Lecture 3 - Evolutionary Significance of Genetic Variation

Question 1

Genotype

Answer 1

a

Question 2

Phenotype

Answer 2

a

Question 3

Heritable variation

Answer 3

a

Question 4

Mutation

Answer 4

a

Question 5

Regulatory gene

Answer 5

a

Question 6

Structural gene

Answer 6

a

Question 7

Genetic polymorphism

Answer 7

a

Question 8

Quantitative inheritance

Answer 8

a

Question 9

Gene-Environment inheritance

Answer 9

a

Question 10

Evolutionary significance of genetic variation

Answer 10

• In the Origin of Species, Darwin provided considerable evidence that biological evolution has occurred.
• More important was that he provided a causal explanation for the origin of biological diversity: The Theory of Natural Selection.

Question 11

Evolutionary significance of genetic variation:

Heritable variation

Answer 11

• The starting point of Darwin’s theory is the occurrence of heritable variation: differences among individuals in morphology, behaviour, and reproductive performance that have a genetic basis.
• For Darwin, this variation was an incontrovertible fact, although he did not understand the mechanisms of inheritance or how hereditary variation arises.
• Darwin argued that certain variations were more advantageous than others for the survival and reproduction of their possessors.
• Thus organisms having advantageous variations are more likely to survive and reproduce than organisms lacking them.
• The process results in evolution by natural selection.
• Natural selection is the fundamental process responsible for organic evolution.
• However, natural selection can only occur if there is hereditary (genetic) variation.
• The more genetic variation there is in a population, the greater the opportunity for the operation of natural selection.

Question 12

R. A. Fisher & heritable variation

Answer 12

• In 1930 the mathematical geneticist R.A. Fisher demonstrated that the amount of genetic variation with respect to fitness is directly correlated with the rate of evolutionary change by natural selection.
• Fitness is the genetic contribution of an individual to succeeding generations, relative to the contributions of other individuals in the population.
• This correlation is stated in Fisher’s Fundamental Theorem of Natural Selection: “The rate of increase in fitness of a population at any time is equal to its genetic variance in fitness at that time.”

Question 13

Kinds of variation:

Genotype & Phenotype

Answer 13

• What kinds of variation occurs in natural populations?
• Genetic polymorphism occurs when two or more alleles at a given gene locus occur in the same population, each with appreciable frequency.
• If the alleles have major effects on phenotype, this can result in discontinuous patterns of variation.
• Such patterns are known as morphological polymorphisms; examples include eye colour and industrial melanism.
• Sometimes polymorphism involves biochemical characters; examples include blood groups and enzymes.
• Quantitative inheritance occurs when a character is controlled by many genes, each of which alone has only a very small influence on the character.
• In general, quantitative traits exhibit continuous distributions and their expression is usually affected by environmental factors.
• As a result it is often not feasible to determine the precise number of genes involved or their individual effects.
• Instead the study of quantitative inheritance rests on the statistical analysis of variation and involves partitioning the observed phenotypic variance into genetic and environmental components:
  Genotype × Environment = Phenotype
• Much of the nature-nurture controversy concerning the biological basis of various human conditions (e.g. race & IQ, sexual preferences, etc.) involves the problem of distinguishing the relative contribution of genetic and environmental factors to a given character.
• In most cases, as in studies of humans, both factors are confounded preventing a clear answer. Experiments with clonal organisms, where an organism can regenerate by asexual means (e.g. most perennial plants, water fleas, aphids), enable biologists to grow different genotypes in a range of environments and thus separate genetic and environmental factors.
• Quantitative characters are often those that are most important to the survival and reproduction of organisms including: size, height, weight, offspring number, growth rate.
• Hence work in quantitative genetics is of prime importance for understanding the evolution of populations.

Question 14

Mutation

Answer 14

• Origin of genetic variation
• Mutation is the ultimate source of genetic variation and thus makes evolution possible.
• Mutation can be viewed as a permanent change in a gene, such as an alteration of its nucleotide sequence.
• These changes can be thought of as random chemical mistakes that occur during the process of replication.
• We will briefly review what defines a gene and introduce the notion of two broad kinds of genes: structural (building proteins in the body) and regulatory genes (genes whose products regulate the operation of other genes) though it should be realized that this is an over-simplification of gene classification.
• Mutations can be classified as (a) gene (or point) mutations, and (b) chromosomal (or structural) mutations.

- Mutation rates vary from less than 10–9 to more than 10–4. Even within the same organism mutation rates for different genes may differ by 3 orders of magnitude.
Mutations can range from 
(a) neutral
(b) deleterious
(c) lethal
(d) beneficial.

• The vast majority of newly arisen mutations are harmful to their carriers; mutations may become beneficial if organisms enter novel environments.
• We will look at a few kinds of mutations: Point mutations, Insertions, Deletions, Gene copy number and Regulatory changes.