Ch. 11 Problems of small populations
Minimum viable population (MVP)
-smallest population that can be predicted to have a 99% chance of
persisting 1000 years
-varies among organisms and takes years of research to determine MVP
Ex. vertebrates may range from 50-5000 individuals per population
invertebrates and annual plants may be 10,000 individuals per population
-one of the best documented cases of MVP size comes from a study of
Bighorn sheep by Berger (1990) where some of these populations have
been studied for 70 years
-100% of populations with fewer than 50 individuals went extinct within
50 years Figure 11.1
11.1 Relationship between (N) of bighorn sheep and the percentage of populations that persist
Field evidence from long–term studies of birds of the California Channel
islands supports large population requirements for persistence of
species because bird populations with more than 100 breeding pairs
had a greater than 90% chance to persist 80 years
Figure 11.2
11.2 Extinction rates of bird species like loggerhead shrike on the Channel Islands
Small populations are subject to rapid decline in numbers and local extinctions
for three reasons
1. Loss of genetic variability and related problems of inbreeding and genetic
drift
inbreeding-mating among close relatives
genetic drift-allele frequencies changing from one generation to the next by
chance
2. Demographic fluctuation due to random variations in birth and death rates
3. Environmental fluctuations due to variation in predation, competition,
disease, food supplies, and natural catastrophes such as fires, floods,
volcanic eruptions, storms, or droughts
1. Loss of genetic variability
genetic variability-results because individuals of a population have
different alleles and due to heterozygosity
-can be affected by genetic drift – changes in alleles due to chance
Ex. A rare allele occurs in 5% of the gene pool in a population of 1000
individuals has 100 copies of the allele. (1000 X 2 copies per
individual X .05 = 100 copies). In a population of 10 individuals (10
X 2 copies per individual X .05 = 1), there is only one copy present. It
is much easier to lose rare alleles in a small population.
-heterozygosity is influenced by population size according to a
relationship worked out by Wright (1931). He proposed a formula to
express the proportion of original heterozygosity (H) remaining after
each generation for a population of breeding adults with an effective
population size (Ne)
H = 1- 1/2Ne
He also expressed the proportion of heterozygosity remaining after tgenerations (Ht) as Ht = Ht
Ex. What would be the proportion of original heterozygosity in a
population of 50 breeding individuals after one generation? After 2, 3,
and 10 generations? What would be the results if the effective
population size was only 10 individuals?
H = 1 - 1/(2) (50)
= 1 - .01 = 0.99
after 2 generations Ht
= Ht
= .992
= 98%
after 3 generations Ht
= Ht
= .993
= 97%
after 10 generations Ht
= Ht
= .9910
= 90%
What would be the results if the effective population size was only 10
individuals?
H = 1 - 1/(2) (10)
= 1 - .05 = 0.95
after 2 generations Ht
= Ht
= .952
= 90%
after 3 generations Ht
= Ht
= .953
= 86%
after 10 generations Ht
= Ht
= .9510
= 60%
Wright’s formula demonstrates significant losses of genetic variability in
small isolated populations due to genetic drift Figure 11.3
Genetic variability within populations will increase over time due to
mutation and immigration and counter losses due to genetic drift if
populations are large enough Figure 11.4
11.3 Genetic variability is lost randomly over time through genetic drift
11.4 Effects of immigration and mutation on genetic variability in 25 simulated populations of size
Ne = 120 individuals
Field data also show that lower effective population size leads to rapid
loss of alleles from a population
Ex. Wind-pollinated conifer from New Zealand was used in a study that
used protein electrophoresis to examine genetic variability in
populations ranging from 10-400,000 individuals. Large populations
had the greatest levels of heterozygosity and percentage of
polymorphic alleles whereas populations smaller than 8000 individuals
suffered a loss of genetic variation
-this has been shown to be true for bird species as well Figure 11.5
11.5 Level of genetic variability is directly correlated with population size in populations of 89
species of birds with Madagascar fish eagle (40 pairs) & willow grouse (10 million) at extremes
How big a population is needed to prevent the loss of genetic diversity?
Franklin (1980) pointed out that 50 reproductive individuals might be the
minimum number needed to avoid loss of genetic variability
H = 1 - 1/(2) (50)
= 1 - .01 = 0.99
after 2 generations Ht
= Ht
= .992
= 98%
-after each generation, only 1% of heterozygosity is lost
-Franklin also suggested that in populations of 500 reproductive
individuals, the rate of new genetic variability arising from mutation
might balance the variability being lost due to small population size
-50-500 rule- isolated populations need to have at least 50 individuals and
preferably 500 individuals to maintain genetic variability
50-500 rule is difficult to apply because it assumes that all individuals
have an equal probability of mating when their are age, health,
sterility, and social factors involved. Just a general rule to try and
determine minimum viable population
-questioned by some that point out mutation rates used in earlier models
may be much lower and several thousand reproductive individuals
need to be protected for long-term species survival
-smaller populations subjected to genetic drift often suffer from
inbreeding and outbreeding depression as well as loss of genetic
flexibility
Inbreeding Depression
-mating among close relatives
-countered by fact that in most large populations outbreeding or mating
with unrelated individuals occurs
Results in more expression of harmful alleles as homozygotes causing:
a. higher mortality of offspring
b. fewer offspring
c. offspring that are weak, have low mating success, or are sterile
Ex. Figure 11.7
Can be prevented by:
a. dispersal
b. sensory cues allowing relatives to identify each other
c. morphological and physiological adaptations to encourage cross
pollination in plants
11.7 A high degree of inbreeding results in a “cost of inbreeding”
Outbreeding depression
-individuals of different species mating
-often occurs when a species is rare or its habitat is damaged and
individuals mate with a related species
-offspring are often weak or sterile because of lack of compatability of the
chromosomes and enzyme systems from different parents
-even when the hybrids are not sterile, the genetic identity of the rarer
species becomes lost as the small gene pool is mixed into the much
larger gene pool of the common species
Ex. grey wolf and coyote in Texas
Ex. 90% of CA threatened and endangered plants occur in close proximity
to other species in the same genus and outbreeding depression is a
factor in their decline
11.8 Mating between unrelated individuals of the same species results in offspring with high fitness
Loss of Evolutionary Flexibility
-idea based on Fundamental Theorem of Natural Selection, which states
that rate of evolutionary change in a population is directly related to
the amount of genetic variation in a population
-small populations will have reduced ability to adapt to environmental
change
Ex. Many plant populations have a small number of individuals that can
tolerate high amounts of lead and zinc so if toxic metals contaminate
the environment, these individuals would survive and reproduce. If the
population has lost these individuals due to genetic drift the population
may become extinct.
