Evolution Flashcards
evolution
change in allelic frequencies in population
mostly takes place over hundreds, thousands, or millions of years
methods of dating Earth:
- studying undisturbed sedimentary rock layers and fossils within can reveal relative age
- absolute age can be accurately mesured by radiometric dating (decay of radioactive isotopes/halflifes)
- if not enough rock source, scientists use paleomagnetic dating (Earth’s magnetic poles shift and sometimes reverse and are recorded in rock layers)
- other methods include change in sea levels, molecular clocks, measurements of continental drift
history of Earth
- Earth has radically changed since forming 4.5 billion years ago
- continents shifted, climate through extreme cooling/warming, oceans risen and lowered repeatedly, volcanic activity brought major change and killed off species, meteorites killed dinosaurs and gave mammals chance to expand around globe
- 5 major extinctions have occured
- 4 billion years ago, non free oxygen: cyanobacteria (oxygen-generation organisms that form rock like structures, stromatolites) provided free oxygen for oceans + atmosphere
- high oxygen killed most anaerobic prokaryotes (oxygen is toxic) but also caused rapid diversification of life on land and in seas
- short period Cambrian explosion (535-525 mil years ago) characterized by sudden appearance of many present-day animal phyla
1. Fossil Record
6 pieces of evidence for evolution
- existence of species that may have gone extinct or evolved
- radiometric dating and half-life measures age of fossils
- prokaryotes first organisms to develop = oldest fossils
- paleontologists discovered transitional forms that link older fossils to modern species
- indicates that organisms living today is tiny fraction of every organism that lived: most life that existed on Earth went extinct
2. Comparative Anatomy
6 pieces of evidence for evolution
study of different structures help understand evolution of anatomical structures and evolutionary relationships
homologous structures
homologous structures have common origin and reflect common ancestry
wing of bat, lateral fin of whale, human arm all have same internal bone structure even if function varies
analogous structures
- bat wings and fly wings have same function, but very different
- similarity is superficial and reflects adaptation to similar environments, not descent from recent common ancestor
vestigial structures
structures with no apparent function, a residual from past ancestor
evidence that structures have evolved
e.g. appendix is a vestige of a structure needed when human ancestors ate a different diet
3. Comparative Biochemistry
6 pieces of evidence for evolution
- organisms with common ancestor will have common biochemical pathways
- more closely related = more similar biochemistry
e.g. medicine can be tested on mice and etrapolate the results to humans
4. Comparative Embryology
6 pieces of evidence for evolution
closely related organisms have similar stages in embryonic development
e.g. all vertebrate embryos go through stage with gill pouches on sides of throats - will deveop into gills in fishs, will develop into eustachian tubes in mammals
5. Molecular Biology
6 pieces of evidence for evolution
- because all aerobic organisms contain cells that carry out aerobic cell respiration, they all contain polypeptide cytochrome c
- comparison of amino acid sequence of cytochrome c in different organisms shows which organisms more closely related
- cytochrome c in human cells almost identical to chimpanzee and gorilla, but different from pig
6. Biogeography
6 pieces of evidence for evolution
- theory of plate tectonics (continents/oceans rest on giant plates of crust that floats on hot mantle) →convection currents in mantle causes continental drift
- plate movement over millions of years changed flora and fauna of Earth
- continental drift changed distributions of life on Earth
- e.g. marsupials only on Australia while other continents home to eutherians (true placental mammals), marsupials originated in Asia and reached Australia from South America and Antarctica while continents were joined, Australia set afloat like raft and carried marsupials and eutherians, marsupials filled every available niche in Australia while true placental mammals went extinct
historical context for evolutionary theory
- Aristotle had theory of Scala Natura: all life-forms can be arranged on ladder of increasing complexity, each with its own rung, species permanent and do not evolve, humans at pinnacle of ladder of increasing complexity
- Carolus Linnaeus/Carl von Linné (1707-1778) specialized in taxonomy (branch of biology for naming and classifying forms of life), believed that scientists should study life + classification system would reveal divine plan, developed binomial nomenclature (genus + species name)
