General Ecology (BIO 160) Dept. of Biological Sciences Worksheet #2 Sacramento State University Worksheet 2: Adaptation and evolution I 1. What is natural selection? (pg 75)Natural selection is the process of differential reproduction and survival whereby alleles in a population that are not adaptive are eliminated. 2. Natural selection requires two conditions. Describe these two conditions and explain how together they lead to the process of natural selection. (pg 75) 1. Variation among individuals of a population in a heritable trait. 2. Differential survival and reproduction among individuals with different heritable traits. Some individuals in a population will have maladaptive traits; these individuals will tend to have lower survival and reproduction rates (i.e., fewer offspring) than individuals with adaptive traits. Thus, maladaptive traits will be eliminated from and adaptive traits will persist in the population. 3. What is evolutionary fitness? How can it be measured? (pg 76) Fitness is the genetic contribution of one generation to the next in the form of offspring. It is measured by quantifying the number of surviving progeny. It is also often estimated by using proxies for number of offspring; for example, numbers of flowers or seeds in plants or eggs in birds. 4. The following is a common definition of an adaptation: An anatomical, physiological, or behavioral trait that increases the ability of an organism to cope with its environment. a. Using a favorite organism of yours, give an example of a characteristic (i.e., trait) that varies among individuals (e.g., hair color, speed) and that likely affects the ability of individuals to cope with their environment. (pgs 75-76) Example (plant): chemical anti- herbivore defense (e.g., telegraph weed); the level of defense can vary from one plant to the next and helps defend against insect herbivores. Examples (animal): running speed of a predator (e.g., African cheetah); maximum running speed of individual cheetahs varies in the population and affects their ability to capture prey. Beak size in birds; varies among individuals and affects ability to crack seeds of different kinds. b. For the trait you chose above, describe a quantitative measurement you could take on each individual of your organism that would quantify how well it copes in its environment. (pgs 76-77) Adaptations: e.g., plants – # spines, conc. of defense chemicals, photosynthesis rate; animals: beak size, claw length, running speed, etc. Fitness: # seeds or seedlings produced (plants); # eggs laid or hatchlings (turtles); # flowers per plant; # coyote pups per litter; etc. 5. Do individuals or populations evolve through the process of natural selection? Explain your answer. (pgs 75-76, and throughout chapter) Populations evolve, not individuals. Evolution is a change in gene (more specifically, allele) frequencies within a population. An individual’s genome is static; that is, it does not change over time. However, in a population, individuals come and go over time and consequently so do their alleles. Collectively, all the alleles that are present in a population constitute its gene pool. It is at the population level that we can identify allele frequencies and that changes in allele frequencies occur generation after generation (i.e., over time). 6. What is the difference between a gene and an allele? (pgs 76-77) A gene is the functional unit of heredity on the chromosome (e.g., flower color), whereas an allele is an alternative form of a gene (e.g., white, purple, etc.). General Ecology (BIO 160) Dept. of Biological Sciences Worksheet #2 Sacramento State University 7. Individuals in populations vary in their genotypes and in their phenotypes. What is the relationship between genotype and phenotype? Is it ever possible for individuals with the same genotype (clones) to have different phenotypes? Explain your answer. (pgs 76-77) The genotype is the total sum of genes in an individual. The phenotype is the outward expression of the genotype and is therefore influenced by both the genetic make-up of the individual and its environment. Yes, it is possible for individuals with the same genotype to differ in phenotype. This is due to the range of influences that the environment can have on the expression of the genotype; the ability of a genotype to give rise to different phenotypes under different environmental conditions is called “phenotypic plasticity.” 8. Peter and Rosemary Grant are scientists who have studied the evolutionary ecology of Darwin’s finches on the Galapagos Islands for many years. Read pages 78-80 in your text and answer the following questions based on this information. a. How did the La Niña weather pattern affect the proportion of small versus large beaked birds in the population of medium ground finches on the Galapagos Islands? A La Nina event occurred that caused a severe drought. The drought caused greater mortality of plants producing small than large seeds. Birds with smaller beaks suffered greater mortality than large beaked birds, thereby increasing the proportion of large beaked birds in the population. b. Why did finches with large beaks have greater fitness than those with small beaks? Large beaked birds were able to crack open the large hard seeds, whereas smaller beaked birds in the population could not. As a consequence, birds with larger beaks survived better and had greater reproduction (i.e., fitness) thank small beaked birds. c. What type of selection (directional, stabilizing, or disruptive) was evidenced in this population of ground finches? What evidence can you site to support your answer? Directional selection. This is evidenced by the observed shift in the frequency distribution of beak size toward one end of the distribution for that trait. 9. Three general types of selection occur in populations. Using a frequency distribution to represent the variation of a trait within a population, illustrate the three types of selection and explain how they occur. (pg 80) Figure 5.10 in your text illustrates frequency distributions for each type of selection. Directional selection occurs when the traits favored by selection are at one end of the distribution causing a shift in allele frequencies to a new optimum. Stabilizing selection occurs when the traits favored by selection are those near the average causing the allele frequency to remain stable. Disruptive selection occurs when the traits favored by selection are alternative traits at either end of the distribution (extremes) and cause the allele frequency distribution to become bimodal. 10. In figure 5.11 in your text (pg 81), black-bellied seedcrackers show a bimodal distribution of bill size. Explain how such a bimodal distribution could have arisen in this species. These birds feed primarily on two species of sedge, one producing a hard-shelled seed and the other a soft-shelled seed. Small billed birds feed on the soft-shelled seeds and large-billed birds feed on the hard-shelled seeds. A bimodal distribution in bill size results because there are two distinct types of seed resources in the environment to which the birds have adapted. 11. Explain how mutation can introduce genetic variation into a population. (pg 81) A mutation gives rise to a new allele and – if selected for – increases the population’s genetic variation.
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