Evolution has various definitions depending on what is being studied. According to (Griffin et al., 2005) it can be defined in this context as a historical transformation of a closely related group of species; phylogeny. It is used by biologists in the study of the evolution of various organisms. The thesis statement in this report is that cooperative breeders adjust offspring sex ratios to produce beneficial helpers.
The issue being investigated in this case is whether birds and mammals adaptively adjust their offspring sex ratios in response to the environmental changes. Many different studies have revealed that there are different patterns as far as sex ratio adjustment is concerned. This has been brought about by the fact that sex ratio adjustment occurs in some cases but with some exceptions. It is much easier to estimate the strength of selection on sex ratio adjustment in cooperatively breeding species and hence they are the best in addressing the problem at hand.
Prior studies of invertebrates have demonstrated strong support for the adaptive manipulation of offspring sex ratios (Griffin et al., 2005). However, this is quite different in the case of birds and mammals, where the evidence is less clear-cut. This is because the different studies have shown varying patterns and hence different conclusions have been drawn. The challenge that arises here is the understanding of how the selection is acting on the sex ratio of invertebrates. This difficulty implies that it is quite hard or utterly impossible to forecast the anticipated pattern of sex ratio adjustment for a given species or how it should differ across the species of birds or mammals. The background information, therefore, justifies the asking of the question for investigation.
Various methods were used to collect and analyze data in this investigation. Several studies were collected as a way of surveying the existing literature. Other secondary sources of information that were used included the searching of the web and the citations in most of the written articles. Direct contacts with the researchers were also an invaluable way of collecting information. The team also had access to additional unpublished studies which focused on three bird species. The three cases indicated that helpers have a negligible effect on parental fitness, and there is no proof for a sex ratio bias in favor of the helping sex (Griffin et al., 2005). The inclusion of these data served to further strengthen the relationship presented in this context. When the helping sex is absent or rare, there is a tendency for parents to produce the helping sex in large numbers depending on the benefit that these helpers provide. In general, it can be predicted that the sex ratio adjustment ought to be positively correlated with the benefit that the helping sexes can provide.
The data that was collected was analyzed using the meta-analysis method (Griffin et al., 2005). This method involves the comparison of standardized measures of two different relationships. First, we have the relationship between the offspring sex ratio, denoted by sex, and the number of helpers in a group. This relationship assesses how far the parents overproduce the helping sex when the particular sex is rare or absent in a given social grouping. The second method is the relationship between the production of offspring or survival and the number of helpers, denoted by help. This one assesses the benefit of having helpers. Extreme positive values show that either there is stronger overproduction of the helping sex when they are rare or a much greater fitness benefit due to the presence of helpers which implies a positive correlation between the two. More advanced versions of the standardized values were also used. These two methods are appropriate for the hypothesis since the approaches are good approximations and can easily help in drawing meaningful deductions especially for the fact that effect sizes vary between species.
The results were as predicted and it was found that greater sex ratio adjustment in the species where the presence of helping sex provides greater benefits as far as fitness is concerned. There was a significant positive correlation between sex and help across the species. The results, however, were slightly different in some species since it was realized that sex ratio adjustment occurs in some species and not in others. The two conclusions drawn from the study included; that comparative statistics is quite significant when testing evolutionary theory and that it can provide an empirical link between the evolution of sex ratio adjustment and cooperation (Griffin et al., 2005). The entire discussion throws some light on the interpretation of the results.
This report has focused on the problem that was under investigation, and the background information about the question asked. It has also identified the methods used to collect and interpret data. Furthermore, it has compared the expected and the actual results of the investigation in the light of their interpretation. The hypothesis states that cooperative breeders adjust offspring sex ratios to produce beneficial helpers was found to be true.
Reference
Griffin, S. A., Sheldon, C. B. & West, A. S. (2005). Sex Ratio Adjustment in Cooperative Breeders. Chicago: The University of Chicago Press, 166 (5), 628-632.