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Lecture 3+4 Evolution.docx

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Lectures 3/4: Evolution Introduction to Adaptations  Adaptations are biological traits or characteristics that help an individual survive and reproduce in its habitat  Can be physical or psychological  Adaptations perform specific functions that make an organism better suited to its environment  Adaptations are always “for” something; they serve identifiable functions in the life of the individual  Adaptations emerge in development as a result of the activation of relevant genes in interaction with relevant aspects of the environment  Scientists categorized as “adaptationists” look for processes that are capable of accomplishing tasks of detecting stimuli o In other words, they look for the relevant adaptations o Scientists describe how hypotheses about adaptive functions guide their investigations  Cognitive psychologists study things like selective attention, memory encoding and retrieval and word recognition o The very names of these tasks refer to tasks that the mind needs to accomplish to do its job o They refer to the adaptive functions of mental activity o Therefore, cognitive psychologists are also adaptationists Evolution By Natural Selection  Natural selection is one of four basic mechanisms of evolution, the others being mutation, genetic drift and migration  The process of natural selection can be described as the differential survival and reproduction of organisms as a result of the heritable differences between them  Three essential components to Darwin and Wallace’s insight o First is that there are significant individual differences  Within any population, there is variation among individuals for any given characteristic o Second, these differences affect individuals chances of surviving and reproducing, causing differential reproduction  Some individuals will have more offspring than others o Lastly, the traits that give rise to differential reproduction have a genetic basis, meaning they are heritable  The offspring of successful reproducers will resemble their parents with respect to these variable characteristics  Ex. Imagine a population of fish that vary in colour o Some individuals are blue and others are red o The blue fish camouflage well in the blue ocean water and the red fish are much more visible to predators o Therefore, the red fish get eaten more often and so on average, the blue fish survive and reproduce better than the red fish o Blue fish tend to have blue coloured offspring because body colour is a heritable trait o Over successive generations, there will be selective transmission of heritable parental traits and the population will be mostly blue o This is because the specific characteristics that are best adapted for survival and reproduction are going to be reproduced at higher rates o Eventually, if this process continues, the entire fish population will be blue Natural Selection In the Wild  What researchers usually observe in wild populations is stabilizing selection, selection against any sort of departure from the species-typical adaptive design o This sort of selection tends to keep traits stable over generations  Ex. Blue would remain the most common colour in the fish population because it is adaptive and minimizes the risk of predation  However, in some cases, especially when there has been a significant change in the environment, selection favours traits that are not typical and evolutionary change can be observed  A classic example of rapid evolutionary change comes from work on the evolution of beak shape and size in a particular species of Darwin’s finches, the medium ground finch, which lives on Daphne Island in the Galapagos  Peter and Rosemary Grant who studied these birds, were able to observe natural selection within only a generation  In 1977, a drought hit the island and decimated the vegetation o Food was scarce and all of the small seeds were quickly eaten up, leaving only large, tough seeds that the finches usually didn’t bother with o The birds that had unusually big, heavy beaks were able to eat the hard seeds that remained and so survived the drought, whereas the birds with small beaks died of starvation o Between 1976 and 1978, the average beak depth increased o The large-beaked survivors went on to reproduce when conditions were again favourable for breeding and because beak size is heritable, their offspring inherited large beaks as well Reproductive Success – Fitness  Natural selection favours those individuals who are not only best at surviving, but also those who are best at reproducing  Fitness in biology refers to how good a particular genotype is at leaving copies of its gene in the next generation relative to other genotypes  Therefore, the fittest individual is not always the smartest, biggest or fastest  Darwinian fitness is the average reproductive success of a genotype relative to alternative genotypes  Because fitness is ultimately about the competition between genotypes to leave copies of themselves in the next generation, some evolutionary biologists like to define evolution as a change in gene frequencies over generations Sexual Selection  In sexually reproducing organisms there is often competition for mates and natural selection acts on mate-finding and reproductive behaviours o Known as sexual selection o The component of natural selection that acts on traits that influence an organism’s ability to obtain mates  Peacock’s tail o Energetically expensive to produce o Makes male more conspicuous to predators and it actually interferes with his ability to escape from a predator o A peacock’s tail is no help at all with respect to physical survival o Increases the risk of dying and for that reason, he’ll shed his tail at the end of the breeding season and grow a whole new one next year o However, the tail contributes to a male’s fitness by increasing his chances of mating  Traits like these that led Darwin to propose a second theory, the theory of sexual selection  Some evolved traits like the tail elevate mortality and are actually being selected against, but they can still evolve and become more elaborate under the countervailing pressure of sexual selection  Stag’s antlers o Only males have antlers and carry them around to fight for females o In some ways, the stags are more vulnerable than the females o Because of their bulk, males don’t have as much stamina as females for running away from predatory wolves and a stag is more likely to get stuck in deep snow o The total effect on survival for all his weaponry is negative o In fact, like the peacock’s tail, stags shed their antlers at the end of the breeding