Evolution

1. Evolution

2. Evolution • Defined • the change in a genetic make-up of a population over time. • Microevolution: genetic changes in a population through recombination and fertilization over time. • Macroevolution: changes in species over large periods of time through the selection of traits best for adaptation. • When scientists speak of evolution as a theory they do not mean that it is a mere speculation.  • It is a theory in the same sense as the propositions that the earth is round rather than flat or that our bodies are made of atoms are theories.  • Most people would consider such fundamental theories to be sufficiently tested by empirical evidence to conclude that they are indeed facts.  • As a result of the massive amount of evidence for evolution accumulated over the last two centuries, we can safely conclude that evolution has occurred and continues to occur.  • All life forms, including people, evolved from earlier species (common ancestors).  • Furthermore, all still living species of organisms continue to evolve today. 

3. Evidence • The evidence for evolution has primarily come from four sources: • the fossil record of change in earlier species • the chemical and anatomical similarities of related life forms (called homologies) • the geographic distribution of related species • the recorded genetic changes in living organisms over many generations

4. Evolution (cont.) • Theories of Evolution • Early theories • Spontaneous Generation • Long ago it was believed that non-living material could produce life. • For example, folks might leave a piece of meat sitting out on the counter. They might come back to find that in time maggots were coming from the meat, meaning that the meat turned into maggots. • Francesco Redi disproved this theory of evolution by conducting a simple experiment where he placed meat into containers that were covered (experimental group) and those that were left open (control group). • In time, flies were observed laying eggs on the meat that contained maggots, while the covered meat bore no flies.

5. G. L. L. Buffon • The 18th century philospher suggested that what most people referred to as species were really just well-marked varieties, modified from an original form by environmental factors. • For example, he believed that lions, tigers, leopards and house cats might all have a common ancestor. • He further speculated that the 200 or so species of mammals then known might have descended from as few as 38 original animal forms. • Buffon's evolutionary ideas were limited; he believed each of the original forms had arisen through spontaneous generation and that each was shaped by "internal moulds" that limited the amount of change.

6. Geologic Expression • Hutton and Lyell published works that were significant in changing the idea of catastrophies being the cause of geologic change and instead, continious gradual change took place • Uniformitarianism—present is the key to the past • Stratigraphy • Geologic time scales

7. Baptiste Chevalier deLamarck • Transformationism and Theory of Aquired Characteristics • Lamarck believed that microscopic organisms appear spontaneously from inanimate materials and then transmutate, or evolve, gradually and progressively into more complex forms through a constant striving for perfection.  • He also believed that an innate life force drove species to become more complex over time, advancing up a linear ladder of complexity that was related to the great chain of being. Lamarck recognized that species were adapted to their environment.

8. Lamarckian Evolution • The ultimate product of this goal-oriented evolution was thought by Lamarck to be humans.  • He believed that evolution was mostly due to the inheritance of acquired characteristics as creatures adapted to their environments.  • That is, he believed that evolution occurs when an organism uses a body part in such a way that it is altered during its lifetime and this change is then inherited by its offspring.  • For example, Lamarck thought that giraffes evolved their long necks by each generation stretching further to get leaves in trees and that this change in body shape was then inherited.  • Likewise, he believed that wading birds, such as herons and egrets, evolved their long legs by stretching them to remain dry.  • Lamarck also believed that creatures could develop new organs or change the structure and function of old ones as a result of their use or disuse.

