Chapter 2: Origins of modern biology and Darwin's Theory of Natural Selection 1
Summary of this week (and next Tuesday) 1. Update on the class 2. Very brief description of evolution 3. Pre-scientific thought 4. Recall the idea of scientific explanation 5. Evaluate Pre-scientific explanations 6. Survey Scientific Revolution's key figures influencing Darwin's theory of natural selection 7. Natural selection 2
2. Very brief description of evolution Evolution: change in the genetic composition of a population over time -framework we use to understand biology Misconceptions: evolution is uncontroversial in science but controversial outside of science -most controversy involves the theories that attempt to explain evolutionary change Darwin's theory of evolution by natural selection is controversial outside of science but not controversial within science. 3
3. Pre-scientific thought Primary explanations of nature before the late 17th century *Fixity of species: all biological organisms were created at the same time with the came features we see today *Young Age of the Earth: Earth was created 6,000 years ago (6 kya) 4
3. Pre-scientific thought Primary explanations of nature before the late 17th century *Fixity of species: all biological organisms were created at the same time with the came features we see today *Young Age of the Earth: Earth was created 6,000 years ago (6 kya) At the start of the 18th century -observations of biological diversity from the New World could not be explained by these accounts 5
Recall from chapter 1 Hypotheses: tentative explanations of facts/observations Scientific method: O bservation -> H ypothesize -> T est x a billion -> T heory Theories: tested and confirmed explanations of facts/observations Hallmarks of good hypotheses Disconfirmable Make testable predictions 6
4. Recall the idea of scientific explanation Abductive reasoning: Inferring the best explanation for specific facts. All mammals have hair. Whales have hair. If whales are mammals, then it makes sense that they have hair. Therefore, whales are probably mammals. Another example https://www.youtube.com/watch?v=ycH9dGXq13w What are the facts? Beavis' hypothesis: this sucks. How can we say this is the best explanation of the facts? 7
Hallmarks of good hypotheses Ask: Does the hypothesis make testable predictions? Ask: Is the hypothesis disconfirmable? The best theories in science are disconfirmable. The best theories in science make predictions. EVERY explanation (hypotheses, theories, postulates, whatever) should have these hallmarks ALL of the topics in this course should rely on explanations with these hallmarks 8
Disconfirmation Disconfirmable: identifying evidence against our hypothesis E.g., Hypothesis: All wood burns If I find wood that doesn't burn then that's evidence against the hypothesis, i.e., it would disconfirm the hypothesis Disconfirmed hypotheses are either modified or rejected New fact: Someone delivers wood to my door which won't catch on fire. Ad hoc modification: all wood burns except if it's been delivered to my doorstep except if it's delivered on Mondays except if it's log-shaped, too Ad hoc modifications only attempt to amend problems undermining the original hypothesis Hypothesis eventually becomes messy or useless Can no longer disconfirm the hypothesis 9
Non-ad hoc modifications make testable predictions Testable predictions: if the hypotheses is correct, then it should be able to predict what observations we could make if 10
5. Evaluate Pre-scientific explanations Set of facts by the end of the 1600s The same animals found in europe are found around the world These animals vary in size These animals vary in color Size and color both tend to match the environment Hypothesis: All living things were created as is and haven't changed -------- Set of facts by the end of the 1600s Some trees grow one ring every year Some trees have 9,000 rings Hypothesis: Everything including the Earth was created 6,000 years ago. 11
Classification - the language John Ray (1627-1705) -Grouped organisms by reproductive viability -First to use "species" and "Genus" Carolus Linnaeus (1707-1778) -Binomial Nomenclature E.g., Homo sapiens , Tyrannosaurus rex , etc. -Taxonomic classification of biological organisms 12
Change - the relationship between environment and organism Comte de Buffon (1707-1788) -Plants and animals interacted with the environment -Environmental changes correlated with changes in plants and animals 13
Precursors - Dynamic Biology Jean-Baptiste Lamarck (1744-1829) -Species alter their characteristics to meet their environment. -First to really attempt and explain evolutionary process 14
Precursors - Ancient age of the Earth Georges Cuvier (1769-1832) -Extinction -Catastrophism Charles Lyell (1797-1875) -Father of modern geology Uniformitarianism: The gradual processes happening today were the same in the past. 15
Precursors - Environment influences organisms Thomas Malthus (1766-1834) -populations grow exponentially -food supplies/production grow arithmetically -populations outgrow food supply - survival of individuals dependent on access to food supply or resources Alfred Wallace (1823-1913) -Independently generated the concept of Natural Selection -Father of Biogeography 16
7. Darwin and natural selection -heavily influenced by these thinkers *5 year voyage on the HMS Beagle -What he observed informed his theory explaining why things change. 17
How Darwin changed the natural sciences Darwin's theory of natural selection explains what guides evolutionary change Took all the best parts of theories posited by the thinkers we just surveyed and made one that we still use today. 18
Natural Selection - Initial requirements I ndividuals with favorable variations --> survive and reproduce at higher rates 1. Populations grow faster than the growth of available resources. 2. Populations have differential variation among members 3. More offspring are produced than survive due to competition for resources. 4. Individuals with favorable traits (speed, disease resistance, size) are more likely to survive* than individuals without those traits. Fitness - A relative measure of reproductive success. *Not who can live the longest but who can survive long enough to reproduce. 19
Natural Selection 5. Favorable traits determined by environmental context 6. Offspring resemble their parents - favorable traits are inherited Reproductive success - favorable traits are inherited and become more common. 7. This favorable variations accumulates in a population over many generations so newer generations are distinct from ancestral generations. New species emerge 8. Geographical isolation - populations become geographically isolated and over time they respond to selective pressures - different ecological contexts - to become distinct species. 20
Natural Selection in Action Reproductive success - favorable traits are passed on with a higher frequency compared to less advantageous traits which decrease in frequency over time. Selective pressures - Environmental forces influencing the reproductive success. Fitness - A relative measure of reproductive success. Adaptations - The evolutionary shifts in the variation of traits in a population in response to environmental changes. 21
Natural Selection in Action Peppered Moths Industrial melanism in populations of peppered moths documented. Shifts of pigment pattern frequencies in response to the change in the environment . Such responses are called adaptations . 22
Natural Selection in Action - Finches WHAT: Galapagos island finch population -Beak thickness changed over time -a favorable trait (thicker beaks) confers a selective advantage in a population over time. -thick beaks had greater reproductive success during droughts . 23
Natural Selection in Action - Finches Finches competed for limited resources Members of finch population varied in beak size E.g., 13 species among the Galapagos Islands 24
Natural Selection - Insights Gained from examples 1. Traits are inherited - otherwise natural selection cannot act 2. Populations of individuals exhibit biological variation of characteristics -Selection also only works on pre-existing variation! 3. Fitness is relative - it changes relative to environmental change -Finch beak size correlated with extended periods of drought on the islands 4. Natural selection only acts on traits that affect reproduction -Traits expressed after an organism reproduces are not influenced by natural selection 25
Natural Selection - Main Points *Darwin was able to recognize that it was variation among the individuals of a population that contributed to the change in a species over time. Think about clones. Natural selection operates on individuals but it is the population that evolves Unit of Natural Selection - Individual Unit of Evolution - Population Individuals don't change genetically but overtime populations do. 26
Common Ancestry - Finches Evolution demonstrated in the finch populations found on the Galapagos islands 27
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