[NARRATOR:] Across the American Southwest, golden deserts, dotted by cacti and brush, stretch for miles. Yet here in New Mexico's Valley of Fire, the landscape changes dramatically. Patches of black rock interrupt the sand, remnants of volcanic eruptions that occurred about 1000 years ago. The eruptions spewed a river of lava more than 40 miles long, across the desert. As the molten rock cooled, it darkened, leaving any creature dependent on camouflage in serious trouble. [DR. CARROLL:] In the complex battle of life, one of the constant struggles is between seeing and not being seen, the evolutionary game of hide and seek. And we've come here to the Valley of Fire in New Mexico, a battlefield, to find one of the tiniest soldiers and what it can teach us about how evolution works. [NARRATOR:] On the desert sands, the rock pocket mouse blends in perfectly, its light-colored fur concealing it from predators. But on dark lava, the same fur makes the mouse stand out, attracting the many creatures that see it as food. [DR. NACHMAN:] These mice are the Snickers bar of the desert. They're eaten by foxes, and coyotes, and rattlesnakes, and certainly by owls and maybe even occasionally hawks. And most of those predators are visual predators. [NARRATOR:] So what happened to the pocket mice that found themselves on this new terrain? When I accompany biologist Michael Nachman onto the lava, it doesn't take long to find out. [DR. NACHMAN:] Oh, this one is closed. [DR. CARROLL:] Does it have something in it? [NARRATOR:] Nachman has been collecting mice, unharmed, in traps. [DR. CARROLL:] And it's a dark one. [DR. NACHMAN:] It is. [DR. CARROLL:] Now are most of the ones you find up here dark? [DR. NACHMAN:] Almost all of them. [NARRATOR:] Not only have the mice here evolved to be as dark as the rock, the color change has occurred precisely where it will conceal them from hunters. [DR. CARROLL:] It has a bit of a white underbelly too. [DR. NACHMAN:] That's right. All of the dark ones here and on other lava flows have a white underbelly and presumably there is no selection for dark on the belly because predators are coming from above. [NARRATOR:] Left to themselves, the mice show no preference for light or dark rocks. It's the predators that have made the difference. [DR. NACHMAN:] The change in color over evolutionary time in the population is driven by predators weeding out the mice that don't match their background. [NARRATOR:] But how did the dark mice arise in the first place? [DR. NACHMAN:] When a black mouse appears in a light population of mice, that is usually going to be due to a new mutation. And those are random and rare events. [NARRATOR:] To fully understand the pocket mouse transformation, Nachman moves from the lava to the lab. He and his team extract DNA from light and dark mice taken from one desert region. The aim? To find one or more genetic mutations that caused dark coloration. A mutation is a change in the chemical letters that make up our genes. It's a copying error that may occur when our cells divide. [DR. CARROLL:] Mutation seems to mean that something bad has happened. Well, mutations are neither good or bad. Whether they are favored, or whether they are rejected, or whether they're just neutral, depends upon the conditions an organism finds itself. So, for the pocket mouse, a mutation that caused the mouse to turn black, that is good if you're living on black rock, and it's bad if you're living out in the sandy desert. [DR. NACHMAN:] The light mice are all on the bottom: here, here, here... [NARRATOR:] Fur color is a trait controlled by many genes. To figure out how dark mice evolved, Nachman focuses on how these genes differ in dark and light mice. One by one the genes prove identical. But at last, something does turn up. The difference between dark and light mice boils down to a difference of four chemical letters in a gene called Mc1r. Because the gene controls the amount of dark pigment in a mouse's hair follicles, a mouse with these mutations grows dark fur, which gives it an advantage on a dark background. But still, that's one mouse. How would its dark fur spread to a whole population? [DR. NACHMAN:] This lava flow is about a thousand years old. And so you might wonder, has there been enough time? It's only been a thousand years. It's a very short period of time for a new mutation to come along and spread, so that all of the mice on this lava flow are black. Because really, they all are. [NARRATOR:] Indeed, such a rapid spread of a mutation may seem unlikely-- until you do the math. [DR. CARROLL:] And the reason is, that while only one new mouse born in 100,000 may be black, hundreds of thousands of mice are born in any given year. And then those mice that are black have enough advantage that their babies do better and they have more offspring. And their offspring have more offspring. And just about a 5% advantage compounded year in and year out can very quickly turn the whole population black as we see today. [NARRATOR:] If dark color gives mice a 1% competitive advantage, and you start with 1% of the population being dark, in about 1000 years, 95% of the mice will be dark. If instead the dark color gives them a 10% advantage, then it only takes 100 years. Thanks to Nachman's mice, science has an example of evolution, crystal clear in every detail. [DR. NACHMAN:] What's exciting about this is that we have a system that's very simple ecologically. You have dark rocks and you have light rocks. And you have dark mice and light mice. It couldn't be simpler. [DR. CARROLL:] We know who the predators are, what the selective force is. We know precisely the genetic basis of what makes the mice have an advantage or a disadvantage depending upon where they live. All the pieces are finally together. It's a perfect illustration of Darwin's process of natural selection. [NARRATOR:] In fact, it's more than that. For Nachman's mice also counter a common misconception: that evolution is a random process. [DR. CARROLL:] Well, there is one random component, and that's the process of mutation. Mutations occur at random throughout our DNA. Every new organism is born with a new set of mutations. But while mutation is random, natural selection is not. Natural selection sorts out the winners and losers and that's really what the whole process of evolution is driven by. [NARRATOR:] But if natural selection is not random, would it produce the same result under the same conditions? It does. And here's proof. Rock pocket mice collected by Nachman from other lava flows in other parts of the Southwest. [DR. NACHMAN:] These are two different black mice and they each evolved on different lava flows. And the lava flows are hundreds of miles apart but the changes, the genetic changes that made these mice black, were different in each case. And what's amazing to me is how similar the black mice are. We didn't know when we started this whether we would find that they were the same genes or different genes. And we were really surprised to find that they were completely different genes. And yet, if you look at the mice they look almost identical. [NARRATOR:] Clearly, there are different genetic ways to make a mouse dark. But once the beneficial mutations appear, natural selection, the non-random part of evolution, can, under very similar conditions, favor very similar adaptations. [DR. NACHMAN:] In effect, each of these lava flows is like rewinding the tape of life and allowing evolution to occur again and again. And in each case, we find the dark mice have evolved. [NARRATOR:] The rock pocket mice show us that evolution can and does repeat itself... and why evolutionary change is never-ending. As environments transform, so must the species that inhabit them, adapting and re-adapting in the great and complex battle of life.