- Georges Cuvier studied fossils and realized each stratum of earth characterized by different fossils, advocated catastrophism (series of events in past occured suddenly and caused by mechanisms different from present, events responsible for changes in organisms on Earth and strong opponent of evolution), influenced Darwin’s theory
- James Hutton (influential geologist) published theory of gradualism (1795), earth was molded and not by sudden events (effects of wind, weather, flow of wateer formed geologic features on earth), theory important because based on idea that earth had long history and change is normal course of events
- Charles Lyell (leading geologist in Darwin era) stated geological change results from slow, continuous actions, believed Earth much older than 6000 years, his work Principles of Geology had influence on Darwin
- Lamarck (contemporary of Darwin) published theory of evolution in 1809 (year Darwin was born) and had idea that inheritance of acquired characteristics + use and disues, stated individual organisms change in response to environment: giraffe has long neck because they stretched neck to reach leaves on tall acacia tree
- Alfred Russel Wallace (naturalist/author) published essay about natural selection similar to Darwin’s (not published yet), credited for theory along with Darwin
- Charles Darwin (naturalist/author, left England aboard HMS Beagle at 22 to visit Galapagos Islands, South America, Africa, Australia) worked out theory of natural selection or descent with modification as mechanism for evolution (1840s), eventually published “On the Origin of the Species” (1859)
Darwin’s Theory: Natural Selection
- major mechanism of evolution: acts on phenotypic variation in populations
- populations tend to grow exponentially, overpopulate, exceed resources
- overpopulation results in competition
- in any population, there is variation and unequal ability of individuals to survive + reproduce
- only best-fit individuals survive to pass on traits to offspring
- evolution occurs as advantageous traits accumulate in population (individual doesn’t change because of pressure in environment, but allelles become more frequent in population)
giraffe + long neck
- according to Darwin: ancestral giraffes were short-necked animals, although neck length varied
- as population of animals competing increased, taller individuals had better chance of surviving than those with shorter necks
- eventually, proportion of giraffes with longer necks increased until only long-necked giraffes existed
insects + resistance to pesticides
- insects do not become resistance, some insects naturally resistant to insecticide
- when environment sprayed, resistant insects have selective advantage
- all insects not resistant die, and remaining resistant ones breed quickly with no competition
- directional selection
1. Stabilizing Selection
5 types of selection
- eliminates extremes, favouring more common intermediate forms
- mutant forms weeded out
aka purifying selection
e.g. human babies weigh 6-8 pounds as larger or smaller results in greater infant mortality, Swiss starlings lay up to 5 has more surviving offspring vs. more or less eggs
2. Disruptive/Diversifying Selection
5 types of selection
- increases extreme types in population at expense of intermediate forms
- results in balanced polymorphism (1 population divided into 2 distinct types, may result in formation of 2 new species over time)
e.g. in environment with very light/dark rocks, very light/dark mice will increase while intermediate mice will die
3. Directional Selection
5 types of selection
- changing environmental conditions
- 1 phenotype replaces another in the gene pool
- can produce rapid shifts in allelic frequencies
e.g. industrial melanism in peppered moths, moths were light in 1845 in England but smog and smoke soon made plants and rocks dark, all moths dark by 1900
4. Sexual Selection
5 types of selection
- based on variation in secondary sexual characteristics to compete for mates
- difference in appearance between males and females known as sexual dimorphism (female birds blend in with environment while males are bright to compete for attention)
e.g. evolution of horns, antlers, large stature, strength: male elephant seals fight for supremacy of harem w/ 50 females, baboons have long canines, colourful birds
5. Artificial Selection
5 types of selection
humans breed organisms by seeking desired traits as breeding stock
e.g. racehorses, laying hens, types of vegetables
preserving variation in population
- variation necessary for population to evolve as environment changes
- even if it seems like natural selection will reduce genetic variation by removing unfavourable genotypes, 8 main mechanisms to preserve variation
1. Balanced Polymorphism
preserving variation in population
presence of 2+ phenotypically distinct forms of trait in a population
e.g. shells of a genus of land snail exhibits wide range of colors + patterns: banded snails living on dark + mottled ground less visible than unbanded ones, unbaned snails have selective advantage in areas where background is uniform