season and grow new ones each year  In both examples o The male trait has a negative effect on survival, but has evolved and persists anyways because it has a big positive effect on the male’s chance of mating o An important difference between these two examples of sexually selected traits  Peacock’s don’t use their tails to fight with  Antlers have evolved to be effective weapons in fights with other males, but the peacock’s tail evolved solely to dazzle the females o The selective force in the evolution of the peacock’s tail was female choice, whereas the selective force in the case of the elk’s weaponry was success in combat with other males o Considering the definition of sexual selection, there are two distinct ways of getting more access to mates than your rivals  Being chosen by the opposite sex (female choice)  More attractive  Beat up your rivals (success in combat)  If you keep your rivals at bay, then your mate can only choose you  It’s been shown experimentally that peahens respond to peacock’s tails o They make very find discriminations o Females discriminate between males based on the number of eyespots (the more the better) and they also prefer males with good left-right symmetry (same number of eyespots on each side) o Female choice picks out male with best resistance to disease o Best genes for being healthy Species-Typical Behaviour and The Comparative Approach  The recognition that behaviour is an evolved characteristic of a given species was the starting point for the modern science of animal behaviour  Ex. Sandpipers o Three different species  Sanderlings  Semipalmated  Dunlin o They all look very similar, but a birdwatcher is likely to recognize these birds at a glance, just by watching their behaviour o These birds can also be differentiated by their vocalizations, dietary and habitat preferences  Sanderlings like sandy beaches, whereas the other species like muddier shores o More obviously, is their signature way of foraging o Little flocks of sanderlings race out after receding waves and furiously peck at tiny insects on the wet sand, then race back towards shore in front of the next wave, with their little legs spinning like wind-up toys o An example of species-typical behaviour  Physical form (typography)  Habitat preference  Group size  Social system  One way we can confirm that behaviour evolves is by doing selection experiments, what are sometimes known as behaviour genetic experiments  You can keep animals in captivity and selectively breed those who are most or least aggressive and you can change the animal’s typical behaviour in a few generations  The popular animal in behaviour genetics is the fruit fly, Drosophilia, because of its short generation times Introduction to Social Behaviours  Organisms evolve to maximize their fitness and reproductive success  Yet, there are many examples in humans and social animals where individuals appear to behave altruistically o Ex. Virtually all honey bees in a colony don’t ever reproduce, they don’t even have functioning reproductive organs  Instead of reproducing themselves, they spend their lives in the service of the colony helping to raise eggs laid by the Queen and many bees often die defending their colony from predators o Ex. In Belding’s ground squirrels, individuals frequently give alarm calls to warn others that there is a predator in the area, giving everyone a chance to flee and hide  By giving the alarm call, the “whistle blower” draws attention to itself, altering the predator to its exact location o Ex. Why do humans spend so much time cooperating and helping family, friends and strangers  Evolution acts at the level of genes and those genes that contribute to an individual’s fitness will consequently get replicated more often, increasing in frequency in successive generations  People sometimes refer to this concept as the “selfish gene” o Means that natural selection will favour the genes and gene complexes that best serve their own interest, namely replication  Table of social behaviours Effect on Actor’s well-being Effect on + -- Recipient’s well- + Cooperation Altruism being -- Selfishness Spite o Sometimes cooperation can contribute more to your own success than being selfish o Ex. Imagine you’re on a hockey team with only six players  One of the players isn’t quite as skilled as the rest of the team and when other teams score, it’s usually because he made a mistake  You’d rather not have him on the team, but to be able to play, you need all six people on the ice, so you can’t send him off  One thing you can do is invest in the time and resources required to teach him to play better  On the surface it looks like you’re unselfishly doing extra work, but it may pay off as you may win more games  Assuming that the cost of teaching him to be a better player is less than the benefit you get from simply joining a better team, it would be in your own interest to help him  What looks like a cost in the short run may in fact be a benefit in the long run because your team performs better, which means that you do better than players on other teams  In the language of evolution, increasing the fitness of others can sometimes improve your own fitness prospects Group Selection  Adaptations aren’t for the good of the group or the species, they are for the good of the gene  In the previous example, helping out the worst player on your team do better may help the group, but in evolutionary terms, that’s not a good enough reason to help someone out  What matters is that the increase in group success translates into better success for the metaphorical helping gene  Ex. Geese foraging for food in groups o Doesn’t seem to make sense, doesn’t having more hungry mouths nearby make foraging more competitive? o Not always, when food is hard to find, it can be useful to have more individuals looking around for it o If you happen to discover some food, then yes, others may come and take some o However, you’ll also have opportunities to take food from others when they find it first o Another advantage to foraging in groups is when animals are busy searching for food, their heads are down and their attention is diverted, giving predators the advantage of a surprise attack o To remain vigilant, foragers need to keep their heads raised, to look around o Called a “trade-off” because you can’t be vigilant and forage at the same time o The trade-off is reduced when animals forage in groups because while some forage, a few can scan for predators  If every individual spends less time scanning and more time eating, group foraging works to everyone’s advantage so long as there is enough food to go around and every indivi
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