Natural Selection Flashcards
Tinbergen’s four ‘why’ questions
- Causation (mechanisms, e.g. sensory, hormonal, and neural)
- Development or Ontogeny (Genetic and developmental mechanisms)
- Adaptive advantage or function (How is the behavior advantageous for the organism?)
- Evolutionary history or phylogeny (how has the behavior evolved?)
Why is this starling singing: example of causation mechanisms
Because increasing day length triggers hormonal changes that result in singing; because the way air flows through the vocal apparatus vibrates a membrane, resulting in singing
Why is this starling singing: example of development of ontogeny
Because it has learned songs from its parents and neighbors and has a genetic disposition to learn the song of its own species
Why is this starling singing: example of adaptive advantage or function
Because singing will attract a mate for breeding, and thus increase this individual’s reproductive success
Why is this starling singing: example of evolutionary history or phylogeny
Because song evolved in starlings from their avian ancestors; the most primitive birds sing simple songs, so the complex songs of starlings evolved from simpler, more ancestral songs
Proximate factors
- Explain how a given individual comes to behave in a particular way during its lifetime
- Causation & development/ontogeny
Ultimate factors
- Explain why and how the individual has evolved the behavior
- Adaptive advantage/function & evolutionary history/phylogeny
Explanation of natural selection
- All organisms have genes that code for proteins. Proteins regulate the nervous system, muscles, animal appearance ,and structure, and so influence its behavior
- Within a population, many genes are present in two or more forms, called alleles, which code for slightly different forms of the same protein or determine when, where, and how much of the protein is expressed. This will cause differences in development and function, resulting in variation in a population.
- Any allele that results in more surviving copies of itself than its alternative will eventually replace the alternative form in the population. Natural selection is the differential survival of alternative alleles through their effects on replication success.
What factors are necessary for behavior to evolve under natural selection? (3 factors)
- There must be, or have been in the past, behavioral variation in the population (alternative behavior expressed by different individuals)
- These differences must be heritable
- Some behavioral alternatives must lead to a greater reproductive success than others
What is an adaptation?
- A trait that becomes common in a population through natural selection
- OR an established trait that is maintained by natural selection
What is the measure of natural selection?
An individual’s reproductive success (i.e. fitness) relative to other members of the same species
Why relative fitness?
- For example, consider an individual who leaves 2 offspring to the next generation
(fitness of 2) - This individual is at a huge fitness advantage if everyone else in the population has a fitness of 1, but a disadvantage if all others have a fitness of 4
- Therefore, we often work with relative fitness: the absolute fitness divided by a fitness reference that we agree on
Arguments against the theory of group selection
- It requires that groups are selected during evolution (with some groups dying out faster than others, or leaving different numbers of “offspring” groups) rather than individuals
In reality:
1) Groups do not usually go extinct fast enough for group selection to be an important evolutionary force
* Individuals nearly always die out at faster rates than groups, so individual selection is more powerful
2) For group selection to work, groups must be isolated
* Otherwise, selfish individuals can migrate into a population of altruists and
destabilize it
* In nature, groups are rarely isolated sufficiently to prevent such immigration
Historical contingency/constraint
- Evolution does not start from scratch in each generation, but builds upon existing genetic variation and phenotypic traits
- Natural selection can only work with variation that is currently present in the population, and current variation depends on genes/traits that evolved in the past
- Evolutionary inertia states that organisms are likely to retain ancestral traits, even if they are no longer adaptive
- This can constrain the rate and direction of evolutionary change
Genetic constraints to evolution
- Pleiotropy: a single gene affects multiple traits
- Genetic correlations: Selection on one trait affects another * Behavioral traits can act in multiple contexts
- Thus, even if selection on one trait would theoretically be favorable, the negative effects on another trait could mean evolution will not favor that change
How can co-evolution constrain evolution?
Other species are also evolving, and tight linkages in relationships like predator/prey, host/pathogen, and mutualism pairs can constrain evolution
