Natural Selection

Question	Answer
Consequences of Drift	1. Allele freqs. fluctuate over time, even in the absence of selection. 2. Some alleles are fixed, others are purified (lost from pop.) and the fraction of heterozygotes decreases. 3. Separate pops. diverge in terms of allele freqs. & in terms of which alleles are present - leads to divergence -> reinforcement -> fixation -> speciation
Red Queen Hypothesis - Host-Parasite Model	This is expected to be a -ve FDS. Hosts adapted to/resistant to the parasite will have a higher fitness, but also a rarer type will have an advantage & benefit from rapid spread. Either the rare/resistant hosts will acquire advantage and maintain polymorphism or a coevolutionary arms race can develop - cycles of evolutionary change between 2 interacting species. This accelerates selection and can lead to speciation
Neutral Theory & Random Selection	KIMURA, 1968 - Most variation in DNA or AA sequences is selectively neutral & differences occur mainly due to genetic drift. Drift is random fluctuation in allele freqs. due to the sampling effect in a finite population & occurs in all natural pops. Drift & NT are 2 causes of allele substitution. BUT... NS ends in adaptation - DRIFT does not.
Genetic Drift	Does not end in adaptation. Alleles are potentially subject to drift but not necessarily subject to NS. The neutralist vs. selectionist debate is not about the effect of typical mutation but whether it is drift or selection that is the primary driver for evolutionary change. It may be the 0-hypothesis to explain differences against which evidence for NS may be tested.
Coat Colour in Pocket Mouse - Hoekstra et al, 2004	Mc1R gene has 2 alleles with a difference of 4 amino acids -> dark colour is dominant to light. Habitats are lava or sandstone, so much easier to model artificially. Used a migration selection model to estimate the selection coefficient & showed selection was acting stronger on light mice in dark habitats so their survival is lower
Selection Coefficient	A dominant allele will increase in frequency each generation depending on the selection coefficient. The larger the selection coefficient, the stronger the action of natural selection and so allele fixation occurs.
Directional Selection	Phenotype is the subject of selection, but the genotype (ALLELES) codes for this. Selection on certain alleles leads to their FIXATION and the elimination (PURIFICATION) of others.
Alleles	DOMINANT = 1 allele will prevail. RECESSIVE = Dominated by other genotypes - this will struggle to survive in natural populations. CODOMINANCE = All genotypes have equal fitness
Dominance Vs. Codominance	Dominant genotypes get established in the populations more rapidly than codominant ones up to a threshold when codominance takes over and reaches fixation more rapidly. Dominance takes much longer to reach fixation after the initial frequency. This is because dominant fitness is as a homozygote, but as a heterozygote (codominant) have 'more to select upon' so fixation is reached faster
Recessive	Recessive genotypes are always present but take a long time to increase in frequency - but once common, fixation occurs quickly
Overdominance	A relationship in which the phenotypic expression of the heterozygote is greater than that of both homozygotes. The direction of selection depends on the allele frequency at the start. If the allele freq. is low it will increase, but if it starts high it will decrease. The allele eventually reaches an intermediate freq. that does not depend on the initial freq. - providing both alleles are present in the pop.
Sickle Cell Anaemia	So homozygoes with 2 sickles alleles have extreme anaemia and die. Homozygotes with 2 normal alleles are not anaemic but have no malarial protection so die. Heterozygotes have slight anaemia and increased malarial protection so survive & increase in pop. The heterozygotes have advantage over both homozygotes.
Underdominance	Heterozygote disadvantage. Very rare and no real world examples. But in theory, if the allele freq is above a threshold level, it will increase in freq. and reach fixation. If it is below the threshold it will be eliminated and purified from the population.
Frequency Dependent Selection	Can be positive or negative & is when the costs/benefits associated with a trait depend on its freq. in the pop.
Positive FDS	The fitness associated with the trait (phenotype) increases as the freq. of the trait (genotype) increases in the pop. If an allele starts above a critical threshold then that phenotype has an adv. and will increase & reach fixation. But if it starts below the threshold then the phenotype will have a disadv. and be purified.
Equilibrium Point of FDS	SEE GRAPH. When phenotypes are dependent on freq. as the freq. of 1 increases, it will dominate and reduce the freq. of the other phenotypes and increase in fitness - the allele combination is selected for. The equilibrium point states the defining moment of allele fixation or elimination.
Examples of Positive FDS	HELICONIUS BUTTERFLIES - the more common you are, the more likely you are to get a mate. EUHADRA SNAILS - left/right coils are genetic and like can only mate with like. So if pops. consist of only one phenotype, it is adv. to be that phenotype. But if the pop. is mixed, the phenotype with the higher freq. has the higher fitness.
Negative FDS	When an allele is at a high freq. it has a low fitness so the phenotype declines in freq. Whhen an allele is at low freq. the phenotype is rare so it has a high fitness and increases in freq. The allele eventually reaches an intermediate freq. and the equilibrium allows 2 polymorphs to coexist.
Examples of Negative FDS	CICHLID CLEANER FISH have 2 morphs, right sided & left sided and they coexist die to the intermediate freq. dependence. As soon as 1 morph becomes rare it becomes more successful and returns to equilibrium. If 1 becomes common it is subject to increased freq. dependent competition and fitness is reduced - returns to equilibrium.
Fisher's Sex Ratio Model	This is the only true example of an ESS & is -ve FDS. Because each individual has both a ma & pa, they both contribute equally to the next generation and so have the same average fitness. Even if female births are rarer, so their fitness is increase, this will be selected upon and the 1:1 sex ratio will be restored. As the 1:1 ratio is reached any advantage is lost.
Physical Constraints	Mass & Strength of Femur - SPACE ELEPHANT. But if the weight is distributed evenly then it could happen (Harvestman) but this is adaptation, not nat. sel. - RACEHORSES. Can only run as fast as their legs will carry them. You can't push a weaker horse to run faster. THIS IS NAT. SEL. & A PHYSICAL CONSRAINT
Teleology	No purpose or foresight. NS cannot predict the future so favours changes that are beneficial immediately, not necessarily in the long run. NS = Gradual
Process	NS acts on the individual. Evolution is the inheritance of the selected trait to the next generation. It depends on the relationship between the phenotype and the genotype. Essentially Environment and Fitness.
Genes x Environment	NS establilshes a relationship between fitness & geneotype through the relationship with the environment which establishes whether or not evolutionary change will occur. Fitness = ability of an indiv. to survive and reproduce. Viabillity = prob. of survival to repro. age Fertility = Avg. no. of offspring per indiv. that survive to repro. maturity
Coat Colour in Oldfield Mice - Hoekstra et al, 2006	Has high levels of within species variation of coat colour due to elevation and geographical transect. Light coloured mice found on beach & dark ones in land. Variation is linked to Mc1R gene. It acts as a switch & promotes either Eumnelanin (dark colour) or Phaeomelanin (agouti colour). Artificially tested whether variation affects fitness - results say yes but is this real life?
History	Developed by Darwin & wallace @ Linnean Society in 1858. Essentially anything is possible with natural selection but it is constrained by teleology & physical constraints.

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	Created by Alice Burke over 11 years ago