Understanding How Genes Make Male and Female Birds Look So Different

For my next series of posts I’ll be focusing on articles from highly reputable peer-reviewed journals on evolution. Evolution covers a very wide range of topics since an evolutionary approach can be used in so many scientific fields. Concepts from evolution help explain phenomena we witness in both the “hard sciences” (like biology and genetics) and the “soft sciences” (like economics and psychology).

The first article I’ll be summarizing is titled “The Genetic Basis of Sexual Dimorphism in Birds.” Sexual dimorphism is the condition where the two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs. Dimorphic traits are interesting to study since they’re only exhibited in one sex. I chose this article after learning that many bird species express quite an interesting dimorphism who’s role we do not quite understand.

Background

Many bird species exhibit dimorphism in their plumage with males having more colorful/ornamental plumage than females. Take a look at the pictures below for a few examples.

I first learned of this dimorphism after watching BBC’s “Planet Earth II” with David Attenborough. In an episode featuring the exotic Birds-of-Paradise, he suggests that the role of these colors/ornaments is “sexual selection.” Sexual selection refers to the idea that these traits were naturally selected for the purpose of finding a suitable mate. A flashier bird with more colors and ornaments is more likely to obtain a mate and pass on their genes than a bird with boring colors.

The researchers believe that sexually dimorphic traits are sexually selected, and they are investigating the genetic origins of dimorphic traits. They want to know if the alleles for dimorphic traits are male-limited from the offset, or are the alleles originally expressed in both sexes, but then modifier genes cause the traits to only be expressed in males?

This debate stems from a disagreement between Charles Darwin, the father of evolution, and Ronald Fisher, the father of statistics. Darwin believed that the genes for male-limited dimorphic traits are only ever expressed in males. Fisher, on the other hand, believed that dimorphic traits are originally expressed in both sexes, but then females acquire suppression modifier genes that “turn off” these traits.

What did the Scientists Do?

To tackle the Darwin vs. Fisher disagreement, the scientists observed the hybrid offspring of bird species who’s males express different levels/forms of sexual dimorphism (ex. distribution of color patches; presence/absence of headwires, elongated feathers, or wattles). Since there are only a few ways (evolutionarily) the alleles for sexually dimorphic traits could become male-limited, the scientists can predict what the offspring of hybrid species would look like.

1) Darwin’s opinion: the traits are male-limited from the beginning, maybe because they arose from genes that were already male limited. If this were the case, female hybrids would not express any male traits.

2) Fisher’s opinion: the traits are expressed in both sexes, but females acquire suppression modifiers. If this were the case, some female hybrids would carry some sexually dimorphic genes. The modifier genes would need to be completely dominant to keep all hybrid females from acquiring sexually dimorphic traits. Otherwise some of the female hybrids would express these traits.

3) Third alternative: the traits are expressed in both sexes, but males acquire modifier genes that promote the expression of these traits. This leaves us with a similar scenario as above: the modifier genes would need to be completely dominant to make sure that only the males acquire sexually dimorphic traits. Otherwise some of the female hybrids would express these traits.

What did the Scientists Find?

The scientists considered over 4,000 hybrid species, but only 13 pairs met their criteria. Of these 13 pairs, not one of the female hybrid offspring expressed the sexually dimorphic traits seen in the males of either parental species. The female hybrids lacked any ornaments and resembled the females of the parental species. However, the male hybrids did express the sexually dimorphic traits seen in the males of the parental species. Therefore, the alleles for male-specific traits are not recessive in hybrids.

The male-limited dimorphic traits observed in these species must have evolved in one of two ways. Either the alleles for these traits were always only expressed in males, or they’re originally expressed in both sexes but become suppressed in females or limited to males by modifiers that are completely dominant in hybrids.

Why this is Important:

For starters, the fact that the dimorphic traits are only expressed in the male hybrids suggests that these traits were sexually selected. The role of the ornaments and colors is to help the male attract a suitable mate.

Unfortunately, this study doesn’t resolve the debate between Darwin and Fisher. We do not know if the alleles for dimorphic traits were at one point expressed in females too. The scientists suggest some breeding strategies that involve backcrossing the hybrids to the parental species to determine if there are repressors or activators in the hybrids that are not dominant.

This paper leaves us with a new strategy for studying the genetic basis of morphological evolution. Studying hybrid species can help us elucidate the genetic origins of dimorphic traits. This will lead us to a better understanding of sexual selection and speciation. This will lead us to a deeper understanding of how such a diversity of species has evolved on this planet.