Evolution, Natural Selection, and Adaptation Nothing in biology makes sense except in the light of evolution. (Theodosius Dobzhansky) Charles Darwin (1809-1882) Voyage of HMS Beagle (1831-1836) Thinking about natural selection & collecting supporting evidence (1837-1858) Origin of Species (1859) (HMS Beagle)
Alfred Russel Wallace (1823-1913) Natural history collecting trips to Brazil and SE Asia Author of The Malay Archipelagio Wrote a letter to Darwin explaining his idea of natural selection - 1858 Evolution via Natural Selection Process whereby heritable traits that confer the highest relative reproductive success increase in frequency over evolutionary time as many more individuals of a species are born than can possibly survive and, as there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new modified form. (Origin of Species, 1859)
Definitions Natural selection: Differential reproduction and survivorship among individuals within a population Fitness: relative lifetime reproductive success Survival & Reproduction Adaptation: trait that increases the fitness of an individual, relative to its competitors, in a population. Artificial Selection Process whereby humans selectively breed some varieties over others, causing one or more varieties to increase in frequency (Dugatkin, 2004) Time to go fishing! Time to go fishing! Start with 5 of each color in your pond Mix them up well Use only chopsticks in one hand to fish Keep caught fish in pile At the end of each round, count the number of each color fish that you caught Each surviving fish replicates at the end of each round Keep track of how many of each color you add to the pond
Fishing for M & M s Why wouldn t populations evolve to have only one color, for example, only red M&M s? Are we stuck with only five colors of M&M s forever? What if the environment changed to a new color, such as black? What would happen? Why is variation useful? Natural selection occurs when 1. Trait is variable within a population 2. Variation contributes to differences in reproductive success 3. Variation is heritable Variation within a population Due to: 1. Differences in genetic composition 2. Differences in environmental conditions 3. Differences in learning Figure 2.4. Honeybee food delivery.
Variation within a population Due to: 1. Differences in genetic composition 2. Differences in environmental conditions 3. Differences in learning Variation may not affect fitness Eastern screech owls show great variation in dispersal direction And 1. Variation in traits may not affect fitness 2. Frequency-dependent selection can maintain different traits in a population Frequency-dependant Selection Frequency-dependant Selection Positive frequencydependent selection: the fitness of a trait increases as it becomes more common Negative frequencydependent selection: the fitness of a trait decreases as it becomes more common
Frequency-dependant Selection Origin of new traits New traits originate by: random changes in genetic material (mutation, sexual recombination) survive or disappear due to selection by environmental factors (thus affecting the composition of the population as a whole). Lamarckism Inheritance of acquired characters Useful characters are acquired during the lifetime of an organism and passed on to offspring. Characters that are not useful are diminished. Continuous, gradual change (Jean-Baptiste Lamarck, 1809, Philosophie Zoologique) Genetics Gene: segment of protein-encoding DNA Allele: variant of a gene Gene pool: sum of all alleles in the population Evolution: change in the allelic frequency of a population Heritable: A genetically-based trait that can be passed from parents to their offspring
Measures of heritability 1. Parent-offspring regression analysis: Examines the similarity between parents and their offspring in terms of the traits they possess If a trait has a genetic basis, then the trait values of offspring should be similar to the trait values of their parents: there should be a positive relationship between offspring and parent trait values 2. Selection experiment method: Different groups of individuals are subjected to differential selection on the trait in question If artificial selection acting on a trait results in changes in that trait value in subsequent generations, then the trait has a genetic basis Selection experiment Over four generations of selection, found strong changes in exploratory behavior Adaptation and Tinbergen s 4 Levels
Adaptationist Approach to Study of Behavior Adaptation What methods are employed to determine if and how a trait that increases the fitness of an individual, relative to others in its population behaviors are adaptive? Aren t all traits adaptive? Constraints on Adaptive Perfection 1. Failure of appropriate mutations to occur Not all traits are adaptive, AND those that are adaptive are not perfect. Selection can t keep up with environmental change