Factors that determine effective population size
Unequal sex ratios
-males and females are not equal due to chance, selective mortality, or harvest
of one sex over the other by people
Ne = 4NmNf/Nm + Nf
Ex. seal population has six breeding males and 150 breeding females with each
male mating with 25 females
Ne = 4(6)(150)/6 + 150 = 3600/156 = 23 Figure 11.9
11.9 Ne declines when number of males and females in a breeding population of 100 individuals is
increasingly unequal
Factors that determine effective population size
Population fluctuations and bottlenecks
-in some species, population size varies greatly from generation to
generation and to estimate effective population size, one must observe
them over time
Ne = t/(1/N1 + 1/N2 + ...1/Nt)
Ex. A butterfly population over five years has 10, 20, 100, 20, and 10
breeding individuals
Ne = t/(1/10 + 1/20 + 1/100 + 1/20 + 1/10)
= 5/31/100
=16.1
-a single year of drastically reduced population numbers can cause
population bottlenecks where rare alleles can be lost if no individuals
possessing those alleles survive
Ex. 100, 10, 100, 100, 100
5/(1/10 + 1/100 + 1/100 + 1/100 + 1/100) = 5/(14/100) = 5/.14 = 35
Factors that determine effective population size
-a special category of a bottleneck is called founder effect- when a few
individuals leave one population to establish another new population.
The new population often has less genetic variability than the larger
original population
Ex. lions of Ngorongoro (N-goro-goro) Crater in Tanzania, Africa
Figure 11.11, 11.12
-when compared to nearby, large population of Serengeti lions, the crater
lions show reduced genetic variability, high levels of sperm
abnormalities, and reduced reproductive rates
11.11 The Ngorongoro Crater lion population in 1961 before crashing in 1962
11.12 Males of an isolated and inbred population of lions exhibit a high level of sperm abnormalities
Normal
2-headed
2-tailed
Coiled tail
Factors that determine effective population size
-in some cases, heterozygosity can be restored after a bottleneck if the
population expands rapidly following the bottleneck
Ex. species of rhino in Asia and Africa with high genetic diversity in
Indian Rhino and Box 11.1
High genetic diversity is explained due to expansion following bottleneck
and migration of individuals into the park area where Indian rhinos are
protected.
Box 11.1 (Part 1) Each of the rhinoceros species occupies only a tiny fraction of its former range
Box 11.1 (Part 2) Each of the rhinoceros species occupies only a tiny fraction of its former range
Small populations are subject to rapid decline in numbers and local
extinctions for three reasons
1. Loss of genetic variability and related problems of inbreeding and
genetic drift
inbreeding-mating among close relatives
genetic drift-allele frequencies changing from one generation to the next
by chance
2. Demographic fluctuation due to random variations in birth and death
rates
3. Environmental fluctuations due to variation in predation, competition,
disease, food supplies, and natural catastrophes such as fires, floods,
volcanic eruptions, storms, or droughts
Demographic Variation
Demographic fluctuation due to random variations in birth and death rates
-variations in reproductive and mortality rates can cause small populations
to fluctuate randomly in size leading to extinction
Ex. dusky sparrow became extinct because the last five survivors were
male
Ex. Number of breeding pairs of Spanish imperial eagles Fig. 11.13
Ex. Allee effect- inability of a social structure to function once a
population falls below a certain size. Pack hunters may need to be a
certain size to be successful in the hunt
11.13 As breeding pairs of Spanish imperial eagles declined, the sex ratio of nestlings changed.
Supplemental feeding (2004)
brings ratios back to normal
Environmental variation and catastrophe
Environmental fluctuations
-random variation in the biological community and physical environment
-variation in predation, competition, disease, food supplies, and natural
catastrophes such as earthquakes, fires, floods, volcanic eruptions,
storms, or droughts
Ex. Considering only demographic variation, Mexican palm MVP was
about 48 mature individuals. When moderate environmental variation
was included the MVP increases to 380 individuals
Figure 11.14
11.14 Effects of three variables on the probability of extinction of a population of the Mexican palm
Extinction vortices
Combined effects of demographic variation, environmental variation, and
loss of genetic variability on small populations create an extinction
vortex that tends to accelerate the drive to extinction
Figure 11.15
11.15 Extinction vortices progressively lower population size, leading to local extinctions of species