Unit 5: Evolution through Natural Selection and other Mechanisms

Natural Selection Natural selection is the process by which those heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations. It is a key mechanism of evolution.

Genetic variation -Genetic variation is essential for natural selection. -The natural genetic variation within a population of organisms means that some individuals will survive and reproduce more successfully than others in their current environment.

-Natural selection acts on the phenotype, or the observable characteristics of an organism, but the genetic (heritable) basis of any phenotype which gives a reproductive advantage will become more common in a population. -Over time, this process can result in adaptations that specialize organisms for particular environment.

Sample Question # 1 The idea that evolution takes place at one point in time, followed by a long period without change is ______. From: Test bank 07-08 A gradualism B punctuated equilibrium C artificial selection D succession

Sample Question 2 Charles Darwin proposed his theory of evolution based on observations of nature. Which observation that contributed to his theory is illustrated by this population of beetles? Species produce more offspring than can survive. Populations remain stable over time. C. Individuals within a population may vary widely. D. Environmental resources are limited.

Genetic Drift -Genetic drift, along with natural selection, mutation, and migration, is one of the basic mechanisms of evolution. -In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.

Genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process. So although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.

Bottleneck and Founder Effects -Genetic drift can cause big losses of genetic variation for small populations. -Population bottlenecks occur when a population’s size is reduced for at least one generation. -A founder effect occurs when a new colony is started by a few members of the original population.

Sample Question 3 A small population of chimpanzees lives in a habitat that undergoes no changes for a long period. How will genetic drift probably affect this population? A It will accelerate the appearance of new traits. B It will promote the survival of chimpanzees with beneficial traits. C It will increase the number of alleles for specific traits. D It will decrease genetic variation.

Sample Question 4 Genetic drift is a ______________ process. A. directed B. selection-driven C. random D. co-evolutionary

Speciation -A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions. -Speciation is a lineage-splitting event that produces two or more separate species.

Causes of Speciation Geographic Isolation Reduction of Gene Flow

Kinds of Speciation Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location. Allopatric speciation is just a fancy name for speciation by geographic isolation

Sample Question 5 This diagram illustrates what kind of speciation? River This diagram illustrates what kind of speciation? A Sympatric B Allopatric C Geographic D Catastrophic

These are two identical cladograms. A cladogram is a diagram used to show ancestral relations between organisms, to represent the evolutionary tree of life. These are two identical cladograms.

Analyzing a cladogram… Which species are most closely related? 1 2 3 Analyzing a cladogram… Which species are most closely related? Which is the common ancestor of flies and bees? Which insects share the most recent ancestor?

Here’s a more complicated one… What conclusions can you draw from this ‘tree of life’?

Sample Question 6 A clubmosses and spikemosses B spikemosses and ferns According to the cladogram above, which two species are most closely-related? A clubmosses and spikemosses B spikemosses and ferns C ferns and flowering plants D flowering plants and clubmosss

Fossil Record Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record.

Fossil evidence indicates that over time organisms of increasing complexity appeared on Earth. Bacteria and blue-green bacteria are the first fossils that were preserved from the Precambrian era. The fossil record contains many examples that could be interpreted to mean that species evolved from more ancient organisms.

Sample Question 7 In a section of the Grand Canyon, scientists have found the fossil remains of several different groups of organisms. The diagram on the left represents the number and age of the fossils the scientists found. The width of each shaded area in the diagram below indicates the relative number of fossils found. Which of the following statements is supported by the fossil record? A Group C is the only group that no longer exists for it’s now extinct. B Group B was the most numerous organism 10 million years ago. C Group D has been in existence the longest. D Group A is the most recent organism to come into existence.

Sample Question 8 Which of the following is usually most helpful in determining the age of the fossils found in rock strata? A the size of the fossils B the color of the fossils C the water content of the rock layer where the fossils were located D the depth of the rock layer where the fossils were found

Sample Question 9 Which of the following best explains how the fossil record provides evidence that evolution has occurred? A It indicates the exact cause of structural and behavioral adaptations of organisms. B It shows that the form and structure of groups of organisms have changed over time. C It shows how the embryos of many different vertebrate species are very similar. D It indicates that forms of life existed on Earth at least 3.5 billion years ago.

Sample Question 10 Orchids were studied to determine if the amount of humidity affected the flowering of these plants. Which of these was the independent variable in this study? A The percentage of humidity B The length of time required for flowering C The amount watered D The number of flowers on each plant This is an I and E Question.

Remember… DEPENDENT VARIABLE -is the variable that is measured at the end of the experiment. INDEPENDENT VARIABLE -is the manipulated variable in the experiment.