Question 1 – Discus fish

There are three species of discus fish (Symphysodon spp.) that are native to the Amazon River basin in Brazil.  Due to their distinctive shape, calm behaviour and bright colour patterns, the discus fish are popular with many fish breeders.

The phenotypic traits of discus fish, such as colouration, body shape, and size, can vary significantly. These traits are influenced by the genotype, leading to distinct colour patterns and different behaviours, including aggression. Minor variations in size and colour are also dependent on food availability and their diet.  

Discuss the effect of genetic variation on the resilience of discus fish populations.

In your answer, you should include the following:

·        define sexual reproduction and mutation

·        explain how both sexual reproduction and mutation contribute to genetic variation  

·        explain the difference between genotype and phenotype within discus fish populations, including evidence of this variation

·        evaluate the importance of genetic variation to the survival of the population of discus fish in a changing environment (e.g. disease resistance, changes in habitat, or food availability).

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The Effect of Genetic Variation on the resilience of Discus Fish populations 

For the discus fish (Symphysodon spp.) in the Amazon River, genetic variation is the key to their long-term survival in a changing world. This variety within the population gives them the strength to survive challenges like disease, changes to their home, and different food options. It is the deciding factor in whether the species can adapt and continue to exist.

Sources of Genetic Variation

The variety in the discus fish gene pool is mainly created and kept up by two key life processes: mutation and sexual reproduction.

• A mutation is a permanent, random change in the DNA of an organism. It is the original source of all new genetic information, creating new versions of genes called alleles. For example, a mutation in a gene that controls colour could create a completely new pattern on a discus fish that has never been seen before. While many mutations have no effect or are harmful, a few can be helpful, providing a new trait that could be an advantage if the environment changes.

• Sexual Reproduction is a process that involves two parents contributing genes to produce a unique offspring. It works by joining special sex cells (gametes), like sperm and eggs. While mutations create new alleles, sexual reproduction is a powerful way to shuffle existing alleles into new combinations. This happens in two main ways when gametes are made (during meiosis):

  1. Independent Assortment: When chromosomes line up in pairs, they are sorted into sperm or egg cells randomly. This means the mix of alleles from the parents' mother and father that ends up in any single gamete is unique.

  2. Crossing Over: Before the chromosomes are sorted, they can swap sections of DNA with each other. This creates new combinations of alleles on the same chromosome.

The random fertilisation of an egg by a sperm then brings these unique combinations from two different parents together. This results in an offspring that is genetically different from its parents and siblings, contributing to the significant genetic variation observed in the entire discus fish population.

Genotype and Phenotype in Discus Fish

The traits of a discus fish, both visible and non-visible, are the result of a combination of its genes and its environment.

• The genotype is the specific set of genes an individual has for a particular trait. For example, a discus fish might have a genotype with alleles for bright red and blue colours.

• The phenotype is how those genes are expressed—the physical traits and behaviours we can actually see. This includes aspects such as the fish's colour, body shape, size, and even its aggression.

You can see the difference between genotype and phenotype in the discus fish. Their genotype directly affects the traits they inherit, like their "distinct colour patterns and different behaviours." However, what they finally look like is not just decided by their genes. The environment is also very important, as "food availability and their diet" can cause "minor variations in size and colour." This means a fish's phenotype is a mix of its genetic code and its surroundings. For example, a fish with genes to be large will only grow to its full size (phenotype) if it gets enough food.

Evaluation: The Importance of Variation for Population Survival

Genetic variation is extremely important for the survival of the discus fish population because it provides the building blocks for adapting to a changing environment. A population with low genetic variety is at risk from environmental changes, as a single problem could affect all the fish in the same way. The importance of this variation can be shown in three possible examples:

1. Disease Resistance: If a disease breaks out, genetic diversity increases the chance that some fish will have alleles that give them natural protection. These protected individuals are more likely to survive, produce offspring, and pass on their helpful alleles, allowing the population to adapt. A population without this variety would be at risk, potentially causing it to collapse or die out.

2. Changes in Habitat: If changes like deforestation make the water murky, the variety in fish colours becomes vital for survival. Fish with genes for dull, camouflaged colours would be harder for predators to see than brightly coloured fish. This gives them a better chance of surviving and passing on the camouflage alleles to the next generation.

3. Food Availability: If the main food source disappears and is replaced by a hard-shelled creature, the variety in jaw shapes would be essential. Fish with genotypes for stronger jaws could eat the new food, giving them a better chance of surviving and having babies. Over time, the population would adapt as the alleles for this helpful trait become more common.

In conclusion, genetic variation gives the discus fish a range of different traits that help them deal with challenges in their environment. It increases the likelihood that some fish will have what it takes to survive unexpected problems, allowing the whole population to adapt. Without the variety created by mutation and sexual reproduction, the discus fish population would be in much greater danger of dying out.