Sexual Dimorphism and Evolutionary Paradox in Sexual Competition

This topic submitted by Brian Lin ( linl2@miamioh.edu) at 11:51 PM on 6/9/06.

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Sexual Dimorphism and Evolutionary Paradox in Sexual Competition

Introduction
Evolutionary patterns, such as genetic drift, speciation, and the development of sexual dimorphism, become increasingly evident along isolated geographic locations, such as islands. What is not as well known is that the underwater communities that isolate these islands are in themselves excellent examples of evolutionary trends. These trends can be best exemplified in marine ecosystems and migration from island to island. The diversity of animal and plant life on tropical ecosystems, both terrestrial and marine, offers insight into sexual behavior among animals, including intrasexual and intersexual competition, female choice, and the “paradoxes” that seem to arise from general observation. These inherently contradictory observations, however, may not be contradictory at all.

Evolution is defined simply as “change through time” (Freeman Herron 35). Charles Darwin called it “descent with modification.” Whatever name it is called, there exists controversy not only between clergy and scientists, but within each group as well. We will leave the religion vs. science controversy aside and focus on the definition of evolution. Ever since, Darwin rushed his Origins to publication, theories have been created to explain its mechanism. Many of the logic gaps in Darwin’s evolutionary theory were actually noted in Origins: Darwin was well aware of the lack of knowledge in the mechanisms of evolution and evolutionary change in his time. Many evolutionary theories are also in competition with each other, such as R. A. Fischer’s Theory on Mass Selection, where mutations gathered in very minute quantities over a great period of time that gave rise to the Red Queen hypothesis; the counter theory of which was Sewall Wright’s Shifting Balance Theory, which disagreed that the entire population of species constituted as the evolving unit, but the structure and size mattered as well, noting the genetic isolation within a population (Merrel 132). Others have notably been proven false. Theories such as Lamarck’s famous answer to the question that is the giraffe’s neck have now been proven wrong. Even modern assumptions on why the giraffe evolved long necks are under reconsideration (Freeman Herron 332). We know now that the work of Mendel in the field of genetics is the defining mechanism in evolutionary change and that mutation is a key factor in macroevolution. Macro and microevolution are oftentimes confused and should be defined: micro is actually the study of populations of species, whereas, macroevolution is the studying of evidence for gradual change over time. In the example of the coral reef, the species and communities that thrive have been around for thousands and thousands of years, but still offer more examples for study on microevolution, because in geological time, the coral communities have not been around for a long period of time (Davidson 37). The search for vestigial organs, transitional fossils, and the study of geographical qualities, the work of paleontologists, are considered studies in macroevolution. Definitions and controversy aside, there is much to be learned from studying the recombination of alleles from one generation to the next, as it is a functioning force of evolution.

Mistakes concerning evolution abound. Individual actions are not “for the good of the species” nor will evolution eventually lead to a “perfect” organism. Adaptation is always one step behind and not “forward looking.” Another misconception is that the traits that are inherited are linked with more successful survival. Survival in evolutionary terms requires not only survival, but the ability to produce viable offspring. This is where sexual dimorphism and the mechanisms of sexual selection become evident in many species, especially with geographical obstacles in the way. Allopatric speciation occurs in these instances, also known as adaptive radiation. The mangroves, sea grass, and coral ecosystems are all very closely linked (Knox 440), but isolation can still occur. Certain species can only maintain survival under certain conditions and a change in currents can cause the isolation and separation required for allopatric speciation. On islands, the effects of isolation lead to a problem of minimum viable population in a minimum area (Whittaker 194). If the population is unable to survive, it will be hard to gather evidence on speciation. Behavioral studies of certain species of amphibians, however, separation at the tadpole stage after only a few generations on different sources of food revealed that these organisms had a significantly less probability with mating with mates of the separated group. The changes in behavior must also be taken into account when studying biogeographic isolation. Sexual dimorphism in marine animals is evident in behavioral studies. Species of water mites have developed an intricate “dance” that serves the purposes of attracting a mate, finding food, and sensing a nearby mate (Freeman Herron 399) Other behavioral traits are even more astounding.
Sexual dimorphism is the difference in phenotypic expressions between members of the opposite sex of the same species (Pauly 187). Many examples are found in avian species, but more on that later. Sex on islands have been studied on plants and animals and how they reproduce, the “simplest dichotomy” being the ability to reproduce sexually or asexually (Whittaker 67). Sex in itself is a curious mode of evolutionary change, but on islands, it is much more interesting, as in the case of the now famous Darwin finches of the Galapagos Islands. 13 species of finches found on the islands today are believed to have evolved from a single species of mainland finch long ago. Islands are the basis of major change in populations, such as the founder effect (genetic drift), adaptive radiation (similar to speciation), and sample cases of asexual/sexual dichotomy. Angiosperms, or flowering plants, may well be known to have the option of such mode of reproduction, but lizard species (Whittaker 65) also have this unique option. This option becomes evident when a founder colonizes a new island. The genetic make-up is less diverse than that of the parent population and hence, the ability to reproduce quickly may come as an evolutionary advantage. However, this may not be so for the most part, bringing us to our first paradox: asexual reproduction vs. sexual reproduction.

