Lecture 21: Adaptation part 1: Study of adaptation Flashcards
What is adaptation? (2)
-Major focus of evolutonary biology
-Often remarkable fitness between organisms and their environment or organisms that interact with each other
Adaptation Definitions (3)
as a noun (thing): A trait (or suite of traits) that increase the fitness of its possessor is called an adaptation, and it is said to be adaptive
As a verb (action word): a process that results in a change in allele frequency that increases population mean fitness
in both cases, it results from natural selection and has a genetic basis
Causes of non-adaptive variation
-Direct effects of the environment
-Genetic drift: Random changes in allele frequency due to sampling error
-Multiple adaptive phenotypes
-Laws of physics or chemistry: Flower color in hydrangeas changes from pink to blue based on the pH of the soil
constraints on adaptation (3)
-trade-offs: Time and resources are limited, developing adaptations is costly and time-consuming
-Pleiotropy: One gene often effects multiple traits and selection of one trait causes change in the second
-Developmental constraints
Demonstrating adaptation is hard! (3)
-Is the trait maintained by natural selection or merely inherited from ancestors?
-Expecting a trait to be adaptive just because it is there can lead to flawed evolutionary thinking
-Our default explanation should be that a trait is not adaptive… this is the null that needs to be rejected
Demonstrating adaptation (3)
- Goal: Show that trait developed or is maintained through natural selection
-Types of study include experimental, observational, or theoretical
-levels of study include population and comparative (within species)
Comparative method (5)
-Each species is a data point
-usually observational
-More general than single species studies
-Greater potential for confounding factors
-Tests for association between a trait and the environment or correlations among traits (ex do carnivore have larger home ranges than herbivores / do self-fertilizing plants invest less into traits that attract pollinators)
Big problem with the comparative method
-Closely related species may resemble one another due to shared inheritance of traits rather than because they experience similar selection pressures BUT phylogenetic info can be used to resolve this issue
Comparative method example (seed size)
-Is large seed size an adaptive trait that allows species to germinate and grow in shady habitats?
-If yes, then species from shady habitats should have larger seed sizes than species from sunny habitats
-Examine 12 species, 6 sunny, 6 shade: all 6 sunny had small and all 6 shady had large.
-heres why phylogeny is important: you could make 2 trees from this. One tree has all the shade species on one branch and all the sunny species on another, this could mean that the shade species only have the large seeds due to shared ancestor. The true tree however has each sunny species as a sister with a shade species, showing that the trait evolved in every case. This means that it is a result of natural selection = true adaptation
Types of Comparative methods (2 with 3 points each)
1) sister species comparisons.
-require many species
-require frequent independent changes in hypothesized causal variable (ex habitat type) .
-Can only involve living species
2). Phylogenetically independent contrasts (PICs)
-Compare changes in causal variable with changes in dependent variable
-Often involves estimation of ancestral phenotypes
-Looks for patterns of divergence as sister species evolve independently away from their common ancestor
Phylogenetics vs comparative method
Phylogenetics:
-Goal is to describe ancestral relationships
-Can be confounded by independent / parallel evolution
-Strategy: use many neutral characters
Comparative method
-goal is to describe adaptation
-Can be confounded by shared ancestry
-Strategy: consider independent / parallel evolution with PICs
-requires phylogeny bases on characters that are independent of the study of focus
