Define directional selection. Under what circumstances?
= Selection favouring individuals that vary in one direction from the mean of the population.
=> changes overall characteristics in the population.
- Change in environmental conditions —> phenotypes that are best suited to new conditions are most likely to survive and reproduce —> mean will then move in direction of offspring over time.
Outline an example of directional selection. Draw normal distribution curves to represent the change.
- Antibiotic resistance with penicillin:
1. Spontaneous mutation in allele of a gene in a bacterium —> new protein (enzyme - penicillinase) produced.
2. Bacterium in situation where penicillin being used to treat an individual (by chance) —> mutation gives bacterium an advantage as it can produce penicillinase and survive.
3. Surviving bacterium divides by binary fission —> small population of resistant bacteria forms.
- Population of resistant bacteria grows at expense of non-resistant population
=> allele frequency of penicillinase allele increases in the population.
=> normal distribution curve shifts in direction of higher penicillin resistance.
=> antibiotic use places a directional selection pressure on the bacterial population.
Define stabilising selection. Under what circumstances?
= Selection favouring individuals with the average characteristic of the population.
=> preserves overall characteristics of the population.
- Stable environmental conditions —> phenotypes closest to mean are favoured => tends to eliminate the phenotypes at the extremes as phenotypes around mean selected for.
Outline an example of stabilising selection. Draw normal distribution curves to represent the change.
- Human birth weights:
1. Infant mortality rate lowest in range 2.5-4.0 kg and increases either side of this range.
2. Mortality rate is greater at the two extremes => population’s characteristics are being preserved rather than changed —> phenotypes around the mean of the population selected for and those at both extremes selected against.
=> normal distribution curve in the same position on x-axis but narrower range of distributions.
What is the consequence of natural selection?
=> Results in species better adapted to the environment in which they live.
Adaptations may be:
- Anatomical.
- Physiological.
- Behavioural
Example of anatomical adaptations.
- Shorter ears and thicker fur in arctic foxes compared to foxes in warmer climates
Example of physiological adaptations.
- Kangaroo rats oxidise fat —> produces water as a by-product in a dry desert environment.
Example of behavioural adaptations.
- Autumn migration of swallows from the UK to Africa to avoid food shortages in UK winter.
Describe how genetic bottlenecks reduce genetic diversity.
= Genetic Bottleneck is an event that causes a big reduction in a population —> when a large number of organisms within a population die before reproducing.
=> reduces the number of different alleles in the gene pool and so reduces genetic diversity.
=> the survivors reproduce and a larger population is created from a few individuals.
Explain the Founder Effect.
= Type of genetic bottleneck.
= Describes what happens when just a few organisms from a population start a new colony and there are therefore only a small number of different alleles in the initial gene pool.
=> the frequency of each allele in the new colony might be very different to those alleles in the original population —> could possibly lead to a higher incidence of genetic disease.
- The founder effect could occur as a result of migration leading to geographical separation or if a new colony is separated from the original population for another reason, such as religion.
Define Phylogeny.
= Study of evolutionary history of groups of organisms.
=> tells which organisms are related to each other and how closely related they are.
What is a phylogenetic tree?
= Tree showing the relationship between organisms.
- First branching point represents common ancestor for all the members of the family - which is now extinct.
- Each of the following branch points represents another common ancestor from which a different group diverged.
- Closely related species diverged away from each other most recently —> branches are closer together.
Define taxonomy.
= Science of classification - involves naming organisms and organising them into groups.
NB => Scientists now take into account phylogeny when classifying organisms and group organisms according to their evolutionary relationships.
Why do we classify organisms?
= Makes it easier to identify, study and compare organisms with others
What are the 8 taxa? How are they arranged?
Taxa = hierarchal levels of groups used to classify organisms.
Domain Kingdom Phylum Class Order Family Genus Species
= Arranged in a hierarchy, with the largest groups on top.
= Organisms can only belong to one organism in the group at each level in the hierarchy - no overlap.
How are organisms classified?
- Organisms first sorted into three large taxa = domains - Eukarya, Bacteria and Archaea.
- Related organisms in a domain are then organised into slightly smaller groups called kingdoms (based on shared characteristics).
- More closely related organisms from that kingdom then grouped into a phylum etc. and so on.
- Hierarchy ends with species - groups that contain only one type of organism.
What changes do we see as we move the down the hierarchy?
= More groups at each level, but fewer organisms in each group —> organisms in each group become more closely related.
Define species.
= Group of similar organisms able to reproduce to fertile offspring.
Why do scientists constantly update classification systems?
= Scientists constantly update classification systems because of discoveries about new species and new evidence about known organisms —> DNA sequence data etc.
Comment on the Binomial Naming system.
= One internationally accepted name - helps to avoid confusion of using common names - over 100 plant species called raspberries etc.
- First part = genus, capital letter.
- Second part = species, lower case.
NB => names always written in italics or underlined if handwritten.
What is Courtship Behaviour? What is its purpose?
= Set of species-specific display behaviours in which an animal attempts to attract a mate of the same species and exhibit their desire to copulate.
- Can be as simple as releasing certain chemicals or as complex as a series of displays.
- Species-specific => only members of the same species will respond to that courtship behaviour.
=> allows members of the same species to recognise each other, preventing interbreeding —> making reproduction more successful (mating with another species will produce infertile offspring).
How can Courtship Behaviour be used to help classify species?
= Species-specific = so we can observe particular display behaviours of two species and compare them - more closely related two species are, the more similar their courtship behaviour will be.
List the 3 ways in which advances in technologies/techniques can clarify evolutionary relationships.
- Genome sequencing.
- Comparing amino acid sequence.
- Immunological comparisons.
