In this lesson we will learn about the importance of genetic diversity for populations to survive and adapt to environmental change.
By the end of this lesson you will be able to:
- Define genetic variation and describe how it is acquired in populations.
- Describe natural selection and elaborate on its role in the adaptation of populations.
- Define adaptive potential and discuss the importance of genetic diversity in the survival of populations.
- Compare natural selection and artificial selection.
- Variation refers to differences in phenotypes (physical and behavioural characteristics) between individual organisms or groups of organisms.
- Variation results from both genetic factors and environmental factors.
- Genetic factors refer to genotypes (allele combinations).
- Environmental factors refer to any non-genetic influences, such as temperature, pH, sunlight and nutrients.
- Variation due to genetic factors is heritable.
- Variation due to environmental factors is non-heritable.
- Genetic variation, therefore, refers to heritable differences between individuals, which is directly related to differences in genotypes.
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Genetic Variation Within Populations
- A species is a group of individuals with similar genetic composition who are capable of interbreeding to produce fertile offspring.
- A population is a group of the same species living and interacting in a defined area.
- Within natural populations, there is usually a range of alleles that exist for each gene.
- As a result, individuals within populations have a range of genetically-determined characteristics, such as size, blood type, disease resistance and aggression.
- The genetic diversity of a population is a measure of the range of different alleles that exist among individuals within that population.
- This is often referred to as the population’s ‘gene pool‘.
- The genetic diversity of a population is related to the diversity of phenotypes within it.
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Sources of Genetic Variation
- Mutation is the original source of all genetic variation, as it is the only way new alleles are created.
- Even though mutations can be beneficial, harmful or neutral, all types have the potential to enter the gene pool.
- Beneficial (advantageous) mutations increase the fitness and survivability of individuals and therefore quickly establish in populations due to natural selection (see below).
- Neutral mutations have no effect on the survivability of individuals so can establish in populations due to chance.
- Negative mutations, even though they decrease the fitness and survivability of individuals, can still establish in gene pools, particularly if they are recessive as natural selection will not affect carriers.
- For sexually reproducing species, genetic variation within populations is enhanced as a result of meiosis and fertilisation, as these can create new combinations of alleles and, consequently, new phenotypes.
- Genetic variation is also influenced by the movement of individuals between populations – known as gene flow.
- In particular, the genetic variation of a population can be enhanced if migrating individuals contribute new alleles to the gene pool.
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- Within a genetically diverse population, there will be certain individuals with characteristics and behaviours that give them a greater chance of surviving and reproducing, known as a selective advantage.
- For example:
- Faster, stronger or more cunning predators have a greater chance of catching prey.
- Prey with better camouflage or defence mechanisms have a greater chance of avoiding predators.
- Individuals with immunity to a particular disease have a greater chance of surviving an outbreak of that disease.
- Individuals with fancier nests, prettier feathers, more elaborate dances or more attractive songs will have a greater chance of finding a mating partner.
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- The process whereby an individual with a selective advantage survives while another individual doesn’t is known as natural selection (also commonly referred to as ‘survival of the fittest‘).
- The environmental factor that leads to natural selection, such as drought, disease or a new predator, is called the selective agent or selection pressure.
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- The fourth example of selective advantage listed above depicts sexual selection – a particular type of natural selection where the selective agent is a potential mating partner.
- Sexual selection can favour practical qualities, such as strength or hunting ability, but can also favour characteristics or behaviours of little practical value beyond helping to find a mating partner.
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Adaptation of Populations
- Individuals favoured by natural selection have a greater chance of surviving and reproducing, and therefore passing their genes on to future generations. Consequently, as environments change in favour of certain individuals, the genetic composition of a population will change.
- Alleles responsible for favourable phenotypes will become more common in the gene pool and the associated phenotypes will become predominant in the population.
- This gradual change in a population, as a result of natural selection, towards phenotypes that are better suited to the environment, is known as adaptation.
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Genetic Variation and Survival
- Environments are always changing. Phenotypes that are favourable in one environment may not be favourable in another.
- Therefore, natural selection is always occurring and populations are constantly adapting to survive.
- The ability of a population or species to survive environmental changes is referred to as adaptive potential.
- Since adaptive potential is directly related to genetic diversity, the greater the genetic variation, the greater the chance of survival, as a genetically diverse population has a greater chance of containing individuals with phenotypes that allow them to survive adverse changes.
- For example, a population will only survive the introduction of a new disease if there are individuals within the population who are immune to the disease (as a result of having alleles that lead to immunity). Even if the majority of the population dies from the disease, the surviving individuals can reproduce and regenerate the population.
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- Artificial selection (selective breeding) is a method used by animal and plant breeders that mimics natural selection.
- Only animals or plants with desired phenotypes are used for producing future generations.
- For example, a dairy farmer who wishes to maximise milk production will only use the highest milk-producing cows for breeding, so that, with each generation, the average rate of milk production increases.
- Artificial selection is used widely not only in agriculture and horticulture, but also in the breeding of domestic animals, such as cats, dogs and horses.
- Since artificial selection favours phenotypes based on human preferences, rather than natural survivability, many of these breeds would be unable to survive in the wild.
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- Genetic variation refers to differences in genotypes (allele combinations) between individuals, which usually results in differences in phenotypes.
- The genetic diversity of a population is a measure of the genetic variation among individuals within that population – often referred to as the population’s ‘gene pool‘.
- Mutation is the source of all genetic variation as it is the only way new alleles are created.
- Genetic variation can be enhanced by sexual reproduction (meiosis and fertilisation) as well as migration of individuals into a population.
- Selective advantage is the increased chance of an individual surviving and reproducing due having a certain phenotype.
- Natural selection (‘survival of the fittest‘) is the process whereby an individual with a selective advantage survives while another individual doesn’t.
- A selective agent (selection pressure) is an environmental factor that leads to natural selection.
- Sexual selection is a type of natural selection where the selective agent is a potential mating partner.
- Artificial selection (selective breeding) is a method used by animal and plant breeders where only individuals with a desired phenotype are used for reproduction, thereby increasing the prevalence of that phenotype in the population.
- Adaptation is the gradual change of a population, as a result of natural selection, towards phenotypes that are better suited to the environment.
- Adaptive potential refers to the ability of a population or species to survive environmental change.
- The greater the genetic variation, the greater the adaptive potential.
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