Lecture 11: combined evolutionary forces Flashcards
Fisher and Wright (3)
-Wright & Fisher disagreed on relative importance of natural
selection, drift & migration.
-Fisher emphasized gradual changes in a single large
population due to selection acting on small variants
- Wright argued that adaptation would be most effective
in a population that was subdivided into many small
subpopulations in his shifting balance theory
Adaptive landscapes (5)
- Provide visual aid to non-mathematical biologists
-One of the most popular metaphors in history of biology”
-complex fitness landscapes are shown as topographic maps with “elevation lines” representing population mean fitness for given allele frequency
-Have adaptive peaks where populations have to cross a “fitness valley” to reach a new peak
ex: how would highly fit a bee pollenated flower evolve to a hummingbird pollenated flower? It would have to first leave its adaptive peak, becoming less adapted to bee pollination. This is the fitness valley the flower must cross. Then once the flower has evolved to be pollenated by humming bids, it can continue to adapt towards a new evolutionary peak
Shifting balance theory (SBT) phases (4)
-Phase I. Drift drives a population down into a fitness valley
-Phase II. Selection drives the population to a new optimum, which is a global optimum (or at least higher than the original optimum)
-Phase III A. The population at the global optimum grows large and sends out many migrants to neighboring populations
-Phase IIIb. The migrants change allele frequency in recipient populations through competition and interbreeding enough to allow them to cross the fitness valley. A new phenotype / species arises
Shifting Balance theory (SBT) (4)
-Proposed in 1932 by Sewall Wright
- Suggests that adaptive evolution proceeds most quickly when a population consists of subpopulations with restricted gene flow.
- Attempts to explain how a population may move across an adaptive valley to a higher adaptive peak.
-empirical evidence often supports one or two phases, but never all three
SBT application (3)
-In many commercial fisheries, the largest, mature fish are being removed preferentially.
-This creates an intense selective pressure to grow rapidly and to a smaller size
-combining natural selection driving body size up, and human predation driving body size down, the salmon end up with a new adaptive peak, where it is most fit to be somewhere between large and small.
SBT and changing environments (3)
-In SBT, the model is often thought of as static, with fixed peaks that populations must work towards
-in real life, since environments are always changing, so do the adaptive peaks
-Because of this, in real life populations are always shifting to “catch up” and move toward the ever changing peaks
Correlated traits (2)
-Many traits influencing fitness are genetically correlated with other traits through things like linkage and pleiotropy
-Correlated traits cannot respond independently to
selection.
multivariate Selection response
-example: if both being tall and having short arms is advantageous, you cannot select for both. If its MORE advantageous to be taller than you’ll see taller people with long arms. If its MORE advantageous to have short arms you’ll see shorter people with short arms
Selection gradient
-Beta = Selection gradient = the slope of the relationship between fitness and phenotype for a given trait
- Beta = slope
-S = Beta * Vpz
