Understanding Sexual Selection Research Paper

Abstract

Why do some avian families have more species than others? Here we do an analysis between various taxa of avian or bird families to test predictions arising out of these six assumptions: that differences between families are due to

1. chance,
2. body size,
3. life history,
4. sexual selection,
5. intrinsic ecological factors and or
6. extrinsic abiotic factors, respectively.

From our analysis, we find no evidence that species richness, among birds, is not due to chance. We find high species richness is due to “plumage dichromatism, generalist feeding habits, and good dispersal capabilities as well as large and fragmented geographical ranges”. We realize that high species richness is not linked to one of the particular assumptions, sexual selection, alone but sexual selection may be linked to species extinction rates. More work is necessary to understand the mechanisms that govern species richness or species extinction rates.

Introduction

Sexual selection was an idea introduced by Charles Darwin (1859) as a special type of natural selection that would take into account the sexual differences between males and females found in many species of animals. According to Darwin (1871), sexual selection depends on the advantage which certain individuals have over others of the same sex and species solely in respect to reproduction. Darwin realized that males could take advantage of females in two different ways. They compete directly to gain control of the females (intersexual selection) or by attracting females by special displays or adornments (intersexual selection).

Among the sexual adornments, a species can have ranged from elaborate and ornate feathers (Peacock: Pavo cristatus) to “cumbersome” antlers (Red deer: Cervus elephus). Sexual displays of the Birds of Paradise (more than 40 species in the family Paradisaeidae) are more elaborate and colorful and well known. These displays and adornments ensure that the females make a definite choice of their sexual partners and the males ensure that these traits are passed on to their offspring. These selective pressures have operated upon these animals in such a way that the sexual embellishments are more likely to ensure that their genes are passed on to the next generation rather than be a “handicap” (Zahavi, 1975).

In this paper, we try to understand why all bird families are not equal since the species richness varies within families and genus. There are around 9672 extant bird species distributed in 145 families with an average of around 67 species. This observation is far from over. “Over half of the species are contained within just 12 species-rich families, each of which contains over 250 species. At the other end of the scale are almost half of the families contain less than ten species each and account for less than 250 species between them.” (Owens et al. 1999).

The question we try to ask is: why is there a lot of variation among bird species’ families. To resolve this we have two explanations. The first says that the uneven distribution of bird species within a family is due to chance alone (Raup et al. 1973) but the second explanation is that the first explanation is not accountable for the species richness but to recognize the issue(s) that influence certain lineages to being species-rich and other lineages to being species-poor (Cracraft 1982, Barraclough et al. 1998). To know more about this paradigm scientists have come out with the theoretical prediction that this high species diversity in certain bird families could be due to intense sexual selection (Barraclough et al. 1995, West-Eberhard 1983).

In this paper, we have collected test data done by other researchers for such bird families. We analyze using this data in two ways. First, we use statistical models to test whether the variation among bird families in species richness could be explained by chance alone. Second, we give attention to the known hypotheses that have been suggested to be important in determining species richness in birds. The five hypotheses are:

1. body size,
2. sexual selection,
3. life history patterns,
4. ecological potential of successful dispersal and
5. geographical potential for speciation.

Results

The species richness across all the bird families (145) was analyzed for the observed frequency histogram, as shown in Figure 1a. “This observed distribution is significantly different from the expected distribution based on the geometric distribution shown in Figure 1b (χ²-test: observations grouped zero to ten species per family, 11-100 species per family, 101-500 species per family and 501-1000 species per family; χ² =57.15, d.f.=3 and p < 0.0001).

The observed distribution is also significantly different from the expected distribution based on the Poisson distribution shown in Figure 1c (χ²=128.41, d.f.=3 and p<0.0001). There are more species-poor and more species-rich families than expected by chance” (Owens et al. 1999, 934)

“The results of our sister-taxon analyses of the relationship between species richness and our various independent variables are shown in Table 1. Contrary to many previous predictions, we found no significant relationship between changes in body size and changes in species richness. Similarly, we found no significant relationship between changes in either of our measures of life history – an age at first breeding and clutch size – and changes in species richness.

However, we did find that increases in species richness were correlated with increases in one of our three measures of the occurrence of sexual selection, plumage dichromatism. In addition, we found that increases in three of our indices of the ecological potential for dispersal are associated with significant increases in species richness. These three indices are the extent of habitat type generalization, food type generalization, and annual dispersal, although in the case of habitat type generalization the association is only significant when using the more powerful Wilcoxon test. Similarly, we found that increases in both of our two indices of the geographical potential for speciation are associated with significant increases in species richness” (Owens et al. 1999, 235-236).

“Most of these results remained qualitatively unchanged when we repeated the analyses with the Ciconiiform and Passeriform families removed (Table 2). For instance, body size and all of the life-history variables remained uncorrelated with species richness. In addition, the extent of plumage dimorphism, annual dispersal and geographical range size remained correlated with species richness irrespective of which statistical test was employed.

