2 What is Evolution?Biological Evolution can be considered a change in attributes within a population over time.Specifically, it is a change in gene frequencyEvolutionary changes lead to adaptation (maximizes fitness)What drives adaptive evolution?Genetic drift (founder effect, bottleneck)?Allopatric speciation (physical separation of two populations)?Natural selection?
3 What is Necessary for Natural Selection to Operate? Variation must occur within a populationBreeding domestic animalsAn excess of offspring must be producedNot all individuals can survive to reproduceIt’s a mean old world!Only those best able to garner limited resources will survive and reproduceCharacteristics must be inheritable and more frequent in the next generation
4 Gypsy Moth Phenotype Selection Soot from the industrial revolution caused light colored trees to become dark.A decrease in the number of light moths and an increase in the number of dark moths was observed.Could it be that light colored moths were more vulnerable to predation?LightDark
5 We have seen the same thing in the United States! The proportion of light to dark colored moths has been changing since about The trees have become noticeably lighter as well!Dark formLight formWe have seen the same thing in the United States!
6 Natural selection acts on phenotypes – the observable attributes of individuals (remember AA and Aa have the same phenotype).Although adaptive evolution is a change in genotype frequency, it is much easier to observe natural selection directly on the phenotype.
7 Three Types of Selection (You should be able to describe these) Original DistributionBefore SelectionAfter SelectionDirectional SelectionStabilizing SelectionDisruptive Selection
8 Directional Selection - Probably accounts for most phenotype changes found in the wild. ImportantA severe drought from caused an 85% drop in the population.Only those with larger beaks could eat the large seeds.Resistance to pesticides can also result in Directional Selection.
9 Stabilizing Selection - phenotypes near the mean are more fit than those at the extremes; most common ecological situation.Human Birth Weight
10 More Stabilizing Selection: Lesser Snow Geese Safety in numbers.Relative Hatch Date = mean date that hatching occurred.In this case, it is best to hatch when there are many others around.
11 Disruptive Selection: not very common Extremes are favored over the meanUnless some form of reproductive isolation occurs, extreme phenotypes may continue to mate and produce intermediate phenotypesOverall: Organisms are adapted to their environment!
12 Four Constraints to Adaptation Genetic ForcesMutation – usually detrimentalGene flow – immigrants can smooth out local adaptationsEnvironments are Continually ChangingMost significant short-term constraintAdaptation is a CompromiseA loon’s wings are efficient for diving, but not flyingHistorical ConstraintsOrganisms have a history and change in small increments
13 Case Study: Clutch Size in Birds Clutch = number of eggs laid and differs among speciesClutch size can be affected by proximate (functional; physiological) factors but is a result of ultimate (evolutionary; genetic) factors.Clutch size does not always = maximum physiological number
14 Determinate layers do not vary the number of eggs they produce, indeterminate layers do: Species Normal MaximumMallard ?? 100Herring GullYellow Shafted FlickerHouse SparrowOvulation is usually stopped before the physiological maximum number of eggs are produced.
15 A cost benefit analysis supports this idea. David Lack (1947) put forward the idea that clutch size was determined by the number of young the parents could provide food for.If this is true, then the highest production of young should be the normal clutch size (optimal size).Clutch SizeOptimality ModelA cost benefit analysis supports this idea.
16 No organism has an infinite amount of energy to spend on its activities! If additional young are placed in a nest, all young will suffer if there is not enough food.Normal clutch size = 11.
17 In the tropics, small clutch size is typical. Smaller clutch size = less parental time away from the nest lower predation rate.Therefore, low clutch size in the tropics is believed to be an adaptation to predation levels. Maximum production.
18 Not All Species Follow Lack’s Hypothesis: However, what is not known is the effect on the parents – can they survive until next breeding season? Or does raising more young exhaust them?Other - gene flow from different habitat qualities.Normal clutch = 3 - 4
19 Coevolution – Specific and Reciprocal Originally described the reciprocal evolutionary influences that plants and plant-eating insects have had on each other (Ehrlich and Raven 1964).Predator prey relationships – ‘arms race’
20 What Is Necessary For Natural Selection To Work? Ability to replicateProduce an excessive number of units above replacement needsSurvival depends on some attribute (size, color, behavior)Attributes must be transmittable to the next generationAn individual meets these requirements, but other units do also: gametic, kin, and group.
21 Units of SelectionGametic Selection (e.g. sperm mobility) – does not directly impinge on ecological relationships.Kin Selection (you stole from my kin!) – increase survival of related individuals b/c they share many of your genes.helps to explain some altruistic behaviorGroup Selection – can occur when populations are broken up into discrete groups.Groups with less adaptive genes may go extinctHighly controversialBird reproduction is limited so that they do not overpopulate an area