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Evolution Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871.

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Presentation on theme: "Evolution Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871."— Presentation transcript:

1 Evolution Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871.

2 Objectives Define evolution. Outline the evidence for evolution. State that populations tend to produce more offspring than the environment can support. Explain that a consequence of over-reproduction of offspring is a struggle for survival. State that members of a species show variation. Explain how sexual reproduction promotes variation in a species. Explain how natural selection leads to evolution. Explain two examples of evolution in response to environmental change.

3 Charles Darwin British; lived from 1809 to Began to develop the theory of evolution on a trip around the world aboard the HMS Beagle.

4 Charles Darwin Came to realize that the earth had changed over a long period of time, which caused some crea- tures to go extinct, opening up space for new creatures to appear. The history of life on earth was similar to a branching tree, tracing back to some common ancestor. Nature selected which creatures survived and passed on their characteristics. Species change!

5 Evolution defined Evolution: the change over time in the frequency of inherited variations in a population over generations. Natural selection amplifies or diminishes inherited (not acquired) variations.

6 Evolution defined Evolution: the change over time in the frequency of in- herited variations in a population (not individuals).

7 Evolution defined Lamarckian vs. Darwinian evolution: change over time Lamarck was a Frenchman who lived before Darwin and first proposed creatures change over time (evolve). Example: giraffes have a long neck because one stretched its neck to reach high branches and passed the trait on.

8 Evolution defined Lamarckian vs. Darwinian evolution: change over time Darwin would say that giraffe’s necks come in various sizes. Those with a longer neck can reach higher branches and survive better during hard times and pass the trait on to their offspring.

9 Evolution defined Lamarckian vs. Darwinian evolution: change over time Common errors “Potato beetles evolved resistance to DDT in order to survive.” There is no intention to evolve. The beetles either had or didn’t have the genes needed to survive the chemical. Offspring of the survivors shifted the population toward a greater likeli- hood of resistance overall. Peppered moths come in various colors, white to black. Population colors shift as colors of tree trunks change over time, and pre- dators choose different colored moths..

10 Natural selection Natural selection: the differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Charles Darwin’s mechanism of evolution. Developed from 5 observations: 1) Species are so fertile that populations would rise exponen- tially if all individuals reproduced successfully. 2) Populations tend to remain stable in size, except for seasonal fluctuations. 3) Environmental resources are limited. 4) Individuals of a population vary; no two individuals are exactly alike. 5) Much of this variation is heritable.

11 Natural selection How natural selection works: Populations tend to produce more offspring than the environment can support. As a consequence, overproduction of offspring leads to a struggle for existence, with only a fraction of the offspring surviving each generation. A generation of mice lasts 45 days, then there are 4 – 7 babies. After 45 days there are perhaps 25 mice, then after another 45 days there could be 125, then 625.

12 Natural selection How natural selection works: Individuals in a species vary.

13 Natural selection How natural selection works: Sexual reproduction promotes variation in a species. Genes are redistributed and mixed among individuals. Asexual reproduction would produce no differences.

14 Natural selection Natural selection leads to evolution Survival is not random, but depends in part on the hereditary make-up of the individuals. Individuals more fit in their environment are likely to leave more offspring than those less fit. Preferential survival & reproduction leads to a gradual change in a population, with favorable characteristics accumulating over the generations.

15 Examples of natural selection Evolution of resistance to in- secticides in insect species. Effectiveness of insecticide decreases with time. Each generation there are more resistant insects. Natural selection edits existing variation. Natural selection favors characteristics that fit the current, local environment.

16 Examples of natural selection Drug-resistant strains of HIV evolve rapidly in the viral population infecting any particular patient. Like pesticide resistance: the drug 3TC interferes with HIV replication in human cells. Resistant strains become 100% of the population in just a few weeks. Bacterial resistance to antibiotic drugs works in the same way.

17 Evolution Satirical cartoon by Thomas Nast, from Harper’s Weekly, August 19, 1871.

18 Evidence for evolution Evidence of evolution is everywhere in biology. Fossil record Homologous structures Biogeography Embryonic development Molecular biology Selective breeding Natural selection within human lifespan

19 Evidence for evolution Fossil record Fossils are “any traces of dead organisms”: bones, tracks (foot-prints), leaf impressions, excrement, actual organisms frozen in ice, in amber, or in tarpits.

20 Evidence for evolution Fossil record Most fossils are found in sedimentary rock where deeper rock is older, formed from sand or clay deposits. Stratigraphy – dating fossils by charting the rock layers. Since the late 1940s, fossils are dated by the decay of radioactive isotopes. This is called radiometric dating.