In the case of the relationships between species richness and habitat type generalization, food type generalization, and geographical range size fragmentation, however, the associations were only significant when the more powerful Wilcoxon test was used. All other associations remained non-significant” (Owens et al. 1999, 235-236).

Discussion

Previous studies have found strong evidence that the observed variation among avian families in species richness is not due to “random branching patterns” (e.g. Bock & Farrand, 1980; Dial & Marzluff, 1989; Nee et al., 1992). If we were to believe in chance alone, then the observed reality is different. We know that there are species-rich and species-poor avian families. So in this case the results support the fact that it is worth seeking correlates of species richness among birds.

It is worth noting that although we have used random models, all the avian families are not of equal age. The discrepancies arising due to the predictions of the random model could be due to this violation of the assumptions. “Of course, it does remain possible that further work may reveal a ‘random’ model that could explain the observed variation in species diversity. At present, however, we feel that this is unlikely given the huge differences between species-rich and species-poor families” (Owens et al. 1999).

During our analysis, we found there was no proof for a significant relationship between species richness and either body size or life history. Are our results different from what others have observed? “We suggest that the two main reasons why other workers have found a correlation between species richness and either body size or life history are that they have, first, failed to identify evolutionarily independent changes and, second, overemphasized the importance of a few speciose groups.

Here, on the other hand, we have used a phylogeny-based method and have repeated all our analyses with the two most speciose groups removed. Hence, we agree with Nee et al. (1992) that the supposed relationship between body size and species richness among bird families is the result of phylogenetic non-independence, and now extend this explanation to the putative association between life history and species richness in birds” (Owens et al. 1999).

“Indeed, most suggestions that small body size is important in determining species richness in birds rest on the crude observation that there are lots of species of passerine and many of them are quite small. Such reasoning ignores the broader picture.

First, certain species-rich passerine lineages are neither unusually small nor unusually short-lived (e.g. crows and allies (Corvidae)). Second, several small-bodied, short-lived passerine groups are not species-rich (e.g. kinglets and crests (Regulidae) and long-tailed tits and bushtits (Aegithalidae)). Third, there is the existence of many species-rich lineages outside the passerines that are neither small nor short-lived (e.g. parrots and allies (Psittacidae), hawks and allies (Accipitridae), and albatrosses and allies (Procellaridae)).

Finally, many small-bodied lineages outside the passerines are species-poor (e.g. todies (Todidae) and mousebirds (Collidae)). Of course, our observations do not challenge the view that body size and life history may be important in determining differences in species richness among higher levels – among kingdoms or classes, for instance. Within the birds though, the effects of variation in body size and life history appear to be swamped by other factors. So what are these other factors?” (Owens et al. 1999)

Summarizing the results we have got so far it is evident that the search for ecological or geographical correlates was indeed fruitful. High species richness is associated with indices of ecological generalism and dispersal ability. The results we have got support Rosenzweig’s (1995) ‘geographical’ model of diversification “whereby the chances of a lineage becoming species-rich is closely associated with its chances of finding and then successfully colonizing new areas”.

The next step we wanted to know was if such differences were the results of different methodologies or if the data indicated more complex interactions. We also realized that the identification of ecological and geographical correlates was not enough. We then found substantial proof that supports Barraclough et al.’s (1995), Mitra et al.’s (1996), and Möller & Cuervo’s (1998) findings that sexual selection may indeed be an important force in driving speciation. This was the most interesting part because we have analyzed a wider range of taxonomic groups in birds. Also worthy of note is the fact that we found that only one index of sexual selection – plumage dichromatism – is associated with differences in species richness, whereas Mitra et al. (1996) found a link between species richness and mating system.

“In addition, since it has recently been shown that sexual dichromatism is probably the most reliable indicator of the occurrence of female choice among birds (Owens and Hartley, 1998), the single correlation between species richness and dichromatism agrees well with Lande’s (1981) original model showing that female choice could drive speciation. Perhaps a high level of sexual selection, operating via cryptic female choice during extra-pair copulations, is the hidden factor’ underlying the great passerine radiation?” (Owens et al. 1999).

We have shown that although body size and life history patterns are important, in so far as to determine extinction risks among birds, they are relatively unimportant for the species richness per se. Also, we realize that sexual selection may not be the key factor that determines species richness.

If that be the case then what does sexual selection lead to? A recent study on the relationship between sexual selection and extinction in birds suggests that “in species with more intense post-mating relative to pre-mating sexual selection, either the absolute population fitness is lower, or the overall selection load is higher. Either of these mechanisms could then have a bearing on how vulnerable a population is to become extinct” (Morrow and Pitcher, 2003, 1797). This only goes to prove that sexual selection is one of the driving forces that sustain natural selection. It is through these ‘vertebrate models’, namely the birds, that we understand sexual selection.

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