Chapters 15, 16, and 17

Evolution

The Theory of Evolution

Theory – well-tested explanation that unifies a broad range of observationsEvolution – change in a kind of organism over time; process by which modern organisms have descended from ancient organismsCan theories change over time?Are theories supported scientifically?

Charles Darwin – 1831 Voyage of the Beagle

Darwin studied:Diversity – he found a wide range of living things well adapted to their environmentFossils – he was curious as to why some were extinct

Ground Sloth Fossil

Galapagos IslandsVolcanic islands off the west coast of South AmericaUnique species found nowhere else in the worldHow did this

happen?

Galapagos

Galapagos

Darwin Returns HomeAfter Darwin returned home, he studied his notes and specimens He struggled with his observations and how they conflicted with his religionHe finally published On the Origin of Species, a book which outlined his theories of natural selection and evolution over time

Darwin’s Influences:James Hutton and Charles Lyell – Geologists whose theories suggested that Earth was extremely old and was always changing slowly over time.Darwin proposed that living things were not fixed in one form, but also changed slowly over time.

Darwin’s Influences:Jean-Baptiste Lamark – proposed the theory of acquired characteristicsUse or disuse…some traits were passed down to help survival of offspringFalse theory

Darwin’s InfluencesThomas Malthus – an English economist who studied human population growthHe believed that if humans became overpopulated, they would struggle to survive without sufficient resourcesDarwin applied this to animals and plants and believed it was the driving force for evolution.

Darwin studied variationDarwin did not know about genes, but did observe traits being passed on Artificial selection – when humans choose which traits are to be passed on in animals and plants by selective breedingExamples: Breeds of dogs, pidgeons, cows, corn, Brassica oleraceae

Darwin proposed Natural Selection

The idea that there is a struggle for survival…need to find food, mates, run away, hide, or protect themselvesDarwin proposed that some individuals were better adapted for their environmentAdaptation - any inherited characteristic that increases an organism’s chance of survival“SURVIVAL of the FITTEST”

Adaptations:

Descent with ModificationDarwin proposed Descent with ModificationLiving things changed over long periods of time due to natural selectionHe believed that living things evolved from a “common ancestor”“Tree of life” links all living things

Darwin’s early drawing of a tree of life…

Evidence of Evolution:The fossil record

Evidence of EvolutionGeographic distribution of living things – different animals on different continents looking similar (marsupial wolf and grey wolf)Animals that are not closely related have similar adaptations

Evidence of EvolutionHomologous Structures – structures that have different mature forms but develop from the same embryonic tissues. Vestigial organs

remnants of legs in skinks,appendix in humans…

Evidence of EvolutionSimilarities in Embryology

Pattern of EvolutionCoevolution: the change in two or more species in close association with each otherEx: Plants and their pollinators

Patterns of Evolution Cont’dConvergent Evolution

Organisms appear similar, but are not relatedThe environment selects similar phenotypesEx: streamlined body of dolphins and sharks

Patterns of Evolution Cont’d

Divergent EvolutionTwo or more related species become more and more dissimilar• Can result in new species

Response to differing habitatsAdaptive radiation: many related species evolve from a single ancestral species

Chapter 16 – Evolution of Populations

Evolution in genetic terms – any change in the relative frequency of alleles in a population.Gene pool – all genes (T, t) present in a populationRelative frequency – number of times one allele appears compared to all alleles in the gene pool. (Example: B = 40%, b = 60%)The population is evolving if the frequency changes

Gene Pool

allele for

brown fur

allele for

black fur

Sources of Genetic Variation:Mutations – a change in the sequence of DNA. Caused by chemicals, radiation, or just randomly. Can be harmful, helpful, or have no effect.Gene shuffling – genes are recombined during the formation of gametes for sexual reproduction. Does not change gene frequencies on its own.How many genes control one trait?

Single Gene and Polygenic TraitsIf a trait is controlled by one gene it is called a single-gene trait. Single-gene traits result in only two phenotypes. Example: Tall or ShortPolygenic traits result in a range of phenotypes. Example: height of a human being.

Which graph represents a polygenic trait?

