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Chapter 22 Descent with Modification: A Darwinian View of Life

Overview: Endless Forms Most Beautiful • Charles Darwin changed biology when he published The Origin of Species in 1859 • Fringe idea  mainstream acceptance How? Decades of evidence • People had talked before about new species forming. Darwin had a method and years of observations to support it.

The “Somebody would have figured it out” principle

Darwin noted that current species are descendants of ancestral species • Evolution can be defined by Darwin’s phrase descent with modification • Evolution can be viewed as both a pattern and a process I know nothing about this bug 

Concept 22.1: Darwin built on earlier ideas of intellectuals • Difference? He said how, and had evidence that difference changed everything

Scala Naturae and Classification of Species • Aristotle: arranged species on scala naturae (natural scale) • Thought species never changed • Linnaeus: taxonomy (classifying organisms) • Each species created individually for a specific purpose

Fossils: life now different from life in past • Fossils: remains or traces of past organisms • Found in strata (layers) • The study of fossils helped to lay the groundwork for Darwin’s ideas

Paleontology: the study of fossils • Georges Cuvier advocated catastrophism • The idea strata boundaries are formed by catastrophies

James Hutton and Charles Lyell: earth shaped by slow gradual processes • Uniformitarianism: the mechanisms of change are constant over time • Lyell’s book Principles of Geology strongly influenced Darwin’s thinking If the processes are slow, the earth must be old Lyell’s book convinced many scientists, not just Darwin

Lamarck: Evolution due to use and disuse • Lamarck: evolution = use, disuse, and inheritance of acquired characteristics • Not much evidence for this method (we think lamarck was wrong) • The book says “no” evidence

Concept 22.2: Descent with modification by natural selection • Early 1800’s: most people still thought species never changed • However, a few scientists were beginning to discuss the idea of new species forming

Darwin: liked to study nature (child adult) • Darwin studied @ Cambridge University: • started in medicine • switched to theology • Graduated took position on the Beagle as a naturalist, around the world,1831-1836

The Voyage of the Beagle: what did darwin do? • Collected South American plants and animals • Saw adaptations to many diverse environments • He read a lot • Lyell’s Principles of Geology: earth > 6000 years old • Malthus: people compete, some win, some loose

Galápagos Islands: many unique species He saw that these species had features that seem to be useful in their environment and their lifestyle Darwin explained this as adaptation due to “Natural Selection”

Fig. 22-6 (a) Cactus-eater (c) Seed-eater (b) Insect-eater

Vampire Finch!!!! • OK, it’s really called the “Sharp beaked ground finch”

Darwin had the idea, but would not publish • 1836: the Beagle trip ends • 1844: Darwin writes essay on species’ origin and natural selection • won’t publish or discuss publicly (probably wanted more evidence: fringemainstream) Discussed privately with other scientists • Showed essay to Joseph Hooker in 1847 • Wrote a letter to Asa Grey about it in 1857 • Wrote to Alfred Russell Wallace about it in 1857 Two times

Darwin had the idea, but would not publish So what was Darwin doing from 1836 until 1859? • Writing books on books on coral reefs, barnacles, geology became a famous scientist for this • Collects info and writes notes Gathering evidence for natural selection. (plans to publish someday)

Alfred Russell Wallace finds same thing 1855: Wallace sent essay to Darwin "On the Law which has regulated the Introduction of New Species“ Darwin wrote back saying that they were thinking similarly, and that he’d have a publication on the same topic within a few years

Alfred Russell Wallace and natural selection 1857: Wallace sent another essay to Darwin “On the Tendency of Varieties to Depart Indefinitely From the Original Type” Same idea’s as Darwin’s “Natural Selection” but didn’t use that term • Darwin quickly finished The Origin of Species and published it the next year • Didn’t want to get scooped

Alfred Russell Wallace had the same idea June 1858: Darwin gets this second paper on speciation from Alfred Russell Wallace Wallace asks Darwin to review, give to Lyell Darwin tells Lyell and Hooker that it is basically the same as what he was going to write Lyell, Hooker and Darwin present Wallace’s paper at meeting of scientific society (along with letters showing Darwin had the same idea earlier)

Why do we talk about Darwin, not Wallace? Wallace wrote a paper. Darwin wrote a book. 1859: Darwin publishes Origin of the Species What: species are different now than past How: same method as selective breeding Evidence: 20+ years of observations

