A high number of alleles (80% ) present in thoroughbred race horses can be traced back to a small number of horses from the 1700's. Therefore, we would expect __ ? a. Abnormally low rates of mutation b. Little varitaion for selection in the present day c. High amounts of random mating d. No evolution over that time period

The correct answer is **b. Little variation for selection in the present day.**<br /><br />Here's why:<br /><br />* **Genetic Bottleneck:** The fact that 80% of alleles trace back to a small number of horses indicates a genetic bottleneck. A bottleneck drastically reduces the genetic diversity of a population. Even as the population grows again, much of the original variation is lost.<br /><br />* **Selection and Variation:** Natural selection acts upon existing variation. With limited variation, there is less opportunity for selection to differentiate between individuals and drive evolutionary change. This doesn't mean evolution *stops*, but it does mean the *rate* of evolution likely slows down considerably, especially for traits linked to those alleles.<br /><br />Let's look at why the other options are incorrect:<br /><br />* **a. Abnormally low rates of mutation:** Mutation rates are generally constant for a given species. While a bottleneck can *indirectly* influence which mutations become fixed in a population, it doesn't directly change the underlying mutation rate.<br /><br />* **c. High amounts of random mating:** The passage doesn't provide information about mating patterns. While random mating can *maintain* existing variation, it doesn't create new variation. Given the bottleneck, even with random mating, the overall variation would still be low.<br /><br />* **d. No evolution over that time period:** Evolution is an ongoing process. Even with reduced variation, some evolution will still occur through mutation, genetic drift, and any selection that can act on the remaining variation. However, the *rate* of evolution is likely to be slower.<br />