A basic statistical analysis would show that asexual reproduction should far outstrip any mode of sexual reproduction. So why has selection preferred sexual reproduction as a viable alternative? Take the lizard species for example. Assuming that the founder lizards have sufficient numbers to form a viable population, asexual reproduction would seem favorable. The founders would have very, very little genetic diversity and be prone to predation and a single microorganism would wipe it out. Sexual reproduction wins the advantage here, where genetic recombination may help the survival of the population. Another possibility is that the lizards will reproduce viable offspring with native lizard populations, hybridizing with the other population.

A few important factors in looking at sexual dimorphism in animals are parental investment, intrasexual/intersexual competition, and female choice. Parental investment (PI) can be defined as the amount of resources by the parent to raise offspring at the detriment of the parents’ future reproductive efforts. A falsehood based on Bateman’s Principle, states that the increased PI found in females and lack of PI in males are inherent in “maleness” and “femaleness.” Prominent examples that prove this to be false includes that of the pipefish of the same family as seahorses: males give more parental investment than the females. Other members of the Sygnathidae family, such as Belontia, males often take charge of taking care of the eggs and may even refuse female intervention in protecting the offspring (Breder 578).
Intrasexual competition is competition for potential mates between members of the same sex, for example fighting among male marine iguanas for territory to mate with females. Intersexual competition involves members of both sexes of a species in their mutual pursuit rearing offspring. Intersexual differs from intrasexual competition, because it usually involves female choice. Thus, this observation leads us to yet another paradox: selection for traits that do not enhance an individual’s survival probability (Hardy 366). According to Ian Hardy:
Many animals have evolved conspicuous traits that seem to reduce their survival. Darwin (1871) proposed the theory of sexual selection to explain the evolution of such traits.

The theory of sexual selection is established as the central explanation for explaining the “paradox” of inherently non-beneficial traits. The traits, such as a male barn swallow’s elongated bright tail or a male peacock’s bright plumage, attract predators, but since female chooses these males over less flashy males, the traits are passed on in a significant quantity. The example of less than optimal to downright detrimental traits exist in marine iguanas and varieties of fish as well: Perilampus perseus, an Indian species of the family of Cyprinidae, has such a bright coloration that this example is called “suicidal altruism.” (Pauly 156). Natural selection, based on female choice, that the “qualities” that increase predation or shorten the lifespan of males are favorable enough to increase reproductive success that it offsets the detriment to predation and life span.
Conclusion

