Ne and sexual selection in extinction risk Flashcards
Extinction is rare. True or false?
False, it is extremely common.
Of the 4bn species that have evolved on Earth in the last 3.5my, what proportion have gone extinct?
Who said this?
Barnosky et al., 2011
99%
Why does extinction occur?
If a lineage cannot adapt fast enough to environmental change
Is extinction bad for everyone?
No; as species disappear, others rise to fill their niche
Extinction is observable in biotic hallmarks. Who described these, what are they?
Barnosky et al., 2012:
Profound changes in species composition (like dominance, rare species become common, new organisms evolve etc.)
Species diversity at any given point is a balance between…
speciation and extinction
Effective population size has a profound effect on extinction risk. True or false?
True
What is the Ne?
The number of individuals who contribute to reproduction and pass on their genes to the next generation.
Ne is the same as the actual population size. True or false?
False; the actual population contains non-reproductive individuals like infants and older animals
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.
- 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.
- 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.
Give a referenced example of a population with a small Ne.
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
Mills, 2013:
What deleterious traits had been fixed in Florida panthers? List 4
Kinked tail
Cowslick
Heart defects
Failure of testes to descend in males
Overall reduction in fitness increases extinction risk.
How can sexual selection increase the extinction risk of a species?
It reduces the Ne
In which kind of species does sexual selection occur?
In sexually dimorphic species
There are two types of sexual selection, what are they?
- Intra-sexual
2. Inter-sexual
What is intra-sexual selection? How does it reduce Ne?
Male-male competition, losing males are excluded from mating.
What is inter-sexual selection? How does it reduce Ne?
Female choice, females preferentially mate with certain males and thus exclude others from reproduction
Both kinds of sexual selection, intra and inter, lead to what?
What does this indicate to the female?
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
What is contest competition and why do males face this?
Competition for a non-sharable resource, fertilisations cannot be shared
Give a referenced example of sexual selection in primates. How does this affect Ne?
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.
Who proposed the Good Genes hypothesis for sexual selection?
Amotz Zahavi, 1975
There are two problems with the Good Genes hypothesis. What are they?
- The Lek Paradox; the ‘best’ male is consistently chosen and gets to mate with all the females, reducing Ne genetic diversity.
- Honesty
The Good Parent Process is another hypothesis for the evolution of mate choice. Who came up with it? Why is this bad for Ne?
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.
The structure of you sexual selection paragraph will be as follows:
- Types of sexual selection, inter and intra
- Both exclude males from reproduction
- Inter-sexual selection (female choice) excludes males. Refer to Good Genes/Lek Paradox/Setchell example and Good Parent Process
- Intra-sexual selection can exclude both males and females; male-male competition, and female reproductive suppression
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.
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.
Reduced Ne reduces genetic diversity. This mean populations are more susceptible to…
drift
The effects of reduced Ne are more severe in sex chromosomes. Why?
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.
Which are a) the major and b)the minor sex chromosomes?
a) X and Z
b) Y and W
Assume monogamy:
Sex chromosomes are always inherited unevenly compared to autosomes. Why?
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.
Assume monogamy:
Reproduction under sexual selection can be seen as two reductions of Ne. How?
- Only some males get to mate
2. Heterogametey reduces the Ne of the major sex chromosomes relative to autosomes
Assume monogamy:
The Ne of major sex chromosomes is smaller than that of autosomes. Why is this a problem?
They are more exposed to selection
Assume monogamy:
Due to their small Ne relative to autosomes, major sex chromosomes undergo faster evolution. What is this called?
Faster-X or Faster-Z syndrome
Under polygyny the Ne of XY and ZW systems is differentially affected. Which is better off?
XY systems
How can polygyny increase the Ne of the X in XY systems?
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.
Why does polygyny not increase the Ne of Z in ZW systems?
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.
Who thinks this has different implications for Faster-X and Faster-Z?
Wright et al., 2015
Faster-X is adaptive whilst Faster-Z is neutral
Why is Faster-X adaptive?
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
Why is Faster-Z neutral?
Relaxed purifying selection has allowed drift the greater potential to fix mildly deleterious alleles
Why do Wright et al. (2015) think Faster-Z is neutral? What did they do?
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.
Who contested the idea of neutral Faster-Z? Why?
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)
Fast-Z is definitely a thing, why?
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.
