Extinction - Why do small population have an increased risk?

Question 1

Population dynamics are influenced by what?

Answer 1

1. deterministic processes (predictable events)

2. Stochastic processes (chance events)

Question 2

What are examples of stochastic processes that influence population dynamics?

Answer 2

• demographic uncertainty
• environmental uncertainty
• genetic uncertainty

Question 3

What is environmental uncertainty?

Answer 3

• random variation in biological and physical environment that affects all members of the population and cases variation in pop size
• habitat and resource, predation and disease , competition, invasive species, catastrophes

Question 4

What is demographic uncertainty?

Answer 4

• random variations in individual reproduction and mortality (ie not all individuals reproduce or have same number of offspring) that creates variations in population size.
• *birth rate, death rate, sex ratio

Question 5

How is genetic drift a stochastic process?

Answer 5

variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce

Question 6

Species go extinct stochastically. Explain the significance of this statement.

Answer 6

Stochastic processes are ones that contain an element of chance or uncertainty. Species extinctions occur due to factors such as demographic uncertainty due to small population sizes, environmental uncertainty (unpredictable environmental changes that lead to pop declines), natural catastrophes that can wipe out small populations, and genetic factors such as genetic drift or inbreeding depression. All populations fluctuate in size due to these factors BUT when the fluctuations are larger than the pop size itself, the population can be wiped out very quickly due to any of the above factors.

Question 7

Describe three situations in which a species might be described as rare.

Answer 7

• species with a narrow geographic range
• species that occupy rare or specialized habitat
• species that exist in small populations

Question 8

How can extinction risk increase?

Answer 8

• less suitable habitat and small populations

- frequent population fluctuations

Question 9

Explain the small population paradigm.

Answer 9

• population viability increases as population size increases
• population viability decreases as population size decreases
• Extinction vortex is the tendency for small populations to decline towards extinction over time

Question 10

Explain how the extinction vortex occurs.

Answer 10

• environmental and demographic uncertainty effects
• positive Population regulation
• genetic factors (inbreeding and drift ))
• interactive effects

Question 11

What is density dependent population regulation?

Answer 11

• can be either positive or negative
• negative: as population size increases, individual fitness decreases via comp for resources, predation and parasitism and pathogens
  L> Typically happens at high population sizes..when birth rate = growth rate (not enough resources to facilitate growth and reproduction) to bring down below the carrying capacity
• positive: as pop increases so does individual fitness: allee effects

Question 12

Explain the allee effect with respect to population regulation

Answer 12

• individual fitness decreases when populations density decreases, generally occurs at very small populations sizes
• why?
  L> inability to find mates
  L> interruptions in behaviour
• mating/courtship
  *predator avoidance (predator swamping via group size)
  *thermoregulation
  *other social behaviours (ex: foraging in groups)
• result = sudden pop crash hen populations size reaches a minimum threshold size
  L> ex: extinction of passenger pigeon

Question 13

Why is genetic diversity important to maintain?

Answer 13

genetic factors influencing small populations:

• inbreeding depression = short term impact (leads to homogenization of gene pool)
• > loss of genetic diversity = longer term impact

*long term impact = inability to adapt in future.

Question 14

What is adaptation?

Answer 14

• a trait that has evolved due to natural selection to perform its current function

Question 15

What does Darwin’s theory of evolution through NS explain?

Answer 15

• variation exits
• variation is heritable
• variation affects fitness
• *neutral variants will not be affected by natural selection

Question 16

What are genetic lethal mutations?

Answer 16

• cause death of individuals before they can reproduce (strong NS against these alleles)
• in very small populations drift plays an important role and leads to fixation of these genetic lethal mutations. Drift is random tho…so it is just as likely to select for positive genes too

Question 17

What is genetic drift?

Answer 17

• change in freq due to random chance
• chance events include:
  L> whether an individual mates
  L> which allele is passed on (in a heterozygote)
• drift is random with respect to fitness
• drift is ALWAYS occurring, but is more pronounced in small pops
  L> natural selection is more efficient in large pops
• consequences:
  L> decrease in heterozygosity (h)
  L> loss of alleles:
• can fix deleterious alleles in populations
• can result in loss of beneficial alleles
  **both of above can lead to a reduction in genetic variation

**in small populations drift is more likely than selection to dictate the rate of fixation / loss of alleles

Question 18

Explain what a selection coefficient is.

Answer 18

• numerical measure of degree of NS against a specific genotype
• measured as relative fitness (individuals number of offspring vs pop average)
• S= 0 (no selection against genotype..same number of offspring produced as pop avg)
• S=1 (genetic lethal mutation, 0 offspring produced before individual dies)
• S= 0.30 (individuals with mutation have 30% less offspring than population avg)

Question 19

What is the relationship between the selection coefficient and natural selection?

Answer 19

• balance between NS and drift = evolution
• bigger differences in selection coefficients of diff alleles can bias evolution dynamics towards NS
• decrease in pop size counterbalances progress towards drift

Question 20

Compare genetic drift in a small population vs large population.

Answer 20

• small pop
  L> low NS, high genetic drift
  L> only alleles with very high S values evolve via NS (ex genetic lethal mutations), ones with low S values evolve via drift instead
  L> genetic drift is main force determining fate of alleles
  L> deleterious alleles can be maintained, advantageous alleles can be moved
• large population
  L> high NS..low genetic drift
  L> even alles with low S values can now evolve through NS
  L> NS is main force determining fate of alleles that have a selection coefficient associated with them
  L> neutral genetic variation is still determined by drift

Question 21

What is inbreeding depression?

Answer 21

• characterized by increased mortality of offspring, production of fewer offspring, unfit or sterile offspring, offspring with reduced main success

Question 22

What are the consequences of inbreeding depression?

Answer 22

• deleterious recessive alleles can hide in heterozygotes and are only expressed in offspring if both parents possess at least on copy of recessive alleles
• relatively unrelated members of a population typically do not carry the same recessive alleles ..therefore unlikely to produce homozygous recessive offspring if they breed with each other
• as pop decreases, inbreeding increases
  L> increases chance of producing unfit offspring

Question 23

What are the mechanisms of inbreeding depression?

Answer 23

• increase homozygosity = increase expression of deleterious recessive phenotypes
• decrease in heterozygosity at loci where heterozygosity is selectively advantages = loss of heterozygote advantage

Question 24

What is genetic purging ?

Answer 24

genetic purging = low frequency of deleterious recessive alleles in pop via NS (recessive alleles can’t hide in heterozygotes anymore ) = increase fitness of overall population but pop now lacks genetic variation

**consequence from inbreeding depression increasing homozygosity

Question 25

Explain interactive effects influencing small populations risk to extinction

Answer 25

• as pop size decreases, factors reinforce each other to increase instability of dynamics
• inbreeding and loss of genetic diversity also contribute
• implications:
  L> factors act on all small pops
  L> interaction btwn stochasticity (genetics environmental, demographic), catastrophes, allele effect is COMPLEX
  L> hence why we talk about RISKS of extinction and not certainty…all extinctions involve stochasticity

Question 26

What are the four main threats to small populations for risk of extinction?

Answer 26

1. loss of heterozygosity , loss go genetic variation, decreased ability of pop to adapt in future
2. inbreeding depression
   L> deleterious recessive mutations are morley to be expressed bc two heterozygotes with same recessive mutations are more likely to mate, reducing fitness of many members of the pop
3. Increased genetic load
   L> accumulation of slightly deleterious mutations that NS cannot get rid of bc there is greater chance of fixation via drift
4. increased risk of extinction due to all factors previously covered and how they interact to increase risk (interactive effects)..extinction vortex