8/25/2017. Chapter 7 Darwinian Evolution:

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1 Chapter 7 Darwinian Evolution: 2017 Pearson Education, Inc. 7.1 Darwin s influences 7.2 Natural selection 7.3 Evolution affects daily life 7.4 Fossil record 7.5 Evidence for evolution 7.6 Populations are the units 7.7 Evolutionary mechanisms 7.8 Macroevolution 7.9 Geological record 7.10 Reproductive barriers 7.11 Speciation 7.12 Taxonomy 7.13 Phylogenetic trees 7.1 Opening Questions: How to explain the unity and diversity of living things? The Earth is filled with a wide diversity of organisms. Yet there is also great unity among living things 2017 Pearson Education, Inc. Chapter Table of Contents 7.1 November 24, 1859, is a landmark date in the history of biology. In 1859, British naturalist Charles Darwin published On the Origin of by Means of Natural Selection. In the Origin of Darwin introduced the concepts of evolution and natural selection Pearson Education, Inc. Chapter Table of Contents 7.1 Until the 1800s most scientists had a different view of life and species. In early history, most thought a young Earth held unrelated and unchanging species. The discovery of fossils (1700s) first suggested that the Earth was very old and that species could change over time. Aristotle believed species were immutable. Darwinius masillae fossil, early primate from around 47 mya 7.1 By the 1800s new views about species and the history of Earth had emerged. Lamarck was one of the first to suggest that species change over time (evolution). Lyell suggested that an old Earth had gradually changed through slow, accumulating processes. 7.1 Darwin s influences and experiences led him to his theory of evolution. As a youth, Darwin spent time observing nature. He did not enjoy formal schooling! His travels allowed him to compare species from different regions. The voyage of the Beagle:

2 7.1 Darwin began an in-depth study of change over time (evolution). Darwin spent decades reading, analyzing his specimens, and discussing ideas with colleagues. Darwin was the first to propose a mechanism to explain how species could evolve: natural selection. 7.2 Opening Questions: Observations on the natural world: True or false? If the answer is false, explain why: True or false: Populations tend to be stable in size. True or false: Resources are unlimited. True or false: All individuals in a species are exactly alike. True or false: Traits can be inherited. Darwin s beetle collection 7.2 In The Origin of Darwin made two important points. First, modern species have descended from common ancestors (evolution). Second, natural selection is the mechanism of evolution. Darwin arrived at the idea of evolution by natural selection through several important observations and conclusions. 7.2 Darwin first observed that populations produce more individuals than can survive. Observation: Overproduction More individuals are born than can be supported by the environment. Observation: Limited resources The amount of resources (such as food, water, shelter, sunlight) stays relatively constant. 7.2 Darwin concluded that competition was a factor for all living things. Conclusion: Competition More offspring are born than can be supported by limited resources; not all individuals survive and reproduce. Observation: Variation Darwin also observed that no two individuals are alike. 7.2 Darwin concluded that favorable variations will be naturally selected. Conclusion: Natural selection Those individuals with variations that make them best suited to their environment will, on average, be more likely to survive and reproduce. 2

3 7.2 Darwin concluded that natural selection can lead to evolution. Observation: Heritability The traits of an organism are likely to be passed to the next generation. Conclusion: Evolution Because traits are passed from one generation to the next, and because certain members are more likely to survive and reproduce, a population will change over time, becoming better suited to its environment. 7.2 Important points about evolution Individuals don t evolve. Natural selection acts on individuals, but only populations evolve. Natural selection works with heritable traits. Only genetically coded traits are subject to natural selection. Evolution does not have a goal. Evolution occurs in response to local environmental conditions, not future ones. 7.3 Opening Questions: How does evolution affect our daily lives? You can witness examples of evolution in action all around you! Darwin noted that humans have been selecting for traits for millennia. He called this artificial selection. 7.3 Artificial selection example: What are some examples of plants or animals that humans have bred for specific traits? 7.3 Natural selection in action Antibiotics were first discovered in the 1940s. Today, some antibiotics have become virtually useless because bacteria have evolved resistance. 7.3 Antibiotic exposure in bacteria is an example of natural selection in action. Pre-antibiotic With antibiotic Post-antibiotic Bacteria non-resistant strain Bacteria antibiotic-resistant strain 3

