Transcript Geospiza fortis

The Genetical Theory of Natural Selection
•
Who wrote this (1858)?
– “It occurred to me to ask the question, why do some die and some live.
– And the answer was clearly, that on the whole the best fitted lived. From the
effects of disease the most healthy escaped; from enemies, the strongest, the
swiftest, or the most cunning; from famine, the best hunters or those with the
best digestion; and so on.
– Then I at once saw that the ever present variability of all living things would
furnish the material from which, by the mere weeding out of those less adapted
to the actual conditions, the fittest alone would continue the race.
– There suddenly flashed upon me the idea of the survival of the fittest.
– The more I thought over it, the more I became convinced that I had at length
found the long-sought-for law of nature that solved the problem of the Origin
of Species.”
Where natural selection occurs
Fundamental patterns of intraspecific natural selection
Disruptive
Rare
Economist. In 1838, Darwin reads essay on population. Rate of population
growth is greater than the rate of increase of the food supply.
An example of natural selection
• The premises
• 1. Populations exhibit phenotypic variation.
• 2. The phenotypic variation has a genetic
component (can be inherited).
• 3. Differential reproductive success among
members of the population.
• Survival and reproduction is, on-average, nonrandom
• 4. Phenotypic variation shifts between generations
in response to a changing environment.
Adaptive radiation
Galapagos finches
Medium ground finch
Geospiza fortis
generation time: 4.5 years
life span c. 16 years
Research of Peter and Rosemary Grant: 1973 - present
120 m
N = c. 1,200
7 cm/yr
Hot spot
2-3 my
4-5 my
1 my
1: Is the population phenotypically variable?
Geospiza fortis
2: Is the variation heritable?
(heritability: proportion of phenotypic variation due to genetic variation; c. 65%)
Evolution!
Effect of natural
selection
1977: drought
130 mm precipitation dropped
to 24 mm
The base level natural selector
Was there differential survival?
The interplay
2.
Number of finches
Seed abundance
1.
3.
The second level natural selector
Seed characteristics of surviving
plants
Had evolution taken place? Significant difference in beak size.
Note: natural
selection is always
one generation
behind the
expression of modified
phenotypes
Natural selection cannot anticipate future
“needs” of a population
• Evolutionary change is based selection on
parents of the present generation.
• 1. Parental population + environment (natural
selectors)
• 2. Part of population selected to reproduce
• 3. Transmit their heritable characteristics to the
new generation.
• But any evolutionary changes were based on
selection from parental phenotypic variation.