Variation

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Key Components

Before we begin, here are some terms which you should familiarise yourself with.

  • Population: a group of organisms of the same species living in a specific region at a given time
  • Gene Pool: the total aggregation of all alleles possessed by reproductive members of a population
  • Species: a group of organisms that can interbreed producing fertile offspring and are reproductively isolated
  • Genotype: the genetic makeup or alleles of an organism
  • Phenotype: the genotype expressed in the environment

Variation

Variation can arise due to mutations, polygenic inheritance, recombination and environmental factors

  • Morphological, physical features of an organism, body shape
  • Biochemical, metabolic characteristics of an organism, concentration of thyroxine
  • Physiological, internal systematic responses to external stimuli, production of venom
  • Behavioural, activities that an organism does to help it survive, hibernation in winter

Allele Frequency

Allele Frequency is the relative frequency a particular allele has within a population. To maintain a constant allele frequency, the population must be large, members of the population must mate at random and be equally fertile, producing equal numbers of viable offspring and the population must be isolated. The allele frequency can be changed by mutations, genetic drift, gene flow, natural selection and artificial selection

Genetic Drift

Genetic drift is the change in allele frequency overtime, caused by unpredictable random events. The potential impact is greater in smaller populations. In genetic drift, no allele is favoured; they are all equally subject to being affected. In small populations, this can lead to the decrease of favourable alleles, and eventual extinct.

The bottleneck effect is when the size of a population is both randomly and drastically reduced, resulting from major disasters such as natural disasters. The gene pool is thus limited to the few survivors and the generations thereafter may have different allele frequencies and the genetic diversity may be greatly reduced

The founder effect is when a small, non-representative sample of a larger population branches off to start a new colony. Thus, the gene pool is likely to have reduced genetic variation and is more subject to genetic drift


Founder Effect


Bottleneck Effect

Gene Flow

Gene flow refers to the movement of alleles into, or out of, a population due migration and interbreeding. Immigration between genetically different populations can increase the diversity of the gene pool, affecting the allele frequency. Emigration of a non-representative sample of a population can also affect the allele frequency. While the genetic diversity increases in the receiving population, the genetic diversity between the two populations exchanging individuals decreases as they become more similar.

Natural Selection

Natural selection occurs when a selective pressure confers a selective advantage upon certain phenotypes in a population in an environment, affecting their survival and reproductive rates.

  • Selective pressures are environmental pressures (resource availability, weather conditions, predation, disease) that give individuals with a specific phenotypic trait an advantage over those without the trait
  • Selective advantage is a feature of an individual that makes them more likely to survive and reproduce

A phenotype that makes a greater contribution to the gene pool of the next generation has a higher fitness value and is at a selective advantage – the more favourable genes are passed on. The less fit phenotype is selected against. Complete selection is where a given phenotype of an organism cannot reproduce because of death or because of sterility and thus makes no contribution to the gene pool of the next generation. Partial selection is where reproduction involving that phenotype produce, on average, fewer viable and fertile offspring

Speciation

Speciation is the formation of a new species. For sexually reproducing organisms, a new species is formed with they are not able to interbreed under natural conditions, or, if they do interbreed, are unable to produce viable offspring, or if they survive, are sterile.

Allopatric Speciation is where speciation occurs under circumstances where a population of a species becomes geographically isolated, resulting in no gene flow between them and other populations. By being geographically isolated they may experience different selective pressures, resulting in changes in their gene pool and allele frequencies, thus, allowing for the differences between the two populations to accumulate.

Isolation Mechanisms

These are factors that keep species separated and prevent interbreeding or gene flow

  • Isolation in time: populations are separated from each other by time, plants flowering at different times of the year
  • Isolation by behaviour: one populations mating behaviour is not attractive to another population
  • Isolation in space: populations are separated by a geographical barrier, a mountain or river
  • Isolation by anatomy: mating may be attempted, but because of physical differences, is not successful
  • Incompatibility of gametes: sperm cannot penetrate the outer coats surrounding the egg

Selective Breeding

Selective breeding or artificial selection is the deliberate human intervention of selecting parents with beneficial phenotypic characteristics to reproduce. In doing so, breeders manipulate the gene pool of their breeding stock, and maintain features in a population that are economically important or aesthetically appealing. This simply allows for traits that are already present to become more common, it does not create new traits. Therefore, it reduces the genetic diversity and decreases the ability to withstand environmental changes

Artificial insemination involves the collecting of semen from a selected animal and then introducing it by artificial means into the reproductive tract of females. Sex selection through sperm sorting is the process of producing offspring predominately of one sex by artificially separating sperm into those with X and those with Y chromosomes