Chapter 17:The Evolution of Life

Principles of Science II

This lecture will help you understand:

• The Origin of Life• Did Life on Earth Originate on Mars?• Early Life on Earth• Charles Darwin and The Origin of Species• How Natural Selection Works• Adaptation• Staying Warm and Keeping Cool• Evolution and Genetics• How Species Form• Evidence of Evolution• Fossils: Earth's Tangible Evidence of Evolution• The Evolution of Humans• History of Science: The Peppered Moth• Science and Society: Antibiotic-Resistant Bacteria

The Origin of Life

• For thousands of years, people believed that life was created through spontaneous generation, the sudden emergence of living organisms from nonliving materials.

The Origin of Life

• Louis Pasteur demonstrated that life did not arise from nonlife.

The Origin of Life

• Life originated billions of years ago on a young Earth very different from the Earth of today.

• The Miller and Urey experiment modeled the young Earth's atmosphere andoceans.

• When Miller and Urey shotelectric sparks through the modelto simulate lightning, complexorganic molecules formed.

The Origin of Life

• There are two alternative hypotheses for the origin of organic molecules on Earth:– Organic molecules came to Earth on incoming

meteorites.

– Organic molecules were synthesized in deep- sea environments.

• But, how do you get from organic molecules to cells?

The Origin of Life

• Liposomes form spontaneously when certain lipids are added to water. Liposomes– have double membranes similar to cell

membranes.

– can grow, shrink, and divide.– run chemical reactions

inside their membranes.

– control what moleculesmove into and out of them.

The Origin of Life

• Living cells also need genes.

• Scientists now believe that the first genes were made of RNA, not DNA.– Short strands of RNA can spontaneously

assemble from individual RNA nucleotides.

– RNA can replicate by itself.• Liposomes may have captured RNA "genes,"

forming the first primitive cells on Earth.

Did Life on Earth Originate on Mars?

• In 1996, scientists found what resembled tiny, fossilized bacteria in a Martian meteorite.

• Mars was once a much warmer and wetter planet.

• The supposed fossils may be too small to be fossilized bacteria.

• NASA continues to explore the possibility of life on Mars.

Early Life on Earth

• The origin of autotrophs– Earth's earliest living organisms

• were marine prokaryotes.• lived with no free oxygen.• were heterotrophs that obtained energy and food from

outside sources.

– The evolution of autotrophs, organisms able to convert inorganic molecules into food and organic molecules, was a key event in the history of life.

• Photosynthesizers use energy from the Sun.• Chemoautotrophs use energy from certain inorganic

chemicals.

Early Life on Earth

• The oxygenation of the atmosphere– Cyanobacteria are photosynthetic bacteria that

produce oxygen as a by-product of photosynthesis.

Early Life on Earth

• The first eukaryotes– Eukaryotes appeared about 2 billion years ago.– The nucleus and most organelles most likely

originated from inward foldings of the cell membrane.– According to the endosymbiotic theory, mitochondria

and chloroplasts evolved from prokaryotes living inside the earliest eukaryotic cells.

Charles Darwin and The Origin of Species

• For thousands of years, people believed that species on Earth did not change.

• But then what are fossils?

Charles Darwin and The Origin of Species

• Jean Baptiste Lamarck argued that modern species were descended from ancestors that had evolved to become better adapted to their environments.

• Lamarck believed that organisms acquired new traits over their lifetimes and then passed these traits to their offspring. He called this the inheritance of acquired characteristics.

Charles Darwin and The Origin of Species

• Charles Darwin argued that evolution—inherited changes in populations of organisms over time—had produced all the living forms on Earth.

Charles Darwin and The Origin of Species

• Darwin was inspired by the observations he made as the naturalist aboard the H.M.S. Beagle as it sailed around South America from 1831 to 1836.

• For example, the 13 species of finches on the Galápagos Islands varied in beak size and shape, depending on their diet. How had the beaks of the finches come to vary in this way?

Charles Darwin and The Origin of Species

• Darwin was also inspired by

– the work of geologist Charles Lyell, who argued that Earth's geologic features were built by gradual processes over millennia.

– the work of economist Thomas Malthus, who wrote about famine and the struggle of humans over resources.

How Natural Selection Works

• In any population of organisms, individuals have many traits that show variation.

• Traits that are determined at least partly by genes are heritable, passed from parents to offspring.

• Organisms that possess variable heritable traits that are advantageous leave more offspring than organisms without the advantageous traits. This is natural selection.

• Advantageous traits become more common in a population, resulting in adaptation.

How Natural Selection Works

Adaptation

• Adaptations are traits that make organisms well suited to living and reproducing in their environments.

• Adaptations can relate to various aspects of an organism's life:– Survival adaptations– Sexual selection, which leads to the evolution of

adaptations that help organisms acquire mates

– Bearing and raising young

Staying Warm and Keeping Cool

• Mammals thermoregulate, maintaininga fairly constant body temperature.

• The surface-area-to-volume ratio is animportant factor in thermoregulation.It affects the size and shape of animals.– Size: Animals found in cold habitats

are often larger than related speciesliving in warm habitats.