Sexual dimorphism and the contradictory claims that observations may infer is a key component in studying individuals, populations, and their environments. To look at a picture and see the animals in the foreground and ignore the environment in the background will lead to false conclusions. The precarious situation of animals that make a home in the sandy beaches would not be recognized if one did not look at effects of the tide (McLachlan 120). The paradoxes are easily explainable if you closer examination warrants it. Take the giraffe example: its long neck was assumed to be advantageous when browsing the tops of the acacia trees of eastern African plains. Lamarck had thought that as its neck grew, the trait was passed on to offspring. Lamarck and his contemporaries were wrong on two accounts. The first was that the trait was passed on through selection of its genes (longer-necked giraffes were more successful in surviving and reproducing), the second, and less well known, is that the giraffes’ neck has nothing to do with their enhanced ability to graze the tops of acacia trees, but are used by males in combat. When searching for mates, the male giraffes taste the females’ urine to determine if they are in estrus. A competing male may challenge the male to the “right” to mate with the female and the long neck is used. Observational studies have shown that it is not finding enough food that caused the selection of long necks, but female choice and sexual competition that was instrumental in the question that is the giraffe’s neck. These questions that may seem hard to answer, in many cases, can be easily clarified by viewing it as sexual competition.

Intro: Evolution defined
-Mendelian Genetics (brief)
-Lamarck
-controversy

Sexual Dimorphism in Marine Species
-Asexual vs. Sexual Reproduction (brief)
-Parental Investment
-Male vs. Male (intrasexual)
-Selection of traits that may/may not increase fitness
-Male vs. Female (intersexual)
-Selection of traits that may/may not increase (Pauly, perilampus perseus)
-Female Choice.
-Why not optimal traits chosen?

Paradox
-Sexual Ratios
-Reproduction Rates vs. Mortality Rates

Conclusion:
-Adaptation, not perfection.
-Sexual competition paradox
-Traits increase LRS: trade-off
==
Original Outline:
Or rather, the Origins of Sexual Dimorphism and Evolutionary Paradox on Sexual Competition
This fits well with island ecology, as evolutionary patterns can be best seen and exemplified in marine ecosystems and migration from island to island (Founder Effect). There are also seemingly contradictatory observations in such cases that seem to be exceptions (or are they?) to the rules governing evolution.
Intro: Evolution defined
-Mendelian Genetics (brief)
-Lamarck
-controversy
Island Geography
-Sandy beaches
-Sea Grass
-Mangroves
-Coral
Sexual Dimorphism in Marine Species
-Asexual vs. Sexual Reproduction (brief)
-Parental Investment
-Male vs. Male (intrasexual)
-Selection of traits that may/may not increase fitness
-Male vs. Female (intersexual)
-Selection of traits that may/may not increase
-Female Choice.
-Why not optimal traits chosen?
Paradox
-Sexual Ratios
-Reproduction Rates vs. Mortality Rates
-Red Queen Hypothesis
-"Now, here, you see, it takes all the running you can do to keep in the same place."
-Evolutionary "Cold War"
Conclusion:
-Adaptation, not perfection.
-Sexual competition paradox
-Red Queen paradox
-Traits increase LRS: trade-off

Bibilography

Breder, Jr., Charles M., Rosen, Donn Eric. (C) 1966. The Natural History Press

Brown, A.C., McLachlan, A. ECOLOGY OF SANDY SHORES. (C) 1990. Elsevier Science Publishers B.V.

Davidson, Osha Gray, The Enchanted Braid (C) 1998. John Wiley & Sons, Inc.

Freeman, Scott; Herron, Jon C. Evolutionary Analysis. (C) 2004 Pearson Education, Inc.

Hardy, Ian C.W. SEX RATIOS CONCEPTS AND RESEARCH METHODS. (C) 2002. Cambridge University Press.

Knox, George !. THE ECOLOGY OF SEASHORES (C) 2001. CRC Press LL.

Merrell, David J. THE ADAPTIVE SEASCAPE THE MECHANISM OF EVOLUTION. (C) 1994. University of Minnesota Press.

Pianka, Eric R. EVOLUTIONARY ECOLOGY. (C) 1971. Harper & Row, Publishers.

Whittaker, Robert J. ISLAND BIOGEOGRAPHY (C) 1998. Oxford University Press.

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