Evolution: Lecture 4

Question 1

What is relative fitness?

Answer 1

The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals.

Question 2

What is selective pressure?

Answer 2

Something that is under selection pressure is going to be removed from the population quicker.

Question 3

A trait under greater selection pressure is?

Answer 3

A trait that is removed from the population quicker.

Question 4

A trait conferring higher relative fitness is?

Answer 4

A trait that is more likely to stay in the generation.

Question 5

In the giraffe example, who had higher relative fitness?

Answer 5

The giraffe with the longer neck had more relative fitness. The longer neck is more advantageous, they have a longer lifespan and produce more offspring.

Question 6

What is polygenic inheritance?

Answer 6

The combination of alleles at several loci determine the genetic component of a trait. When your phenotype is influenced by several genes (alleles at several loci).

Ex. human height or mouse coat colour.

Question 7

Polygenic inheritance measures?

Answer 7

Quantitative character, a smooth range of phenotypes.

• We measure it, but it varies along the phenotypic range.

Question 8

What graph typically do we see when looking at frequencies of quantitative character that could be acted on by selection?

Answer 8

We would normally see a normal distribution (bell curve). It is commonly seen if there is heritable variation at several loci.

Question 9

Directional selection?

Answer 9

There is greatest selection pressure on either extreme of the curve. We decide to shift away from the selection pressure.

• One end of distribution selected against
• Classic response to a changing environment

Question 10

Examples of directional selection?

Answer 10

• Pesticide/drug treatments
• Climate change

Question 11

Soapberry bugs?

Answer 11

In the area where there are new, flatter fruits, there is greater selection pressure falling on individuals with longer beaks. This is because they are getting less nutrition due to inefficient feeding.

Question 12

Soapberry bugs - directional selection?

Answer 12

There is directional selection favouring the shorter beaks, therefore the average beak length in this population falls.

Question 13

Stabilizing selection?

Answer 13

Both extreme phenotypes are selected against (under greater selection pressure). This will then stabilize the average, making the mean higher and both sides shorter.

Question 14

What is stabilizing selection due to?

Answer 14

It is often due to different, opposing, selective forces.

Question 15

Examples of stabilizing selection?

Answer 15

Birth weight in humans:

• heavy or light = selection pressure
• middle/avg. = most fit

Question 16

Disruptive selection?

Answer 16

Intermediate phenotypes are selected against or the selective pressure falls in the middle of the distribution. Both extremes are favoured.

Question 17

Disruptive selection has?

Answer 17

A role in some speciation events, which occur when a species diverges enough to become separate species.

Question 18

How do we measure fitness?

Answer 18

Fitness is measured by the number of viable offspring an individual has on average.

Question 19

What is sexual selection?

Answer 19

Effectively, a special case of natural selection that involves:

• Competition for mating opportunities by the number or quality of mating opportunities

Question 20

Result of sexual selection?

Answer 20

• Results in adaptations that increase
  mating success
  – But (sometimes) actually reduce survival

Question 21

Intrasexual Selection?

Answer 21

Competition within one sex (usually males) for mating opportunities

Question 22

Example of intrasexual selection?

Answer 22

Having ritual fights.

• Male with antlers will win fights

Question 23

Intersexual Selection?

Answer 23

One sex (usually females) chooses mate from (competing members of) other sex

Question 24

Example of intersexual selection?

Answer 24

Good features are chosen more.

• A male peacock’s tail (ornament)

Question 25

Intrasexual and Intersexual selection increase?

Answer 25

Many remarkable adaptations increase success during intrasexual or intersexual selection
- Antlers and peacock’s tails

Question 26

Disadvantages of sexual selection?

Answer 26

• Yet, some disadvantages are brought by sexual selection like how predators could see peacock’s tails easier the bigger they are and how it may be harder to move quickly without antlers.

Question 27

Sexual Dimorphism?

Answer 27

Adaptation benefits only one sex, while both sexes would suffer any survival cost.

• Males and females will have different features as the females do not need them.

Question 28

Sexual dimorphism can evolve?

Answer 28

Sexual dimorphism can also evolve via “normal” natural selection.

Question 29

Example of sexual dimorphism evolving via natural selection?

Answer 29

Female birds having larger bodies to help protect her children/nest

Male birds are smaller and more suited to flight to provide food

Question 30

What does diploidy do?

Answer 30

Diploidy ‘hides’ recessive alleles from
selection when they are rare. This means that they are seen less and less but bad at being fully eliminated. This is an example of preserving allelic variation.

• Natural selection cannot drive recessive alleles to complete extinction

Question 31

What is balancing selection?

Answer 31

Natural selection can sometimes favour allelic variation, which is called balancing selection.

Question 32

Examples of balancing selection?

Answer 32

• Heterozygote advantage
• Frequency-dependent selection

Question 33

Sickle-cell anemia: heterozygote advantage

Answer 33

Sickle cell anemia is a single locus recessive genetic disease.

ss homozygotes have a high mortality while young, therefore the fitness of ss is much lower than SS or Ss.

• Yet, Ss confers resistance to malaria (deadly) and prevalent.
• With malaria absent, both are similar fitnesses
• With malaria present, Ss is higher fitness than SS

This means that there is a higher chance of there being ss individuals overall.