Summary: Bos et al. (2009)

Mate selection is important in many organisms and occurs through multiple trait assessment in a context or condition-dependent manner. Theoretically, mate choice may be based on an individual's genotype, but this requires organisms to self-reference their own genotype. MHC genes may be used in self-referencing as they can be assessed through ligand binding in the vomeronasal organ. Due to overdominance, major histocmpatibility complex (MHC) alleles are likely to be adaptive and there is likely to be negative assoratative mating between individuals. Bos et al. (2009) assessed MHC-based mating preferences in wild, outbred eastern tiger salamanders (Ambystoma tigrinum tigrinum) by genotyping adults and larvae at microsatellite and MHC loci. They aimed to address 4 questions about MHC-based mate choice: 1) Do morphological characteristics increase reproductive success? 2) Is reproductive success in males mediated by the MHC? 3) If yes, are the mating patterns context-dependent? 4) If yes (2 and 3), is variation in male reproductive success linked to MHC-based cues only when large differences are apparent? Bos et al. (2009) found that male reproductive success was positively correlated with increased tail length.They found that MHC-similar males (to females) sired more offspring, indicating that negative assortative mating is not occurring in the population and that MHC-diverent males may be at a reproductive disadvantage. Bos et al. (2009) suggest that MHC-based choice is likely to be influenced by many factors.

Summary: Honda-Sumi (2005)

Cricket songs are species-specific signals important for species recognition, mate choice and male advertisement. Honda-Sumi (2005) compared the calling songs of 3 Japanese crickets to validate the function of acoustic signals (i.e. calling song) as a premating isolation mechanism between sympatric species using playback experiments. Teleogryllus emma is widely distributed and is sympatric with both T. yezoemma (North) and T. taiwanemma (South), while T. yezoemma and T. taiwanemma are allopatric. Hondu-Sumi (2005) compared three temporal parameters, 1 frequency parameter and two structural parameters to test for species divergence in signals and also tested whether female T. taiwanemma distinguish conspecific males. Sympatric species had divergent songs, while allopatric species had similar songs. T. taiwanemma females could discriminate conspecifics (i.e. showed positive phonotaxis). This suggests that, where hybridization is costly, calling song is an important premating isolation barrier in sympatric Teleogryllus species.

Summary: Ahearn et al. 1974

Ahearn et al. (1974) identified two kinds of reproductive isolating mechanisms: 1) premating isolation barriers (in the form of behavioural/ethological isolation); and 2) postmating isolation barriers (male hybrid sterility). They suggest that reproductive isolation arises as a by-product of genetic divergence or is induced by natural selection as a gene exchange barrier. Thus, premating isolation should occur more often between sympatric species. Drosophila heteroneura and D. silvestris are sympatric fly species on the island of Hawaii, while D. planitibia is allopatric on nearby Maui. Ahearn et al. (1974) found pronounced behavioural isolation between the sympatric, but not the allopatric, species. They did note that D. planitibia females do discriminate against D. heteroneura males. Male bybrid sterility is seen in allopatric crosses and is not correlated with behavioural isolation. They suggest that this is expected if premating isolation between closely related species is an ad hoc production of natural selection, while postmating isolation is an incidental result of genetic divergence.

Summary: Olsson et al. (2005)

Animals can learn to associate negative events with environmental stimuli (classical fear conditioning). Race bias and fear condition potentially rely on overlapping neural systems. Therefore, Olsson et al. (2005) aimed to further understanding of basic learning theory and social group interaction by investigating how the mechanisms of fear conditioning apply when humans learn by classical fear conditioning in a sociocultural context. They also assessed whether a bias in fear conditioning extends to social groups defined by race. Prepared fear learning is generally demonstrated as persistence to fear-relevant conditioned stimuli in the learned fear response. They conducted two experiments to test whether fear responses acquired to outgroup and ingroup faces persists during extinction (relative to each other). Their results show that individuals from a racial outgroup are more readily associated with an aversive stimulus. However, Olsson et al. (2005) indicate that this response might be reduced by close, positive interracial contact. They postulate that humans might have evolved a more general preparedness to fear others who are dissimilar to themselves or their social group because such individuals were, historically, more likely to pose a threat.