MBIO162 Genetic Diversity

10 March 2025: Genetic diversity


  • What is genetic diversity?

    • Definition 

      • The variety of genetic information contained in all living organisms

        • Mass of biological information carried out by genes

          • DNA sequence variants = genes

    • Gene pool

      • Collection of all the genes and the various alternate or allelic forms of those genes within a population

    • Subject to natural selection and therefore the raw material for evolution to act on

  • How does genetic diversity arise?

    • Mutations

      • Hereditary changes 

        • Every mitosis is a potential mutation event 

        • Mutations are the ultimate origin of all genetic variation 

          • Intermediate level of mutation needed

      • Frequency of mutations

        • Rare when considering a per-locus or per-nucleotide level

          • BUT from a genome perspective, mutations are quite common due to the large number of base pairs

        • Rate of mutation in nuclear DNA

          • Estimated to be 10^-9 per nucleotide (per meiosis)

            • Increases to 0.1-10 in a eukaryotic genome

        • Most mutations have no phenotypic effect, but most mutations with phenotypic effects tend to reduce fitness

    • Genetic recombination (during sexual reproduction)

      • Reassortment of genes provided by the two parents of the offspring 

        • has an impact on variation by creating new combinations of existing genes

  • Why is genetic diversity important?

    • Loss of diversity is associated with a reduction in reproduction and survival 

      • Founder effect

        • A few indictable from a larger population establish a new population 

      • Population bottleneck

        • A sharp reduction in the size of a population as a result of an environmental impact 

    • Populations with low genetic variation are more vulnerable to changing environmental conditions than diverse populations are

      • Example: Irish potatoes

        • Lack of genetic variation in genetic variation contributed to the severity of the Irish potato famine 

          • By the 1800s the Irish began planting potatoes of the “lumper” variety 

            • Lumpers were clones and susceptible to a water mold that caused potato blight and affected the crops in the 1840s

    • Genetic variation is the raw material for evolution to act upon

      • In a population with low genetic diversity, there might not be enough genomic flexibility to allow it to evolve and adapt

      • The rate of evolution is dependent on how much variation there is in a population 

        • Example: Red squirrel population in Canada (n=325)

          • Breeding at the right time is essential 

            • Strong selection

          • High levels of genetic variation for parturition in the population

          • Warmer (2 C) over 10 years, earlier springs + more food availability (35%)

            • Breeding advanced 18 days over 10 years (6 days per generation)

  • How do human activities affect genetic diversity?

    • Environmental change

      • Pushing organisms beyond their tolerance limits

    • Exploitation of populations

      • Removing specific genotypes from the environment 

      • Changes in allele frequencies 

        • Example: Icelandic stock of Atlantic cod

          • Coastal cod are more likely to have the AA genotype, whereas deep-migrating animals tend to have the BB genotype

          • Cod with different genotypes also have different phenotypes

          • During a period of intense fishing, the frequencies changed 

            • BB declined from 26% in the 1930s to 5% in the 1990s 

            • AA increased to above 50% in the same period

          • Likely cause is the change in exploitation patterns i of the fishing fleet

            • Larger fleets fishing in deeper water following changes in jurisdiction

    • Degradation and fragmentation of habitats 

      • Leading to a reduction of total stocks and thus increasing the likelihood of inbreeding

    • Release of farmed fish into the wild

      • Genetic variation can be reduced in farmed populations, potentially resulting in a population less capable of coping with changes in the environment, disease, etc. 

  • How do we conserve genetic diversity?

    • In situ conservation 

      • Habitat protection

        • High latitude coral reefs (HLR) are potentially vulnerable marine ecosystems

          • Geographically isolated, thus poor larval connections to tropical reefs and a reduced genetic diversity and capacity to respond to environmental change 

        • MPAs system , established to provide sources of colonizing larvae

          • Pocillopora damicornis

            • Brooding 

              • Internal fertilization and embryogenesis

              •  Release larvae ready for settlement 

            • Genetic diversity decreases with latitude 

            • Show high genetic differentiation between MPA populations and between GBR and HLRs, so these temperate populations are effectively closed 

          • Goniastrea australensis

            • Broad spawning

              • Release eggs and sperm in mass-spawning events

              • Larvae reside as plankton for several weeks 

            • Genetic diversity was more consistent across reefs

            • Show no differentiation between regions, implying that HLRs and GBR are strongly interconnected 

      • Habitat restoration 

    • Ex situ conservation 

      • Captive breeding

        • For highly endangered populations, captive breeding programs may be needed’

          • Provide demographic/genetic support for wild populations 

          • Sources of new populations in the wild

          • Prevent extinction where survival in the wild is not possible

      • Genome libraries 

      • Cryo-conservation of cells/tissues

      • Sperm/oocyte banks

      • Seed banks

      • Embryo banks