Bio 10 Lecture Notes 6: Evolution A. Carranza

Transcription

1 (1.) Historical Perspective Plato ( BC) and Aristotle ( BC) Aristotle first to classify living animals Western culture life forms unchanged (immutable) since creation Early Beliefs Chain of Beings- Life extended from lowest forms to humans, spiritual beings were highest. Single Creation- All species were links created at the same time at one center of creation. Age of Exploration led to new discoveries 15th century world travel began New species of plants and animals were discovered Naturalists cataloguing them identified patterns in where species lived and whether they seemed related Among first to ponder about ecology and evolution Evidence against concept of immutable species Biogeography Comparative morphology Geologic discoveries Biogeography Study of patterns in geographic distribution of species and communities Accepted beliefs did not explain discovery of new organisms in previously unknown places Comparative Morphology Study of similarities and differences in body plans of major groups Puzzling patterns: Animals as different as whales and bats have similar bones in forelimbs Some parts seem to have no function (vestigial structures) Geological Discoveries Similar rock layers throughout world Certain layers contain fossils Deeper layers contain simpler fossils than shallow layers Some fossils resemble known species A Carranza Page 1 7/25/2015

2 19th Century: New Theories Scientists attempt to reconcile evidence of change with traditional belief in a single-creation event Theories Georges Cuvier ( ) French anatomist Fossil species from Paris Basin older strata had flora and fauna increasingly dissimilar from modern life extinction common occurrence new species appear, others disappear theory of catastrophism following creation global disasters destroyed species repopulated from other areas James Hutton (Scottish Geologist) James Hutton: gradualism Theory of Gradualism (1795) profound change cumulative product of slow but continuous processes Jean Baptiste Lamarck Theory of Evolution (Philosophie Zoologique,1809) Organisms had a drive to perfection evol. responds to felt needs teleological - evolution has a goal, or directed purpose species changed over time because they "wanted" to be "better" 1st to combine evolutionary and ecological thinking Proposed mechanism: Use and disuse body parts used extensively become larger and stronger, those not used deteriorate Inheritance of acquired characteristics -modifications an organism acquires during its lifetime can be passed to offspring Credit deserved for his recognition that: Evolution best explanation for both fossil record and current diversity Recognition of great age of the Earth Emphasis on adaptation to the environment Charles Lyell (Scottish geologist) Principles of Geology (1830) Uniformitarianism (Theory of Uniformity) Natural laws are constant in space and time. Most geological change occurs slowly and gradually, not through sudden, catastrophic events. Scientists should attempt to explain events of the past through the same sorts of natural processes that we observe today. Challenged the view that Earth was only 6,000 years old A Carranza Page 2 7/25/2015

3 Charles Darwin ( ) Father was wealthy Physician Natural History interested in collecting insects, hunting fishing and reading College University of Edinburgh studied medicine (age 16) transferred to Christ College, University of Cambridge studied theology BA in 1831 natural theology most naturalist and scientists belonged to the clergy Darwin s History Rev. John Henslow, Professor of Botany 1831 H.M.S. Beagle (Captain FitzRoy) Professor Henslow recommended him as naturalist Gave him Lyell's book Darwin s Voyage At age 22, Charles Darwin began a five-year, round-the-world voyage aboard the HMS Beagle As ship s naturalist, he collected and examined species that inhabited regions the ship visited Voyage of the Beagle 5 years Galapagos Islands Many Endemic Species Fossil Evidence Darwin found Glyptodont (giant armadillo-like) fossils in Argentina Proposed descent with modification Voyage Ended in 1836 Upon return home to England: Darwin began discussing his observations as possible evidence for evolution with other naturalists Also read Thomas Malthus An Essay of Population (1798) Human population growth would eventually be limited by: War Famine Disease Reproductive Capacity and Competition All populations have the capacity to increase in numbers No population can increase indefinitely Eventually, individuals of a population end up competing for resources Galapagos Finches Darwin observed finches with a variety of lifestyles and body forms On his return, he learned that there were 13 species He attempted to correlate variations in their traits with environmental challenges A Carranza Page 3 7/25/2015

