The Diversity of LifeHumans share the Earth with millions of

other kinds of organisms of every imaginable shape, size, and habitat.

• The process by which modern organisms have descended from ancient organisms is evolution.

• Evolution is the change in populations over time.

• Many explanations about how species evolve have been proposed, but the ideas first published by Charles Darwin are the basis of modern evolutionary theory.

• A theory is a well-supported testable explanation of phenomena that have occurred in the natural world.

Darwin on HMS Beagle

• It took Darwin years to develop his theory of evolution.

• He began in 1831 at age 22 when he took a job as a naturalist on the English ship HMS Beagle, which sailed around the world on a five-year scientific journey.

• Darwin studied and collected biological and fossil specimens from several continents and many remote islands.

• His observations and data led Darwin to propose a revolutionary hypothesis about the way life changes over time.

Darwin in the Galápagos• On the Galápagos Islands, Darwin studied

many species of animals and plants that are unique to the islands but similar to species elsewhere.

• The Galapagos Islands were unique in that although they were close together, each island had very different climates.

• Darwin observed that the characteristics of many animals and plants varied noticeably among the different islands of the Galapagos.

• Many of the fossils that Darwin discovered resembles living organisms but were not identical to them.

• The glyptodon, an extinct animal known only from fossil remains is an ancient relative of the armadillo of South America.

Giant Tortoises of the Galápagos Islands• Among the tortoises, the shape of the shell corresponds to different habitats. • The Hood Island tortoise (right) has a long neck and a shell that is curved and

open around the neck and legs, allowing the tortoise to reach the sparse vegetation on Hood Island.

• The tortoise from Isabela Island (lower left) has a dome-shaped shell and a shorter neck. Vegetation on this island is more abundant and closer to the ground.

• The tortoise from Pinta Island has a shell that is intermediate between these two forms.

• These observations led Darwin to consider the possibility that species can change over time.

Influences on Darwin’s Work• Influenced by the work of many others, Darwin worked to refine his

explanation for how species change over time.

• Hutton and Lyell helped scientists recognize that Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present.

– This led Darwin to think that if the Earth could change over time the organisms on it probably changed over time as well.

– Also, if it took the Earth many, many years for life to change, then the Earth must be extremely old.

• An English economist Thomas Malthus believed that the human population grows faster than Earth’s food supply.

– Applying this concept to biology, Darwin knew many species produce large numbers of offspring and that these species had not overrun Earth.

– He concluded that individuals struggle to compete in changing environmental conditions.

– Also, only some individuals survive the competition and produce offspring.

• Unfortunately not all scientists that influenced Darwin were completely accurate in their beliefs.

Acquired Characteristics and Use/Disuse

• Lamarck was one of the first to develop a theory about evolution and realize that organisms adapted to their environment.

• He believed that selective use or disuse of a body part would cause it to change and that this change would be passed on to offspring.

1) The male crab uses its small claws in front to attract mates and ward off predators.

2) Lamarck believed that because it was used repeatedly it became larger.

3) According to Lamarck’s theory , the acquired characteristic, a larger claw, would be passed on to the crab’s offspring.

This theory has been shown to be incorrect!

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Inherited Variation and Natural Selection

• Darwin observed that the traits of individuals vary in populations.

• Variations are then inherited.

• Breeding organisms with specific traits in order to produce offspring with identical traits is called artificial selection

• Darwin hypothesized that there was a force in nature that worked like artificial selection.

• Natural selection is a mechanism for change in populations.

– It occurs when organisms with favorable variations survive, reproduce, and pass their variations to the next generation.

– Organisms without these variations are less likely to survive and reproduce.

– As a result, each generation consists largely of offspring from parents with these variations that aid survival.

• Alfred Russell Wallace, another British naturalist, reached a similar conclusion.

Darwin explains natural selection

• Darwin proposed the idea of natural selection to explain how species change over time.

• In any population, individuals have variations. Fishes, for example, may differ in color, size, and speed.

Darwin explains natural selection

• Individuals with certain useful variations, such as speed, survive in their environment, passing those variations to the next generation.

• Over time, offspring with certain variations make up most of the population and may look entirely different from their ancestors.

Adaptations: Evidence for Evolution

• Recall that an adaptation is any variation that aids an organism’s chances of survival in its environment.

• Darwin’s theory of evolution explains how adaptations may develop in species.

Structural adaptations arise over time• According to Darwin’s theory, adaptations

in species develop over many generations.• Learning about adaptations in mole-rats

can help you understand how natural selection has affected them.

• The ancestors of today’s common mole-rats probably resembled African rock rats.

• Some ancestral rats may have avoided predators better than others because of variations such as the size of teeth and claws.

Structural adaptations arise over time• Ancestral rats that

survived passed their variations to offspring.

