Evolution and selection of quantitative traits (continuous traits)

Question 1

what are the three forms of selection that act of quantitative traits?

Answer 1

stabilising, directional and disruptive

Question 2

what is the consequence of directional selection on the frequency distribution of quantitative traits?

Answer 2

shifts the mean value to the left or right depending the trait and selection

Question 3

how can you measure the heritability of a trait?

Answer 3

look at the resemblance between relatives- work out the covariance

Question 4

what is heritability?

Answer 4

• if you plot a graph showing the correlation between the mid parent value and the mean value of the offspring- the gradient of the regression line if the heritability.
  you can calculator the heritability by calculating the covariance and then divide it by the variance of mid parents
• the covariance of the offspring and mid parental values- 1/2 Va
• the variance of th mid parents values= 1/2 Vp

Question 5

what does the variance of the mid parents values vs offspring mean?

Answer 5

1/2 Vp

Question 6

what is the covariance between offspring and mid parent values equal?

Answer 6

1/2 Va

Question 7

how is heritability and directional selection linked?

Answer 7

if you know the heritability of the trait (which can be calculated) and you know the s value then you can calculate the response to selection - the different between mean of progeny and mean of population

Question 8

what is S in terms of selection?

Answer 8

Selection differential- difference between mean of parents compared to mean of population

Question 9

what is R?

Answer 9

The response to selection- difference between mean of progeny and population mean

Question 10

what is the definition of directional selection

Answer 10

selection that causes a directed change in a character. In the case of 2 alleles at a single locus this looks like. it will lead to a fixation of one allele and a loss of the other

Question 11

given that you have found that you can use the heritability of a trait and the s value to calculate heritability, was experiment can you now perform?

Answer 11

artificial selection

Question 12

when performing an artificial experiment, what two types of response can you look at?

Answer 12

direct response- changes to the trait under selection
correlated response- changes to other traits as a result of selection e.g sexual selection- can see if a change in sexual dimorphism changes female preference

Question 13

what experiment can be performed to look at direct responses and correlated responses in selection?

Answer 13

can look at the stalk eyed fly and how selecting the length of eye span causes a correlated response in female choosiness.

Question 14

explain the correlated and direct response stalk eyed fly experiment

Answer 14

• form two lines- those that had the longest eye span relative to body size and those that had the shortest eyespot relative to body size
• also had a control where mating partners were randomly picked

Question 15

explain the correlated and direct response stalk eyed fly experiment

Answer 15

• form two lines- those males that had the longest eye span relative to body size and those males that had the shortest eyespot relative to body size
• also had a control where mating partners were randomly picked
• this showed that there was a selective response in eye span and that therefore the trait was heritable
• so you know that there is a direct response, but is there a correlate response
• puts females in choice chambers and see whether there is a correlation between female choice and eye span size
• yes- control females and large females went for large males
• but small females when for small males

Question 16

describe an experiment other than the stalk eyed fly experiment that shows correlated response with explanation

Answer 16

selection for cold shock, starvation and lifespan was showed to increase ability to cope with starvation – this is thought to be as a result of selection for the ability to accumulate metabolic energy reserves.

• selection for dessication resistance also improved resistance to heat shock tests
• correlated responses can be attributed to the selection of pathways that have multiple roles or contribute to multiple mechanisms for example there is a great correlation response for selection for starvation resistance and dessication resistance- this has been attributed to increased levels of glycogen in both phenotyps suggesting that selecting for this increases selection for other.

Question 17

explain an experiment which looks at directional selection in the natural field

Answer 17

look at bill size in darwins finches

• there is a great diversity in this species mostly through their beaks for the different sources of food
• see variation in the thickness of the bill- different abilitis to crack open certain types of seeds
• in 1977 there was a draught- draught resistant plants have large seeds- imposes directional selection on stronger beaks
• this leads to an increase in average beak depth
• however when there is a very wet season there is a selectin for thinner beaks
• all this variation in the beaks sacts in concert
• these changeable enviroments gives episodes of directional selection that ae constantly reversing- fluctuating directional selection can maintain variability in traits

Question 18

what is stabilising selection?

Answer 18

selection that favours intermediate values of quantitative traits e.g. clutch size

Question 19

explain an experiment which looked at the forces of stabilising selection

Answer 19

utch size

• the more eggs a bird raises the more progeny that she will have- but individual parenst are limited by the amount of food that they can bring to the next
• so need an intermediate number
• scientists changed the number of eggs within the nest and in some cases they removed them
• they did this to minimse environmental effects that could confound natural variation in clutch size such. Exemplar effects: variation in parental reering ability or variation in quality of the breeding territory. They could also control the ability of parents to look after their progeny and the size of the natural clutch that they laid. This removed as many variables as possible
• they found that more eggs produced by fledgelings but they also looked average weight and size of fledglings was reduced as this number increased. This suggests that larger broods were less well nourished. This show that middle values are best. This showed that the net effect was increasing brood size saw a steady decline in probability of amembers becoming a breeding recruit
• overall pattern: net recruitment highest for original natural clutch size

Question 20

what is disruptive selection?

Answer 20

selection that favours extreme values of the quantitative phenotypes

Question 21

give an example of dsiruptive selection in nature and give an explanation for it

Answer 21

black bellied seed cracker- there are two peaks in bill width- bimodal distribution

• Each morph Is not reproductively isolated- they interbreed, th morphs are not segregated by habitat the co occur in flocks. There is no phenotypic plasticity either- the bill morph is clear before they birds start to feed.
• An empirical study:
• Thought that this difference in morph is due to a mutation in a gene coding for jaw muscles. It is postulated that there is a close developmental integration between jaw musculature, skeletal morphology and bill size. Mechanotransduction results in the mechanical stress caused by increased contractile jaw musculature being translated into a skeletal tissue reorganisation- causing a chain of events which increases beak size and width.
• Birds with one of the types of beaks can feed on soft or hard seeds better than the other