Here’s some food for thought. Most animal populations have a 50:50 ratio of male
ID: 72597 • Letter: H
Question
Here’s some food for thought. Most animal populations have a 50:50 ratio of males to females. This does not have to be so: it is theoretically possible for parents to produce predominantly male offspring, or predominantly female offspring. Imagine a monogamous population with a male-biased sex ratio, say 70% males and 30% females. Which sex will have the easier time finding a mate? As a result, which sex will probably have higher fitness? Accordingly, which parents will have higher fitness – those that produce offspring of the more common or the less common sex? One could imagine the same population with a female biased sex ratio and ask the same questions.
a. Heterozygote advantage. b. Disruptive selection. c. Positive frequency dependent selection. d. Negative frequency dependent selection. e. Random genetic drift.
Explanation / Answer
a. Heterozygote advantage.
Heterozygous advantage, also called overdominance, refers to the fitness superiority of heterozygotes over homozygotes.
When heterozygotes are fitter than the homozygotes, natural selection will maintain a polymorphism.
The classic example of heterozygous advantage is sickle cell anemia where humans who are homozygotic for sickle shaped cells suffer from a near lethal condition.
Heterozygote advantage, frequencydependent selection and some modes of selection in fluctuating environments are all forms of balancing selection, which actively maintains genetic variation.
b. Disruptive selection.
Disruptive selection is a type of natural selection that selects against the average individual in a population. The make up of this type of population would show phenotypes of both extremes, but have very few individuals in the middle. Disruptive selection is the rarest among the different types of natural selection.
c. Positive frequency dependent selection.
Positive frequency-dependent selection selects for common phenotypes in a population and decreases genetic variance.
In positive frequency dependent selection, the fitness of a phenotype increases as it becomes more common.
an example of positive frequency-dependent selection might be in the development of warning colouration, in plants or insects, to prevent predation by birds or mammals. Warning colouration is only effective when it is 'common knowledge' that the colouration symbolises that the plant or insect is poisonous. While the colouration is a rare phenotype, it is unrecognised by predators and the plant or insect will be eaten regardless. Hence, the colouration phenotype requires an abundant frequency in the population in order for it to have increased fitness.
d. Negative frequency dependent selection.
Negative frequency-dependent selection selects for rare phenotypes in a population and increases a population's genetic variance.
In negative frequency dependent selection, the fitness of a phenotype decreases as it becomes more common. In other words, less frequent phenotypes have higher fitnesses than common ones.
Ex. Maintenance of a 50:50 sex ratio: If one sex becomes more common, some of its members will not be able to mate whereas al members of the less frequent sex will mate. This will result in higher fitness for the rare sex and the sex ratio will return to the balance.
e. Random genetic drift.
Genetic drift is the change in the frequency of an allele in a population due to random sampling and the random events that influence the survival and reproduction of those individuals.
Genetic drift, also called genetic sampling error or Sewall Wright effect, a change in the gene pool of a small population that takes place strictly by chance. Genetic drift can result in genetic traits being lost from a population or becoming widespread in a population without respect to the survival or reproductive value of the alleles involved. A random statistical effect, genetic drift can occur only in small, isolated populations in which the gene pool is small enough that chance events can change its makeup substantially.