Evolution How species arise

Transcription

1 Evolution How species arise 9.1 What is Evolution? -The Process of Evolution Natural selection the differential survival and reproduction of individuals in a population. Process by which populations adapt to varying environments Populations evolve, NOT individuals Reproductive isolation over time leads to incompatible mating = New Species 9.1 What is Evolution? - The Process of Evolution Microevolution changes that occur within a species and the characteristics of a population. Easily observed, relatively non-controversial. Macroevolution changes that occur, as a result of microevolution, over long periods of time and result in the origin of new species. Controversial among nonbiologists. 1

2 11.1 What is a Species - Speciation: an Overview Three steps necessary for one species to give rise to a new species 1. Isolation of gene pools of populations 2. Evolutionary changes in gene pools of populations 3. Evolution of reproductive isolation between populations 1 Populations become isolated (no gene flow). 3 Enough differences accumulate so that reproduction between the populations becomes impossible. 2 Evolutionary changes accumulate over time, and the populations diverge in their characteristics. Time Figure What is a Species - The Biological Species Concept The biological species concept states that species are reproductively isolated from one another. In nature, members of the same species can potentially interbreed Members of different species cannot interbreed Sum total of alleles in a species is called gene pool Movement of alleles within a gene pool is called gene flow. Gene flow does not occur between species, due to reproductive barriers. Two general kinds of reproductive barriers: Prefertilization prevent fertilization from occuring Postfertilization fertilization occurs, but hybrid cannot reproduce 2

3 Five different prefertilization reproductive barriers Spatial Behavioral Mechanical Temporal Gamete incompatibility Spatial reproductive isolation Species are separated by distance Example: polar bear (Arctic) and spectacled bear (South America) Behavioral reproductive isolation Differences in mating behavior may interfere with reproduction Example: many birds have mating songs or dances Figure

4 Mechanical reproductive isolation Sexual organs are incompatible Example: many insects have lock and key genitals Temporal reproductive isolation Difference in timing of reproduction Example: organisms might have different mating or flowering times Figure 11.3 Gamete compatibility reproductive isolation Eggs and sperm of different species unable to fuse Common among organisms that release gametes into the environment Wolbachia, a bacteria that causes reproductive isolation between invertebrate species. 4

5 Hybrid inviability Hybrid sterility Postfertilization barriers to reproduction: 1. Hybrid inviability Zygote unable to develop because genetic instructions are incomplete Example: sheep crossed with goat produces an embryo, but it dies early in development Postfertilization barriers to reproduction: (a) A mule results from the mating of a horse (b) Why mules are sterile and a donkey. 2. Hybrid infertility Interspecies cross is unable to reproduce Example: mule x Horse cells: Donkey cells: 64 chromosomes 62 chromosomes Meiosis Mule cell Horse Donkey chromosome chromosome Metaphase I of meiosis Horse egg has 32 chromosomes. Donkey sperm has 31 chromosomes. The chromosomes are from different species, so they are unable to pair during the first part of meiosis. Mule: 63 chromosomes Figure

6 11.1 What is a Species - Speciation: an Overview Once reproductively isolated, how long does the process of evolution take? Two general explanations Gradualism slow accumulation of small changes over long period of time Punctuated equilibrium rapid change followed by long periods of no change Evidence that both processes are at work 11.1 What is a Species - A Closer Look: Speciation Isolation and divergence of gene pools Migration can lead to isolation of a population Examples include oceanic islands Because migrant populations are small, genetic changes can occur rapidly 11.1 What is a Species - A Closer Look: Speciation Species separated by barriers or distance are allopatric Species occupying the same area are sympatric 6

7 11.1 Allopatric Speciation More than 50 species of Hawaiian silversword are descended from a migrant population of California tarweed. California tarweed Hawaiian silversword 3700 km (2300 miles) North America Hawaii Figure Allopatric Speciation Geographic barriers can also intrude between populations Isthmus of Panama connects North and South America, but divides an ocean gulf 6 pairs of snapping shrimp species exist. One species pair is on the Carribean side and the other is on the Pacific side Genetic evidence indicates that each species pair is descended from ancestral species separated by rise of Panama 11.1 Sympatric Speciation Apple maggot flies appear to be speciating sympatrically. Apples are not native to N. America, introduced by colonists Apple maggot flies infest hawthorns and apples Flies mate on fruit where they will lay their eggs Hawthorns fruit 1 month after apples Apple-preferring and hawthorn-preferring flies appear to have little gene flow 7

