Chapter 22 Descent with Modification: A Darwinian View of Life

Darwin made two major points in his book: Many current species are descendants of ancestral species Natural selection is a mechanism for this evolutionary process Darwin’s theory of evolution by natural selection integrates diverse areas of biological study and stimulates many new research questions

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 Video: Grand Canyon Video: Grand Canyon

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

Lamarck’s Theory of Evolution Lamarck hypothesized that species evolve through use and disuse and the inheritance of acquired traits The mechanisms he proposed are unsupported by evidence Rats and giraffs

LE 22-6 Cactus eater. The long, sharp beak of the cactus ground finch (Geospiza scandens) helps it tear and eat cactus flowers and pulp. Seed eater. The large ground finch (Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground. Insect eater. The green warbler finch (Certhidea olivacea) used its narrow, pointed beak to grasp insects.

The Origin of Species Darwin developed two main ideas: Evolution explains life’s unity and diversity Natural selection is a cause of adaptive evolution The phrase descent with modification summarized Darwin’s perception of the unity of life The phrase refers to the view 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 branches representing life’s diversity

LE 22-7 Hyracoidea (Hyraxes) Sirenia (Manatees and relatives) Years ago 0 10, Millions of years ago Moeritherium Barytherium Deinotherium Mammut Platybelodon Stegodon Mammuthus Elephas maximus (Asia) Loxodonta africana (Africa) Loxodonta cyclotis (Africa)

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 Observation #2: 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

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

Artificial Selection In artificial selection, humans have modified other species over many generations by selecting and breeding individuals with desired traits

LE Cabbage Flower clusters Terminal bud Lateral buds Brussels sprouts Leaves Kale Stem Kohlrabi Wild mustard Broccoli Cauliflower Flowers and stems

Summary of Natural Selection Natural selection is differential success in reproduction from interaction between individuals that vary in heritable traits and their environment Natural selection produces an increase over time in adaptation of organisms to their environment If an environment changes over time, natural selection may result in adaptation to these new conditions Video: Seahorse Camouflage Video: Seahorse Camouflage

LE A flower mantid in Malaysia A stick mantid in Africa

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: the effect of differential predation on guppy populations and the evolution of drug- resistant HIV Researchers have observed natural selection leading to adaptive evolution in guppy populations- Differential Predation in Guppy Populations

LE 22-12a Predator: Killifish; preys mainly on small guppies Guppies: Larger at sexual maturity than those in “pike-cichlid pools” Experimental transplant of guppies Predator: Pike-cichlid; preys mainly on large guppies Guppies: Smaller at sexual maturity than those in “killifish pools” Pools with killifish but no guppies prior to transplant

LE 22-12b Control population: Guppies from pools with pike-cichlids as predators Experimental population: Guppies transplanted to pools with killifish as predators 92.3 Females Age of guppies at maturity (days) Males Females Mass of guppies at maturity (mg) Males

The Evolution of Drug- Resistant HIV The use of drugs to combat HIV selects for viruses resistant to these drugs The ability of bacteria and viruses to evolve rapidly poses a challenge to our society

LE Patient No. 3 Patient No. 2 Patient No Weeks Percent of HIV resistant to 3TC

Homology, Biogeography, and the Fossil Record Evolutionary theory provides a cohesive explanation for many kinds of observations Homology is similarity resulting from common ancestry-Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor

LE Human Cat Whale Bat

Comparative embryology reveals anatomical homologies not visible in adult organisms

Vestigial organs are remnants of structures that served important functions in the organism’s ancestors

Molecular Homologies Examples of homologies at the molecular level are genes 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 homologies Anatomical resemblances among species are generally reflected in their molecules, genes, and gene products

LE Percent of Amino Acids That Are Identical to the Amino Acids in a Human Hemoglobin Polypeptide 100% 95% 87% 69% 54% 14% Rhesus monkey Species Human Mouse Chicken Frog Lamprey

Biogeography Darwin’s observations of biogeography, the geographic distribution of species, formed an important part of his theory of evolution Some similar mammals that have adapted to similar environments have evolved independently from different ancestors

LE Sugar glider Flying squirrel NORTH AMERICA AUSTRALIA

The Fossil Record 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