Learning Objectives

1. Characterize the patterns of Natural Selection
2. Define evolutionary fitness
3. Explain the different components of fitness
4. Demonstrate how the success of different phenotypes can be compared by calculating their relative fitness 

Chapter Summary

     24.2 focuses highly on Natural Selection. Back in Chp. 23, each "teacher" briefly talked about Natural selection and gave their own definition, so here's mine. Natural Selection is the ability of an organism to develop beneficial traits based solely on its environment. From there, those beneficial traits are heritable and become expressed in the following generations. In addition, as beneficial traits become expressed, detrimental traits become less common. From Chp. 23, we learned about the theory of evolution and how Darwin posed his idea of "Natural Selection." One thing you should know is that Natural Selection ≠ Evolution. Rather that Natural Selection helps lead towards evolution through changes called adaptations (changes in an organisms body to ensure survival in its specific environment). 

     We know now what Natural Selection is, now we have need to relate it to reproductive fitness. Reproductive success is the likelihood of an individual to provide gametes to produce fertile offspring for the next generation. In relation with Natural selection, individuals with beneficial traits have a higher fitness than those with detrimental traits. Reproductive success is achieved in 2 ways: having beneficial traits that make organisms better adapted for their environment to ensure survival to the age to reproduce, and involving traits directly related to reproduction, as the ability to find a mate and produce viable gametes. 

     How does this relate to "fitness" ? Well fitness is the probability that an organism can provide a genotype to the gene pool of the next generation. Going back to Chp. 16, we know that genotypes affect the phenotype that is produced. Ultimately, fitness will show which genotype/phenotype combinations will be shown over time in a population. Fitness is measured in the success of reproduction. So if we had an individual with high fitness and and individual with low fitness, the high fitness individual will have a higher chance to produce offspring, even if the low fitness individual is more physically fit. 

     Measuring fitness isn't as bad as you might think. First, we need to know the average reproductive success of each type of individual (typically 3 due to variations in alleles). So if there is the TT, Tt, and tt gene and we're given that TT = 7 rs (reproductive success), Tt = 5 rs, and tt = 3 rs, we'll be able to determine fitness. First, we find the highest average, and set it as the common denominator for all averages. Since 7 is the largest, it will be the denominator. By that, we should have 7/7, 5/7, and 3/7. The largest will always have a fitness of 1.0 or 100%. Tt will have 71.4% f and tt will have 42.9% f. From this, we can find the mean fitness, which is the average reproductive success of a population. Typically, high fitness values are expressed levels, increasing the mean fitness. From that, natural selection takes place by having organisms better suited for their environment and are able to survive and reproduce!

     Today, there are 4 types of natural selection can be studied: Directional, Stabilizing, Diversifying, and Balancing. 

     Directional selection is when individuals on one extreme of a phenotype shift to the other extreme. This occurs when a new allele is introduced in a population and has a high fitness value. What that means is that the allele would, in time, be more expressed than the current alleles. For example, lets say we have white rabbits in a forest. White isn't the color of choice when trying to survive in a forest. So, we introduce a few brown rabbits into the population with high fitness values. Now what would happen? Well, the introduction of the new allele will affect the gene pool. With high fitness values, the brown allele will have a high expression rate. Ultimately, shifting the the population from the white phenotype to the brown phenotype. Stabilizing selection is when the intermediate phenotypes are favored, which causes a decrease in genetic diversity. Diversifying selection is when two or more phenotypes are favored in an environment. Unlike stabilizing selection, Balancing selection is when a population maintains its alleles. There's no direct favortism of any phenotype. Typically, the heterozygous allele is favored in certain situations. For example, sickle cell anemia is prevalent in Africa where malaria is abundant. However, in the US, malaria isn't prevalent. The heterozygous allele provides the best of both worlds. So, by being heterozygous, you would have sickle cells as well as normal blood cells. What this will do is help protect you from malaria and keep you healthy from experiencing the negative effects of being a full sickle cell anemiac.

Useful Materials

Primary Literature

The Young, the weak, and the Sick: Evidence of Natural Selection by Predation

     This article talks about evolution, mainly natural selection by the process of predation. As we know, predation helps organisms select beneficial traits to ensure survivability. In the natural world, we know that predators normally hunt the young, old, and sick because of survival of the fittest. This will leave the healthy-aged individuals survive, reproduce, and adapt to his or her environment. In the article, the author does a study of yellow-legged gulls to learn information about predation: if they're killed randomly or if there are specific features at play.

     There were two tests performed: predation by trained birds like falcons and hawks, and by shooting them. From these tests, the dead gull would show the sex and age of the individual, the individual's body condition, and any sign of parasitism (internal and external), infection, malformation or chronic disease. Out of the 628 gulls killed (506 shot and 122 killed by predation), a majority of the birds we're either juveniles, elders, or sick. The shooting tests are only used to compare the hunting results, since shooting can't necessarily determine what you've killed is in the wanted category. For each characteristic tested, the hunted category consisted of males that were young, having a weak muscle condition, and are healthy. 

     Now, we can dive into the relationship between predation and natural selection. Based on the results, the author states that unlike shooting, predation isn't randomized, it's a selection based on traits (not necessarily linked). Predators focused mostly on individuals based on age, muscle condition and sickness, which strongly supports the hypothesis that predators prey primarily on substandard individuals. In regards to natural selection, the author states that there could be a couple of traits that affect the fitness of the yellow-legged gulls. Additionally, results showed preferential predation on those individuals with poor muscle condition, suggesting that stabilizing selection could be operating on traits linked to body mass.

Video Lectures

Part 1

Part 2

Powerpoint

Practice Quiz

Grading Sheet

Grading Sheet (Nick)