21 Evidence for evolution Fossil record The fossil record in rocks provides relative ages. Radiometric dating can determine absolute ages. Organisms accumulate radio-active isotopes when alive. Isotopes decline after death - they decay (transform) into another element. Most carbon is 12 C, but there is a small yet constant amount of 14 C in the air, and therefore in our living bodies – 1 part per trillion. This amount declines after death. Most carbon is 12 C, but there is a small yet constant amount of 14 C in the air, and therefore in our living bodies – 1 part per trillion. This amount declines after death.

22 Evidence for evolution Half-life Half-life: time for ½ of the isotope atoms to decay. Use 40 K to date old rocks: half-life = 1.3 billion yr. Use 235 U for early vertebrates: half-life = 700 mil. yr. Use 14 C for recent fossils: half-life = 5,730 years.

23 Evidence for evolution Half-life of 14 C is 5,730 years. Using 14 C dating to determine the age of organic materials. Half-life of 14 C is 5,730 years. Work backward from the amount present today to a time when there was maximum 14 C, 1 part per trillion.

24 Evidence for evolution Fossil record Fossils give evidence about the major branches of descent in the tree of life. Order established Ex: fossil fishes predate other vertebrates; amphibians are next, followed by reptiles, then mammals and birds.

25 Evidence for evolution Fossil record Transitional forms link old & new species. Evolution of horse’s hooves from 5 toes. Evolution of whale from horse-like animal: whale retains a pelvis where hind legs once attached; forelegs now flippers

26 Evidence for evolution Homologous structures Features of new species are altered versions of ancestral features. Similarity in characteristics resulting from common ancestry is known as homology. All cats have a common ancestor. All cats have a common ancestor.

27 Evidence for evolution Homologous structures For example, the forelimbs of human, cats, whales, and bats share the same skeletal elements, but different functions because they diverged from the ancestral tetrapod forelimb. They are homologous structures.

28 Evidence for evolution Vestigial organs are homologous structures that have marginal, if any, importance to a current organism, but which had important functions in ancestors. Skeletons of some snakes & fossil whales retain vestiges of pelvis and leg bones of walking ancestors. In humans - wisdom teeth, tailbone, appendix.

29 Evidence for evolution Biogeography - the study of the distribution of life forms over geographical areas. If evolution is true, then we should expect to find related species living near each other Except in cases of great mobility (like sea animals, birds, and animals distributed by humans) or over long periods of time (due to plate tectonics). If, however, we find that species are distributed in a random geographic manner, with closely related species no more likely to be found close to each other than unrelated species, then this would be strong evidence against evolution and common descent.

30 Evidence for evolution Biogeography Plate tectonics – the continents are on plates that glide over the surface of the earth carrying life with them.

31 Evidence for evolution Biogeography Identical fossils in parts of the world now widely separated indicate that the continents were once joined. The southern part of Pangaea

32 Evidence for evolution Biogeography Australian example: marsupials vary widely but are more closely relat- ed to each other than to similarly- appearing animals on other continents. All have a pouch! Placental predators out-competed them on other continents, and they disappeared. Australia (with no placentals) was isolated.

33 Evidence for evolution Biogeography Marsupials ori- ginated in SA 70 million yrs ago then spread to Australia.

34 Evidence for evolution Biogeography Species tend to be more closely related to other species from the same area than to other species with the same way of life, but living in different areas. The sugar glider from Australia is more closely related to other marsupial mammals in Australia than to the flying squirrel, a pla- cental mammal of North America. This is an example of convergent evolution.

35 Evidence for evolution Embryonic development Genes for embryonic development are conserved in many different species making the embryos similar. All vertebrate embryos have structures called pharyngeal pouches in their throat at some stage in their development. These develop into different, but still homologous, adult structures: gills of fish or Eustachian tubes in mammals.

36 Evidence for evolution Molecular biology corroborates evolutionary trees. Evolutionary relationships among species are documented in their DNA and proteins. Ex: the Cytochrome c protein is more similar when crea- tures are closely related: Human & chimp have the same 104 amino acids, dog has 13 differences, rattle- snake has 20 changes.

37 Evidence for evolution Selective breeding Humans have domes- ticated many animals, giving them new char- acteristics over time (they evolved). Dogs domesticated from wolves about 15,000 years ago. Sheep, cattle, horses goats, pigs, chickens

38 Evidence for evolution Selective breeding Plants: corn, wheat, potato, bean, cabbage, etc.


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