Natural Selection Flashcards
What are the four modes of selection?
Directional selection, stabilising selection, purifying selection, frequency-dependent selection.
Define directional selection using an example.
Directional selection occurs at the phenotypic level. it changes the average value of a trait because heights reproductive success is found in individuals above the average.
E.g. being taller is an advantage and taller people will have the most reproductive success. the pop average will increase over time due to the selective advantage of being taller.
Define stabilising selection using an example.
Stabilising selection occurs at the phenotypic level. it occurs when the current pop average is also the optimum from a fitness perspective. Individuals at the average will have high fitness than those above or below it. It maintains the pop mean for the characteristic in the same place.
E.g. babies born with a high/low birthweight may have more problems, selection stabilises this by sleeting for a medium birthweight.
What is purifying selection?
Purifying selection occurs at the genotypic level, it occurs when an allele is fixed at a locus. When a mutation occurs at this locus, they will have lower fitness than the existing allele and so will be weeded out.
How does directional selection act as an agent of change?
Directional selection can move the average for polygenic characteristics. As allele frequencies change across loci, new genetic combinations are made. New combinations make new variations for selection to work on. Directional selection can eventually create new phenotypes never seen in the pop before.
What is negative frequency-dependent selection? Describe Gross’s (1991) study on this.
This is when the phenotype has higher fitness when it is rare and lower fitness when it is common.Gross (1991) studied this in Bluegill Sunfish. There are 2 types of males; parental (build nests for females to deposit eggs for fertilisation) and cuckolders (sneak into nests to deposit their sperm for fertilisation). He manipulated the amount of cuckolders - success was heighest when they were rare and increased when they became common.
What is the mutation-selection balance?
Mutation introduces genetic variation, selection filters variation out.
What does a ‘heterozygous advantage’ mean?
When individuals with 1 copy of an allele have higher fitness than those with none or two copies.
Describe inconsistent selection.
When the selective optimum moves around from time to time, there is still an optimum phenotype that selection moves towards. But selection’s ability to counteract mutation and variation is weakened. This is because it decreases frequencies of alleles in some years and increases them in others.
Describe sexually antagonistic selection.
The optimal phenotype may not be the same for males and females. Some alleles that improve the fitness of males may decrease that of females and vice versa.
What are the 2 assumptions of how selection produces design?
1 - the final structure must have higher fitness that the original state.
2 - there must be a continuous sequence of intermediate phenotypes each separated by a small change brought about by 1 genetic mutation.
What is the adaptionist stance and what is the problem with this view?
If some feature/behaviour is common in an organism, it is probably an effective design solution to a problem, if it wasn’t then the alleles building the feature would be outcompeted.
Describe ultimate and proximate explanaitions.
Ultimate = how the design has increased ancestral fitness Proximate = the genetic/developmental mechanisms leading to the formation of that characteristic in an individual
Describe a study relating to genetic corrleations.
Trut (1999) - taming foxes.
Captive but undomesticated foxes were assessed for tameness, the tamest were bred. This was repeated and 30-35 generations later the foxes were unafraid, affectionate, and responsive. However, other features changed too, e.g. floppy ears, piebald colouring, shorter heads etc.
What are the four ways to test the adaptionist hypothesis?
Reverse engineering, optimality modelling, experimental manipulation, and comparative evidence.