• After many generations, most of the population’s individuals would have these adaptations.

• Over time, natural selection produced modern mole-rats.

• Their blindness may have evolved because vision had no survival advantage for them.

Structural adaptations arise over time• Some other structural adaptations are subtle.• Mimicry is a structural adaptation that enables one species

to resemble another species.• In one form of mimicry, a harmless species has adaptations

that result in a physical resemblance to a harmful species.

• Predators that avoid the harmful looking species also avoid the similar-looking harmless species.

• In another form of mimicry, two or more harmful species resemble each other.

• For example, yellow jacket hornets, honeybees, and many other species of wasps all have harmful stings and similar coloration and behavior.

Structural adaptations arise over time• Predators may

learn quickly to avoid any organism with their general appearance.

• Another subtle adaptation is camouflage, an adaptation that enables species to blend with their surroundings.

• Because well-camouflaged organisms are not easily found by predators, they survive to reproduce.

Physiological adaptations can develop rapidly• In general, most structural adaptations develop over millions of years.

• However, there are some adaptations that evolve much more rapidly.

• Physiological adaptations are changes in an organism’s metabolic processes.

• For example, do you know that some of the medicines developed during the twentieth century to fight bacterial diseases are no longer effective?

1) The bacteria in a population vary in their ability to resist antibiotics.

2) When the population is exposed to an antibiotic, only the resistant bacteria survive.

3) The resistant bacteria live and produce more resistant bacteria.

• Today, penicillin no longer affects as many species of bacteria because some species have evolved physiological adaptations to prevent being killed by penicillin.

• In addition to species of bacteria, scientists have observed these adaptations in species of insects and weeds that are pests.

Non-resistant bacterium

Resistant bacterium

Antibiotic

Other Evidence for Evolution• Physiological resistance in species of bacteria, insects, and plants is direct

evidence of evolution.

• However, most of the evidence for evolution is indirect, coming from sources such as fossils and studies of anatomy, embryology, and biochemistry.

includes

Evidence of Evolution

Physical remains of organisms

Common ancestral species

Similar genes Similar genes

which is composed of which indicates which implies which implies

The fossil recordGeographic

distribution of living species

Homologous body structures

Similaritiesin early

development

Fossils- Clues to the Past• About 95 percent of the species that have

existed are extinct—they no longer live on Earth.

• Fossils, which come in many forms such as leaf imprint, a worm burrow, or a bone, are an important source of evolutionary evidence because they provide a record of early life and evolutionary history.

• For example, fossils can help to predict whether an area had been a river environment, terrestrial environment, or a marine environment. They also provide information on ancient climate.

• Paleontologists are detectives of the past that study ancient life and events

• By studying fossils, scientists learn about the diversity of life and about the behavior of ancient organisms.

• Although the fossil record provides evidence that evolution occurred, the record is incomplete.

Camel Evolution

• For example, you can see how paleontologists have charted the evolutionary path that led to today’s camel after piecing together fossil skulls, teeth, and limb bones.

Camel EvolutionCamel EvolutionAge

Organism

Skull and teeth

Paleocene 65 million years ago

Eocene 54 million years ago

Oligocene 33 million years ago

Limb bones

Miocene 23 million years ago

Present

Comparing Relative and Absolute Dating of Fossils Chart

Relative Dating

Can determine

Is performed by

Drawbacks

Absolute Dating

Comparing Relative and Absolute Dating of Fossils

Imprecision and limitations of age data

Difficulty of radioassay laboratory methods

Comparing depth of a fossil’s source stratum to the position of a reference fossil or rock

Determining the relative amounts of a radioactive (C, K) isotope and nonradioactive isotope in a specimen

Age of fossil with respect to another rock or fossil (that is, older or younger)

Age of a fossil in years

Water carries small rock particles to lakes and seas.

Dead organisms are buried by layers of sediment, which forms new rock.

The preserved remains may later be discovered and studied.

The Fossil Record• Earth’s history is divided into the geologic time scale, based on

evidence in rocks and fossils.• The four major divisions in the geologic time scale are the

Precambrian, Paleozoic Era, Mesozoic Era, and Cenozoic Era. • The eras are further divided into periods.

• By comparing older rock layers (near the bottom) with fossils from younger layers (near the top), scientists can document the fact that life on earth has changed over time.

Movement of the Earth’s Crust

Sea level

Sedimentary rocks form in horizontal layers.

When part of Earth’s crust is compressed, a bend in a rock forms, tilting the rock layers.

As the surface erodes due to water, wind, waves, or glaciers, the older rock surface is exposed.

New sediment is then deposited above the exposed older rock surface.

Geographic Distribution of Living Species

• Darwin realized that similar animals in different locations were a result of evolutionary descent.

• Even though animals were on different continents, if they were living under similar ecological conditions, they were exposed to similar pressures of natural selection.