8 9.1 What is Evolution? - The Theory of Evolution All species present on earth today are descendents of a single common ancestor, and all species represent the product of millions of years of accumulated evolutionary changes. 9.1 What is Evolution? - The Theory of Evolution Organisms observed today Time (thousands of gen nerations) Evolution Evolution Common ancestor Figure Charles Darwin and the Theory of Evolution - The Voyage of the Beagle Galapagos islands tortoises vary with their environments. Figure 9.5 8

9 9.3 Examining the Evidence for Evolution - An Overview of Evidence for Evolution Biological classification implies common ancestry. Bread yeast Morels Leaf rusts Mushrooms Class Chitridomycota (water molds) Zygomycota (bread molds) Ascomycota Basidomycota Phylum Fungi Kingdom Common ancestor Figure Examining the Evidence for Evolution - An Overview of Evidence for Evolution Anatomical similarities between organisms. Mammalian forelimbs have the same set of bones. The underlying structure is similar despite the very different functions. Humerus Radius and ulna Carpals Metacarpals Phalanges Bat Sea lion Lion Chimpanzee Human Figure Examining the Evidence for Evolution - An Overview of Evidence for Evolution Useless traits in modern species. Vestigial, but similar, structures in ferns and flowering plants. (a) Fern gametophyte (b) Flower ovary Gametophyte generation is found here. Figure

10 9.3 Examining the Evidence for Evolution - An Overview of Evidence for Evolution Shared developmental pathways. Similarity among chordate embryos. Snake Chicken Possum Cat Bat Human Early embryo Pharyngeal slits Tail Intermediate embryo Late embryo Figure Examining the Evidence for Evolution - An Overview of Evidence for Evolution DNA similarities. Birds in same genus have DNA that is more similar to one another, while distantly-classified birds have DNA that is less similar. Ruddy turnstone Black turnstone Red knot Caspian tern % DNA similarity to Ruddy turnstone 100% 90% 82% 72% Same genus (Arenaria) Same family (Scolopacidae) Same order (Charadriiformes) Figure Examining the Evidence for Evolution - An Overview of Evidence for Evolution Distribution of organisms on earth. Different species of mockingbird found on Galapagos all resemble another species found on the mainland. Figure

11 9.3 Examining the Evidence for Evolution - An Overview of Evidence for Evolution Divergence Species arise with new functions to existing structures. Forelimb of vertebrates Convergence Species arise with similar structures but are NOT related. Habitat creates selection pressure. Spines and thorns of desert plants Figure Examining the Evidence for Evolution - An Overview of Evidence for Evolution Fossil evidence. Horse fossils provide a good sequence of evolutionary change within a lineage. Equus Merychippus 1 toe Mesohippus Hyracotherium (Eohippus) 3 toes 4 toes 3 toes Horse ancestor Modern horse Millions of years ago Figure Biological Classification - A Closer Look: Kingdoms and Domains Recently, some scientists have begun to suggest that organisms should be classified based on evolutionary relationships. Major groups under this system correspond to divergences early in life s history. Determining evolutionary relationships requires comparing DNA. DNA of closely related organisms should be more similar than DNA of distantly related organisms. 11

