Part III: Evolution and Population-Community Ecology Summary
Lecture 11 - Evolution by Natural Selection, 10/21
The gradual change of living things from one form to another over the course of time.
o The origin of species and lineages by decent of living forms from ancestral forms.
o The generation of diversity.
Darwin’s Intellectual Environment
o Uncertainty of the earth’s age
o Uniformitarianism on the rise
Present conditions are a key to the past
o Fossils, suggest extinctions had occurred
o Organism similarities suggested relatedness
o Natural Selection: the mechanism by which evolution might take place
Not “God directed”, but a natural process
Natural Selections “BIG IDEAS”
o Struggle for existence: Individuals produce more offspring than needed for replacement.
o Individuals vary in physical & behavioral traits
o Many traits are inherited
o Traits that benefit survival and reproduction will be passed on to a greater proportion of the future generation.
Natural Selection requires:
o There must exist variation in a trait. i.e. color, size, speed…
o That variation must be heritable.
o There must exist a greater (or lesser) probability of survival due to the possession of that trait.
The “FULL” Theory.
o Organisms change over time.
o Organisms are derived from a common ancestor.
o Similarities of traits suggest a recent, common ancestor.
o Change is gradual and slow.
o Natural Selection is the mechanism of evolutionary change.
o Branching of species generates the tree of life.
Natural Selections “Dangerous Ideas”.
o Human evolution is part of the same process
o Evolution does not push organisms to an “Ideal Form”
o Humans are not the “Pinnacle” , but one twig on the tree of life.
• Types of Natural Selection
Narrow Bell Shifting Bell Bi Modal
If Natural Selection eliminates genetic variation, what generates it?
o Environmental Variation
Ultimate source of genetic variation.
Occurs at a low rate.
Typically, “not good”
Does not arise out of need
Exchange of individual among discrete populations.
Akin to mutation
Most advantageous to small populations.
Migrants washed out in large populations.
Assumes genetic variations among discrete populations.
o In sexual reproduction: no two individuals genetically alike.
Scrambling and recombination.
o Union of egg and sperm creates genotypes.
o Some are genetically superior, some not.
o When natural selection favors one genotype in different years or different locations.
Wet vs. dry, sandy vs. stony.
o Energy expended on one character trait cannot simultaneously be expended on another
o “Perfect” strategies cannot evolve.
Allocation to one trait precludes maximum allocation to another.
“Other” kinds of evolution
o Genetic drift
In small populations individual loss can cause a dramatic change in genotype ratios.
o Artificial Selection
Selection (harvesting) of organisms with desirable features.
o Genetic Engineering
o The number of offspring over an individuals lifetime that survive to reproduce.
o Over time, the progeny of fitter parents will come to dominate a population.
o Fitness is ALWAYS relative to other genotypes and current environment.
o Direct &indirect fitness
Direct personal reproduction
Indirect additional reproduction of relatives made possible by an individuals actions.
2 Brothers vs. 8 Cousins (think!)