Effects of different forms of selection on evolution

Question 1

What are environmental factors?

Answer 1

• environmental factors help create variation within a population
• these environmental factors may be an agent for constancy or an agent for change according to the type of selection pressure they exert

Question 2

What are the three main types of selection that affect the characteristics of a population?

Answer 2

• stabilising selection
• directional selection
• disruptive selection

Question 3

How does stabilising selection affect the characteristics of a population?

Answer 3

-stabilising selection preserves the average phenotype (phenotypes around the mean) of a population by favouring average individuals, in other words, selection against the extreme phenotypes

Question 4

How does disruptive selection affect the characteristics of a population?

Answer 4

-disruptive selection favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population

Question 5

What is stabilising selection?

Answer 5

• stabilising selection tends to eliminate the extremes of the phenotype range within a population and with it the capacity for evolutionary change
• it tends to occur where the environmental conditions are constant over long periods of time

Question 6

What is directional selection?

Answer 6

• within a population there will be a range of genetically different individuals in respect of any one phenotype
• the continuous variation among these individuals forms a normal distribution curve
• this curve has a mean that represents the optimum value for the phenotypic character under the existing conditions
• if the environmental conditions change, so will the optimum value for survival
• some individuals, either to the left or the right of the mean, will possess a combination of alleges with the new optimum for the phenotypic character
• as a result there will be a selection pressure favouring the combination of alleles that results in the mean moving to either the left of the right of its original position

Question 7

What is disruptive selection?

Answer 7

• although the least common form of selection, it is the most important in bringing about evolutionary change
• disruptive selection occurs when an environmental factor, such as temperature, takes two or more distinct forms

Question 8

What is selection in the peppered moth?

Answer 8

• some species of organisms have two or more distinct forms
• these different forms are genetically distinct but exist within the same interbreeding population
• this situation is called polymorphism
• one example is the peppered moth
• it existed almost entirely in its natural light form until the middle of the nineteenth century
• around this time a melanic (black) variety arose as the result of a mutation
• these mutant moths had undoubtedly occurred before (one existed in a collection made before 1819) but they were highly conspicuous against the light background of the lichen-covered trees and rocks on which they normally rest
• as a result, the black mutants were subjected to greater predation from insect-eating birds, for example, robins and hedge sparrows, than were the better camouflaged light forms
• when in 1848, a melanic form of the peppered moth was captured in Manchester, most buildings, walls and trees were blackened by the soot of 50 years of industrial development
• the sulphur dioxide in smoke emissions killed the lichens that formerly covered trees and walls
• against this black background the melanic form was less, not more, conspicuous than the light form
• as a result the light form was eaten by birds and more frequently than the melanic form and by 1895, 98% of Manchester’s population of the moth was the melanic type
• this is an example of selective predation by birds favouring individuals that lie at one extreme or the other of a range of different colour types
• it illustrates directional selection of different types in different populations
• the melanic form is selected for in industrial areas while the natural form is selected for in rural areas
• it also shows evolution in action whereby there has been a change in the allelic frequency in populations of moths in industrial areas
• however, as the two populations overlap and interred they are still one species
• to become two distinct species, the two populations would need to become reproductively separated from one another

Question 9

what is an example of stabilizing selection?

Answer 9

• -one example is fur length in a particular mammalian species
• in years when the environmental temperatures are hotter than usual, the individuals with shorter fur length will be at an advantage because they can lose body heat more rapidly
• in colder years the opposite is true and those with longer fur length will survive better as they are better insulated
• therefore, if the environment fluctuates from year to year, both extremes will survive because each will have some years when it can thrive at the expense of the other
• if, however, the environmental temperature is constantly 10oC, individuals at the extremes will never be at an advantage
• they will therefore be selected against in favour of those with average fur length
• the mean will remain the same, but there will be fewer individuals at either extreme
• an example of stabilising selection is the body mass of human children at birth
• babies born with a body mass greater or less than the optimum of 3.2kg have a higher risk of dying in the few months after birth

Question 10

what is an example of directional selection?

Answer 10

–an example of this is antibiotic resistance in bacteria

Question 11

what is an example of disruptive selection?

Answer 11

• -in our example this might arise if the temperature alternated between 5oC in winter (favouring long fur length) and 15oC in summer (favouring short fur length)
• this could ultimately lead to two separate species of the mammal - one with long fur and active in winter, the other with short fur and active in summer
• an example of disruptive selection is the Coho salmon where large males and small males have a selective advantage over intermediate-sized males in passing on their alleles to the next generation
• the small males are able to sneak up to the females in the spawning grounds
• the large males are fierce competitors
• this leaves intermediate-seized males at a disadvantage

Question 12

How does directional selection affect the characteristics of a population?

Answer 12

-directional selection changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population, in other words, selection for one extreme phenotype