Microevolution

1. Chapter 16 Microevolution

2. Questions about fossils • From the 1600s on, geologists began mapping layers of rocks at sites where there were deep cuts in the earth. • They found similar layers around the world and decided that sand and sediments had deposited at different times to form the layers.

4. Within certain layers were distinctive fossils. • Some deeper layers had simple marine organisms, where, other layers had organisms similar to those found at the time.

5. They decided that maybe all organisms had not left one central location, but had originated in more than one place. • If the body structure was different in different areas, then perhaps they had evolved, or modified over time.

6. Old Beliefs • Georges Cuvier was one of the leaders in the naturalist field at this time. • He founded the belief of catastrophism

7. Catastrophism • At the time of creation, the world was populated with all species, but some great catastrophy had destroyed some of the species, and the survivors had repopulated the world. • Other catastrophies through time had wiped out other species, thus leading to the species in existence.

8. Jean Lamarck Acquired Characteristics • Environmental pressure and internal needs bring about a permanent change in body form and function. • This gradual improvement should lead to perfection in time.

9. For example the giraffe: short neck giraffes needed to reach leaves higher in trees, so they stretched their neck to reach them. • This stretched neck was passed on to offspring, who in turn, stretched their own necks and passed on that neck to their offspring.

11. Thomas Malthus • A clergyman and economist, documented human survival and the influences of famine, disease, and war. • Humans would run out of food, living space, and other resources because they reproduce too much. • When this happens, they compete for the remaining resources

12. Charles Lyell • Lyell traveled through Europe to find more evidence that gradual changes, the same we can see happening today, had produced the features of the Earth's surface. • He found evidence for many rises and falls of sea level, and of giant volcanoes built on top of far older rocks. • Processes such as earthquakes and eruptions, which had been witnessed by humans, were enough to produce mountain ranges. • Valleys were the work of the slow grinding force of wind and water.

13. Uniform Processes of Change • Lyell's version of geology came to be known as uniformitarianism, that the processes that alter the Earth are uniform through time. • Lyell could never have grasped the mechanism — plate tectonics — that makes this kind of geological change happen.

14. Lyell had an equally profound effect on our understanding of life's history. • He influenced Darwin so that Darwin envisioned evolution as a sort of biological uniformitarianism. • Evolution took place from one generation to the next before our very eyes, he argued, but it worked too slowly for us to perceive.

15. Charles Darwin • In 1831, he boards the HMS Beagle as the ship’s naturalist. • It sailed to South America to map the coastline and crossed the Atlantic. • Darwin collected much information during the stops at the islands in between. • When he returned in 1836, Darwin began discussing his ideas with other scientists.

16. Darwin wondered if he could apply Malthus’ idea to other animals. • He assumed the environment would restrict the number of individuals, and then what would decide who would live, and who would die. • He then realized that one individual’s traits could determine its survival.

17. He realized that the Galapagos Island finches were representative of this idea. • Thirteen species of finch inhabited the islands. • Each species had a variation in traits that made it suitable to the environment in which it lived.

19. In 1859, he published On the Origin of Species. • He discussed how artificial selection could be used to quickly mold species, and how natural selection could do the same thing in a longer time.

20. Populations evolve, not individuals • Population is a group of the same species occupying a given area. • Certain features characterize a species: morphology (a unique body plan), physiology (the cells operate in the same way), behavior (the organisms respond in the same basic way to stimulus), and they reproduce with members of their own species.

21. There are other small variations within the population, but they are basically the same. • Variations (polymorphism) include differences in color and size.

22. Within a population is a gene pool – all of the genes within the population. • Each gene within the pool exists in two or more slightly different forms (alleles). • Individuals inherit different combinations of alleles leading to different phenotypes.

23. Offspring inherit the genotype not the phenotype • Which alleles are passed on depends on five things: a. Gene mutations – produce new alleles b. Crossing over at meiosis I – produces new allele combinations c. Independent assortment at meiosis I – randomly puts mixes of parental alleles into egg or sperm

24. d. fertilization – combines two parents e. change in chromosome number or structure – can lead to loss, duplication or relocation of genes

25. All five criteria are rarely in play at one time. • Mutations are infrequent, but they are inevitable. • These changes lead to microevolution – small-scale changes in allele frequency as brought about by mutation, genetic drift, gene flow, and natural selection.

26. Natural Selection • Darwin explained natural selection after associating inheritance of certain features and the environment. • He made four observations to explain his ideas:

27. 1. All populations can increase in number over generations 2. No population can increase indefinitely – it will run out of resources Inference: Individuals will end up competing for resources

28. 3. All individuals have the same genes, which specify the assortment of traits 4. Most genes occur in alleles which give rise to different phenotypes

29. Inference: Some phenotypes are better for resource competitions and therefore help them to survive and reproduce. • These alleles increase and the genetic change leads to fitness - an increase in adaptation to the environment

30. Conclusion: Natural selection is the difference in survival and reproduction among individuals that differ in heritable traits. • Adaptation is one outcome of the microevolutionary process.

31. Directional Selection • Allele frequency shifts in a consistent direction. • This shift occurs in response to changes in the environment. • A trait at one end of the range becomes dominant over the midrange traits.

32. Stabalizing Selection • Favoring of the intermediate form of a trait. • This mode counteracts gene flow, mutations and genetic drift to keep the common phenotype.

33. Disruptive Selection • Forms at both extremes are favored over the intermediate form.

35. Gene Flow • Individuals of a species may move about. • Alleles from those individuals are lost when they leave the population. • Physical flow of genes (gene flow) tries to counteract the loss and genetic differences. • It helps keep populations genetically similar.

37. Genetic Drift • Random changes in allele frequency brought about by chance alone.

38. Fixation • Only one kind of allele remains in the population and all individuals are homozygous for it. • It happens in all populations eventually, but quickly in small populations.

39. Bottleneck • Severe reduction in population size brought about by a catastrophy such as disease, habitat loss, hunting, or natural disaster.

40. Inbreeding • Nonrandom mating among closely related individuals. • Human populations discourage such things. • In animal populations, it has lead to rare inheritable diseases that can cause painful death in the animals.

41. http://www.dkimages.com/discover/previews/740/87636.JPG • http://www.cwrl.utexas.edu/~bump/E603B/web06/Mita/Spring%20Portfolio/Lamarck_files/image011.gif • http://cache.eb.com/eb/image?id=54911&rendTypeId=4 • http://www.evoled.org/images/modes.gif • http://evolution.berkeley.edu/evosite/evo101/images/geneflow_beetles.gif • http://fig.cox.miami.edu/~cmallery/150/evol/23x4.jpg