10. Baron George Cuvier--1769-1832 • Lamarck did not invent the idea of inheritance of acquired characteristics but stated it clearly and publicly in 1809. • It was relatively easy for the French scientist, George Cuvier, and other critics of Lamarck to discredit his theory.   • If it was correct, the children of cowboys who have developed bowed legs as a result of a lifetime of riding horses would be born with bowed legs as well.  That, of course, does not occur.  Likewise, the children of professional weight lifters are not born with enlarged muscles. • While Lamarck's explanation of evolution was incorrect, it is unfair to label him a bad scientist.  In fact, he was at the cutting edge of biological research for his time.  He and George Cuvier were largely responsible for making biology a distinct branch of science. • Despite his criticism of Lamarck, Cuvier did not reject the idea that there had been earlier life forms.  • In fact, he was the first scientist to document extinctions of ancient animals and was an internationally respected expert on dinosaurs.  • However, he rejected the idea that their existence implied that evolution had occurred • Cuvier advocated the theory of catastrophism. • This held that there have been violent and sudden natural catastrophes such as great floods and the rapid formation of major mountain chains.  • Plants and animals living in those parts of the world where such events occurred were often killed off according to Cuvier.  • Then new life forms moved in from other areas.  As a result, the fossil record for a region shows abrupt changes in species.  • Cuvier's explanation relied solely on scientific evidence rather than biblical interpretation.

11. Later Theories (cont.) • Alfred Wallace • Co-author of natural selection with Charles Darwin. • A very tenacious explorer who was infected with malaria and lost everything more than once. • Believed in “selectionism” where natural selection could account for every form and structure of living things on earth except for the human brain which was spiritually created (where he and Darwin disagreed). • Did most of his work in the jungles of Borneo and South America while Darwin sat at home.

12. Later Theories (cont.) • Charles Darwin • Co-author of natural selection with Alfred Wallace. • Explored South America and the world via the HMS Beagle. • The voyage changed his life and the way he looked at nature. • Most early work was done by studying animal breeding (artificial selection) and later applying it to nature. • Did most of his work by studying the isolated species on the Galapagos Islands. • Found that through isolation, many species (originally from South America) adapted their behavior and feeding methods depending on what kind of food was available on each island. • Darwin had no idea of genetics when he formulated his theory of natural selection. • Darwin came to understand that any population consists of individuals that are all slightly different from one another.  • Those individuals having a variation that gives them an advantage in staying alive long enough to successfully reproduce are the ones that pass on their traits more frequently to the next generation.   • Subsequently, their traits become more common and the population evolves.  Darwin called this "descent with modification."

14. The Principles of Natural Selection • Every organism produces many more offspring than will survive and reproduce. • There is a great amount of variety in individuals in populations, some of which we can see and some we cannot see. • This variety is inherited over generations. • Resources are limited, so organisms must compete for those resources. • Those individuals who inherit traits that allow them to survive will compete best for the resources available.

15. Adaptation • Nature provides “pressure” on organisms through inorganic processes (weather, heat, etc) and organic processes (competition, etc.). • The pressures on organisms “select” for those best suited to withstand these pressures. Those best suited are that way due to genetics. He called this adaptation. • As the earth changes over millions of years, organisms are continually adapt to these pressures. Those that adapt prosper, those that do not, perish. • Each population is a collection of alleles. This is called the gene pool. • The gene pool will change depending on what “selection pressures” act upon it, and thus the species will change with it.

16. Evidence for Evolution Homology and Analogy Homology: similarity in structure between species Bird wings and bat wings share the same arm structure (built from the same bones) Example: Limbs of all tetrapods. Example: embryo development Example: molecular homology (hemoglobin) Example: Vestigial structures (like our appendix) Homologous similarities provide the best evidence of the relationship between fossil and living species having evolved from a common ancestor. It makes more sense that these homologies arose from common ancestors rather than having developed on their own. Analogy: similarity in function between species Bird wings and bat wings have the same function: flying Not evolutionary, but just a shared way of life.

17. Example: Tetrapod Limbs • Tetrapods are groups of organisms with four legs (mammals, amphibians, reptiles, birds, etc). • They live in a wide variety of environments and use their limbs for many different functions. • Each of them has a 5 digit limb, even though a mole digs, man grasps, a porpoise swims, and a bat flies. • The evolutionary explanation is that long ago there was a common ancestor that had a 5 digit limb that all tetrapods descended from. • This common ancestor was the branching off point for other species to evolve. • This is a much simpler explanation that why different functions would evolve separately.

19. Example: Comparative Embryology • Individual development of an organism is called ontogeny. Its line of ancestors defines its phylogeny. • Human embryos, chick embryos and fish embryos have much in common in their early developmental stages. • Whenever two or more different organisms have complex but similar developmental programs, it must be assumed that they inherited their programs from a common ancestor, rather than that the programs arose independently, by chance. • All vertebrate embryos follow a common development plan due to having the same set of genetic instructions.

