Genetics and Environment

1. Genetics and Environment • We’ve learned about DNA, a genotype gives you a phenotype… • But why do we all have different DNA? • Why did you have a postcaudal tail as an embryo? • Why don’t most of us have the allele for Huntington’s disease? • Why are some alleles more common in people from one continent? • Why is more than 99.9% of your DNA the SAME as everybody else in the world? • Why is 80% of your DNA the same as a cow’s? Why is 50% of your DNA the same as a banana tree’s?

2. Genetics and Environment • Certainly the single most foundational idea in all of biology, and perhaps the greatest biological discovery… • All life is connected.

3. What is Evolution? • Evolution • changes in living organisms over time • explains how modern organisms have descended from ancient organisms, and continue to change today

4. DNA Evolution explains two significant questions • unity of life • Why do all living things share fundamental similarities? • diversity of life • Why, in spite of fundamental similarities, are living things so different?

5. Early Ideas on Evolution • Darwin was influenced by: • Lyell and Hutton: geologists who said that the Earth was very old and described uniformitarianism: explaining that the geological structures changed continuously over time • Malthus: if the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone.

6. Earlier ideas on Evolution • LaMarck • “acquired characteristics” • creatures developed traits during their lifetime in response to need and activity • give those traits to their offspring • example • “in reaching higher leaves giraffes stretch their necks & give the acquired longer neck to offspring”

7. Refuting Lamarck • Experiment: • Cut off a mouse’s tail • Allow mouse to breed, see if offspring are born tailless • …They are not. • Conclusion: Acquired traits are not responsible for change. ?

8. Voyage of the HMS Beagle • Charles Darwin invited to travel around the world • 1831-1836 (22 years old!) • makes many observations of nature • main mission of the Beagle was to chart South American coastline Robert Fitzroy

9. Voyage of the HMS Beagle • Stopped in Galapagos Islands • 500 miles off coast of Ecuador

10. Galapagos Recently formed volcanic islands. Most of animals on the Galápagos live nowhere else in world, but they look like species living on South American mainland. 800 km west of Ecuador

11. Darwin found…many unique species Many of Darwin’s observations made him wonder… Why? Darwin asked: Why were these creatures found only on the Galapagos Islands?

12. present day Armadillos ancient Armadillo Darwin found…clues in the fossils Darwin found: Evidence that creatures have changed over time Darwin asked: Why should extinct armadillos & modern armadillos be found on same continent?

13. Darwin found: Different shells on tortoises on different islands Darwin asked: Is there a relationship between the environment & what an animal looks like?

14. Darwin found… birds Darwin found: Many different birds on the Galapagos Islands. He thought he found very different kinds… Finch? Sparrow? Warbler? Woodpecker?

15. But Darwin found… a lot of finches Darwin was amazed to find out: All 14 species of birds were finches… But there is only one species of finch on the mainland! Large ground finch Small ground finch Finch? Sparrow? Darwin asked: If the Galapagos finches came from the mainland, why are they so different now? Warbler finch Tree finch Warbler? Woodpecker?

16. Darwin’s view of Evolution • Darwin • giraffes that alreadyhave long necks survive better • leave more offspring who inherit their long necks 

17. “Equation” Variation +Inheritance +Struggle for life Differential reproductive success +Time Evolution – Change over time

18. Darwin’s “Ingredients”: Variation • Variation is the raw material for natural selection • there have to be differences within population • some individuals must be more fit than others

19. Wet year Beak depth Dry year Dry year Dry year 1977 1980 1982 1984 11 10 Beak depth of offspring (mm) 9 Medium ground finch 8 8 9 10 11 Mean beak depth of parents (mm) Where does Variation come from? • Mutation • random changes to DNA • errors in mitosis & meiosis • environmental damage (rare) • Sexual Reproduction • mixing of alleles • Independent assortment • recombination of alleles • new arrangements in every offspring

20. Variation of Traits in a Population • Within a population individuals vary in traits • Measuring the frequency of most traits will produce a bell curve graph. • This shows that few individuals have the extreme phenotype • Most individuals have the average or medium phenotype

21. Darwin’s “Ingredients”: Inheritance • Evolution-vulnerable traits have a genetic basis • Gene Pool: Total genetic information available in a population • Allele Frequency: how common an allele is within a gene pool of a population • Determined by dividing the number of a certain allele by the total number of alleles of all types

22. Darwin’s “Ingredients”: Struggle for Life • A pair of barn swallows arrived at my house in summer 1983. Barn swallows have an average of 10 chicks a year, and chicks return to their birthplace to start their own nests the next year. • Adults live for 10 years, and siblings will breed. How many barn swallows were at my house by 1990?