• Due to the similar selective pressures, different animals ended up evolving striking features in common.

Beaver

NORTH AMERICA

Muskrat

Capybara SOUTH AMERICA

Coypu

Beaver

Muskrat

Beaver andMuskrat

Coypu

Capybara

Coypu andCapybara

Homologous Body Structures

• Structural features with a common evolutionary origin are called homologous structures.

• Homologous structures can be similar in arrangement, in function, or in both.

Turtle Alligator Bird Mammal

Ancient lobe-finned fish

Homologous Structures• Although homologous

structures show common ancestry, each organism’s limbs developed according to their environmental niche.

• Structures were modified due to adaptations in their individual surroundings over time.

• Bird bones show an adaptation to flying that the bones of the flightless organisms, though homologous, do not have.

• Bird bones have evolved to be delicate, lightweight, and elongated to make flight much easier.

Whale forelimb

Crocodileforelimb

Birdwing

Analogous Structures

• The body parts of organisms that do not have a common evolutionary origin but are similar in function are called analogous structures.

• Although analogous structures don’t shed light on evolutionary relationships, they do provide evidence of evolution.

• For example, insect and bird wings probably evolved separately when their different ancestors adapted independently to similar ways of life.

• The fangs of a rattlesnake and the fangs of a spider are analogous structures. They share the same function in each organism, to deliver venom, but the organisms do not share a common evolutionary origin.

• Analogous structures show the way dissimilar organisms adapted independently to similar ways of life by developing functionally similar structures.

Vestigial Structures• Another type of body feature that suggests an evolutionary relationship is a

vestigial structure—a body structure in a present-day organism that no longer serves its original purpose, but was probably useful to an ancestor.

• A structure becomes vestigial when the species no longer needs the feature for its original function, yet it is still inherited as part of the body plan for the species. Many organisms have vestigial structures.

• Vestigial structures, such as pelvic bones in the baleen whale, are evidence of evolution because they show structural change over time.– Pelvic bones are evidence that whales once possessed hind limbs. Since whales now

have no hind limbs, their loss must be the result of an evolutionary change.• In humans, the appendix is vestigial because

it carries out no function in digestion.• In some skinks legs have become vestigial.

– They are reduced because they no longer function because they are no longer used in walking.

Embryology• An embryo is the earliest

stage of growth and development of both plants and animals.

• The embryos of a fish, a reptile, a bird, and a mammal have a tail and pharyngeal pouches.

• It is the shared features in the young embryos that suggest evolution from a distant, common ancestor.

Fish Reptile Bird Mammal

Pharyngealpouches

Pharyngealpouches

Tail Tail

Summary of Darwin’s Theory

1) Individual organism’s differ, and some of this variation is heritable. 2) Organisms produce more offspring than can survive, and many that do

survive do not reproduce3) Since more organisms are produced than can survive, they compete for

limited resources. 4) Each organism has different advantages and disadvantages in the

struggle for existence. – Individuals best suited for their environment will survive and

reproduce more successfully. – These organisms pass their heritable traits to their offspring. Other

individuals die or leave fewer offspring. – This process of natural selection causes species to change over time.

5) Species alive today are descended with modifications from ancestral species that lived in the distant past. – This process by which diverse species evolved from common

ancestors, unites all organisms on Earth into a single tree of life.

Evidence of Evolution Flow Chart

includes

Evidence of Evolution

Physical remains of organisms

Common ancestral species

Similar genes Similar genes

which is composed of which indicates which implies which implies

The fossil recordGeographic

distribution of living species

Homologous body structures

Similaritiesin early

development

Biochemistry• Biochemistry also provides strong evidence for evolution.• Nearly all organisms share DNA, ATP, and many enzymes among

their biochemical molecules.

• One enzyme, cytochrome c, occurs in organisms as diverse as bacteria and bison.

• Biologists compared the differences that exist among species in the amino acid sequence of cytochrome c.

• The data show the number of amino acid substitutions in the amino acid sequences for the different organisms.

• Organisms that are biochemically similar have fewer differences in their amino acid sequences.

Comparison of OrganismsPercent Substitutions

of Amino Acids in Cytochrome c Residues

Two orders of mammals

Birds vs. mammals

Amphibians vs. birds

Fish vs. land vertebrates

Insects vs. vertebrates

Algae vs. animals

Biochemical Similarities of Organisms

5 and 10

8-12

14-18

18-22

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Interpreting Evidence after Darwin

• Since Darwin’s time, scientists have constructed evolutionary diagrams that show levels of relationships among species.

• In the 1970s, some biologists began to use RNA and DNA nucleotide sequences to construct evolutionary diagrams.

• Today, scientists combine data from fossils, comparative anatomy, embryology, and biochemistry in order to interpret the evolutionary relationships among species.