The Evidence for Evolution Chapter 21 1 Evidence of Natural Selection Darwin collected a closely related group of 14 finch species in the Galápagos Islands All similar except for beak characteristics Darwin hypothesized that different beak shapes were related to food gathering Darwin wrote one might really fancy that one species has been taken and modified for different ends. 2
Evidence of Natural Selection Darwin s finches 3 Evidence of Natural Selection Modern research verified Darwin s selection hypothesis 3 conditions of natural selection Variation must exist in population Variation must lead to differences among individuals in reproductive success Variation among individuals must be genetically transmitted to the next generation 4
Evidence of Natural Selection Peter & Rosemary Grant studied medium ground finch beak depth variation among members of population Average beak depth changed from year to the next in a predictable fashion - Droughts: birds with deeper, more powerful beaks survived better - Normal rains: average beak depth decreased to its original size 5 Evidence of Natural Selection Evidence that natural selection alters beak shape 6
Evidence of Natural Selection When environment changes, natural selection often favors different traits in a species Biston betularia: peppered moth Light gray with black specks to jet black coloration Black individuals have dominant allele Dominant allele rare in population until 1850s 7 Evidence of Natural Selection J.W. Tutt hypothesized that light-colored moths declined because of predation Light moths were easily seen by birds on darkened (sooty) trees 8
Evidence of Natural Selection Bernard Kettlewell tested hypothesis Dark tree trunks = more dark-colored moths survived Light tree trunks = more light-colored moths survived When environmental conditions reverse, so does selection pressure 9 Evidence of Natural Selection Industrial melanism: phenomenon in which darker individuals come to predominate over lighter ones Pollution control resulted in lichen growing on trees and bark color being lighter again Light-colored peppered moths now are dominant in population 10
Evidence of Natural Selection Agent of selection may be difficult to pin down Could poisoning by pollution be agent of natural selection? Selection against melanism 11 Artificial Selection Laboratory Experiments Drosophila melanogaster (fruit fly) Selected fruit flies with many bristles on abdomen Chose only those with most bristles to reproduce 86 generations later: average # of bristles 4X 12
Artificial Selection Artificial selection in the laboratory 13 Agriculture Artificial Selection Corn looks very different from its ancestor 14
Artificial Selection Domestication of silver foxes result of artificial selection 15 Can selection produce major evolutionary changes? Artificial Selection Breeds of dogs: Differences among dog breeds are greater than differences displayed among wild species of canids. 16
Fossil Evidence of Evolution Fossils: preserved remains of once-living organisms Rock fossils: created when three events occur organism buried in sediment Ca ++ in bone/hard tissue mineralizes surrounding sediment hardens to form rock 17 Fossil Evidence of Evolution Absolute dating: age of fossils estimated by rates of radioactive decay Relative dating: fossil s position in sediment Isotopes, like U 238, transform at precisely known rates into nonradioactive forms Rate of decay isotope s half-life C 14 half-life: 5,280 yrs K 40 half-life: 1.28 million yrs 18
Fossil Evidence of Evolution Radioactive Decay 19 5 Major Extinctions
Hypotheses for Cambrian Explosion Filling in ecological barrel: many habitats/niches to exploit Unrestricted genetic design: freedom to try new/different morphological forms; some successful/others do not work extinct Stealth predator: Anomalocaris canadensis natural selection for 20 million yrs pressuring prey to adapt/evolve new structures to escape predator Cambrian Explosion Anomalocaris canadensis Burgess Shale
Geological Studies I. Continental Drift Environmental Changes II. Plate Tectonics- 6 Major Plates 1) North American 2) South American 3) Eurasian 4) African 5) Antarctican 6) Pacific A. Subduction Zones: plates collide formation of islands, mountains, volcanoes B. Transform Faults: plates slide past earthquakes C. Oceanic Ridges: plates pull apart deep sea vents
Fossil Evidence of Evolution Fossil records document the course of life through time 25 Fossil Evidence of Evolution Fossils document evolutionary transition Oldest known bird fossil: Archaeopteryx found in 1859 intermediate between bird & dinosaur possesses some ancestral traits & some traits of present day birds 26
Archaeopteryx reptilian bird Fossil Evidence of Evolution Recent discoveries Four-legged aquatic mammal Important link in evolution of whales & dolphins from land-dwelling, hoofed ancestors Fossil snake with legs Tiktaalik: a species that bridged gap between fish & first amphibian Oysters: small curved shells to large flat shells 28
Tiktaalik Fish 375 million yr old fossil head of a crocodile & gills of a fish Fossil Evidence of Evolution Whale missing links 30
Fossil Evidence of Evolution Evolutionary change: horse body size & toe reduction 31 Anatomical Evidence for Evolution Homologous structures: structures with different appearances & functions that all derived from same body part in a common ancestor Bones in forelimb of mammals: homologous structures Different functions, same ancestor structure 32
Anatomical Evidence for Evolution I. Anatomical Homology: mammalian forelimb bones 33 Anatomical Evidence for Evolution Strongest anatomical evidence supporting evolution comes from comparisons of how organisms develop. Early vertebrate embryos possess pharyngeal pouches that develop into: In humans: glands & ducts In fish: gill slits 34
Anatomical Evidence for Evolution II. Embryological homology: descent from common ancestor 35 Anatomical Evidence for Evolution Neck vertebrae Geese: 25 Plesiosaurs: 76 Mammals: 7 Giraffe: 7 vertebrae, very large in size, to make up for length of neck 36
Anatomical Evidence for Evolution Eyes Molluscs: photoreceptors face forward focusing lens move forward-backward Vertebrate: photoreceptors face backward focusing lens scrunched by muscles 37 Anatomical Evidence for Evolution Eyes of vertebrates 38
Anatomical Evidence for Evolution Eyes of Mollusks 39 Anatomical Evidence for Evolution Vestigial structures: no apparent function but resemble structures their ancestors possessed Vestigial structures of a whale 40
Anatomical Evidence for Evolution Humans Muscles for wiggling ears Boa constrictors Hip bones & rudimentary hind legs Manatees Fingernails on their fins Blind cave fish Nonfunctional eyes 41 III. Biochemical Homology bacteria/yeast to humans same codons & amino acids 1953 Miller & Urey Chemical Evolution 7 days 4 amino acids urea fatty acids
Convergent Evolution Biogeography: study of the geographic distribution of species Some plants & animals have similar appearance but are only distantly related Convergent evolution: independent development of similar structures in organisms that are not directly related Convergent evolution: usually seen in animals & plants that live in similar environments 43 Convergent Evolution Marsupials & placentals Marsupials: young born in an immature condition & held in a pouch until they develop Placentals: young are not born until they can safely survive in the external environment 44
Convergent Evolution 45 Convergent Evolution Convergence among fast-swimming predators 46
Biogeographical Record Darwin noted on his voyage that Islands: often missing plants & animals common on continents Species present on islands often diverged from continental relatives Island species usually more closely related to species on nearby continents 47 Biogeographical Record Darwin concluded: Species arrive on islands by dispersing across water Dispersal from nearby areas more likely than distant sources Species that can fly, float or swim can inhabit islands Colonizers often evolve into many species 48