Lecture 8: Small Populations

What is genetic drift?

In small populations, allele frequencies may change substantially from one generation to the next simply due to chance, based on random differences in the fitness of individuals

Is genetic drift a different process than natural selection?

Yes; evolutionary pressures focusing on traits

Assume a relatively rare allele occurs in 5% of the population (gene pool)

If the population has 1000 individuals, then there are 100 copies the allele present (1000 individuals x 2 copies x 0.05)

With random breeding, this allele should maintain in the population for many generations.

Population bottleneck in elephant seals

Declines to an effective population size of <20 individuals ca 1880s-1890s (census 10-30)

now >220k

• Original population had high genetic diversity

• Hunting in late 1800s greatly reduced population

  • ‘Bottleneck’ only allows a few individuals through

• Survivors had little genetic diversity

  • Survival was not based on selection of any trait or characteristics

• Today’s population reflects the lack of genetic diversity

The original population of elephant seals had high genetic diversity, but hunting in the late 1800s led to?

greatly reduced population; ‘Bottleneck’ only allows a few individuals through

Due to a greatly reduced population of elephant seals, survivors had little?

genetic diversity; survival was not based on selection of any trait or characteristics

Today’s population of elephant seals reflects the lack of?

genetic diversity

More diversity means?

more chance of survival

Following substantial population decline, what do surviving individuals do not represent (have)?

the genetic diversity of the original population

Chance of surviving is random

Effective population size (Ne):

Population size based on the number of breeding individuals, which is generally much smaller than total population size

What are some factors that affect Ne?

Individuals in a population that do not breed

Variation in reproductive output among individuals

Social structure

Unequal sex ratios

How do Individuals in a population that do not breed affect Ne?

some individuals may be too young, too old, in poor health, malnourished or sterile

How does Variation in reproductive output among individuals affect Ne?

fitness varies among individuals

How does Social structure affect Ne?

limits mating; i.e. wolves might have 20 individuals in a pack, but only 2 breed

How does Unequal sex ratios affect Ne?

differential mortality

Does genetic diversity decline rather rapidly?

Yes

How does Migration limits genetic drift?

immigration of even a few successful individuals from a genetically diverse population will limit genetic drift

works against evolution; negative effect minimizing

Immigrants come from a large, stable population with the same original heterozygosity

How does Population fluctuations greatly affect Ne?

for most species, population size fluctuates over time, and in some species, dramatically

Population fluctuations greatly affect Ne

Over time, Ne is somewhere between the highest and lowest numbers of breeding individuals

In general, the greater the variation in population size the lower the Ne (often the most important factor when estimating Ne)

Ne = t/(1/N1 + 1/N2 + 1/N3...)

On average Ne is only 11% of total population size

Over time, Ne is somewhere between the highest and lowest numbers of breeding individuals

often closer to the lowest

In general, the greater the variation in population size:

the lower the Ne (often the most important factor when estimating Ne)

What is Ne = t/(1/N1 + 1/N2 + 1/N3... + 1/N4…) reflect?

population estimates of effect population size

On average, how much of Ne is the total effective population size?

only 11%

Assume a population of rare frogs monitored for 4 years results in the following counts of breeding individuals: 20,100,100,10

Ne = t/(1/N1 + 1/N2 + 1/N3... + 1/N4…)

4/(1/20+1/100+1/100+1/10) = 23.529

Demographic variation - affects persistence of small populations

Individuals do not produce the average number of offspring (fitness of individuals varies)

Deaths vary (not everyone lives through reproductive ages)

This randomness in birth and death rates (demographic variation), becomes more important when populations are small (<50)

How does Demographic variation affects small populations?

affects persistence

Ideally in a stable environment, a population would increase to carrying capacity:

at which time it stabilizes and continues with little net change in numbers (deaths = births)

Example of Demographic variation:

Assume a plant population of 3 hermaphroditic individuals; each lives for 1 year, then dies

Assume each plant has a 33.33% chance of having 0, 1, or 2 offspring

Average is 0+1+2/3=1 (average birth rate is 1, which is replacement for a hermaphrodite)

When this population breeds, however, there is a 3.6% chance the population goes extinct (0.33 + 0.33 + 0.33)

Environmental variation and stochasticity affect all individuals in a population

Variation in nutrients

Variation in predation, parasitism, disease

Larger stochastic environmental events (drought, freezes, fires)

Often, such events are unpredictable in the short-term, but often inevitable over the long-term

If the chance of a large event is 1% per year, then you should see one of these events?

every 100 years