Descent with Modification: A Darwinian View of Life Chapter 22 Descent with Modification: A Darwinian View of Life

Overview: Darwin Introduces a Revolutionary Theory A new era of biology began on November 24, 1859 The day Charles Darwin published On the Origin of Species by Means of Natural Selection

The Origin of Species Focused biologists’ attention on the great diversity of organisms Figure 22.1

Darwin made two major points in his book He presented evidence that the many species of organisms presently inhabiting the Earth are descendants of ancestral species He proposed a mechanism for the evolutionary process, natural selection

In order to understand why Darwin’s ideas were revolutionary Concept 22.1: The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species In order to understand why Darwin’s ideas were revolutionary We need to examine his views in the context of other Western ideas about Earth and its life

The historical context of Darwin’s life and ideas Figure 22.2 Linnaeus (classification) Hutton (gradual geologic change) Lamarck (species can change) Malthus (population limits) Cuvier (fossils, extinction) Lyell (modern geology) Darwin (evolution, nutural selection) Mendel (inheritance) Wallace (evolution, natural selection) 1750 American Revolution French Revolution U.S. Civil War 1800 1850 1900 1795 Hutton proposes his theory of gradualism. 1798 Malthus publishes “Essay on the Principle of Population.” 1809 Lamarck publishes his theory of evolution. 1830 Lyell publishes Principles of Geology. 1831–1836 Darwin travels around the world on HMS Beagle. Darwin begins his notebooks on the origin of species. 1837 Darwin writes his essay on the origin of species. 1844 Wallace sends his theory to Darwin. 1858 The Origin of Species is published. 1859 Mendel publishes inheritance papers. 1865

Resistance to the Idea of Evolution The Origin of Species Shook the deepest roots of Western culture Challenged a worldview that had been prevalent for centuries

The Scale of Nature and Classification of Species The Greek philosopher Aristotle Viewed species as fixed and unchanging The Old Testament of the Bible Holds that species were individually designed by God and therefore perfect

Carolus Linnaeus Interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose Was a founder of taxonomy, classifying life’s diversity “for the greater glory of God”

Fossils, Cuvier, and Catastrophism The study of fossils Helped to lay the groundwork for Darwin’s ideas Fossils are remains or traces of organisms from the past Usually found in sedimentary rock, which appears in layers or strata Figure 22.3

Paleontology, the study of fossils Was largely developed by French scientist Georges Cuvier Cuvier opposed the idea of gradual evolutionary change And instead advocated catastrophism, speculating that each boundary between strata represents a catastrophe

Theories of Gradualism Is the idea that profound change can take place through the cumulative effect of slow but continuous processes

Geologists Hutton and Lyell Perceived that changes in Earth’s surface can result from slow continuous actions still operating today Exerted a strong influence on Darwin’s thinking

Lamarck’s Theory of Evolution Lamarck hypothesized that species evolve Through use and disuse and the inheritance of acquired traits But the mechanisms he proposed are unsupported by evidence Figure 22.4

As the 19th century dawned Concept 22.2: In The Origin of Species, Darwin proposed that species change through natural selection As the 19th century dawned It was generally believed that species had remained unchanged since their creation, but a major change would challenge this thinking

As a boy and into adulthood, Charles Darwin Darwin’s Research As a boy and into adulthood, Charles Darwin Had a consuming interest in nature Soon after Darwin received his B.A. degree He was accepted on board the HMS Beagle, which was about to embark on a voyage around the world

The Voyage of the Beagle During his travels on the Beagle Darwin observed and collected many specimens of South American plants and animals Darwin observed various adaptations of plants and animals That inhabited many diverse environments

Darwin’s interest in the geographic distribution of species Was kindled by the Beagle’s stop at the Galápagos Islands near the equator west of South America Figure 22.5 England EUROPE NORTH AMERICA Galápagos Islands Darwin in 1840, after his return SOUTH Cape of Good Hope Cape Horn Tierra del Fuego AFRICA HMS Beagle in port AUSTRALIA Tasmania New Zealand PACIFIC OCEAN Andes ATLANTIC

Darwin’s Focus on Adaptation As Darwin reassessed all that he had observed during the voyage of the Beagle He began to perceive adaptation to the environment and the origin of new species as closely related processes

From studies made years after Darwin’s voyage Biologists have concluded that this is indeed what happened to the Galápagos finches Figure 22.6a–c (a) Cactus eater. The long, sharp beak of the cactus ground finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp. (c) Seed eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground. (b) Insect eater. The green warbler finch (Certhidea olivacea) uses its narrow, pointed beak to grasp insects.

In June 1858 Darwin received a manuscript from Alfred Russell Wallace In 1844, Darwin wrote a long essay on the origin of species and natural selection But he was reluctant to introduce his theory publicly, anticipating the uproar it would cause In June 1858 Darwin received a manuscript from Alfred Russell Wallace Who had developed a theory of natural selection similar to Darwin’s Darwin quickly finished The Origin of Species And published it the next year

Darwin developed two main ideas The Origin of Species Darwin developed two main ideas Evolution explains life’s unity and diversity Natural selection is a cause of adaptive evolution

Descent with Modification The phrase descent with modification Summarized Darwin’s perception of the unity of life States that all organisms are related through descent from an ancestor that lived in the remote past

In the Darwinian view, the history of life is like a tree With multiple branchings from a common trunk to the tips of the youngest twigs that represent the diversity of living organisms Figure 22.7 Hyracoidea (Hyraxes) Sirenia (Manatees and relatives) Years ago Millions of years ago Deinotherium Mammut Stegodon Mammuthus Platybelodon Barytherium Moeritherium Elephas maximus (Asia) Loxodonta africana (Africa) cyclotis

