Nile Perch from Lake Victoria

Genetic Diversity

Fitness

• evolutionary fitness is a measure of the number of offspring an individual produces

Loss of Fitness

• Another important aspect of polymorphism is that it tends to maintain fitness -

• populations of animals in zoos, which are typically low in genetic diversity, often have low fitness - low fertility and high mortality among offspring

Fitness of Zoo Animals

Reasons for Loss of Fitness

1. increased incidence of deleterious recessive homozygous individuals

2. lack of heterosis – heterosis (hybrid vigor) is the phenomenon where heterozygous individuals have higher fitness than do homozygotes - often heterozygotes are more resistant to disease

3. lack of evolutionary potential - with all homozygotes there is lack of variation and all individuals will be susceptible to the same problems

Inbreeding Depression

• Inbreeding depression is the loss of fitness resulting from the breeding of closely related individuals - it occurs due to the three reasons listed before

Ngorongoro Crater

Lions at Ngorongoro Crater

Vipera berus - adder

Glanville Fritillary Butterfly

Outbreeding Depression

• The loss of fitness that occurs when distantly related individuals breed –

• This occurs because certain populations may have been selected for traits that are successful in their environment, so that introducing novel traits may reduce fitness for that environment

Austrian Ibex – Capra ibex ibex

Turkish Ibex – Capra ibex aegagrus

Nubian Ibex – Capra ibex nubiana

Optimum outbreeding in Japanese Quail

Fitness

• evolutionary fitness is a measure of the number of offspring an individual produces

Maintenance of Polymorphism

without natural selection -

• random mating tends to maintain polymorphism – due to the benefits of sexual reproduction – recombination, independent assortment, and crossing over

Maintenance of Polymorphism

• The effects of nonrandom mating are variable - species may either mate assortatively (like with like) or disassortatively (like with unlike)

• assortative mating results in many homozygous individuals

• disassortative with many polymorphic, heterozygous individuals

Assortative Mating - Three spined stickleback

Disassortative Mating – Nonbreeding Ruff

Disassortative Mating - Breeding male ruff and variations on head pattern

Maintenance of Polymorphism

• environmental variance - the environment may affect development of different genotypes so that which genotype dominates changes with the environment - if the environment varies or different habitats exist within the species range, then different genotypes will exist

Backswimmers – winged or wingless forms

Maintenance of Polymorphism

With Natural Selection

with selection, we would expect the most fit genotype to come to dominate the population, but polymorphism may still occur:

1. selection acts to maintain stable polymorphism so that different genotypes are most fit under different situations

2. fixation of a particular genotype is counteracted by mutation

3. fixation of a particular genotype in one population is counteracted by gene flow from another population

Polymorphism under selection –in the Grove Snail - Cepaea

Clines

• in many species, local populations have little variation, but the entire species exhibits much variation as local populations are adapted to different conditions - if these changes in genes change in response to certain environmental variables, we may see a cline - a gradual change along a geographic transect

Clines with Body Size

• Bergmann's rule - many animals get larger in size as the species range approaches the poles - it is related to ability to keep warm - larger bodies maintain warmth better

• Allen’s Rule – size of extremities decreases towards the poles – heat is lost through things like large ears

Bergman’s Rule in same aged White-tailed Deer

Allen’s Rule in Foxes

Arctic Fox Desert (Kit) Fox

Allen’s Rule in Hares

Cline in Cyanide Production in White Clover

Cline incyanideproductionby whiteclover

Greater Racquet-tailed Drongocline in crest size

Reductions in Polymorphism

• Gene Flow - the movement of alleles from one population to another tends to maintain genetic similarity among populations

African Wild Dog

Rates of Gene Flow – Ne (effective population size) = 120

Minimum Viable Population

• The smallest population for a species which can be expected to survive for a long time

• Many factors effect MVP – the study of those factors is often called Population Viability Analysis – or Population Vulnerability Analysis – or PVA

Factors that make populations vulnerable to extinction

• Environmental fluctuations

• Catastrophes

• Demographic uncertainties

• Genetic problems

• Habitat fragmentation

Environmental Fluctuations

Kirtland’s Warbler

Cheetah

Habitat Fragmentation

• Fragmentation is the transformation of large expanse of habitat into a number of smaller patches of smaller total area isolated from each other by a matrix of habitat unlike the original

Habitat Fragmentation

Habitat fragmentation occurs due to:

• Natural climatic shifts

• Human caused habitat loss: logging, agriculture, urbanization, dams, road construction, etc.

• Overexploitation of species

• Species introduction

• Secondary effects due to extinctions

Domesday Book – 1085-86

Selection from the Domesday Book

Factors that make populations vulnerable to extinction

• Environmental fluctuations

• Catastrophes

• Demographic uncertainties

• Genetic problems

• Habitat fragmentation

Heath Hen – Extinction Vortex

Minimum Viable Population Size

• Another definition - often defined as 95% probability of 100 year survival, but can also plan for longer survival (500 or 1000 years)

• MVP is usually determined by modeling

Forces which may cause extinction

1) deterministic - something essential is removed (habitat loss) or something lethal is added (pollutant, disease, introduced species) - presumably we can act to minimize these risks

Forces which may cause extinction

2) stochastic (random) - environmental, catastrophic, demographic and genetic - this is what we need to worry about and what is hardest to prevent

• environmental randomness effects resources and conditions and we can't do much about it

• catastrophic randomness - floods, fires, hurricanes, volcanoes - can't really prevent but can spread individuals around to minimize the impact

• demographic - just natural random variation in birth and death rates can lead to extinction

• genetic - lack of genetic variability can lead to problems of inbreeding and poor response to diseases and environmental change

Bighorn Sheep and MVP

Bighorn Sheep and MVP

Grizzly Bear and 50/500 Rule

MVP – 50/500 Rule?

Reductions in Polymorphism

Reductions in population size can lead to losses of genetic polymorphism

Two special cases of reductions in population size are:

1. A few individuals move to a new area and start a new population that is isolated from other populations – founder effect

2. We can also experience a population bottleneck where a formerly large population is drastically reduced in size

Founder Effect – Galapagos Tortoise

Founder effect – Amish and Polydactyly

Population Bottleneck – Northern Elephant Seal