Ne and sexual selection in extinction risk

Question 1

Extinction is rare. True or false?

Answer 1

False, it is extremely common.

Question 2

Of the 4bn species that have evolved on Earth in the last 3.5my, what proportion have gone extinct?

Who said this?

Answer 2

Barnosky et al., 2011

99%

Question 3

Why does extinction occur?

Answer 3

If a lineage cannot adapt fast enough to environmental change

Question 4

Is extinction bad for everyone?

Answer 4

No; as species disappear, others rise to fill their niche

Question 5

Extinction is observable in biotic hallmarks. Who described these, what are they?

Answer 5

Barnosky et al., 2012:

Profound changes in species composition (like dominance, rare species become common, new organisms evolve etc.)

Question 6

Species diversity at any given point is a balance between…

Answer 6

speciation and extinction

Question 7

Effective population size has a profound effect on extinction risk. True or false?

Answer 7

True

Question 8

What is the Ne?

Answer 8

The number of individuals who contribute to reproduction and pass on their genes to the next generation.

Question 9

Ne is the same as the actual population size. True or false?

Answer 9

False; the actual population contains non-reproductive individuals like infants and older animals

Question 10

If the Ne is very small only a subset of the total genetic variation will be passed on. Why is this bad? Give 2 reasons.

Answer 10

1. Less variation reduces the adaptive capacity of an organism, or the chance that it will have the optimal genes to cope in a changed environment.
2. A reduction in variation reduces the efficacy of selection, as there is little differential in fitness between genotypes. This allows deleterious alleles to accumulate in mutational meltdown.

Question 11

Give a referenced example of a population with a small Ne.

Answer 11

Mills, 2013:

Puma concolor, or the Florida panther
Population reduced to 20-25 breeding adults in the 1990s due to fragmentation and hunting.
Genetic rescue programme saw introduction of 8 adult females from Texas to increase Ne size

Question 12

Mills, 2013:

What deleterious traits had been fixed in Florida panthers? List 4

Answer 12

Kinked tail
Cowslick
Heart defects
Failure of testes to descend in males

Overall reduction in fitness increases extinction risk.

Question 13

How can sexual selection increase the extinction risk of a species?

Answer 13

It reduces the Ne

Question 14

In which kind of species does sexual selection occur?

Answer 14

In sexually dimorphic species

Question 15

There are two types of sexual selection, what are they?

Answer 15

1. Intra-sexual

2. Inter-sexual

Question 16

What is intra-sexual selection? How does it reduce Ne?

Answer 16

Male-male competition, losing males are excluded from mating.

Question 17

What is inter-sexual selection? How does it reduce Ne?

Answer 17

Female choice, females preferentially mate with certain males and thus exclude others from reproduction

Question 18

Both kinds of sexual selection, intra and inter, lead to what?

What does this indicate to the female?

Answer 18

The evolution of costly male ornaments and weapons so males can compete in contest competition, either in combat or display

The male is of good heritable quality, Good Genes Hypothesis

Question 19

What is contest competition and why do males face this?

Answer 19

Competition for a non-sharable resource, fertilisations cannot be shared

Question 20

Give a referenced example of sexual selection in primates. How does this affect Ne?

Answer 20

Setchell, 2005:

Mandrills (Mandrillus sphinx) are cercopithecine monkeys from Central Africa.

Female mandrills will only groom and mate with the most colourful male, regardless of his rank. They live in polygynous systems, meaning only one male fertilises each female, thus reduces Ne.

Question 21

Who proposed the Good Genes hypothesis for sexual selection?

Answer 21

Amotz Zahavi, 1975

Question 22

There are two problems with the Good Genes hypothesis. What are they?

Answer 22

1. The Lek Paradox; the ‘best’ male is consistently chosen and gets to mate with all the females, reducing Ne genetic diversity.
2. Honesty

Question 23

The Good Parent Process is another hypothesis for the evolution of mate choice. Who came up with it? Why is this bad for Ne?

Answer 23

Hoelzer, 1989

Females choose males based on non-heritable epigamic traits that convey direct benefits to them, like territory quality.
This then ends up with multiple females living in polygyny with a single male in the best territory, again reducing Ne and genetic diversity.