Natural Selection and Adaptation Evolutionary biologist Ernst Mayr Has dissected the logic of Darwin’s theory into three inferences based on five observations

Observation #1: For any species, population sizes would increase exponentially If all individuals that are born reproduced successfully Figure 22.8

Observation #2: Nonetheless, populations tend to be stable in size Except for seasonal fluctuations Observation #3: Resources are limited Inference #1: Production of more individuals than the environment can support Leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

Observation #4: Members of a population vary extensively in their characteristics No two individuals are exactly alike Figure 22.9

Observation #5: Much of this variation is heritable Inference #2: Survival depends in part on inherited traits Individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals

Inference #3: This unequal ability of individuals to survive and reproduce Will lead to a gradual change in a population, with favorable characteristics accumulating over generations

In the process of artificial selection Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits Figure 22.10 Terminal bud Lateral buds Brussels sprouts Cabbage Flower cluster Leaves Cauliflower and stems Broccoli Wild mustard Kohlrabi Stem Kale

Summary of Natural Selection Natural selection is differential success in reproduction That results from the interaction between individuals that vary in heritable traits and their environment

In the adaptation of organisms to their environment Natural selection can produce an increase over time In the adaptation of organisms to their environment Figure 22.11 (a) A flower mantid in Malaysia (b) A stick mantid in Africa

If an environment changes over time Natural selection may result in adaptation to these new conditions

Concept 22.3: Darwin’s theory explains a wide range of observations Darwin’s theory of evolution Continues to be tested by how effectively it can account for additional observations and experimental outcomes

Natural Selection in Action Two examples Provide evidence for natural selection

Differential Predation in Guppy Populations Researchers have observed natural selection Leading to adaptive evolution in guppy populations Reznick and Endler transplanted guppies from pike-cichlid pools to killifish pools and measured the average age and size of guppies at maturity over an 11-year period (30 to 60 generations). EXPERIMENT Pools with killifish, but not guppies prior to transplant Experimental transplant of guppies Predator: Killifish; preys mainly on small guppies Guppies: Larger at sexual maturity than those in “pike-cichlid pools” Predator: Pike-cichlid; preys mainly on large guppies Guppies: Smaller at sexual maturity than those in “killifish pools” Figure 22.12

Control Population: Guppies from pools with pike-cichlids as predators RESULTS After 11 years, the average size and age at maturity of guppies in the transplanted populations increased compared to those of guppies in control populations. 161.5 185.6 67.5 Weight of guppies at maturity (mg) Age of guppies at maturity (days) 92.3 48.5 Control Population: Guppies from pools with pike-cichlids as predators Experimental Population: Guppies transplanted to pools with killifish as predators 76.1 Males Females 85.7 58.2 CONCLUSION Reznick and Endler concluded that the change in predator resulted in different variations in the population (larger size and faster maturation) being favored. Over a relatively short time, this altered selection pressure resulted in an observable evolutionary change in the experimental population.

The Evolution of Drug-Resistant HIV In humans, the use of drugs Selects for pathogens that through chance mutations are resistant to the drugs’ effects Natural selection is a cause of adaptive evolution

Percent of HIV resistant to 3TC Researchers have developed numerous drugs to combat HIV But using these medications selects for viruses resistant to the drugs Patient No. 1 Patient No. 2 Percent of HIV resistant to 3TC Patient No. 3 Weeks Figure 22.13

The ability of bacteria and viruses to evolve rapidly Poses a challenge to our society

Homology, Biogeography, and the Fossil Record Evolutionary theory Provides a cohesive explanation for many kinds of observations

Homology Homology Is similarity resulting from common ancestry

Anatomical Homologies Homologous structures between organisms Are anatomical resemblances that represent variations on a structural theme that was present in a common ancestor Figure 22.14 Human Cat Whale Bat

Comparative embryology Reveals additional anatomical homologies not visible in adult organisms Figure 22.15 Pharyngeal pouches Post-anal tail Chick embryo Human embryo

Vestigial organs Are some of the most intriguing homologous structures Are remnants of structures that served important functions in the organism’s ancestors

Molecular Homologies Biologists also observe homologies among organisms at the molecular level Such as genes that are shared among organisms inherited from a common ancestor

Homologies and the Tree of Life The Darwinian concept of an evolutionary tree of life Can explain the homologies that researchers have observed

Anatomical resemblances among species Are generally reflected in their molecules, their genes, and their gene products Figure 22.16 Species Human Rhesus monkey Mouse Chicken Frog Lamprey 14% 54% 69% 87% 95% 100% Percent of Amino Acids That Are Identical to the Amino Acids in a Human Hemoglobin Polypeptide

Biogeography Darwin’s observations of the geographic distribution of species, biogeography Formed an important part of his theory of evolution

Some similar mammals that have adapted to similar environments Have evolved independently from different ancestors Sugar glider AUSTRALIA NORTH AMERICA Flying squirrel Figure 22.17

The succession of forms observed in the fossil record Is consistent with other inferences about the major branches of descent in the tree of life

The Darwinian view of life Predicts that evolutionary transitions should leave signs in the fossil record Paleontologists Have discovered fossils of many such transitional forms Figure 22.18

What Is Theoretical about the Darwinian View of Life? In science, a theory Accounts for many observations and data and attempts to explain and integrate a great variety of phenomena

Darwin’s theory of evolution by natural selection Integrates diverse areas of biological study and stimulates many new research questions