Genetic Variation, Natural Selection, and Allele Frequency Changes, Slides of Biology

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12th grade

SELECTION &

EVOLUTION

Chapter 17 – A Level Biology

Variation

• (^) Phenotypic variation can also be explained by a combination of genetic and environmental factors - (^) For example, the recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria
• (^) The complete phenotype of an organism is determined by the expression of its genotype and the interaction of the environment on this: Phenotype = Genotype + Environment

Genetic variation

• (^) Organisms of the same species will have very similar genotypes, but two individuals (even twins) will have differences between their DNA base sequences
• (^) Considering the size of genomes, these differences are small between individuals of the same species
• (^) The small differences in DNA base sequences between individual organisms within a species population are called genetic variation
• (^) Genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population
• (^) The following processes cause genetic variation as they result in a new combination of alleles in a gamete or individual: - (^) Independent assortment of homologous chromosomes during metaphase I - (^) Crossing over of non-sister chromatids during prophase I - (^) Random fusion of gametes during fertilization

Sources of genetic variation table

Environmental factors

• (^) The environment that an organism lives in can also have an impact on its phenotype
• (^) Different environments around the globe experience very different conditions in terms of the following: - (^) Length of sunlight hours (which may be seasonal) - (^) Supply of nutrients (food) - (^) Availability of water - (^) Temperature range - (^) Changes in the factors above can affect how organisms grow and develop - (^) For example, plants with a tall genotype growing in an environment that is depleted in minerals, sunlight and water will not be able to grow to their full potential size determined by genetics - (^) Variation in phenotype caused solely by environmental pressures or factors cannot be inherited by an organism's offspring

Discontinuous variation •^ Qualitative differences fall

into discrete and distinguishable categories , usually with no intermediates (a feature can’t fall in between categories)

• (^) For example, there are four possible ABO blood groups in humans; a person can only have one of them
• (^) It is easy to identify discontinuous variation when it is present in a table or graph due to the
• (^) Qualitative differences in the phenotypes of individuals within a population give rise to discontinuous variation

Continuous variation

• (^) Continuous variation occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics
• (^) Quantitative differences do not fall into discrete categories like in discontinuous variation
• (^) Instead for these features, a range of values exist between two extremes within which the phenotype will fall - (^) For example, the mass or height of a human is an example of continuous
• (^) The lack of categories and the presence of a range of values can be used to identify continuous variation when it is

Genetic basis of discontinuous variation

• (^) This type of variation occurs solely due to genetic factors
• (^) The environment has no direct effect
  • (^) Phenotype = genotype
• (^) At the genetic level:
  • (^) Different genes have different effects on the phenotype
  • (^) Different alleles at a single gene locus have a large effect on the phenotype
  • (^) Remember diploid organisms will inherit two alleles of each gene, these alleles can be the same or different
• (^) A good example of this is the F8 gene that codes for the blood-clotting protein Factor VIII - (^) The different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the

Genetic basis of continuous variation

• (^) This type of variation is caused by an interaction between genetics and the environment
• (^) Phenotype = genotype + environment
• (^) At the genetic level:
  • (^) Different alleles at a single locus have a small effect on the phenotype
  • (^) Different genes can have the same effect on the phenotype and these add together to have an additive effect
  • (^) If a large number of genes have a combined effect on the phenotype they are known as polygenes

Natural & Artificial Selection 02

• (^) Every individual within a species population has the potential to reproduce and have offspring which contribute to population growth
• (^) If the offspring for every individual survived to adulthood and reproduced then the population would experience exponential growth - (^) This type of growth only happens when there are no environmental factors or population checks acting on the population (for example, when there are plentiful resources and no disease) - (^) One well known but rare example of exponential growth in a population is the introduction of 24 European rabbits into Australia in the 1800s. The rabbits had an abundance of resources, little or no competition and no natural predators.

Environmental factors

• (^) Environmental factors limit population sizes by reducing the rate of population growth whenever a population reaches a certain size
• (^) Environmental factors can be biotic or abiotic
• (^) Biotic factors involve other living organisms
  • (^) This includes things like predation, competition for resources and disease
• (^) Abiotic factors involve the nonliving parts of an environment
  • (^) Examples of abiotic factors include light availability, water supply and soil pH
• (^) When biotic and abiotic factors come into play not all individuals within a population will survive - (^) For example, if a food source is limited some animals within a population will not get enough to eat and will starve to

Population limitation by environmental

factors

• (^) For African lions living in the wild there are several environmental factors that limit their population growth rate:
• (^) 1. Competition for food
  • (^) There is a limited supply of prey: other lions and carnivores will also be hunting the same prey. If a lion is not able to hunt and feed then they will die from starvation
• (^) 2. Competition for a reproductive mate
  • (^) Female lions will often outnumber male lions in a population. This means the males compete with each other to mate with the females. When one male is in a contest with another male one (or both) could be injured or killed. Whoever loses the contest won’t be