Lecture 7.docx

Lecture 7: The Functions of Play – The ‘why’ of Adaptive Value What is Function? The observation that there is a good fit between structure and function Giraffe’s have long neck – why? So they can reach the leaves of tall trees But how does structure come to fit the functions for which that structur...

Lecture 7: The Functions of Play – The ‘why’ of Adaptive Value What is Function? The observation that there is a good fit between structure and function Giraffe’s have long neck – why? So they can reach the leaves of tall trees But how does structure come to fit the functions for which that structure is used? Particularly, since structure changes over evolutionary time The Currently Accepted Mechanism Was Developed in the 1800s by Darwin and Wallace Natural Selection By identifying why some individual in a population survive and reproduce better than others, Darwin and Wallace recognize that they could explain evolutionary changes Natural selection is a key driving force of evolution, shaping the diversity of life on Earth by favoring traits that enhance an organism's ability to survive and reproduce in its environment. It is important to note that natural selection does not involve conscious decision-making or foresight but rather operates through the differential success of individuals with certain heritable traits. Extend natural selection over many generations and populations within species can diverge into new species (speciation) Natural selection can also explain why different organisms can converge on having similar features. Convergence of Features due to Adapting to Similar Niches Mole Marsupial mole Wolf Tasmanian wolf What are the key components of natural selection? 1. Over Production 2. Variation 3. Struggle for Survival Selective pressures – the parts of the environment that make an organism better adapted 4. Differential Reproduction Three Types of Natural Selection: 1. Directional Selection Individuals with a particular trait are favored over individuals with other traits, causing the frequency of the favored trait to shift in one direction over time. 2. Disruptive Selection Individuals with extreme phenotypes at both ends of a trait distribution have higher fitness than individuals with intermediate phenotypes. In disruptive selection, the population becomes divided into two or more distinct phenotypic groups, with the intermediate phenotypes being selected against. 3. Stabilizing Selection Favors the intermediate phenotype of a trait while acting against extreme phenotypes at both ends of the distribution. In stabilizing selection, the average value of a trait remains relatively unchanged over time, and the population becomes more homogeneous in terms of that trait. Problem 1. Incremental Change What is the point of looking like 5% of a turd? You outcompete neighbors who look 4% like a turd Problem 2. Inheritance Parents pass on their beneficial traits Pangenesis Hereditary information is carried by particles called "pangenes" or "gemmules" from various parts of an organism's body to its reproductive organs, where they are then passed on to offspring. Through reproduction adaptations become diluted not strengthened Since pangenesis works against natural selection, many biologist at the time rejected natural selection as the mechanism for evolutionary change Gregor Mendel: By Breeding plants so that they always produced the same traits (‘breed true’) and then cross-breeding and back breeding, Mendel discovered that ‘traits’ can be dominant or recessive To account for the appearance and disappearance of traits he had to hypothesize the existence of ‘heritable particles’ The heritable particles (i.e., genes) do not blend and so solves Darwin’s problems The Modern Synthesis: Combines Darwin’s Natural Selection with Mendel’s Genes Evolution by natural selection can be conceptualized as changes in the proportion of versions of genes (i.e., alleles) in a population Given that changes in the spread of alleles in a population are associated with adaptations, how do we identify adaptations? Trait to explain: Egg-shell removal of black headed gull Nesting black-headed gulls Camouflaged gull eggs Predation by crows Tinbergen’s Hypothesis Once a chick hatches the white interior of the eggshell is exposed, making the next visible to crows flying overhead and endangering the chick and the eggs yet to be hatched. To test the hypothesis, Tinbergen placed broken eggshells at different distances from nests, then sat in a hide and watched Prediction: Nests closer to broken eggs should be more likely to be predated Prediction was supported