Was Darwin a Jerk? Probably not It may look like it now, but probably not Famous naturalist: already had good reputation Wallace was an unknown Wealthy, respected family Wallace: a lot less money (less likely to be listened to in Victorian England) Could easily have taken all the credit Later helped Wallace find a job

The Origin of Species (1859) • Darwin developed two main ideas: • “Descent with modification” explains life’s unity and diversity • What we now call “evolution” • He didn’t use that term when writing book • Natural selection is a cause of adaptive evolution

Descent with Modification • The phrase ”descent with modification” • organisms are related: common ancestor in past Old, but uncommon, idea Zoonomia, Erasmus Darwin C. Darwin explained HOW, had EVIDENCE

Darwinian view of natural world • the history of life is like a tree with branches representing life’s diversity • Darwin’s theory can explain hierarchy of Linnaeus

Darwin: Natural Selection is like Artificial Selection • artificial selection: humans modify by selecting and breeding individuals with desired traits

Darwin: Natural Selection is like Artificial Selection • Darwin then described four observations • Variation exists • Traits can be inherited from parents • Too many offspring for environment • Competition to survive, not all will • Darwin made two inferences Will discuss in a second

Observation #1: Members of a population often vary greatly in their traits

Observation #2: Traits are inherited from parents to offspring Observation #3: All species are capable of producing more offspring than the environment can support • If all the spores grew up we’d live in mushroom world

We don’t live in mushroom world!!!! • Observation #4: Owing to lack of food or other resources, many of these offspring do not survive • Most spores die 

Darwin’s two inferences • Inference #1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals The ones that survive get to make babies!!!

Darwin’s two inferences • Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations • The ones with good traits live to breed, so good traits accumulate

Darwin applied Malthus to nature • Thomas Malthus: humans might reproduce so fast that they run out of food and resources • Good traits accumulate, and individuals with good traits are will become more common • This process explains the match between organisms and their environment • Wallace also based his ideas on Malthus!!!

Fig. 22-12 the match between organisms and their environment (a)A flower mantid in Malaysia (b)A stick mantid in Africa

Natural Selection: A Summary • Good traits = survival = live to breed • # of kids: how evolution keeps score • Good traits build up, and organisms appear to adapt to their environment • Populations evolve, not individuals!!!!! • natural selection + new environmental conditions can cause new species to form

Note that individuals do not evolve; populations evolve over time • Evolution: A change in the genetic makeup of a population over time • Natural selection can only increase or decrease heritable traits in a population • Works on existing traits. • Mutation, not natural selection, makes new traits Adaptations vary with different environments

Lamarck vs. Darwin/Wallace Thought individuals evolved populations evolve because those bad traits don’t have kids

Concept 22.3: Evolution has LOTS of evidence • New discoveries continue add more evidence • Guppies • Drug resistant HIV

Predation and Coloration in Guppies : Scientific Inquiry • John Endler has studied the effects of predators on wild guppy populations • Brightly colored males are more attractive to females • However, brightly colored males are more vulnerable to predation • Guppy populations in pools with fewer predators had more brightly colored males What happens to a group if you move them?

Results match prediction: support for evolution • Endler put drab colored guppies (many predators) in a pool with few predators • As predicted, the population changed dullbright More bright spots More area Covered in Bright spots

Results match prediction: support for evolution • Endler transferred brightly colored guppies (with few predators) to a pool with many predators • As predicted, over time the population became less brightly colored 

Direct Observations of Evolutionary Change • the evolution of drug-resistant HIV • drugs to fight HIV selects for viruses resistant to these drugs • The drug 3TC is supposed to interfere with HIV’s reverse transcriptase enzyme Virus needs enzyme for RNA  DNA

The Evolution of Drug-Resistant HIV • there are different forms of Reverse Transcriptase • Some variations are immune to 3TC • Can make DNA without errors • Viruses with 3TC resistant enzymes are better are reproducing, and become more common than other variants • Now HIV strains have resistance to 3TC • The ability of bacteria and viruses to evolve rapidly poses a challenge to our society

Fig. 22-14 100 Patient No. 1 Patient No. 2 75 Percent of HIV resistant to 3TC 50 Patient No. 3 25 0 2 0 4 6 8 10 12 Weeks

Natural Selection: like pruning shears • Natural selection does not create new traits • It chooses between existing traits in the phenotype • local environment determines what is selected for or against

The Fossil Record – records of past living things • Extinctions • New groups • Changes in groups

Transitional Fossils Exist • Archeopteryx Lizard and bird features

Transitional Fossils Exist • 11 specimens found, various sizes

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