Question

1 Explain at least 3 ways that selection can preserve genetic variation.

2 Explain how selection affects differently: dominant vs recessive traits; homozygotes vs heterozygotes; and when frequency-dependence is a factor.

3 What is the source of genetic variation? How does meiosis contribute to maintenance of genetic diversity?

4Describe the preconditions for natural selection to operate and what the results of natural selection will be.

5 Use the HW equations to solve simple population genetics problems with and without selection.

6 Hardy-Weinberg equilibrium tells us the conditions under which populations will not evolve. So what types of things may lead to a population evolving? How do these things lead to populations evolving (gene frequencies changing)?

7 When is genetic drift the strongest?

Answer 1

1) Three ways of seceltion procedure preseves the gentic variation.

a) Directional selection: This occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting a population's frequency curve for the phenotypic character in one direction or the other.

b) Disruptive selection: This occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes.

c) Balancing selection: Form of natural selection that maintains two or more phenotypic forms in a population.

2) Dominant vs recessive traits: The traits which are dominant eg. curly hair in human over recessive trait straight hair, because of natural selection process recessive-straight hair is commonly seen in individuals.

Homozygotes vs heterozygotes - A recessive trait will only be expressed when the genes are in recessive form . eg 'aa' and heterozygotes or dominant carrier will be expressed when they are in 'Aa form'. So genes for curly hair shall be expressed when heterozygous condition is seen but straight hair will only be expressed when 'aa' recessive trait is expressed.

Frequency dependence - this factor works in selection process according to the frequency of the individuals present. More the frequency more will be the same individuals seen.

3) Two sources of variation: Mutation and Sexual reproduction. Meiosus being a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. Two process involved in Meiosis help in maintaining genetic diversity.

a) Crossing over: occurs at Prophase I of Meiosis I stage where, homologous chromosomes cross over to exchange the genetic information

b) Random orientation of homologue pairs: Random orientation of homologue pairs in metaphase I allows for the production of gametes with many assortments of homologous chromosomes.

4) The preconditions are excess fecundity i.e to process many offsprings and competition for limited resources. Although weeds do not survive but still they procude more seeds. And among the survived seeds only successful competitors will reproduce. Natural selection will result in population which are dominant in nature as without competitive behavious the weak population will not survive.

7) Genetic drift can impact a population in many ways: it is significant in small population, can cause allele frequencies to change at random, can lead to loss of gentic variation within population, can cause harmful alleles to become fixed. Considering all these implications, genetic drift is the strongest factor.