29. Overreproduction • A single bacterium dividing every 20 minutes would have 40,000,000,000,000,000,000,000 offspring by the end of one day. After two days, it would have enough offspring to cover the entire Earth in a 2 meter layer of bacteria. • In one year, a single pair of fruit flies would have so many offspring, their descents would weigh more than the planet. • The elephant is one of the slowest breeding species. But one pair of elephants would have 19 million descendents 750 years later. By contrast, there are only 700,000 elephants alive in the whole world today.

30. Darwin’s “Ingredients”: Struggle for Life • Living things “overreproduce.” There are more offspring born than can be sustained. • Therefore, there is competition for resources to survive and reproduce. • “Nature, red in tooth and claw” - Tennyson

31. Competition • Why aren’t we buried in barn swallows, bacteria, fruit flies, elephants, and everything else? • Limiting factors = things that limit equal survival and reproduction for all individuals include… Limited resources (examples: oxygen, nitrogen, water, food, space) Predation (being eaten, disease) Dangerous environments (climate, disasters) Available mates

32. Darwin’s “Ingredients” • Variation – Everyone’s different • Inheritance – Differences are passed down to offspring • Struggle for Life – Everyone’s in competition to have offspring • Result: Differential reproductive success • Individuals with an ADVANTAGE in the competition will leave more offspring on average

33. Darwin’s “Ingredients” • Repeated over generations, differential reproductive success results in natural selection = Traits that confer a reproductive advantage become more common, traits that confer a disadvantage become less common

34. Populations evolve • Natural selection pressures act on individuals… • differential survival • “survival of the fittest” (not a great term, but famous now ) • differential reproductive success • “someone has more babies” • …but it’s populations that evolve • genetic makeup of population changes over time • Population: collection of individuals of the same species that inhabit the same area and interbreed • Smallest unit in which evolution occurs Mummichog Presence of lactate dehydrogenase

35. Body size & egg laying in water striders Fitness • Fitness: A measure of reproductive success • Equal to average contribution to the gene pool by individuals bearing the trait

36. Mutation Gene Flow Non-random mating Genetic Drift Selection Expanding Upon Darwin’s Ingredients: Agents of evolutionary change

37. 1. Mutation & Variation • Mutation creates variation • Mutation is “rare, regular, and random” • Rare = Happens few million base pairs • Regular = Genome – billions of base pairs, so mutation always happens • Random = Where it occurs, what kind of mutation – random • Mutation changes DNA sequence • changes in protein may change phenotype & therefore change fitness

38. 2. Gene Flow • Gene Flow: the process of genes moving from one population to another • seed & pollen distribution by wind & insect • migration of animals • sub-populations may have different allele frequencies • Immigration = movement in, emigration = movement out • causes genetic mixingacross regions • + variety within pop • - differences between pops

39. 3. Non-random mating • Sexual selection

40. Warbler finch Tree finches Ground finches 4. Genetic drift • Effect of chance, including • Founder effect • Bottleneck effect

41. Founder effect • When a new population is started by only a few individuals • some rare alleles may be at high frequency; others may be missing • skew the gene pool of new population • human populations that started from small group of colonists • example:colonization of New World

42. Distribution of blood types • Distribution of the O type blood allele in native populations of the world reflects original settlement

43. Distribution of blood types • Distribution of the B type blood allele in native populations of the world reflects original migration

44. Out of Africa Likely migration paths of humans out of Africa 10-20,000ya 10-20,000ya 50,000ya Many patterns of human traits reflect this migration

45. Bottleneck effect • When large population is drastically reduced by a disaster • famine, natural disaster, loss of habitat… • loss of variation by chance event • alleles lost from gene pool • not due to fitness • narrows the gene pool

46. Cheetahs • All cheetahs share a small number of alleles • less than 1% diversity • as if all cheetahs are identical twins • 2 bottlenecks • 10,000 years ago • Ice Age • last 100 years • poaching & loss of habitat

47. Peregrine Falcon Conservation issues • Bottlenecking is an important concept in conservation biology of endangered species • loss of alleles from gene pool • reduces variation which reduces adaptability Breeding programs must consciously outcross Golden Lion Tamarin

48. 5. Natural selection • Illustrated example of evolution by natural selection:

49. Natural Variation and Inheritance Elephants within the population have different alleles causing variation in trunk length. Trunk length is genetic. Short Very Short Very Long Long

50. Overreproduction and Struggle for Existence More elephants are born than can survive and reproduce. Food and water are limited, and elephants are in competition with each other for these resources.