What a subtopic title indeed – organisms! I didn’t even make it up myself, it’s the spec. This topic is really about evolution. The basics of evolution, such as mutation, genetic drift, natural and sexual selection and allele frequency are covered in the Scottish Higher topics Evolution and Mutations, so this topic builds further on those concepts.
Previously we learned that evolution is the change in heritable traits through generations, brought about by random shifts such as genetic drift, as well as directed shifts such as selection.
In small populations, genetic drift is more powerful because the relatively small background fluctuations in allele frequency can actually result in some alleles being lost altogether rather than just slightly decreasing, as may be the case in a large population.
The variation present between organisms at the DNA sequence level is enabled through mutations of the code in genes, or the change in a chromosome part, or even whole chromosomes and chromosome sets.
As different genotypes change in frequency over generations and between them, quantitative analysis in terms of absolute fitness and relative fitness can be implemented.
In biology, fitness (w) refers to the prevalence and change in frequency of a given trait.
Absolute fitness refers to a standalone trait and how it changes over generations. For example, a ratio of 563 counts of genotype Ee (second generation) to 563 counts of the same genotype Ee (first generation) is 1, meaning no change.
A less than 1 absolute fitness indicates a decline in genotype frequency, while a greater than 1 indicates an increase in frequency.
Relative fitness takes into account the values in frequency between multiple genotypes. They are no longer compared absolutely with themselves over generations, but they are compared with each other. Relative fitness can also account for the surviving offspring associated with each genotype.
In this case, the ratio is obtained by comparing a given genotype frequency to the highest frequency genotype in the comparison. For example, the Ee frequency is 563, while ee is 482 and EE is 622.
The highest frequency of 622 EE becomes the reference, and is given the value of 1.
Ee compared to EE is 563 / 622 = 0.905.
ee compared to EE is 482 / 622 = 0.775.
ee has the lowest relative fitness. If in the previous generation, ee was at 284, its absolute fitness (compared to itself) would be 482 / 284 = 1.70. Almost doubling! So in itself, the ee genotype is on an astronomical increase, but relative to the other genotypes (EE and Ee) it’s still got the lowest fitness.
The evolution of different genotypes can take place over short or long timescales, according to the selection pressures present as part of natural selection.
The rate of evolution depends on multiple factors such as the strength of selection, shorter generation times, increased temperature and sharing DNA between individuals horizontally (e.g. bacterial conjugation) or vertically (sexual reproduction).
Stronger selection pressures result in faster evolution because relative fitness is challenged more. This results in faster selection of genotypes with a higher fitness, and results in a steeper gradient between the fitness of different genotypes.
Shorter generation times simply speed up the frequency of reproduction and the opportunity for DNA recombination in sexual selection. Increased temperature itself results in shorter generation times because the reproductive function of organisms matures quicker and develops faster.
Therefore, these factors affect the rate of evolution in overlapping ways.
Antibacterial resistance has evolved very quickly in bacteria, as seen with the resistance of MRSA (methicillin-resistant Staphylococcus aureus) against vancomycin – an up to 30% increase in less than a decade. Climate change is making some areas inhabitable and causing species migration, while in the hotter areas that are hospitable to life, faster generation times are seen. Some insects rely completely on temperature to cue their reproduction and life cycle, and even determine offspring sex.
Many types of organism have tight relationships with other species, whether in mutually beneficial ways or as parasites. That’s why their path of evolution often converges with their neighbour species, and starts moving together as part of co-evolution.
One change in one species becomes a selection pressure for the other. For example, herbivores and plants co-evolve to interact closely with each other e.g. plants make attractive food sources for animals, while the animals ingest them and help spread their seed.
Pollinators spread the pollen of flowering plants to maximise their dispersal, while the plants offer nectar as food for insects. Prey evolve escape mechanisms against their predator, while predators evolve detection mechanisms for their prey. Parasites evolve invasion methods for their host, while the host evolves protection mechanisms against the parasite.
This pattern of close connection that relies on both parties keeping up with each other is known as the Red Queen hypothesis because it follows the “keep running in order to stay still” line.
While prey and parasite hosts derive better fitness through evolving away from predators and parasites, the latter equally evolve increased fitness to counteract these changes, and be able to continue surviving using their prey and hosts.
Co-evolution has led to some species being entirely dependent on each other for survival. Human head lice, for example, cannot live anywhere else except a human scalp. Many flowers have evolved to cater to a specific insect as its exclusive pollinator.