2 - Natural Selection Flashcards
Mechanism for organic evolution
Adaptation by natural selection
Key points of natural selection
- Species can change
- Species can evolve from other species through mechanism of natural selection
Darwins three postulates
- Competition: Individuals compete because resources are finite
- Variation: Individuals vary in ways that affect their ability to survive (i.e. fitness)
- Heritable: Some of this variation is heritable
Competition
- Populations can expand indefinitely, but resources are always finite
- Individuals compete for limited resources (e.g. food) within a particular habitat
- Not all individuals survive long enough to reproduce
Variation
- There is a variation among individuals in a population
- Some individuals will possess traits that make them more successful
(i.e. higher fitness) - Those traits allow them to survive and reproduce, or produce more offspring
Heritable
- Differences among individuals are transferred from parents to offspring
- Those advantageous traits will become more common in successive generations
- In Darwin’s time mechanisms of inheritance still unknown
Conclusion of Darwin’s postulates
- Those individuals who compete well pass on their traits, including those that
help them to survive/reproduce - Those that don’t survive or reproduce less leave fewer of their characteristics
(disadvantageous traits disappear) - Those traits that help individuals to survive and reproduce → adaptations
Natural selection
The process that leads to
adaptations when these 3 postulates hold
Adaptation
A trait that is shaped by natural selection and allows the individual to survive and reproduce more successfully
Peppered Moth (Biston betularia)
- First appears in mid-1800’s
- By 1895, 95% B. carbonaria
- Darker moths (due to variation) less visible to bird predators on trees blackened by soot
- With Clean Air laws, B. carbonaria declines
Galapagos Finches postulates
- Competition for limited resources in environment (droughts)
- Variation in traits important for survival (beak depth)
- Variation heritable
Changing environments causes competition
- Drought leads to failure of seed crops
- Overexploitation of small seeds
- Large hard seeds remain causing struggle for survival
Variation in beak depth
- Large beaks confer advantage (differential
survival) - Note that fewer total
individuals now in population - Success of large beaks leads to shift in mean
beak size in population - Variation is heritable
Types of selection
- Directional selection
- Stabilising selection
- Disruptive selection
Directional selection
Mean shifts after selection
Stabilising selection
- Mean remains the same
before and after selection - Preserves equilibrium
Stabilising selection in finches
- Selection that preserves status quo
- Some disadvantages to larger beaks
- Equilibrium
- Those at tails eliminated (selection still active)
Disruptive selection
- May eventually lead to two populations with distinct means
Natural selection can’t explain the evolution of complex traits misconception
- Complex adaptations don’t evolve all at once
- e.g. A computer simulation of the evolution of the eye generates this sequence of
forms - Demonstrates each change, building on previous small changes, can produce complex adaptations
Convergent evolution
Live in similar habitats and so are subject to similar selective pressures
Natural selection occurs at the level of the individual misconception
- Occurs at level of population
- Changes in gene frequencies
- More interested in how average beak size changes over time (not how any one individual’s beak looks)
All traits are adaptive misconception
- Not all traits are adaptations
- Only an ‘adaptation’ if it contributes to fitness
- Some traits are former adaptations (e.g. appendix)
- Others are just traits (e.g. human chin)
Evolution is not ‘progress’
- Evolution does not always progress in one direction
- No such thing as “better”
- ONLY better suited to a particular environment.
- So, if the environment changes then gg
