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 ECB4e-fig-11-01-0.jpg

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 ECB4e-fig-11-02-0.jpg

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 t-generations (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

2-headed Normal 2-tailed Coiled tail 11.12 Males of an isolated and inbred population of lions exhibit a high level of sperm abnormalities 2-headed Normal 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

Supplemental feeding (2004) brings ratios back to normal 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