4 Variation and Artificial Selection Artificial Selection Domesticated plants and animals bred for specific traits Heritable variation is random, purposeless, and in no way subject to control. Farmers could not cause a variation to arise. Variation in Populations All individuals have the same genes that specify the same assortment of traits Most genes occur in different forms (alleles), which produce different phenotypes Some phenotypes compete better than others (fitness) Change over Time Over time, alleles that produce the most successful phenotypes will increase in the population Less successful alleles will become less common Change leads to increased fitness Increased adaptation to environment Natural Selection (Darwin s Conclusion) Natural selection for various traits among individuals of a population affects which individuals survive and reproduce in each generation Process results in adaptation to the environment (increases fitness) Darwin held off on publishing his findings out of fear Alfred Wallace British Naturalist Malay archipelago 1858 Darwin received manuscript with developed theory of Natural Selection essentially identical to Darwin s Prompted Darwin to publish On the origin of Species Summary Of Darwin s Publication: Natural selection is differential success in reproduction. Natural selection occurs through an interaction between the environment and the variability inherent among individual organisms making up the population. The product of natural selection is the adaptation of populations of organisms to their environment. Darwin s Uncertainty Lack of Transitional Forms: Darwin was unable to explain gaps in fossil record No mechanism for the origin of a new species from a parental one. Gradual evolution of species was fiercely debated natural selection is too slow to be observed A Carranza Page 4 7/25/2015

5 Adaptation Some heritable aspect of form, function, or behavior that improves the odds for surviving and reproducing Environment specific Outcome of natural selection Salt-Tolerant Tomatoes An example of adaptation Common tomato plants originated in Ecuador and Peru These will wilt when irrigated with salty water One species Adaptation to What? Llamas live at high altitude and have hemoglobin with a high oxygen affinity Is this an adaptation to altitude? Probably not Llamas are related to camels, which live at low altitudes Camels also have hemoglobin with high oxygen-binding capacity Common Ancestors Llama and camel Populations Evolve Biological evolution changes populations, not individuals Traits in a population vary among individuals Evolution: change in the frequency of traits The Gene Pool All the genes in a population Genetic resource that is shared (in theory) by all members of population Variation in Phenotype Each gene in gene pool may have two or more alleles Individuals inherit different allele combinations leading to variation in phenotype Offspring inherit genes, not phenotypes What Determines Alleles in a New Individual? Mutation Crossing over at meiosis I Independent assortment Fertilization Change in chromosome number or structure Genetic Equilibrium Allele frequencies at a locus are not changing Population is not evolving Five Conditions of Genetic Equilibrium No mutation Random mating Gene doesn t affect survival or reproduction Large population No immigration/emigration A Carranza Page 5 7/25/2015

6 (2.) Microevolution Microevolutionary Processes Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by Natural selection Gene flow Genetic drift Gene Mutations Infrequent but inevitable Each gene has own mutation rate Lethal mutations Neutral mutations Advantageous mutations Polymorphism Variability in form within or among populations of same species Results of Natural Selection Three possible outcomes: A shift in the range of values for a given trait in some direction Stabilization of an existing range of values Disruption of an existing range of values Directional Selection Directional Selection Allele frequencies shift in consistent direction over time Speed in grasshoppers Pinpointing the Target of Selection Populations of rock pocket mice have fur that matches the rocks on which they live Black basalt: dark fur Tawny granite: light fur DNA comparisons show that the two populations differ in Mclr gene sequence Pesticide Resistance Pesticides kill susceptible insects Resistant insects survive and reproduce Speed in Grasshoppers If resistance has heritable basis, it becomes more common with each generation Antibiotic Resistance Antibiotics first came into use in the 1940s Overuse has led to increase in resistant forms Most susceptible cells died out, while resistant forms multiplied Number of Grasshoppers A Carranza Page 6 7/25/2015

7 Stabilizing Selection Intermediate forms are favored and extremes are eliminated Human Birth Weight Number of Babies Stabilizing Selection Birth Weight in Humans Disruptive Selection Happens when forms at both ends of the range of variation are favored Intermediate forms are selected against African Finches Selection favors birds with very large or very small bill Birds with intermediate-sized bill are less effective feeders Disruptive Selection 14 Sexual Selection 12 Selection favors certain secondary sexual characteristics 10 Through nonrandom mating, alleles for 8 preferred traits increase 6 Leads to increased sexual dimorphism 4 Sexual selection common in birds 2 Balanced Polymorphism 0 Polymorphism: having many forms Occurs when two or more alleles are maintained at frequencies greater than 1 percent Sickle-Cell Trait: Heterozygote Advantage Allele HbS causes sickle-cell anemia when heterozygous Heterozygotes are more resistant to malaria than homozygotes 16 Number of Birds Size of Beak A Carranza Page 7 7/25/2015