4 7.3 Evolution is witnessed as adaptation. Individual with traits that enhance survival and reproduction will have, on average, more offspring. Adaptation is the accumulation of these favorable traits in a population over time. 7.4 Opening Questions: How are Darwin s observations and conclusions connected? Replicate the figure below on a full page in your notes. Identify Darwin s observations and conclusions. Draw connections between the ideas. For each connecting line, write an explanation. Limited resources Competition Natural selection Overproduction Variation Heritability Evolution 7.4 The fossil record provides important evidence for evolution. Fossils form when organisms die, fall into accumulating sediment, and are compressed into rock. 7.4 Fossils provide a glimpse into the past. Fossils can be dated using their geological position or through radiometric dating. 7.4 How does the fossil record provide evidence of evolution? The fossil record reveals an ordered appearance of life on Earth, from prokaryotes to today s life forms. Transitional forms provide evidence of change within lineages. 7.5 Opening Questions: Can we predict evolution? Write a short answer to the question below: What characteristics must be present in a population and the environment in order for natural selection to occur in a population? Fossil whales with rear legs are examples of transitional forms. 4

5 7.5 The geographic distribution of species provides much evidence of evolution. Biogeography is the study of the geographic distribution of species. For example, the geographic isolation of Australia accounts for the dominance of marsupial mammals. 7.5 Comparative anatomy provides much evidence of evolution. Comparisons of the body structures of modern organisms is called comparative anatomy. 7.5 Comparative anatomy can provide insight into evolutionary history. Examination of animal forelimbs shows they are all constructed from similar bones. 7.5 DNA and bioinformatics provide much evidence of evolution. All life uses DNA for genetic code. Closely related species will have similar DNA and protein sequences. Such as in primates Bioinformatics employs computational tools to process genetic data. 7.6 Opening Questions: What exactly is a population? Populations are the smallest unit that can evolve. But what is a population? Answer the following questions: 1. How would you define a population? Is your class a population? Is your country a population? Is the entire human species a population? 2. How might biologists define a population? Explain. 7.6 Populations are the units of evolution. Natural selection acts on individuals. However, evolution is defined only in terms of changes in a population over time. A population is a group of individuals of the same species living in the same place at the same time. 5

6 7.6 Members of a population are capable of meeting and mating. Birds: Same population Fish: Same population Squirrels: Different population Squirrels don t swim! 7.6 The members of a population may carry different gene versions. The gene pool consists of all versions of all the genes carried by all the individuals in a population. Genetic variation in a gene pool can arise through mutation. Sexual reproduction ensures that genes are randomly mixed. 7.6 Natural selection acts on the gene pool. Traits that enhance survival and reproduction will be represented with increasing frequency in the gene pool. A generation-to-generation change in the gene pool is called microevolution, which is evolution occurring on its smallest scale. 7.6 Microevolution is a generation-togeneration change in the gene pool. Taken over many generations, microevolution can result in the gradual adaptation of species to the local environment. 7.7 Opening Questions: Can you breed a Chihuahua? Imagine you have a pack of wolves. How could you turn your wolves into Chihuahuas? Explain why your strategy would work. Extra thought question: Is body size the only trait that matters? 7.7 Changes to the genetic makeup of a population can arise via two mechanisms. 1. Mutations: Random changes to DNA can create new genes. 2. Sexual recombination: During the formation of sperm and eggs, chromosomes can exchange pieces of DNA, shuffling genes. Crossing over 6

7 7.7 Natural selection and fitness 7.7 Mechanisms of evolution Darwinian fitness is the contribution that an individual makes to the gene pool of the next generation in comparison to the contributions from other individuals. The fittest individual is not always the strongest. There are many sorts of adaptations that can improve fitness. Camouflage is an adaptive trait. What can lead to changes in a gene pool over successive generations? Hint: We ve already discussed one important mechanism that can result in gene pool change. Natural selection is the primary mechanism that can lead to a change in a gene pool, also known as evolution. 7.7 Mechanisms of evolution In addition to natural selection, there are other mechanisms that can also contribute to evolution in gene pools. Genetic drift Bottleneck and founder effect Gene flow Sexual selection 7.7 Genetic drift can lead to changes in a gene pool over successive generations. Genetic drift is a change in a gene pool due to chance. For example, genes may be lost if a few individuals die or migrate at random. This is important in small, or isolated, populations. In evolving populations, some combination of all the mechanisms operates. 7.7 The bottleneck and founder effects can change gene pools. If a population is drastically reduced in numbers, that is a bottleneck. If a few individuals migrate to a new isolated habitat, that is a founder effect. In either case, by chance, some genes will be lost from the gene pool. In the 1800s, cheetah numbers were drastically reduced. 7.7 Gene flow tends to reduce differences among gene pools. Most populations are not isolated. Gene flow is the genetic exchange among populations due to migration. Drifting pollen may transfer genes between distant populations, causing them to become more genetically similar over time. 7