12 12.3 Learning About Species - Understanding Ecology Understanding an organism s ecology how it lives in its environment can be helpful in evaluating potentially beneficial compounds Survival in extreme environments High levels of competition with bacteria and fungi Susceptibility to predation Ability to live on or in other organisms Survival in high population densities 12.3 Learning About Species - Reconstructing Evolutionary History A classification system reflecting evolutionary relationships is more useful to scientists An organism s chemical traits will probably be similar to those of its closest relative If looking for new compounds, could begin by screening close relatives of organisms already known to produce compounds 12.3 Learning About Species - Reconstructing Evolutionary History The challenge in making an evolutionary classification is that organisms do not always resemble their closest living relatives New World vultures are more closely related to storks than they are to Old World vultures (a) Old World vulture (b) New World vulture (c) Stork Hooded vulture (Necrosyrtes monachus) Turkey vulture (Cathartes aura) Wood stork (Mycteria americana) Figure

13 12.3 Learning About Species - Developing Evolutionary Classifications An evolutionary classification of sparrows Dark-eyed junco Harris s sparrow Golden-crowned sparrow White-crowned sparrow White-throated sparrow White throat patch Seven stripes on crown Black stripes on crown of head Ancestral sparrow Ancestral bird Figure Learning About Species - Developing Evolutionary Classifications Reconstructing evolutionary history not always as easy as the sparrow example Can be confounded by loss of traits, and convergence DNA provides a means of testing evolutionary hypotheses 12.3 Learning About Species - Testing Evolutionary Classifications Scientists can test evolutionary hypotheses with data from fossils and from living organisms Fossils provide information about the genealogy of different living groups Comparison of DNA from living organisms can also validate or refute proposed classifications DNA supports vulture classification, but does not support sparrow classification DNA can give clues to near relatives, but lab process can take time 13

14 9.3 Examining the Evidence for Evolution - A Closer Look The same lines of evidence that support common descent can be used to look for the closest relatives of humans. Are humans related to apes? Figure Examining the Evidence for Evolution - A Closer Look Anatomical and developmental similarities Tail great apes and humans have tailbone, but no tail Useful trait in primate relative Vestigial trait in human (a) Tail bone Figure 9.18a 9.3 Examining the Evidence for Evolution - A Closer Look Evidence from molecular homology African monkey Gorilla Chimpanzee Human 96.66% 98.90% 99.01% 100% Figure

15 9.3 Examining the Evidence for Evolution - A Closer Look Bipedal humans have some unique anatomical traits, such as features of hips, knees, and skull. Chimpanzee Human Foramen magnum Base of skull Back of skull Pelvis Accommodate four-legged stance Modified for upright stance Larger arms Limbs relative to body Larger legs Feet Grasping Weight bearing Figure Examining the Evidence for Evolution - A Closer Look Early hominim fossils such as Lucy provide evidence that the earliest human ancestors arose in Africa. Figure Examining the Evidence for Evolution - A Closer Look Evidence from fossils Radiometric dating Used to determine age of rocks Relies on decay of radioactive isotopes into daughter products 15

16 9.3 Examining the Evidence for Evolution - A Closer Look Trends in human evolution Larger brains Flatter face Reduced jaw size Australopithecus afarensis Australopithecus africanus Homo habilis Homo sapiens Age of fossil as Ancient hominin Modern hominin determined by radiometric dating (million years ago) Figure Examining the Evidence for Evolution Theory of common descent is controversial. There are some possible alternative hypotheses that can be tested against available data (each explained in subsequent slides): Static model hypothesis Transformation hypothesis Separate types 9.3 Examining the Evidence for Evolution Graphical representations of theory of common descent and alternative hypotheses: (a) Static model Species arise separately and do not change over time. (b) Transformation Species arise separately but do change over time in order to adapt to the changing environment. Time Time Difference in form (c) Separate types Species do change over time, and new species can arise; but each group of species derives from a separate ancestor that arose independently. Difference in form (d) Common descent Species do change over time, and new species can arise. All species derive from a common ancestor. Time Time Difference in form Difference in form Figure

17 9.4 Are Alternatives to the Theory of Evolution Equally Valid? Weighing the Alternatives Static model Transformation Separate types Common descent Rejected Rejected Rejected Earth is far older than Evidence of relationships Universality of DNA, 10,000 years, and among organisms abound. genetic code, and cell species have clearly components are changed over time. evidence of a single origin of all life. Supported by a wide variety of evidence, including comparative anatomy, DNA sequences, and the fossil record. Figure