THE EVOLUTION OF POPULATIONS THE EVOLUTION OF POPULATIONS

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1 WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 8 THE OF POPULATIONS Figure 8-10, part 1 Evolution defined. THE OF POPULATIONS TIGER POPULATION Allele frequencies: Proportion of orange fur-pigment alleles in the population Proportion of white fur-pigment alleles in the population Evolution is a change in the allele frequencies of a population over time. For example, a change in the proportion of pigment alleles in the population of tigers means that evolution has occurred. Figure 8-10, part 2 Evolution defined.

2 CHAPTER 8 AND NATURAL SELECTION, PAGE 9 MUTATION A mutation can create a new allele in an individual. When this happens, the population experiences a change in its allele frequencies and, consequently, experiences ARY CHANGE: MUTATION DNA Mutagen Normal basepair sequence Mutated basepair sequence Normal protein Mutated protein Normal phenotype Mutated phenotype Despite mutation s vital role in the generation of variation, mutations almost always cause early death or lower the reproductive success of an organism. Figure 8-11 Agents of evolutionary change: mutation.

3 WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 10 POPULATION BEFORE Allele frequencies: cleft chin (dominant) smooth chin (recessive) Neither allele is related to reproductive success. Inheritance is based solely on chance. POPULATION AFTER There are now more recessive alleles in the population than before. REPRODUCTION In this example, a heterozygous couple (Cc) could have two children that are homozygous recessive (cc), causing an increase in the proportion of recessive alleles in the population. A population can experience random changes in allele frequency that do not influence reproductive success and, consequently, the population experiences FIXATION Genetic drift leads to fixation when an allele s frequency becomes 100% in a population. If this occurs, there is no longer genetic variation for the gene. Figure 8-12 Agents of evolutionary change: genetic drift. SOURCE POPULATION Allele frequencies: 5 digits per hand (recessive) >5 digits per hand (dominant) : FOUNDER EFFECT A group of individuals may leave a population and become the founding members of a new, isolated population. FOUNDER EFFECT The founding members of a new population can have different allele frequencies than the original source population and, consequently, the new population experiences NEWLY FOUNDED POPULATION The new population will be dominated by the genetic features present in the founding members. Figure 8-13 One way that genetic drift occurs: founder effect.

4 CHAPTER 8 AND NATURAL SELECTION, PAGE 11 Occasionally, famine or disease or rapid environmental change may cause the deaths of a large, random proportion of the individuals in a population. SOURCE POPULATION : BOTTLENECK EFFECT BOTTLENECK EFFECT The surviving members of a catastrophic event can have different allele frequencies than the source population and, consequently, the new population experiences EXTREME AND RAPID ENVI- RONMENTAL CHANGE NEW POPULATION The new population will be dominated by the genetic features present in the surviving members. All cheetahs living today can trace their ancestry back to a dozen or so individuals that happened to survive a population bottleneck about 10,000 years ago! Figure 8-14 Another way that genetic drift occurs: bottleneck effect. 1 MIGRATION (GENE FLOW) BEFORE MIGRATION Two populations of the same species exist in separate locations. In this example, they are separated by a mountain range. Population 1 Population 2 MIGRATION After a group of individuals migrates from one population to another, both populations can experience a change in their allele frequencies and, consequently, experience 2 MIGRATION A group of individuals from Population 1 migrates over the mountain range. Population 1 Population 2 3 AFTER MIGRATION The migrating individuals are able to survive and reproduce in the new population. Population 1 Population 2 Figure 8-15 Agents of evolutionary change: migration (gene flow).

5 WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 12 Figure 8-16 part 1 Necessary conditions for natural selection: 1. Variation for a trait. Figure 8-16 part 2 Necessary conditions for natural selection: 1. Variation for a trait.

6 CHAPTER 8 AND NATURAL SELECTION, PAGE 13 Figure 8-17 Necessary conditions for natural selection: 2. Heritability. The tiniest dog in a litter has reduced differential reproductive success. Its more robust siblings prevent access to the food it needs to grow and thrive. Figure 8-18 Necessary conditions for natural selection: 3. Differential reproductive success.

7 WHAT IS LIFE? A GUIDE TO BIOLOGY, ART NOTEBOOK, PAGE 14 BY NATURAL SELECTION: A SUMMARY 1 VARIATION FOR A TRAIT Different traits are present in individuals of the same species. 2 HERITABILITY Traits are passed on from parents to their children. 3 DIFFERENTIAL REPRODUCTIVE SUCCESS In a population, individuals with traits most suited to reproduction in their environment generally leave more offspring than individuals with other traits. NATURAL SELECTION When these three conditions are satisfied, the population s allele frequencies change and, consequently, evolution by natural selection occurs. Figure 8-19 Agents of evolutionary change: natural selection. NATURAL SELECTION IN NATURE 1 VARIATION FOR A TRAIT Running speed in rabbits can vary from one individual to the next. Speed 2 HERITABILITY The trait of running speed is passed on from parents to their offspring. 3 DIFFERENTIAL REPRODUCTIVE SUCCESS In a population, rabbits with slower running speeds are eaten by the fox and their traits are not passed on to the next generation. Figure 8-20 Removing the losers.