experiment 7

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Campbell Essential Biology
Week 7

Evolution

CHARLES DARWIN AND THE ORIGIN OF SPECIES

  • Charles Darwin published On the Origin of Species by Means of Natural Selection, November 24, 1859.
  • Darwin presented two main concepts:
  • Life evolves
  • Change occurs as a result of “descent with modification,” with natural selection as the mechanism

Natural Selection

  • Natural selection is a process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are individuals with other characteristics.
  • Natural selection leads to:
  • A population (a group of individuals of the same species living in the same place at the same time) changing over generations
  • Evolutionary adaptation
  • Jean Baptiste Lamarck suggested that organisms evolved by the process of adaptation by the inheritance of acquired characteristics, now known to be incorrect.
  • If we cut off the tails of mice for many generations, we would eventually have mice with no tails born.

Darwin’s Theory of Natural Selection

  • Darwin based his theory of natural selection on two key observations:
  • All species tend to produce excessive numbers of offspring
  • Organisms vary, and much of this variation is heritable
  • Observation 1: Overproduction
  • All species tend to produce excessive numbers.
  • This leads to a struggle for existence.
  • Observation 2: Individual variation
  • Variation exists among individuals in a population.
  • Much of this variation is heritable.
  • Inference: Differential reproductive success
    (natural selection)
  • Those individuals with traits best suited to the local environment generally leave a larger share of surviving, fertile offspring.

Mechanisms of Evolution

  • The main causes of evolutionary change are:
  • Genetic drift
  • Bottleneck effect
  • Founder effect
  • Gene flow
  • Natural selection
  • Mutations
  • Nonrandom Mating
  • Migration

Three General Outcomes of Natural Selection

  • Directional selection:
  • Shifts the phenotypic “curve” of a population
  • Selects in favor of some extreme phenotype
  • Disruptive selection can lead to a balance between two or more contrasting phenotypic forms in a population.
  • Stabilizing selection:
  • Favors intermediate phenotypes
  • Is the most common

Original

population

Evolved

population

Phenotypes (fur color)

Frequency

of individuals

Original

population

(a) Directional selection

(b) Disruptive selection

(c) Stabilizing selection

Figure 13.28

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Figure 13.28 Three general effects of natural selection on a phenotypic character

Sexual Selection

  • Sexual dimorphism is:
  • A distinction in appearance between males and females
  • Not directly associated with reproduction or survival
  • Sexual selection is a form of natural selection in which inherited characteristics determine mating preferences.

Sexual Selection

Click on the links below to view presentations on Sexual Selection:

Sexual selection peacock

Sexual selection: pheasant

What is a Species?

  • The biological species concept defines a species as
  • “A group of populations whose members have the potential to interbreed and produce fertile offspring”

Reproductive Barriers between Species

  • Prezygotic barriers prevent mating or fertilization between species.
  • Include:
  • Temporal isolation
  • Habitat isolation
  • Behavioral isolation
  • Mechanical isolation
  • Gametic isolation

  • Postzygotic barriers operate if:
  • Interspecies mating occurs and
  • Hybrid zygotes form
  • Postzygotic barriers include:
  • Reduced hybrid viability
  • Reduced hybrid fertility
  • Hybrid breakdown

Mechanisms of Speciation

  • A key event in the potential origin of a species occurs when a population is severed from other populations of the parent species.
  • Species can form by:
  • Allopatric speciation, due to geographic isolation
  • Sympatric speciation, without geographic isolation

Allopatric speciation

Simpatric speciation

Figure 14.6

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Figure 14.6 Two modes of speciation

What is the Tempo of Speciation?

  • There are two contrasting models of the pace of evolution:
  • The gradual model, in which big changes (speciations) occur by the steady accumulation of many small changes
  • The punctuated equilibria model, in which there are
  • Long periods of little change, equilibrium, punctuated by
  • Abrupt episodes of speciation