Question 24

The structure of you sexual selection paragraph will be as follows:

Answer 24

1. Types of sexual selection, inter and intra
2. Both exclude males from reproduction
3. Inter-sexual selection (female choice) excludes males. Refer to Good Genes/Lek Paradox/Setchell example and Good Parent Process
4. Intra-sexual selection can exclude both males and females; male-male competition, and female reproductive suppression

Question 25

Intra-sexual selection between females can exclude females from mating. This occurs in eusocial insects and primates by the dominant female producing pheromones. Give a reference example in primates.

Answer 25

Abbott, 1984:

Studied common marmosets (Callitrichix jacchus), fertility suppression as dominant female released a pheromone that caused hypothalamic inhibition of LHRH, causing inadequate gonadotrophin secretion.

Question 26

Reduced Ne reduces genetic diversity. This mean populations are more susceptible to…

Answer 26

drift

Question 27

The effects of reduced Ne are more severe in sex chromosomes. Why?

Answer 27

Heterogametey means that there are less major sex chromosomes relative to autosomes. This is further exacerbated by breeding system and the method of sex determination, as XY and ZW are differentially affected.

Question 28

Which are a) the major and b)the minor sex chromosomes?

Answer 28

a) X and Z

b) Y and W

Question 29

Assume monogamy:

Sex chromosomes are always inherited unevenly compared to autosomes. Why?

Answer 29

In both XY and ZW systems, 3 major chromosomes and 1 minor chromosome are brought together during sex. This means there are 3 major sex chromosomes for every 4 autosomes.

Question 30

Assume monogamy:

Reproduction under sexual selection can be seen as two reductions of Ne. How?

Answer 30

1. Only some males get to mate

2. Heterogametey reduces the Ne of the major sex chromosomes relative to autosomes

Question 31

Assume monogamy:

The Ne of major sex chromosomes is smaller than that of autosomes. Why is this a problem?

Answer 31

They are more exposed to selection

Question 32

Assume monogamy:

Due to their small Ne relative to autosomes, major sex chromosomes undergo faster evolution. What is this called?

Answer 32

Faster-X or Faster-Z syndrome

Question 33

Under polygyny the Ne of XY and ZW systems is differentially affected. Which is better off?

Answer 33

XY systems

Question 34

How can polygyny increase the Ne of the X in XY systems?

Answer 34

Males are heterogametic; although there is a one-step reduction in NeX as only some males get to mate, they have numerous XX females. This means NeX > 3/4NeA, thus increasing the Xs adaptive potential slightly.

Question 35

Why does polygyny not increase the Ne of Z in ZW systems?

Answer 35

Females are hetergametic; there is a one-step reduction in NeZ as only some males get to mate, and this is not offset by having multiple females as they only have a single Z each. This means NeZ is not increased.

Question 36

Who thinks this has different implications for Faster-X and Faster-Z?

Answer 36

Wright et al., 2015

Faster-X is adaptive whilst Faster-Z is neutral

Question 37

Why is Faster-X adaptive?

Answer 37

The single X in males is not masked by a second copy, leading to greater fixation of beneficial alleles as it is more exposed to selection

Question 38

Why is Faster-Z neutral?

Answer 38

Relaxed purifying selection has allowed drift the greater potential to fix mildly deleterious alleles

Question 39

Why do Wright et al. (2015) think Faster-Z is neutral? What did they do?

Answer 39

Looked at the evolution of 6 bird species over 90million years to assess chromosomal change.

Found that natural selection was less effective on the Z chromosome in general, something that was more pronounced in polgynous species where the Ne of Z is greatly reduced.

Question 40

Who contested the idea of neutral Faster-Z? Why?

Answer 40

Dean et al., 2015

In response to Wright et al. (2015) they also looked at birds, concluded that Fast-Z is due to positive selection in females, much likes Fast-X is in males

(basically don’t look at the Z from the homogametic perspective, look at it in the heterogametic perspective)

Question 41

Fast-Z is definitely a thing, why?

Answer 41

As both Wright et al. (2015) and Dean et al. (2015) cfound higher rates of NS-mutation on the Y. It is just contentious what Fast-Z is driven by.