution: A change in allele frequencies of a population. The gradual process in which something changes into a different and usually more complex or better form. Four mechanisms of evolution:
1. Mutation a. An alteration of the base-pair sequence of an individual’s DNA. i. We cannot predict which individuals will have which mutations ii. we cannot predict whether the consequences of a mutation will be neutral (no effect), harmful, or useful. b. Mutation is critical to natural selection: all variation—the raw material for natural selection—must initially come from mutation. 2. Genetic drift a. A random change in allele frequencies in a population. 3. Migration (gene flow) a. The movement of some individuals of a species from one population to another i. This movement distinguishes migration from the founder effect, in which individuals migrate to a new habitat previously unpopulated by that species. 4. Natural selection (three conditions that need to occur) a. Variation for a trait b. Heritability c. Differential reproductive success i. “individuals with traits more suited to survival and reproduction in their environment generally leave more offspring than do individuals with other traits” ii. “Survival of the fittest” is necessarily correct because sexual selection is also natural selection but doesn’t affect survival Fixation: When an allele’s frequency in a population reaches 100% (and the frequency of all other alleles of that gene becomes 0%) Founder effect: A small number of individuals may leave a population and become the founding members of a new, isolated population. This founder population may have different allele frequencies than the original, source population.
Bottleneck effect: A famine, natural disaster, or rapid environmental change causes the deaths of a large proportion of individuals in a population. For this reason, the remaining population may not possess the same allele frequencies as the original population. Sexual selection: Traits that make the individual more attractive to the opposite sex. Hardy-Weinberg equilibrium: A principle stating that the genetic variation in a population will remain constant from one generation to the next, recessive genes won’t be wiped out and the proportion stays the same. (Assumes that random mating occurs, with alleles randomly coming together in all possible genotypes, and that there is no evolution.) Fitness: a measure of the amount of reproduction (the reproductive output) of an individual with a particular phenotype relative to the reproductive output of individuals of the same species with alternative phenotypes. Three elements of fitness:
- An individual’s fitness is measured relative to other genotypes or phenotypes in the population.
- Fitness depends on the specific environment in which the organism lives.
- Fitness depends on an organism’s reproductive success compared with other organisms in the population. Adaptation: the process by which organisms become better matched to their environment and to the specific features that make an organism more fit. Artificial selection: A special case of natural selection; the three necessary and sufficient conditions for natural selection are satisfied, but the differential reproductive success is determined by humans rather than by nature and so is typically goal-oriented. Individual selection: individuals with one extreme of the range of variation in the population have higher fitness. Stabilizing selection: when individuals with intermediate phenotypes are the fittest. Disruptive selection: individuals with extreme phenotypes experience the highest fitness Biogeography: the study of the distribution patterns of living organisms around the world. Homologous feature: a feature inherited from a common ancestor. Vestigial feature: a structure that was once useful to the organism but has lost its function over evolutionary time.
10.11: “Adaptation refers both to the process by which organisms can become better matched to their environment and to the specific traits that make an organism more fit. Although natural selection can produce adaptation, it doesn’t necessarily lead to organisms perfectly adapted to their environment.” 10.12: “Animal breeders and farmers undertaking artificial selection are making use of natural selection when they modify their animals and crops through selective breeding. In directional selection, the average value for the trait increases or decreases; in stabilizing selection, the average value of a trait remains the same while extreme versions are selected against; and in disruptive selection, individuals with extreme phenotypes have the highest fitness.” 10.13: “Finding answers can require creative exploration of multiple alternative hypotheses. We don’t always find definitive answers, but as evidence accumulates, we may increase our confidence in one hypothesis and reject others.” 10.14: “Natural selection can change allele frequencies for genes involved in complex physiological processes and behaviors. Sometimes a trait that has been selected for a particular function is later modified to serve a completely different function.” 10.15: “By analyzing fossil remains, paleontologists can reconstruct what organisms looked like long ago, learn how organisms were related to each other, and understand how groups of organisms evolved over time.” 10.16: “Observing geographic patterns of species distributions—noting similarities and differences among species living close together but in very different habitats, and among species living in similar habitats but located far from one another—helps us understand the evolutionary histories of populations.” 10.17: “Similarities in the anatomy and development of different groups of organisms and in their physical appearance can reveal common evolutionary origins.” 10.18: “All living organisms share the same genetic code. The degree of similarity in the DNA of different species can reveal how closely related they are and the amount of time that has passed since their last common ancestor.” 10.19: “Replicated, controlled laboratory selection experiments and long-term field studies of natural populations allow us to watch and measure evolution as it occurs.”
