L1 Adaptation by natural selection Flashcards
What is an adaptation?
A trait that increases the survival or reproduction of organisms that bear it, relative to alternative character states
What is the Hardy-Weinberg equilibrium?
States that alleles and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. Population isn’t evolving
What four evolutionary forces do you need to disrupt HW equilibrium?
Mutation, Genetic drift, Migration, Natural selection
What is natural selection?
The differential survival and/or reproduction of classes of entities that differ in one or more characteristics, it is fitness focussed,
What are the different entities section can act on?
Genotype, subset if genotype, individual populations, species etc.
What is fitness?
The probability of survival x average number of offspring for a class of individuals
How do we know natural selection exists?
Correlation between trait and environment
Responses to experimental change in the environment
Correlations between trait and fitness component
Describe the experiment with the guppies that illustrates natural selection is acting on the populations
Poecillia reticulate, Guppies in Trinidad and Tobago, colour of guppies differs between the streams, the difference is mainly seen in males. Predation pressure differs between populations, one targets all fish, one only juveniles. Colour/no. of spots seems to correlate with environment. To test, fish were bred in two different environments, moved around to stop genetic isolation, and then introduced to predator. See a difference in blue and iridescent spots- not related to the environment, until a predator is introduced, and then spots change to match the background.
What is sexually antagonistic coevolution?
Where beneficial trait evolves in one sex that isn’t beneficial in the other sex
What problems are encountered when trying to detect natural selection?
Could just be a consequence of physics or chemistry - there by chance but not adaptive
Genetic drift can spread traits in a population - if neutral than selection won’t act so may spread by chance
Ancestral state, may not be under natural selection any more, just present from the past
- Selection might not cause any change, may just act to keep proportion of alleles the same if at max fitness
-might not be working at the individual level
-Linkage
What is linkage disequilibrium?
When two alleles are linked together, appearing in the genome together more often than you would expect by chance
What are the reasons why linkage disequilibrium may occur?
Beneficial alleles grouping together
Hitchhiking alleles
Structural changes in DNA
Why do beneficial alleles group together
Wont be selected for if weakly beneficial, but if multiple weak alleles in same spot in the genome, section will become more beneficial, so selection acts to keep them together more
How might DNA structural changes result in linkage disequilibrium?
When inversion happens, an inversion loop forms. Recombination cannot happen and so the genes in the loop always appear in the genome together, whether or not they are beneficial
What are hitchhiking alleles?
If next to a beneficial allele then chance of recombination is lower, a selection works to keep that allele in the population. A selective sweep - would appear to be under selection when actually only tin linkage disequilibrium with a beneficial allele.