8 7.7 Sexual selection can lead to changes in a gene pool over successive generations. Sexual selection is a form of natural selection that depends on an individual s ability to obtain a mate. Females may choose males for their traits. Males may compete with each other for access to mates. 7.7 Review Questions: Survival of the prettiest? While working on his theory of natural selection, Darwin was quite troubled about peacocks. A long tail makes them vulnerable to predators. Why might a peacock, with its long, beautiful tail, initially have troubled Darwin? 7.8 Opening Questions: Do only the strong survive? Do you agree or disagree with the following statement? Explain your answer. Only the strongest individuals in a population will survive to reproduce. 7.8 Macroevolution encompasses the major changes in the history of life. Macroevolution is genetic change on a large scale. Speciation is the evolutionary formation of new species. Earth s incredible diversity represents a long history of evolution, as ancestral species gave rise to one or more new species. 7.8 Speciation may occur through two different mechanisms. In nonbranching evolution, an ancestral population changes gradually. In branching evolution, an ancestral population splits into two or more populations. 7.8 Novel features may spur large-scale evolution. Throughout the history of life on Earth, novel features have evolved. The evolution of feathers and flight in birds is an example of how structures that serve one role can gradually change to serve another. 8

9 7.8 In mass extinctions, the majority of living species suddenly die out. There have been five documented mass extinctions in the history of life. 7.8 Rapid species diversification follows mass extinctions. Following the mass extinction of the dinosaurs, mammals diversified. 65 mya the large dinosaurs went extinct. Many biologists think we are entering the sixth great extinction. What might be the cause? 7.9 Opening Questions: Can you trust Fred Flintstone? Did any of our human ancestors ever ride a dinosaur to work? Did they ever fry up a dinosaur egg for breakfast? Or have a cute little dino for a pet? 7.9 The geological record ties together the history of Earth and its life. Geologists recognize four broad eras in the history of Earth, each marked by the appearance of distinctive life. Each era represents a distinct period in the history of life. As fun as it is to imagine, what is wrong with the above scenarios? Explain. Precambrian Paleozoic Mesozoic Cenozoic 7.9 Precambrian era: 4.6 bya to 541 mya 7.9 Paleozoic era: 541 to 251 mya Highlights from the Precambrian: 4.6 bya: Earth forms 3.5 bya: Oldest known prokaryote (primitive-celled) fossils 2.1 bya: Oldest known eukaryote (moderncelled) fossils Highlights from the Paleozoic: 541 mya: Explosion in animal diversity within the oceans 420 mya: Plant life begins on land 370 mya: Animals migrate to land 251 mya: Mass extinction event 9

10 7.9 Mesozoic era: 251 to 65 mya 7.9 Cenozoic era: 65 mya to today Highlights from the Mesozoic: 230 mya: First dinosaurs 100 mya: Flowering plants begin to dominate the land Highlights from the Cenozoic: 65 mya: Extinction of dinosaurs and diversification of mammals 200,000 years ago: Appearance of anatomically modern humans 7.9 Earth s geology has had a profound impact on the history of life. The Earth s crust is composed of large tectonic plates floating atop a very hot layer of rock called the mantle. Plate movement continuously rearranges the geography of the continents. 7.9 The Earth is a dynamic planet. Geological upheavals can be catastrophic in the short term and can alter the evolution of life on Earth in the long term. Earthquakes, mountain building, volcanoes Changes in the Earth s geology over time have intertwined with the history of life on Earth Opening Questions: Lions and tigers! Oh, my! Imagine you are visiting the zoo, and in the Big Cats exhibit you see lions from Africa and tigers from Asia. We consider lions and tigers different species. Why? 7.10 What is a species? The word species is derived from a Latin word meaning appearance. However, appearance alone cannot be used to tell one species from another. In captivity, mating between lions and tigers may lead to hybrid ligers. Are ligers a species? Why or why not? 10

