(a) Drift and selection

• Evolution is the change over time in the proportion of individuals in a population differing in one or more inherited traits

• During evolution, changes in allele frequency occur through the non-random processes of natural selection and sexual selection, and the random process of genetic drift

• Natural selection acts on genetic variation in populations

  • Variation in traits arises as a result of mutation. 

  • Mutation is the original source of new sequences of DNA. 

  • These new sequences can be novel alleles. 

  • Most mutations are harmful or neutral, but in rare cases they may be beneficial to the fitness of an individual.

• Populations produce more offspring than the environment can support

• Individuals with variations that are better suited to their environment tend to survive longer and produce more offspring, breeding to pass on those alleles that conferred an advantage to the next generation

  • Selection results in the non-random increase in the frequency of advantageous alleles and the non-random decrease in the frequency of deleterious alleles.

• Sexual selection is the non-random process involving the selection of alleles that increase the individual’s chances of mating and producing offspring

• Sexual selection may lead to sexual dimorphism

• Sexual selection can be due to male-male rivalry and female choice

  • Male-male rivalry: large size or weaponry increases access to females through conflict. Female choice involves females assessing the fitness of males.

• Genetic drift occurs when chance events cause unpredictable fluctuations in allele frequencies from one generation to the next

• Genetic drift is more important in small populations, as alleles are more likely to be lost from the gene pool

• The importance of bottleneck and founder effects on genetic drift

  • Population bottlenecks occur when a population size is reduced for at least one generation.

  • Founder effects occur through the isolation of a few members of a population from a larger population. 

  • The gene pool of the new population is not representative of that in the original gene pool.

• A gene pool is altered by genetic drift because certain alleles may be under-represented or over-represented and allele frequencies change

• Where selection pressures are strong, the rate of evolution can be rapid

  • Selection pressures are the environmental factors that influence which individuals in a population pass on their alleles.

  • They can be biotic: competition, predation, disease, parasitism; or abiotic: changes in temperature, light, humidity, pH, salinity.

• The Hardy-Weinberg (HW) principle states that, in the absence of evolutionary influences, allele and genotype frequencies in a population will remain constant over the generations

  • The conditions for maintaining the HW equilibrium are: no natural selection, random mating, no mutation, large population size and no gene flow (through migration, in or out).

• The HW principle can be used to determine whether a change in allele frequency is occurring in a population over time

  • Use the HW principle to calculate allele, genotype and phenotype frequencies in populations.

(b) Fitness

• Fitness is an indication of an individual’s ability to be successful at surviving and reproducing

  • Fitness is a measure of the tendency of some organisms to produce more surviving offspring than competing members of the same species.

• It refers to the contribution made to the gene pool of the next generation by individual genotypes

• Fitness can be defined in absolute or relative terms

• Absolute fitness is the ratio between the frequency of individuals of a particular genotype after selection, to those before selection

• If the absolute fitness is 1, then the frequency of that genotype is stable. 

• A value greater than 1 conveys an increase in the genotype and a value less than 1 conveys a decrease.

• Relative fitness is the ratio of the number of surviving offspring per individual of a particular genotype to the number of surviving offspring per individual of the most successful genotype

(c) Co-evolution

• Co-evolution is the process by which two or more species evolve in response to selection pressures imposed by each other

• A change in the traits of one species acts as a selection pressure on the other species

• Co-evolution is frequently seen in pairs of species that have symbiotic interactions

  • Symbiosis: co-evolved intimate relationships between members of two different species.

• The impacts of these relationships can be positive (+), negative (-) or neutral (0) for the individuals involved

• Mutualism, commensalism, and parasitism are types of symbiotic interactions

  • Mutualism: both organisms in the interaction are interdependent on each other for resources or other services. As both organisms gain from the relationship, the interaction is (+/+).

  • Commensalism: only one of the organisms benefits (+/0).

  • Parasitism: the parasite benefits in terms of energy or nutrients and the host is harmed as the result of the loss of these resources (+/-).

• The Red Queen hypothesis states that, in a co-evolutionary relationship, change in the traits of one species can act as a selection pressure on the other species

• This means that species in these relationships must adapt to avoid extinction