Natural Selection and Evolution

1
Natural Selection and Evolution

• Modern Biology
• Chapters 15-16
• Biology Exploring Life
• Chapter 14 Sections 15.1-15.2

2
Evolution

• Orderly succession of changes in populations of
  organisms over generations
• Scientific fact, with theories to explain it
• Jean Baptiste de Lamarck
• Charles Darwin
• Microevolution / macroevolution

3
Lamarcks Explanation

• Living species descended from similar extinct
  species evident in the fossil record.
• Species change over time by inheriting acquired
  traits.

4
Darwins Theories

• On the Origin of Species by Means of Natural
  Selection (a.k.a. The Origin of Species)
• Descent with modification
• Evolution occurs.
• All species descended from one or a few original
  types of life.
• Similar organisms arise in the same geographic
  location.
• Organisms give rise to others similar to
  themselves.

5
Darwins Theories (continued)

• Modification by natural selection
• The environment limits the growth of populations
  by increasing the rate of death or decreasing the
  rate of reproduction, or both.
• Organisms best suited to their environment (those
  with adaptive advantages) reproduce more
  successfully (have higher fitness) than other
  organisms.
• Over generations, the proportion of organisms
  with favorable traits increases in a population
  (the population adapts).

6
Population Genetics

• The study of evolution from a genetic point of
  view
• Populations are
• Interbreeding single-species groups
• The smallest units in which evolution (gradual
  change in genetic material) occurs

7
Genetic Variation

• Bell curve common
• Causes
• Mutation
• Recombination
• Random fusion of gametes

8
The Gene Pool

• The total genetic information available in a
  population
• Phenotype frequency
• The number of individuals with a particular
  phenotype divided by the total number of
  individuals in the population
• Allele frequency
• The number of a certain allele divided by the
  total number of alleles of all types in the
  population

9
The Gene Pool (continued)

• Genotype frequencies (x y)2
• If x dominant allele frequency and y
  recessive allele frequency, then
• Homozygous dominant genotype frequency x2
• Heterozygous genotype frequency 2xy
• Homozygous recessive genotype frequency y2

10
Hardy-Weinberg Genetic Equilibrium

• Theoretical state in which the allele frequencies
  in a non-evolving population tend to remain the
  same from generation to generation unless acted
  on by outside influences
• Assumptions
• No net mutations occur.
• Individuals neither enter nor leave the
  population.
• The population is large.
• Individuals mate randomly.
• Selection does not occur.

11
Disruption of Genetic Equilibrium

• Mutation
• Can produce totally new alleles for a trait
• Migration
• Genetic drift
• Nonrandom mating
• Natural selection

12
Migration

• Immigration or emigration
• Gene flow
• The process of genes moving from one population
  to another

13
Genetic Drift

• Change in allele frequencies in a population as a
  result of random events
• Inversely proportional to population size
• Founder effect
• If one of two allele frequencies reaches zero,
  then the population is in danger of extinction.

14
Nonrandom Mating

• Geographic proximity / kinship
• Assortative mating
• Selection of a mate based on similarity of
  characteristics

15
Natural Selection

• The genes of successful reproducers, rather than
  those of merely successful survivors, are
  amplified.
• Types of natural selection
• Stabilizing selection
• Directional selection
• Disruptive selection
• Sexual selection

16
Stabilizing Selection

• Individuals with the average form of a trait have
  the highest fitness.

17
Directional Selection

• Individuals with an extreme form of a trait have
  greater fitness than those with the average form.
  

18
Disruptive Selection

• Individuals with either extreme form of a trait
  have the greatest fitness.

19
Sexual Selection

• Females tend to choose the males with whom they
  mate based on certain traits.
• Ex. Peacocks gaudy plumage

20
Speciation

• Morphological and biological species concepts
  provide our modern definition of species
• A species is a single type of organism.
• Members of a species are morphologically similar
  and can interbreed to produce fully fertile
  offspring.
• The many species alive today diverged from a
  smaller number of earlier species, through a
  process beginning with isolationin which two
  parts of a formerly interbreeding population stop
  interbreeding.
• Gradualism vs. punctuated equilibrium

21
Isolating Mechanisms

• Geographic isolation
• Reproductive isolation
• Prezygotic isolation
• Behavioral isolation
• Temporal isolation
• Postzygotic isolation
• Death
• Sterility

22
Patterns of Evolution

• Coevolution
• Convergent evolution (analogous structures)
• Divergent evolution (speciation)
• Adaptive radiation
• Artificial selection

23
Evidence for Evolution

• The fossil record
• Law of superposition
• Relative age vs. absolute age
• Mass extinctions
• Biogeography
• Homologous structures (not analogous structures)
• Vestigial structures
• Similarities in embryology
• Similarities in macromolecules