Polygenic traits continued…Directional Selection

Polygenic traits continued…Stabilizing selection (Example: Birth weight)

Polygenic traits continued…Disruptive selectionExample: Beak Size

Genetic DriftGenetic drift is a random change in gene frequency.Common in small populationsFounder effect – a small group migrates away from the original group and has a unique gene poolExamples: Galapagos, Hawaii..

Hardy-Weinberg principleIf a population does not evolve, it is in genetic equilibrium. It must:1. Have random mating2. Consist of a large population3. No members move in or out of the group4. Have no mutations5. No natural selection occursExample? Coelacanth?

SpeciationSpecies – a group of organisms that can breed with one another and have fertile offspringSpeciation – formation of a new species due to natural selection or chance events

Isolating MechanismsIn order for new species to evolve, groups of organisms must be separated or isolated.If the two groups change enough, and can no longer breed and create fertile offspring, then new species were formed.

Causes of reproductive isolation - Geographical isolation – Groups are physically separated.Ecological isolation- Groups occupy different habitats.Temporal isolation- reproduce at different times of the day.Behavioral isolation- no attraction.Mechanical isolation- structural differences.Reproductive failure- no fertile offspring.

Speciation in Darwin’s Finches:

17-2 Earth’s Early History and 17-4 Patterns of Evolution

Earth is about 4.6 billion years oldEarth’s early atmosphere probably contained:Hydrogen cyanideCarbon dioxideCarbon monoxideNitrogen Hydrogen sulfideWater

Where did organic compounds come from?

1950s – Stanley Miller and Harold Urey designed experiments that showed organic compounds could be formed from elements in Earth’s early atmosphere if an electric current was introducedLightning could provide the electric currentAmino acids, cytosine, and uracil could be formed in these experiments.

Miller/Urey’s experimentMixture of gases

simulating atmospheres of

early Earth

Spark simulating lightning storms

Cold water cools

chamber, causing

droplets to form

Liquid containing

amino acids and other organic

compounds

Organic molecules to cells?Organic molecules can form tiny bubbles called proteinoid microspheres, almost like oil in waterRNA may have been the first hereditary material, as it can self-replicate and act as catalysts.These components may have been the first primitive cells…but the exact origin of life is a MYSTERY!

Early Bacteria created OxygenCyanobacteria (blue green algae) were probably the first living things to manufacture oxygenOxygen changed the atmosphere, turned the sky blue, and allowed organisms to respire aerobically (more efficient)

Endosymbiotic TheoryEukaryotic cells may have formed when larger cells engulfed smaller prokaryotesMitochondria and chloroplasts may have been free-floating bacteriaEvidence: They have DNA and ribosomes similar to bacteria and reproduce by binary fissionThe ability to respire aerobically and the ability to reproduce sexually increased diversity and influenced evolution

Endosymbiotic Theory

Other Topics:Fossils and ancient lifeIndex fossilsRadioactive datingGeologic time scaleEvolution of

multicellular lifeMass extinctions

Patterns of EvolutionMacroevolution – large scale evolutionary patterns and processes over long periods of timeExtinctionAdaptive RadiationConvergent EvolutionCoevolutionPunctuated EquilibriumChanges in developmental genes

ExtinctionWhen a species no longer exists on earthMost extinctions were natural until recent timesCan be gradual for individuals or can be catastrophic mass extinctions that affect multiple life formsScientists believe an asteroid impact caused a mass extinction event in the Cretaceous

Adaptive RadiationWhen a single species or small group of species evolve into many diverse forms over time.Examples: Mammals evolved, finches evolved

Convergent evolutionWhen unrelated organisms that live in similar environments develop similar adaptations to survive

CoevolutionThe process by which two species evolve in response to changes in each other over time

Punctuated EquilibriumLong, stable periods with little change in species interrupted by brief periods of rapid changeResults from small populations, isolated populations, following mass extinctions…

Developmental Genes and Body Plans

In some cases, small changes in just a few genes can cause major changes in an organismHox genes are “master control” genes for animal body plans One change in a gene can lead to wingless, one pair, or two pairs