experiment 7
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:
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