Lecture 3- Mechanisms of evolution III Flashcards
Maintenance of genetic variation, constraints of evolution, human influences
What are the three major forces that maintain genetic variation within populations?
Neutral mutations, sexual recombination, frequency dependent selection
What are neutral alleles?
An allele that does not affect the fitness of an organism as a result of mutations that do not effect the function of the resulting proteins.
These are unaffected by natural selection, and tend to accumulate over time to provide significant genetic variation.
Much phenotypic variation that can be observed is not neutral. How is neutral variation measured?
Molecular techniques allow neutral alleles to be identified. These can be used to estimate rates of evolution.
How does sexual recombination increase genetic variation?
It amplifies the number of possible genotypes by crossing over and independent assortment of chromosomes during meiosis, as well as combination of different gametes.
What are the short-term disadvantages of sexual reproduction?
- Recombination breaks up adaptive combinations of genes
- Sex reduces the rate at which a female passes on her genes to their offspring
- Dividing the population into genders greatly reduces the overall reproductive rate.
What are the advantages of sexual reproduction that possibly led to this type of reproduction arising?
- Facilitates the repair of damaged DNA
- Permits elimination of deletrious mutations
- Defense against pathogens and parasites
What is Muller’s Ratchet?
The accumulation of deletrious mutations in asexual lineages- each mutation is passed on to its offspring, the number of such mutations can only increase
Why does sexual recombination defend organisms from pathogens and parasites?
Parasites and pathogens have much shorter life cycles than their hosts so can evolve counter-adaptations against their host’s defense mechanisms. Sexual recombination gives the host a chance to keep up.
What effect does sexual recombination have on allele frequency?
Allele frequency does not change. Sexual recombination generates new combinations of alleles on which natural selection can act.
What is frequency dependent selection?
The maintenance of polymorphism when the fitness of a genotype or phenotype depends on its frequency in a population.
This maintains genetic variation within populations.
Give an example of frequency-dependent selection.
The mouth of Perissodus microlepis- a small fish in Lake Tanganyika, opens on either the right side or left side due to an asymmetrical jaw joint.
The scale eater approaches its prey from behind and notes off several scales.
Vigilance by prey favors an equal number of left and right mouthed scale-eaters- if attacks from one side were more common, prey would pay more attention to potential attacks from that side.
Environmental variation favors genetic variation because no single genotype is likely to perform well under all conditions. Explain how genetic variation is maintained by environmental variation in Colias butterflies in the Rocky Mountains.
Dawn temperatures are cold, afternoon temperatures are hot. The Colias butterfly is polymorphic for the enzyme phosphoglycose isomerase which influences how well a buttefly flies at different temperatures.
At different temperatures, heat or cold tolerate genotypes are favoured.
Heterozygous Colias butterflies fly over a greater temperature range- giving them a foraging and mating advantage.
Why do subpopulations vary genetically?
They are subjected to different selective pressures in different environments.
Temperature and soil moisture vary between north and south facing mountain slopes. How does this effect the Pinus ponderosa in the rocky mountains?
On south facing slopes, there are more heterozygous Pinus pnderosa for peroxidase enzyme where temperatures fluctuate dramatically.
On north facing slopes, temperatures are cooler and less fluctuating, peroxidase homozygote is more frequent (this has a lower optimum temperature)
Explain the geographic variation in white clovers across Europe.
Subpopulations of Trifolium repens produce cyanide depending on the winter temperature of their environment. Poisonous clover plants are less appealing to herbivores, but are more likely to be killed by frost (freezing damages the cell membrane and releases cyanide into the plants own tissue).