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5/25/2005 Summer BIO152 Lectures 5 & 6 Key topics this week | Natural selection Darwinian fitness (and adaptations) z Artificial selection z Types (directional, stabilizing, disruptive) z | Alternative explanations: inheritance of acquired traits (‘Lamarckian’) 1 5/25/2005 Natural selection Write out the two conditions which must be met for natural selection to occur: 1. Individuals in a population must vary in characteristics which are heritable. 2. In a particular environment, certain versions of the heritable trait(s) help individuals survive & reproduce more offspring. Natural selection: 4 steps (Chapter 23) 1. 2. 3. 4. Individuals in a population must vary in their characteristics (traits). Some of these variable traits are heritable (passed on to their offspring). Only some offspring survive long enough to reproduce. Individuals with certain traits are more likely to produce more offspring in a given environment. (differential reproductive success) 2 5/25/2005 Evolution (= change in the characteristics of a population over time) Results when differential reproductive success is based on heritable variation. Darwinian fitness The ability of an individual to produce more offspring relative to the ability of other individuals in a population to reproduce. | Darwinian fitness is measurable. Count the offspring How does the meaning of Darwinian fitness differ from common English usage of fitness? | 3 5/25/2005 Adaptation A heritable trait that increases the fitness of an individual in a particular environment relative to individuals without that trait. Adaptations increase [Darwinian] fitness---the ability to produce offspring. Tutorials 23.1 & 23.2 | Check your understanding Compare these two examples of change in beetle populations. Which one is an example of evolution by natural selection? Evolution = change in the [heritable] characteristics of a population over time. http://evolution.berkeley.edu/evosite/evo101/IIIADescent.shtml 4 5/25/2005 Beetles on a ‘diet’ Imagine a year or two of drought in which there are few plants that these beetles can eat. The beetles in the population are a little smaller than the preceding generation of beetles All the beetles have the same chances of survival and reproduction, but is the change in size due to food restrictions? How would you test to see if evolution of the beetles had taken place. | Stated differently, how would you test to see if the size change was due to environmental conditions or to a change in the genetic makeup of the population perhaps due to natural selection? | 5 5/25/2005 4 steps of natural selection Variability | Heritability | Some beetles have more offspring | Beetles with a specific trait have more offspring | Different coloured beetles Most of the beetles in a population (say 90%) are bright green but a few of them (10%) are brown. 6 5/25/2005 Some number of generations later, things have changed: brown beetles are more common than they used to be and make up 70% of the population. Is this evolution? What questions do you need to ask? | Is colour heritable—how would you find out? | Have the proportion of heritable traits changed in the population? 7 5/25/2005 Is this evolution? To see if natural selection had taken place what other questions do you need to ask? 1. 2. 3. 4. | Are there variable traits (yes-green and brown) Is colour heritable (yes) Do some beetles have more offspring? Do brown beetles have more offspring than green? Which example illustrates descent with modification—a change in gene frequency over time? Smaller beetles following limited food Proportion of brown/green beetles 8 5/25/2005 (e.g., 1) The difference in weight due to environmental influences—the low food supply—likely not because of a change in the frequency of genes. Example 1 is not evolution because the small body size in this population was not genetically determined, this generation of small-bodied beetles will produce beetles that will grow to normal size if they have a normal food supply. (2) The changing proportion of color in the population of beetles is evolution: the population of beetles has changed over time from predominately green to predominately brown. 9 5/25/2005 Level of action Natural selection acts on INDIVIDUALS within a population (some individuals survive and reproduce more than other individuals) | Evolution acts on POPULATIONS-the characteristics of the population changes over time. | Natural Selection in action Chapter 23 pp 506-508 Also The Beak of the Finch, John Weirner Peter & Rosemary Grant Galapagos Islands: Daphne Major During severe drought in 1977: The struggle to survive favored the larger birds with deep, strong beaks for opening the hard seeds. Smaller finches with less-powerful beaks perished. 10 5/25/2005 The Grants found that the offspring of the birds that survived the 1977 drought tended to be larger, with bigger beaks. Hypothesis: the adaptation to a changed environment led to a larger-beaked finch population in the following generation. 11 5/25/2005 Did natural selection take place? Variable beak size—yes | Heritable- (yes, birds with larger beaks had offspring with larger beaks) | Differential reproductive success-yes, huge die back, only birds with larger beaks survived | Unusually rainy weather in 1984-85 ► More small, soft seeds & fewer of the large, tough ones. The birds best adapted to eat those seeds because of their smaller beaks were the ones that survived and produced the most offspring. Evolution had cycled back the other direction. It is possible to see evolution happen! 12 5/25/2005 More evolution in action Chapter 23 Mycobacterial tuberculosis resistance to antibiotics (also lab 3) plus tutorial 23.1 Alpine skypilot plants plus tutorial 23.2 13 5/25/2005 Natural Selection: Case Study 3 http://evolution.berkeley.edu/evosite/evo101/IIIE1aTegula .shtml Some species live only in deep water, and some only live much higher up the shore. Tegula funebralis, a snail, can be found in both (California): In Southern California, Tegula live high up on the shore, while in Northern California, they live in deeper water. NS: Snails in California cont. Can natural selection explain the different niches? (Michael Fawcett study) He found that predators, such as octopi, starfish, and crabs, were more abundant in southern California than in northern California. 14 5/25/2005 Snail distribution Hypothesis: Intense predation in the south selected for snails that lived higher up the shore, out of reach of many predators. In the north, selection might not have been as strong, so the snails were not selected to live high on the shore. Snail Experiment M. Fawcett took northern and southern snails, released them in deep water in the south and watched what happened. If predators were around, all the snails high-tailed it towards higher ground (snails can probably sense the chemicals exuded by predators). 15 5/25/2005 Results Southern snails moved further up the shore faster than northern snails. Because the northern snails were slower and didn’t move high enough, they were more likely to be eaten by predators. What did this experiment show? 1. 2. There is a difference between southern and northern snails (i.e., some difference that is not merely a function of being on a southern or northern shore). This difference is probably genetic (but we would need to do more experiments to be absolutely sure). This difference can lead to differential survival. If predation is intense, snails that move higher faster are more likely to survive. 16 5/25/2005 Natural selection again? | Variation: There is variation in a trait between and within populations. | Heredity: The variation probably has a genetic basis. | Differential reproduction: The variants of the trait have different probabilities of surviving to reproduction. These three features define natural selection. Without them, natural selection does not happen. http://evolution.berkeley.edu/evosite/evo101/IIIE1aTegula.shtml 23.4 The Nature of Natural Selection and Adaptation Although natural selection appears to be a simple process, research has shown that it is often misunderstood. | 17 5/25/2005 Selection acts on individuals, but evolutionary change occurs in populations Individuals do not change during natural selection. Those that are selected simply produce more surviving offspring than other individuals do, causing a change in the genetic makeup of the population. 18 5/25/2005 Acclimation occurs when an individual changes in response to changes in the environment (e.g., beetles changed in size from less food) |, but adaptation occurs only when a population changes in response to natural selection. | Preparation for Thursday Lab 2: print, read, prepare table (2 copies!) also Major Project- copy of log Friday-Wednesday; write up your method | Read Chap 24.2 types of selection: | Directional z Stabilizing z Disruptive Do tutorial 24.2 modes of Natural Selection z 19