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Modern Biology II Who Are You? • • • • • Your major Your year, part time or full time Your plans Your objectives Your interests Evolution • Please read Chapter 20 (Genes Within Populations) if you have not done so already • Know who Charles Darwin is! Evolution • Evolution is descent with modification; living species are descendents of ancestral species that were different from present-day ones • Evolution describes the genetic changes in a population over time Evolution • Organisms are not perfectly fit – a good fit, but not perfect • As descendents of a remote ancestor spread into various habitats over millions and millions of years, they accumulated diverse modifications (adaptations) that fit them to specific ways of life; descent with modification evolution Evolution • Natural variation among individuals is based on heredity (and mutation). These variations enable organisms to become adapted to their environment over time Natural Selection • Natural selection is the process by which favorable, inherited traits become more numerous in successive generations of a population of reproducing organisms Natural Selection • Over time, natural selection leads to species that are well adapted (highly evolved) to their environments The Principles of Natural Selection • Struggle for existence/Competition – More offspring are produced than can be supported by resources King Penguin Rookery © Momatiuk - Eastcott/Corbis The Principles of Natural Selection • Variation – Some individuals, due to heredity or mutation, possess characteristics which make them better adapted to their environment The Principles of Natural Selection • Inheritance of Traits – Best-suited organisms will survive to produce more individuals that share same adaptation Survival of the Fittest • Organisms are adapted to their environment through natural selection • Natural selection is a pessimistic process 1Population with varied inherited traits 2Elimination of individuals with certain traits Reproduction of survivors 3 Certain individuals with a distinct, inherited characteristic will be selected against, while others with a (different) distinct, inherited trait will survive Evidence for Evolution • Microevolution – can be observed in nature; small changes in organism, generations changing over time • Artificial selection – evolution can be experimentally produced Microevolution • A well-known example of microevolution involves the peppered Moth, Biston betularia in England during the industrial revolution • Prior to the industrial revolution, light variants of the peppered moth survived better than dark variants because they blended well with the light colored trees – caused by the presence of a light-colored lichen on the darkcolored bark Microevolution • Pre-industrial era – only light variant known • During the industrial revolution, poor air quality killed the lichens which covered the (otherwise dark) trees and camouflaged the light moths against predation • In 1848, the first dark variant collected • By mid-1900’s, the dark variant made up 90% of population in industrial areas! Microevolution Artificial Selection • Darwin got idea of natural selection by artificial selection! • Modern corn looks very different from its ancestor • Tumbler pigeons!!! www.flickr.com/photos/rinalia/ 3285371111/ www.flickr.com/photos/ terryandchristine/ 2399227035/ Artificial Selection • All dogs are domesticated breeds of the Gray wolf, Canis lupus; “Fido” is actually a subspecies of the wolf! Artificial Selection Evidence for Natural Selection • Darwin’s fishes of the Galapagos 14 species of finch; 1 common ancestor (from the mainland); different beaks Evidence for Evolution • Homologous similarities: similarities between 2 species that is NOT functionally necessary • Provide clues to common ancestry • Constraint is not there, may look the same, but doesn’t have to • Example: Tetrapods; do not need 5 digits to make flying wings, swimming structure, etc. • Common pentadactyl ancestor; limb adapted into various ‘means’ Evidence for Evolution • Homologous structures – similar characteristics that result from a common ancestry Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat The Panda’s Thumb • The panda’s thumb is a homologous trait; modification of the wrist bone, not anatomically a finger (or thumb) at all • Constructed from the radial sesamoid, normally a small component of the wrist • May have originated from a single genetic change (mutation) The Panda’s Thumb Homologies • Fossil evidence of evolution: Whale ‘missing link” • Vestigial structures – no apparent function, but resemble structures ancestors possessed Homologies Vestigial structures of a whale http://www.edwardtbabinski.us/mpm/mpm_whale_limb.html Evidence for Evolution Evolutionary View of Homology • Use the starting materials and processes already available • Fashion adaptations, rather than starting from scratch • Evolution is a tinkerer, not a master engineer Evolutionary View of Homology • Adaptations are NOT perfect – it is the imperfection of adaptation that, instead, gives evidence for evolution (a tinkerer uses tools already there to improve) • Different homologies are correlated – similar patterns between human, ape, and monkey for many proteins • Similarities stem from common ancestor Anatomical Evidence of Evolution • • • • Orchid petals - used as pollinator lure Snake with 2 leg bones Manatee fingernails Humans – muscles for wiggling ears Anatomical Evidence of Evolution • Developmental similarities reflect descent from a common ancestor • “Ontogeny recapitulates phylogeny”! • (Ontogeny = growth and development of an organism; Phylogeny = evolutionary history of a species) Evidence for Evolution • In contrast to homologous structures, analogous structures also provide evidence for evolution • Analogous structures are structures that share similarities by a way of life, not by a common ancestry • Analogous structures arise among unrelated organisms in response to similar needs or similar environmental factors Analogous structures Examples: wings of insects (a) and birds (b); flippers of seals (c) and penguins (d) Origin of New Species • Individuals do NOT evolve • Populations are the smallest units that can evolve • A population is a group of interbreeding individuals belonging to a particular species sharing a common geographic area Origin of New Species • What is a species? • Biological species concept: a population or group whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot interbreed with other such groups • Species are based on ability to interbreed – NOT on physical similarities Origin of New Species • Example: Eastern and Western Meadowlarks – 2 different species with similar shape and coloration, but differences in song help prevent interbreeding • Barriers to breeding can be behavioral Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Origin of New Species • In contrast, humans have considerable diversity, but we all belong to the same species because of our ability to interbreed Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Speciation happens • It takes a reproductive barrier to keep individuals of closely-related species from interbreeding • Reproductive barriers may be behavioral, geographical, anatomical, or temporal Allopatric Speciation • How do reproductive barriers arise? • One of the clearest forms of species is allopatric speciation caused by a geographic barrier • When a geographic barrier occurs, the isolated populations each become adapted to their own environment, such that over time, they may no longer interbreed, even if they were to come into contact with one another again Speciation • Reasons for geographic (allopatric) isolation • Example of allopatric isolation: antelope squirrels on south and north rim of Grand Canyon Speciation Speciation • Prezygotic vs. Postzygotic • Prezygotic – mechanisms preventing formation of a zygote (ecological, behavioral, temporal, mechanical) • Postzygotic – mechanisms preventing organisms from developing into a reproducing adult Prezygotic examples • Ecological – Lions and Tigers • Behavioral – bluefooted boobies • Temporal – Wild lettuce (different blooming periods) • Mechanical - insects Postzygotic example • Hybrid inviability or infertility – mule • Mules are the reproductive result of a horse and a donkey breeding • Mules are sterile • Therefore, a horse and a donkey must represent 2 distinct species Sympatric Speciation • Sympatric speciation – the process by which new species arise within the range of another species • More controversial • In this case, a new species does not arise from geographic isolation • Instead, a new species may arise by accident when errors during cell division resulted in organisms with extra sets of chromosomes (very common in plants) Sympatric Speciation • New species formed by having extra sets of chromosomes are considered to be polyploid • Polyploid organisms have more than 2 complete sets of chromosomes • For example, a polyploid containing 4 sets of chromosomes will produce diploid (2n) gametes! This species would be unable to mate with normal diploid species (which produce haploid gametes) Error in cell division Polyploid cells undergo meiosis Selffertilization 3 2 1 Parent species Diploid Polyploid (“tetraploid”) Viable, fertile tetraploid species Diploid gametes Polyploidy by error in cell division and selffertilization Polyploidy by errors in cell division of a ‘sterile’ hybrid Chromosomes not homologous (cannot pair) 1 Species A 2n = 4 2 Gamete n=2 3 Sterile hybrid n=5 Species B 2n = 6 Gamete n=3 Viable, fertile hybrid species 2n = 10 3. However, ‘sterile’ hybrid can reproduce asexually (as many plants do), and if subsequent errors in cell division occur, chromosome duplications can result in a fertile polyploid species! Polyploid speciation • Remember, polyploidy is a type of sympatric speciation • As many as 80% of all living plants today are believed to have arisen by polyploidy! • A polyploid contains twice (or sometimes more) the genetic diversity as its diploid predecessors, which provides an adaptive advantage! Polyploid speciation • Many of the plants grown for food are polyploids – – – – – – – Oats and Barley Potatoes Bananas Peanuts Plums and Apples Wheat Coffee! Speciation Macroevolution • Origin of taxonomic groups higher than the species level • Evolutionary change substantial enough to view its products as new genera, families or phyla • Has a random component • Considers major evolutionary innovations – bird feathers, insect wings • Extinctions and Radiations Macroevolution Adaptive Radiation • The evolution of many diverse species from a common ancestor is called adaptive radiation • The adaptations of these species allow them to fill new habitats or roles in their communities (“niches”) • New phenotypes arise in response to the environment, driven by natural selection Adaptive Radiation • Example: Development of a fourth cusp in mammalian tooth – increases range of food which can be utilized • Adaptive radiation typically occurs when a few organisms colonize new, unexploited habitats, or when environmental changes open up new opportunities for the survivors Adaptive Evolution • The Galapagos Islands is one of the world’s greatest showcases of adaptive radiation • Each island arose ‘naked’ from underwater volcanoes and were gradually clothed by plants, animals and micro-organisms which strayed from the South American mainland Darwin’s finches are a prime example of adaptive evolution Rise of the mammals • The extinction of the dinosaurs provided a tremendous evolutionary opportunity to mammals, who once lived in their shadows Macroevolution • Mass extinctions Macroevolution • Fossil record provides clues for the outline of macroevolution • Extant (living) species also supply clues • What about soft-bodied organisms which do not leave a fossil record? • Does evolution occur in ‘fits’ and ‘starts’ or is our understanding of historical accounts skewed? Suggested Readings • • • • The Selfish Gene, Richard Dawkins The Blind Watchmaker, Richard Dawkins Unweaving the Rainbow, Richard Dawkins The Panda’s Thumb (and any other collection of stories by), Stephen J. Gould