Summary Lecture 11 Slides

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
Darwin’s Contributions
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 Mutation
o Migration
o Environmental Variation
o Sex
o Trade-offs

o Mutation
 Ultimate source of genetic variation.
 Occurs at a low rate.
 Typically, “not good”
 Does not arise out of need
o Migration.
 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.
Environmental Variation
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

Survival  Fitness
o The number of offspring over an individuals lifetime that survive to reproduce.
 Relative term
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.

Inclusive Fitness
o Direct &indirect fitness
 Direct  personal reproduction
 Indirect  additional reproduction of relatives made possible by an individuals actions.
 2 Brothers vs. 8 Cousins (think!)

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