21. Example: Molecular Development • One homology shared by all living things is a genetic code. • All organisms have DNA that is translated to amino acids. • One such example is the mRNA for hemoglobin. • If we look at different species, all have a different number of amino acids in the polypeptide that makes hemoglobin. • Those species that have the least difference in the number of amino acids are more related, and those that have the biggest number of differences are less related. • Scientists use hemoglobin as a sort of “molecular clock” to tell how long ago species shared common ancestors. • Human vs. Mouse hemoglobin: 16 amino acid diffrences • Human vs. Horse hemoglobin: 18 amino acid diffrences • Human vs. Bird hemoglobin: 35 amino acid diffrences • Human vs. Frog hemoglobin: 62 amino acid diffrences • Human vs. Shark hemoglobin: 79 amino acid diffrences • Using this data it is possible to construct new “family trees” showing the common ancestors between species.

24. Example: Vestigial Structures • Vestigial organs are those that have lost their function, or have reduced function. • If we look at whales we find that they do not have 4 legs. • However, if we look at the whale skeleton we will find that the pelvic bones are there but they serve no function in the whale. • We can then make the assumption that whales most likely evolved from a tetrapod ancestor rather than evolving independently.

26. Phylogenetic Trees • Using homology evidence, it is easy to develop a line of common ancestors in a “tree-like” form. • Scientists use common features like homologies to establish timelines and histories of evolution called cladograms. • On the board is one such example.

27. Evidence for Evolution Fossils 3 lines of evidence Transition sequences: finding fossils that show a change from one form to another (horses). Transitional forms: “missing links” between species (Archeoptryx) Extinct forms: where lineages dies out. Geology relies on the “principle of uniformitarianism” which states: The present is the key to the past.

33. Evidence for Evolution Biogeography Species living in the same area often appear to be related. Example: the Galapagos finches Isolation allows for new species to develop.

36. Example: Galapagos Finches

39. Evidence for Evolution Direct Observation Evolutionary change can be directly observed in nature. Examples: Dog breeding Plant hybrids Peppered moths

40. Example: Dog Breeding • There are more than 350 different breeds of dog, each one--from the Chihuahua to the Great Dane--descended from the grey wolf. • The domestication of dogs by man dates back at least twelve thousand years. • Domestication of the wolf may have begun with orphaned wolf cubs brought into villages and raised by children. • Ancient man may have also started the domestication process by befriending wolves which helped them hunt and drive herds. • There were advantages at that time to having semi-tame wolves. • The acute hearing of the wolf would alarm humans of imminent danger at night, for example. • Selective breeding has allowed human manipulation to create the traits that you see in dogs today, such as the pushed-in face of the pug, or the speed of the greyhound to catch its prey.

41. Example: Plant Hybrids • The citrus family (Rutaceae) contains some of the world's most delicious fruits, including numerous hybrid crosses between species. • The popular tangelo grown in San Diego County is a hybrid produced by crossing a grapefruit (C. x paradisi) with a tangerine.

42. Melanistic Moths • A good example of natural selection was discovered among "peppered" moths living near English industrial cities.   • These insects have varieties that vary in wing and body coloration from light to dark.  • During the 19th century, sooty smoke from coal burning furnaces killed the lichen on trees and darkened the bark.  • When moths landed on these trees, the dark colored ones were harder to spot by birds who ate them and, subsequently, they more often lived long enough to reproduce.  • Over generations, the environment continued to favor darker moths.  As a result, they progressively became more common.  • By 1895, 98% of the moths in the vicinity of English cities like Manchester were mostly black.  • Since the 1950's, air pollution controls have significantly reduced the amount of heavy particulate air pollutants reaching the trees.   As a result, lichen has grown back, making trees lighter in color.  • Now, natural selection favors lighter moth varieties so they have become the most common. 

43. Dark moths on light colored bark are easy targets for hungry birds but are hidden on pollution darkened trees.