Chapter 15.3, Problem 3MI

Summary Introduction

To write:

Correct vocabulary term for the given definitions.

Introduction:

Charles Darwin in his theory of natural selection proposed that natural selection could modify a population to produce new species. Mechanisms of evolution like genetic drift, gene flow, nonrandom mating, mutation and natural selection violate the Hardy- Weinberg Principle.

Answer to Problem 3MI

1. Hardy- Weinberg Principle
2. Founder effect
3. Bottleneck
4. Prezygotic isolating mechanism
5. Genetic drift
6. Stabilizing selection
7. Postzygotic isolating mechanism
8. Directional selection
9. Disruptive selection
10. Sexual selection
11. Allopatric speciation
12. Sympatric speciation

Explanation of Solution

1. Hardy- Weinberg Principle- Hardy and Weinberg showed mathematically that evolution will not occur in a population unless allelic frequencies are acted upon by forces that cause change. When these forces are absent, the allelic frequencies remain the same and evolution does not occur. This idea came to be known as the Hardy- Weinberg principle.
2. Founder effect- Genetic drift may occur when a small population separates from a big population and settles in a new location. Because the small population is a random subset of the original population it carries a random subset of the population’s genes. Uncommon genes may become common in the new population and the offspring will carry those alleles. This can result in large genetic variations in the separated populations.
3. Bottleneck- This occurs when a population declines to a very low number and then rebounds. The gene pool of the rebound population is genetically similar to the population at its lowest level. Hence it has reduced diversity.
4. Prezygotic isolating mechanism- This is type of reproductive isolation mechanism which operates before fertilization occurs. It prevents reproduction by making fertilization unlikely. This mechanism prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral, or other differences.
5. Genetic drift- In large populations we know that there are enough alleles that “drift” to ensure that the allelic frequency of the entire population remain constant from one generation to another. In smaller populations, the genetic drift is more pronounced and there is high chance of losing an allele.
6. Stabilizing selection- It is the most common forms of natural selection. It occurs to eliminate the extreme expressions of a trait when the average expression leads to higher fitness.
7. Postzygotic isolating mechanism- This occurs when fertilization has taken place but a hybrid offspring cannot develop or reproduce. The hybrid remains infertile. It prevents offspring survival or reproduction.
8. Directional selection- If an extreme version of a trait makes an organism more fit, directional selection might occur. This form of selection increases the expression of the extreme version of a trait in a population.
9. Disruptive selection- It is a process that splits a population into two groups. It tends to remove individuals with average traits but retains those expressing extreme traits at both ends of a continuum.
10. Sexual selection- This is a type of selection which functions in a population where males and females are quite different in appearance. Usually males are largest and most colorful in the population. The changes in traits are due to competition for mates.
11. Allopatric speciation- When a species evolve into a new species without a physical barrier, it is called sympatric speciation. The ancestor species and the new species live side by side during the speciation process. This method is quite common in plants especially through polyploidy.
12. Sympatric speciation- In allopatric speciation a physical barrier divides one population into two or more populations. This is the most common form of speciation.