8 Genetic Drift Random change in allele frequencies brought about by chance Effect is most pronounced in small populations Sampling error: fewer times an event occurs, greater the variance in outcome Genetic Drift: Small Populations Frequency of b+ allele Genetic Drift: Large Populations Frequency of b+ allele Bottleneck A severe reduction in population size Causes pronounced drift Example Elephant seal population hunted down to just 20 individuals Population rebounded to 30,000 Electrophoresis revealed there is now no allele variation at 24 genes Founder Effect Effect of drift when a small number of individuals starts a new population By chance, allele frequencies of founders may not be same as those in original population Effect is pronounced on isolated islands Inbreeding Nonrandom mating between related individuals Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed Gene Flow Physical flow of alleles into a population Tends to keep the gene pools of populations similar Counters the differences that arise from mutation, natural selection, and genetic drift Gene Flow Example Blue jay carries acorn between oak populations A Carranza Page 8 7/25/2015

9 (3.) Evolutionary Patterns, Rates, and Trends Macroevolution Major patterns and trends among lineages Rates of change in geologic time Comparative Morphology Comparing body forms and structures of major lineages Guiding principle: When it comes to introducing change in morphology, evolution tends to follow the path of least resistance Morphological Divergence Change from body form of a common ancestor Produces homologous structures Morphological Convergence Individuals of different lineages evolve in similar ways under similar environmental pressures Produces analogous structures that serve similar functions Comparative Development Each animal or plant proceeds through a series of changes in form Similarities in these stages may be clues to evolutionary relationships Mutations that disrupt a key stage of development are selected against Altering Developmental Programs Some mutations shift a step in a way that natural selection favors Small changes at key steps may bring about major differences Molecular Evidence Biochemical traits shared by species show how closely they are related Can compare DNA, RNA, or proteins Comparing Proteins Compare amino acid sequence of proteins produced by the same gene Human cytochrome c (a protein) Identical amino acids in chimpanzee protein Chicken protein differs by 18 amino acids Yeast protein differs by 56 Sequence Conservation Cytochrome c functions in electron transport Deficits in this vital protein would be lethal Some sequences are identical in wheat, yeast, and primates Nucleic Acid Comparison Use single-stranded DNA or RNA Hybrid molecules are created, then heated The more heat required to break hybrid, the more closely related the species Molecular Clock Assumption: Ticks (neutral mutations) occur at a constant rate Count the number of differences to estimate time of divergence A Carranza Page 9 7/25/2015

10 Biological Species Concept Species are groups of interbreeding natural populations that are reproductively isolated from other such groups. -Ernst Mayr Variable Morphology Reproductive isolation can cause genetic divergence leading to new species Genetic Divergence Gradual accumulation of differences in the gene pools of populations Natural selection, genetic drift, and mutation can contribute to divergence Gene flow counters divergence Reproductive Isolation Cornerstone of the biological species concept Speciation is the attainment of reproductive isolation Reproductive isolation arises as a by-product of genetic change Reproductive Isolating Mechanisms Prevent pollination or mating Block fertilization or embryonic development Cause offspring to be weak or sterile Prezygotic Isolation Mechanical isolation Wasp and zebra orchid Temporal isolation Cicada Behavioral isolation Albatrosses Ecological isolation Populations residing in different ecosystems Gametic mortality Mutation driven gamete incompetence or incompatibility Postzygotic Mechanisms Early death Sterility Low survival rates A Carranza Page 10 7/25/2015

11 Models for Speciation Allopatric Speciation Speciation in geographically isolated populations Some sort of barrier arises and prevents gene flow Effectiveness of barrier varies with species Extensive Divergence Prevents Inbreeding Species separated by geographic barriers will diverge genetically If divergence is great enough it will prevent inbreeding even if the barrier later disappears Archipelagos Island chains some distance from continents Galapagos Islands Hawaiian Islands Colonization of islands followed by genetic divergence sets the stage for speciation Adaptive radiations: Honeycreepers: in absence of other bird species, they radiated to fill numerous niches Speciation without a Barrier Sympatric speciation Species forms within the home range of the parent species Sympatric Speciation in African Cichlids Studies of fish species in two lakes Species in each lake are most likely descended from single ancestor No barriers within either lake Feeding preferences localize species in different parts of lake Speciation by Polyploidy Change in chromosome number (3n, 4n, etc.) Offspring with altered chromosome number cannot breed with parent population Common mechanism of speciation in flowering plants Parapatric speciation Neighboring populations become distinct species while maintaining contact along a common border A Carranza Page 11 7/25/2015