Exploring Natural Selection^[1]

1. Lab Overview and Background

In this study, we will attempt to simulate natural selection of cryptic colouration in a species that occupies a diversely coloured habitat and is under predation pressures. To do this, we will evaluate survival selection (i.e., the number of individuals that survive) associated with the colour phenotypes of the population within this diverse habitat. We will be running a predation simulation on a species of herbivorous invertebrate with 7 colour morphs, specifically focusing on survival rates to adulthood, an indication of reproductive success (fitness) in this species. In this simulation, we are interested in seeing if the activity of a visual predator in a visually complex habitat provides the required conditions for natural selection to occur in an herbivorous invertebrate population. The independent variable in this simulation is the coloured fabric by estimating the proportion of the fabric that matches the invertebrate colour morphs (e.g., proportion of light green) and the dependent variable is proportion of the population of each colour morph that survive to the end of the simulation. Therefore, we will be addressing the question: Is there a relationship between the proportion of habitat colour and associated colour morph in an herbivorous invertebrate population undergoing predation pressure?

1.1 Overview of Natural Selection

Natural selection is the mechanism Charles Darwin proposed to explain the extensive morphological diversity that he observed in populations of various species. He used his observations from the Beagle trip plus learnings about constraints on human population growth to formulate the mechanism. With scientific advances, his original hypothesis was revised and readily accepted by the scientific community as the only evolutionary mechanism (of the 5) that consistently increases the match of a population to its environment, resulting in adaptations. This match occurs because natural selection is a non-random process where individuals with heritable traits that allow for successful survival and reproduction will leave a disproportionately higher number of viable offspring in the next generation when compared to those without that trait. Therefore, in a particular environment over several generations, there will be an increasing proportion of the population that possess the beneficial, heritable characteristics (i.e., organisms have higher fitness due to natural selection). The complexity of natural selection during a sexually reproducing organism’s life cycle is evident through simultaneous selection events it encounters such as:

• Its survival (e.g., evading predation events)
• Mating success (i.e., find a mate in order to reproduce)
• Fecundity (i.e., number of offspring an individual can produce)
• Viability of its offspring

One thing to remember is that there can be a variety of trade-offs for a given adaptation (e.g., being a highly colourful male Bird of Paradise may increase odds of procuring a mate, but also increase the odds of predation).

1.2 Overview of the Simulation Scenario

In this survival selection simulation, we are modeling an herbivorous invertebrate species found throughout the understory of habitat with diverse vegetation. The population of this prey species is made up of long-lived females that reproduces parthenogenically (i.e., a form of asexual reproduction in animals). Each female only produces one female offspring per year. In this scenario, the predator is a visual hunter which must consume 35 individuals within the population to ensure survival, and returns to its den to consume each prey item.

We will be using foam dots that have 7 colour phenotypes to represent the invertebrate species, and a patterned fabric with the same colours but in varying proportions (other colours are also present in the habitat). Students equipped with forceps will simulate the visual predator.