Table of contents
- 1. Introduction to Biology2h 40m
- 2. Chemistry3h 40m
- 3. Water1h 26m
- 4. Biomolecules2h 23m
- 5. Cell Components2h 26m
- 6. The Membrane2h 31m
- 7. Energy and Metabolism2h 0m
- 8. Respiration2h 40m
- 9. Photosynthesis2h 49m
- 10. Cell Signaling59m
- 11. Cell Division2h 47m
- 12. Meiosis2h 0m
- 13. Mendelian Genetics4h 41m
- Introduction to Mendel's Experiments7m
- Genotype vs. Phenotype17m
- Punnett Squares13m
- Mendel's Experiments26m
- Mendel's Laws18m
- Monohybrid Crosses16m
- Test Crosses14m
- Dihybrid Crosses20m
- Punnett Square Probability26m
- Incomplete Dominance vs. Codominance20m
- Epistasis7m
- Non-Mendelian Genetics12m
- Pedigrees6m
- Autosomal Inheritance21m
- Sex-Linked Inheritance43m
- X-Inactivation9m
- 14. DNA Synthesis2h 27m
- 15. Gene Expression3h 20m
- 16. Regulation of Expression3h 31m
- Introduction to Regulation of Gene Expression13m
- Prokaryotic Gene Regulation via Operons27m
- The Lac Operon21m
- Glucose's Impact on Lac Operon25m
- The Trp Operon20m
- Review of the Lac Operon & Trp Operon11m
- Introduction to Eukaryotic Gene Regulation9m
- Eukaryotic Chromatin Modifications16m
- Eukaryotic Transcriptional Control22m
- Eukaryotic Post-Transcriptional Regulation28m
- Eukaryotic Post-Translational Regulation13m
- 17. Viruses37m
- 18. Biotechnology2h 58m
- 19. Genomics17m
- 20. Development1h 5m
- 21. Evolution3h 1m
- 22. Evolution of Populations3h 52m
- 23. Speciation1h 37m
- 24. History of Life on Earth2h 6m
- 25. Phylogeny2h 31m
- 26. Prokaryotes4h 59m
- 27. Protists1h 12m
- 28. Plants1h 22m
- 29. Fungi36m
- 30. Overview of Animals34m
- 31. Invertebrates1h 2m
- 32. Vertebrates50m
- 33. Plant Anatomy1h 3m
- 34. Vascular Plant Transport2m
- 35. Soil37m
- 36. Plant Reproduction47m
- 37. Plant Sensation and Response1h 9m
- 38. Animal Form and Function1h 19m
- 39. Digestive System10m
- 40. Circulatory System1h 57m
- 41. Immune System1h 12m
- 42. Osmoregulation and Excretion50m
- 43. Endocrine System4m
- 44. Animal Reproduction2m
- 45. Nervous System55m
- 46. Sensory Systems46m
- 47. Muscle Systems23m
- 48. Ecology3h 11m
- Introduction to Ecology20m
- Biogeography14m
- Earth's Climate Patterns50m
- Introduction to Terrestrial Biomes10m
- Terrestrial Biomes: Near Equator13m
- Terrestrial Biomes: Temperate Regions10m
- Terrestrial Biomes: Northern Regions15m
- Introduction to Aquatic Biomes27m
- Freshwater Aquatic Biomes14m
- Marine Aquatic Biomes13m
- 49. Animal Behavior28m
- 50. Population Ecology3h 41m
- Introduction to Population Ecology28m
- Population Sampling Methods23m
- Life History12m
- Population Demography17m
- Factors Limiting Population Growth14m
- Introduction to Population Growth Models22m
- Linear Population Growth6m
- Exponential Population Growth29m
- Logistic Population Growth32m
- r/K Selection10m
- The Human Population22m
- 51. Community Ecology2h 46m
- Introduction to Community Ecology2m
- Introduction to Community Interactions9m
- Community Interactions: Competition (-/-)38m
- Community Interactions: Exploitation (+/-)23m
- Community Interactions: Mutualism (+/+) & Commensalism (+/0)9m
- Community Structure35m
- Community Dynamics26m
- Geographic Impact on Communities21m
- 52. Ecosystems2h 36m
- 53. Conservation Biology24m
1. Introduction to Biology
Natural Selection and Evolution
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