11 7.10 What is a species? 7.10 What is a species? The most commonly used definition of species is a population that is capable of interbreeding to produce healthy, fertile offspring. What types of species might not fit the definition above? Our earlier definition focused on interbreeding doesn t work for all species. Bacteria reproduce asexually. For extinct organisms, we can t know if they were capable of mating. For some organisms, we have to use appearance, or another means, to determine species groups Reproductive barriers maintain species. For species that we can define as a group of individuals capable of successfully interbreeding, what keeps them separate? One or more reproductive barriers prevent members of different species from breeding. What might prevent Eastern and Western meadowlarks from interbreeding? 7.10 Reproductive barriers maintain species. Behavioral isolation: Members of a species often identify each other through specific rituals. Mating time differences: Many species are able to reproduce only at specific times. Habitat isolation: If species live in slightly different habitats, they may never meet Reproductive barriers maintain species. Mechanical incompatibility: Members of different species often cannot mate because their anatomies are incompatible. Gametic incompatibility: The gametes (sperm and egg) of different species usually cannot fertilize each other. Hybrid weakness: Offspring of two species may be unfit, or they may be sterile Review Questions: Lions and tigers! Oh, my! Lions and tigers can produce a hybrid liger offspring. Does this mean that lions and tigers are the same species? Explain. 11

12 7.11 Opening Questions: How do we get new species? Imagine that a small flock of forest birds gets blown off course during a hurricane. The flock lands on a small, dry, and grassy island. You locate this population 300,000 years later. Do the current birds look the same or different from the original colonists? Draw pictures of your imagined birds. Do they have the same behaviors? Would you consider them the same species or different from the original species? Explain How do we get new species? Speciation occurs when one ancestral species evolves into one or more new species. Some event separates a population: Time, space, or genetics A B Original Population Populations then diverge along their own evolutionary path New species may form over long periods of time. In the graduated model, a species acquires small adaptations to its environment over millions of years New species may form relatively rapidly. In the punctuated equilibrium model, there are periods of stasis interrupted by occasional bursts of speciation. 530 million years ago during a period called the Cambrian explosion, the rate of evolution was an order of magnitude higher than the normal rate. (It still required millions of years.) 7.11 Geographic isolation may separate populations and lead to new species. Allopatric speciation may occur when a physical barrier isolates populations New species may arise due to other types of isolation, such as genetic. Sympatric speciation occurs with no geographic isolation. The Grand Canyon separated a single population into two. Speciation in plants can occur suddenly due to large-scale genetic changes. (There is no physical barrier.) 12

13 7.12 Opening Questions: What do we know about evolution anyway? Write a short response to the following question: What are at least three things you know that provide supporting evidence for evolution? 7.12 Taxonomy is the classification of life. Taxonomy is the identification, naming, and classification of species. All life is classified into one of three large groups called domains based on cell type Where does a tiger fit in the taxonomic hierarchy? Starting with a domain, every organism can be placed into the taxonomic hierarchy, an ordered series of progressively smaller categories. The hierarchy ends with the species name. Panthera tigris 7.12 Where does a tiger fit in the taxonomic hierarchy? Domain Kingdom Phylum Class Order Family Genus 7.12 Where does a tiger fit in the taxonomic hierarchy? are identified using the last two groups in the hierarchy: Genus species. This binomial (two-part) is sometimes called a scientific name. Panthera tigris 7.13 Opening Questions: How can we map our ancestry? Sketch out a quick family lineage for your immediate family. How far back can you go? Your grandparents? Great-grandparents? Great-great-grandparents? What shape best describes your family sketch? Explain. We often refer to our family tree when discussing our ancestry. Why might trees be a useful term to represent relationships? 13

14 7.13 Evolutionary relationships may be represented by branching trees. Phylogenetic trees are one way to reflect the evolutionary history of organisms. Phylogenetic trees present a hypothesis about the evolutionary history of related species. A B C Time 7.13 Clades can be thought of representing a branch on the tree of life. A clade is any group of species that consists of an ancestral species and all its descendants. The analysis of clades is called cladistics Reading phylogenetic trees can provide insights into the interrelationships of life. The tips of the tree represent groups of the most recently evolved species. To determine how closely related two species are, find their most recent common ancestor. A B C Time 7.13 Review Question: Which species is most closely related to the polar bear? 14