Answers to Problem Sets
1. Describe natural selection and distinguish it from evolution. Give an example of natural selection and an example of evolution that is not also natural selection. Over the course of several decades, plant breeders working with corn crops significantly increased the kernel size and nutrition. Why did this work? If the rate of improvement slows down or stops,
what might you recommend they do to improve the results from their selective breeding programs? Why? While every case of natural selection is considered a type of evolution, not every instance of evolution can be considered natural selection. Natural selection requires three factors: genetic variation, trait heritability, and a difference in survival/reproduction capabilities. An example of natural selection is the gradual increase of white fur mice compared to dark fur mice in a snowy environment, as the mice with white fur are more likely to survive against predators and pass on their trait of white fur to their offspring. An example of evolution is a mutation, a random change in the animal’s DNA where they grow a patch of dark spots on their neck. This doesn’t necessarily have to be heritable, or tied into survival/reproduction. In this case, these dark spots count as evolution but not natural selection. For the example of corn, the size of kernels and nutrition are hereditary factors, so farms can selectively breed the corn of each generation that has the largest kernel size and the best nutrition. This way, future generations will inherit the larger kernels and better nutrition. If the rate of improvement perhaps slows down, the farmers can be more picky in choosing which corn are bred, and be more selective in the size and nutrition. They can experiment with focusing only on size or nutrition first, and then incorporate the other trait when they are satisfied with what they had originally bred.
2. What is evolutionary fitness? Describe three important components of an organism’s fitness. Suppose that you measure the fitness of one hundred blue jays within a population, evaluating each individual’s rank. Within that population will those birds all have the same relative fitness over the course of five years? Why or why not? Evolutionary fitness is a measure of the amount of reproduction (the reproductive output) of an individual organism. The three components of fitness are:
- An individual’s fitness is measured relative to other genotypes or phenotypes in the population.
- Fitness depends on the specific environment in which the organism lives.
- Fitness depends on an organism’s reproductive success compared with other organisms in the population. Most likely, the relative fitness of these blue jays will either stay the same or increase over the course of five years. Most likely, the blue jays with poorer fitness will die early and not reproduce as often, leading to the blue jays with better traits being more successful in having offspring. These offspring will then carry the superior traits, and repeat the process. It is possible that the environment is changing faster than the blue jays can adapt, leading to no change in fitness or even a decrease in fitness. Also, it is possible that 5 years is not enough time for blue jays to adapt, as they may not have much genetic variation or they may have a small population. 3. Among the Old Order Amish of Lancaster, Pennsylvania, there is an unusually high frequency of polydactylism. Since the group was founded by a small group of separatist German Mennonites in the early 1770s, some 61 cases of this rare trait have been reported, about as many cases as in the entire rest of the world since that time.
Yes, I believe that natural selection led to the anatomical and physiological differences in people from these elevations. As aforementioned, the higher infant mortality rates due to respiratory infections imply that many people who were born without the ability to deal with these harsh conditions, would not live past infancy. This means that in this environment, the traits of having more blood cells and larger ventricles of the heart would prove to be advantageous in living in this environment. The people born with these characteristics would have a higher chance of surviving birth, and therefore lead to a higher frequency of these traits appearing in the future. This was probably the product of several evolutionary agents, such as the initial mutation of these traits, possibly migration of humans to these remote areas who already have these traits, etc. But, all in all, it is most likely a form of natural selection that caused these changes. Source From What Is Life? a Guide to Biology with Physiology Phelan, Jay;