– Shape: Animals found in coldhabitats tend to have short legs andsmall ears. Animals found in warmhabitats have long legs and largeears.

Evolution and Genetics

• The incorporation of modern genetics into Darwin's theory of evolution took place in the middle of the 20th century.

• This led to a focus on evolution as changes in the allele frequencies of genes over time.

• Allele frequencies describe how common different alleles for a gene are in the population.

Evolution and Genetics

• Mechanisms of Evolution

– Natural selection• A specific allele may be advantageous and

allow organisms with the allele to reproduce more than organisms with a different allele.

• More copies of the advantageous allele are passed to the next generation, and the frequency of the advantageous allele increases in the population.

Evolution and Genetics

• Mechanisms of Evolution (continued)– Mutation pressure

• A genetic mutation may be more likely to turn one allele into a second allele than vice versa.

– Genetic drift• Genetic drift is the evolution of populations due

to chance.

• Genetic drift is particularly important in small populations.

Evolution and Genetics

• Mechanisms of Evolution (continued)

– Gene flow

• Changes in allele frequency result from the movement of alleles into or out of a population, such as through migration.

How Species Form

• A species is a group of organisms whose members can breed with one another but not with members of other species.

• Speciation is the formation of new species.• The key to speciation is the evolution of

reproductive barriers that prevent two groups of organisms from interbreeding.

How Species Form

• There are two kinds of reproductive barriers:– Prezygotic reproductive barriers prevent

individuals of different species from mating or prevent fertilization from occurring if they do mate.

– Postzygotic reproductive barriers occur when mating results in hybrids that do not survive or are sterile, unable to reproduce.

How Species Form

• Two ways speciation can occur:– In allopatric speciation, speciation occurs after a

geographic barrier divides a population into two isolated populations.

How Species Form

• Two ways speciation can occur (continued):– In sympatric speciation, speciation occurs without

a geographic barrier.

• Examples: hybridization and chromosomal changes such as polyploidy

Evidence of Evolution

• Observations of natural selection in action– Resistance to myxoma virus in Australian rabbits– Peppered moth coloration and camouflage

– Antibiotic-resistant bacteria

– Evolution of the beaks of Darwin's finches after a drought

– Many other documented examples

Evidence of Evolution

• Artificial selection– In domesticated animals and crops, such as

dogs, racehorses, and corn

• Similarities in body structures

Evidence of Evolution

• Vestigial organs

• DNA and molecular evidence– Similar DNA sequences in related species

• Patterns of development

Evidence of Evolution

• Hierarchical organization of living things– Nested groups, or "groups within groups"

• Biogeography

– The study of how species are distributed on Earth

– Organisms evolved in a certain place and then left descendants in the places where they were able to spread.

Fossils: Earth's Tangible Evidence of Evolution

• Fossils allow us to follow the evolution of certain groups of organisms over time.

• The evolution of the whale blowhole

Fossils: Earth's Tangible Evidence of Evolution

• The evolution and loss of whale hind legs

Fossils: Earth's Tangible Evidence of Evolution

• Archaeopteryx, an early bird

The Evolution of Humans

• Humans are– primates—we share an ancestry with monkeys

and apes.

– hominids—the primate group that includes Homo sapiens and extinct relatives.

The Evolution of Humans

• The fossil record has allowed us to document some aspects of human evolution.

History of Science: The Peppered Moth

History of Science: The Peppered Moth

• Peppered moths in England had always been light with a scattering of peppery flakes. This made them well camouflaged from birds (their main predators) in a habitat of lichen-covered trees.

• During the Industrial Revolution, pollution caused trees to become darkened with soot.

• More and more dark moths were seen in the peppered moth population.

• After antipollution laws were passed, the soot disappeared.

• Light moths increased in number. The dark moths have all but disappeared.

History of Science: The Peppered Moth

• Kettlewell's experiments confirmed that natural selection was responsible for changes in coloration in peppered moth populations.

• Kettlewell released and recaptured marked moths. He recaptured more dark moths in polluted habitats and more light moths in unpolluted habitats.

• Kettlewell also placed moths on trees and filmed birds eating the moths. Birds ate more light moths in polluted habitats and more dark moths in unpolluted habitats.

• Challenges to Kettlewell's work resulted in his experiments being repeated by Michael Majerus. Kettlewell's results were confirmed.

Science and Society: Antibiotic-Resistant Bacteria

• Antibiotic resistance has become a serious health issue.

• Antibiotic resistance is the result of natural selection.• When a patient takes a course of antibiotics, a few

naturally resistant bacteria may survive the treatment. These reproduce. Eventually, strains of bacteria exist that cannot be controlled by the antibiotic.

• All antibiotic use contributes to antibiotic resistance.• We must learn to take antibiotics only for bacterial

infections, and then to complete the course of treatment.

• Antibiotics should not be used to promote the growth of livestock.

Science and Society: Antibiotic-Resistant Bacteria

• Ways to slow the development of antibiotic resistance in bacteria:

– Take antibiotics only when needed.– Take the entire course of antibiotics.– Use antibiotics responsibly in agriculture.