Evolution: Lecture 4 Flashcards
What is relative fitness?
The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals.
What is selective pressure?
Something that is under selection pressure is going to be removed from the population quicker.
A trait under greater selection pressure is?
A trait that is removed from the population quicker.
A trait conferring higher relative fitness is?
A trait that is more likely to stay in the generation.
In the giraffe example, who had higher relative fitness?
The giraffe with the longer neck had more relative fitness. The longer neck is more advantageous, they have a longer lifespan and produce more offspring.
What is polygenic inheritance?
The combination of alleles at several loci determine the genetic component of a trait. When your phenotype is influenced by several genes (alleles at several loci).
Ex. human height or mouse coat colour.
Polygenic inheritance measures?
Quantitative character, a smooth range of phenotypes.
- We measure it, but it varies along the phenotypic range.
What graph typically do we see when looking at frequencies of quantitative character that could be acted on by selection?
We would normally see a normal distribution (bell curve). It is commonly seen if there is heritable variation at several loci.
Directional selection?
There is greatest selection pressure on either extreme of the curve. We decide to shift away from the selection pressure.
- One end of distribution selected against
- Classic response to a changing environment
Examples of directional selection?
- Pesticide/drug treatments
- Climate change
Soapberry bugs?
In the area where there are new, flatter fruits, there is greater selection pressure falling on individuals with longer beaks. This is because they are getting less nutrition due to inefficient feeding.
Soapberry bugs - directional selection?
There is directional selection favouring the shorter beaks, therefore the average beak length in this population falls.
Stabilizing selection?
Both extreme phenotypes are selected against (under greater selection pressure). This will then stabilize the average, making the mean higher and both sides shorter.
What is stabilizing selection due to?
It is often due to different, opposing, selective forces.
Examples of stabilizing selection?
Birth weight in humans:
- heavy or light = selection pressure
- middle/avg. = most fit
Disruptive selection?
Intermediate phenotypes are selected against or the selective pressure falls in the middle of the distribution. Both extremes are favoured.
Disruptive selection has?
A role in some speciation events, which occur when a species diverges enough to become separate species.
How do we measure fitness?
Fitness is measured by the number of viable offspring an individual has on average.
What is sexual selection?
Effectively, a special case of natural selection that involves:
- Competition for mating opportunities by the number or quality of mating opportunities
Result of sexual selection?
- Results in adaptations that increase
mating success
– But (sometimes) actually reduce survival
Intrasexual Selection?
Competition within one sex (usually males) for mating opportunities
Example of intrasexual selection?
Having ritual fights.
- Male with antlers will win fights
Intersexual Selection?
One sex (usually females) chooses mate from (competing members of) other sex
Example of intersexual selection?
Good features are chosen more.
- A male peacock’s tail (ornament)
Intrasexual and Intersexual selection increase?
Many remarkable adaptations increase success during intrasexual or intersexual selection
- Antlers and peacock’s tails
Disadvantages of sexual selection?
- Yet, some disadvantages are brought by sexual selection like how predators could see peacock’s tails easier the bigger they are and how it may be harder to move quickly without antlers.
Sexual Dimorphism?
Adaptation benefits only one sex, while both sexes would suffer any survival cost.
- Males and females will have different features as the females do not need them.
Sexual dimorphism can evolve?
Sexual dimorphism can also evolve via “normal” natural selection.
Example of sexual dimorphism evolving via natural selection?
Female birds having larger bodies to help protect her children/nest
Male birds are smaller and more suited to flight to provide food
What does diploidy do?
Diploidy ‘hides’ recessive alleles from
selection when they are rare. This means that they are seen less and less but bad at being fully eliminated. This is an example of preserving allelic variation.
- Natural selection cannot drive recessive alleles to complete extinction
What is balancing selection?
Natural selection can sometimes favour allelic variation, which is called balancing selection.
Examples of balancing selection?
- Heterozygote advantage
- Frequency-dependent selection
Sickle-cell anemia: heterozygote advantage
Sickle cell anemia is a single locus recessive genetic disease.
ss homozygotes have a high mortality while young, therefore the fitness of ss is much lower than SS or Ss.
- Yet, Ss confers resistance to malaria (deadly) and prevalent.
- With malaria absent, both are similar fitnesses
- With malaria present, Ss is higher fitness than SS
This means that there is a higher chance of there being ss individuals overall.
