Darwin's Evolutionary Theory: Natural Selection Explained

1. Chapter 13 Section 1 The Theory of Natural Selection Grade 10 Biology Spring 2011

2. Objectives • Identify several observations that led Darwin to conclude that species evolve • Relate the process of natural selection to its outcome • Summarize the main points of Darwin’s theory of evolution by natural selection as it is stated today • Contrast the gradualism and punctuated equilibrium models of evolution

3. Bell Ringer • Write down what you already know about evolution

4. Biography of Darwin • Darwin was a poor student, disinterested in subjects his father wanted him to engage in • Attended medical school but was repelled by surgeries and often skipped lectures • Was often found outdoors collecting biological specimens • Completed a degree in theology but was interested in natural sciences

5. Biography of Darwin • In 1831, Darwin was recommended for a position as a naturalist on the HMS Beagle

6. Science Before Darwin • Idea that each species is a divine creation that exists, unchanging, as it was originally created • Scientists had begun to seek to explain the origins of fossils • Tried to explain their observations by altering traditional explanations of creation • Others proposed various mechanisms to explain how living things change over time

7. Science Before Darwin • Lamarck’s theory • Over the lifetime of an individual, physical features increase in size because of use or reduce in size because of disuse • These changes are then passed on to offspring • Is incorrect, but correctly pointed out that change in species is linked to “physical condition of life”

9. Darwin’s Observations • Read Lyell’s book Principals of Geology, that proposed that the surface of the Earth changed slowly over many years • Visited different places, where he saw things that he thought could be explained only by a process of gradual change • Extinct Armadillo fossils similar but not identical to present species

10. Darwin’s Observations • Galapagos Islands • Many plants and animals resembled those of coast • Hypothesized that ancestors of Galapagos species migrated to islands from South America long ago and changed after they arrived • Called it decent with modification- evolution

11. Growth of Populations • Malthus • Wrote that human populations are able to increase faster than the food supply can • Unchecked populations grow by geometric progression • Food supplies grow by arithmetic progression • Proposed human population do not grow unchecked because death caused by disease, war, and famine slows population growth

12. Growth of Populations • Population: consists of all the individuals of a species that live in a specific geographical area and that can interbreed

13. Evolution by Natural Selection • Darwin realized Malthus’s hypothesis about human population applied to all species • Every organism has potential to produce many offspring during its lifetime • In most cases, only a limited number of offspring survive to reproduce

14. Evolution by Natural Selection • Darwin connected Malthus’s ideas with his own observations and ideas • Individuals that have physical or behavioral traits that better suit their environment are more likely to survive and will reproduce more successfully than those that do not have such traits • Natural Selection

15. Natural Selection

16. Evolution by Natural Selection • In time, the number of individuals that carry favorable characteristics that are also inherited will increase in a population • Nature of population will change- evolution

17. Evolution by Natural Selection • Proposed organisms differ from place to place because their habitats present different challenges to and opportunities for survival and reproduction • Adaptation: is an inherited trait that has become common in a population because that trait provides a selective advantage

18. Publication of Darwin’s Work • 1844, Darwin wrote down his ideas about evolution and natural selection • Published in 1858 • Presented with Alfred Wallace who was coming to the same conclusion as Darwin was about natural selection and evolution

19. Darwin’s Theory • Inherited variation exists within the genes of every population or species (the result of random mutation and translation errors) • In a particular environment, some individuals of a population or species are better suited to survive (as a result of variation and have more offspring- natural selection)

20. Darwin’s Theory • Over time, the traits that make certain individuals of a population able to survive and reproduce tend to spread in that population • There is overwhelming evidence from fossils and many other sources that living species evolved from organisms that are extinct

21. Darwin’s Ideas Updated • Change Within Populations: • Now know genes are responsible for inherited traits • Certain forms of a trait become more common in a population because more individuals in the population carry the alleles for those forms • Natural selection causes the frequency of certain alleles in a population to increase or decrease over time • Mutations and recombination of alleles that occurs during sexual reproduction provide endless sources of new variations of natural selection to act upon

22. Darwin’s Ideas Updated • Species Formation: • The environment differs from place to place • Populations of the same species living in different locations tend to evolve in different directions • Reproductive isolation: condition in which two populations of the same species do not breed with one another because of geographic separation, difference in mating periods, or other barriers to reproduction • May eventually become unable to breed together

23. Darwin’s Ideas Updated • The Tempo of Evolution: • Gradualism: model of evolution in which gradual change over a long period of time leads to species formation • Suggested that successful species may stay unchanged for long periods of time (Gould and Eldredge) • Major environmental events in the past have caused major evolution to occur in spurts • Punctuated equilibrium: model of evolution, in which periods of rapid change in species are separated by periods of little or no change • Evidence for both

25. Review • T/F Malthus and Lyell influenced Darwin in his ideas of natural selection • T/F The number of individuals that carry favorable characteristics that are also inherited will increase in a population • T/F The smallest scale that evolution happens on is the individual

26. Answers • T/F Malthus and Lyell influenced Darwin in his ideas of natural selection • T/F The number of individuals that carry favorable characteristics that are also inherited will increase in a population • T/F The smallest scale that evolution happens on is the individual • Smallest scale is at the population level, individuals don’t evolve, populations do