1. Natural Selection and the Origin of Species
Chapter 5

2. On the Origin of Species
Literature published by Charles Darwin on 24 November, 1859
Full title: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life
Most likely the most important piece of scientific biological literature ever written
Designed to be understood by those not fully versed in biology or naturalism.
Popularized Darwin’s notions on the diversity of life through patterns of descent
Outlaid evolutionary adaptation through natural selection where individuals less suited to the environment are less likely to survive and less likely to reproduce

3. Ecology and Adaptation
Adaptation is always relative to the environment
Variation
Natural selection changes allele frequency
There are several mechanisms of variation
Mutation
Independent assortment
Recombination
Crossing over during meiosis

4. Habitat
Anything and everything external to an organism is its environment
Physical or cultural
Habitat - The specific place where an animal lives
The specific habitat in which an animal lives along with how it exploits this habitat is its ecological niche

5. Mechanisms of Natural Selection
Any factor that brings about a difference in fertility among members of a population is termed a selective agent
Places a selective pressure on certain individuals in the population
Results in a change in allele frequencies in the next generation
Individuals, or populations, that have higher fertility rates in a particular ecological niche are said to display a greater fitness to that niche
Fitness is a measure of how well-adapted a particular individual or group is to the requirements of the niche

6. Types of Natural Selection
Directional –– long-term selection of an extreme, most adaptive, trait
Advantageous allele frequency increases at expense of others
Stabilizing – two or more forces favor different traits
Favors balanced polymorphism – alleles maintain a constant frequency in a population over time
Decreases diversity
Disruptive - natural selection favors both directional and stabilizing extremes

8. Sexual Selection Male Reproduction
Success is limited by his access to females
Sexual Selection - Characteristics that increase males’ success will increase in frequency over time
Intersexual selection is selection for traits that make males more attractive to females
Intrasexual selection occurs in cases where males directly compete with one another
Sexual selection will favor traits that make the male a stronger fighter
Thus larger size is favored in males, resulting in sexual dimorphism

9. Kin Selection Relatives share many of the same genes
Kin Selection - an individual assists a relative in reproducing and passing on its genes
Also assisting in passing on its own genes
Explains altruistic behavior that decreases the reproductive success of the individual
Inclusive fitness - number of offspring produced plus the offspring produced by relatives who share some of the same genes

10. Evolution and Time
Microevolution vs. Macroevolution
Evolution takes time
Appearance of a new species is rarely, if ever, “witnessed”
Demes
A local population of a species where individuals tend to resemble one another closely
As a result of gene flow, nearby demes resemble one another more than demes that are further away
Subspecies occur when something is present that hinders or prevents gene flow

11. Geographic Isolation
Allopatric Speciation - new species evolves in geographic isolation from its ancestor
Geographic isolation is a primary initiator of evolution
Sympatric Species – new species evolve from a single ancestral species while inhabiting the same geographic location
There are mechanisms that serve to separate closely related species living side by side:
seasonal isolation - reproductively active at different times
sexual isolation – prevents certain individuals form reproducing
mechanical isolation – individuals have incompatible sex organs
gametic mortality – destroyed before fertilization
hybrid inviability - low health of these reduces fertility
hybrid sterility - suppression of reproductive capacity

12. Species and Adaptation
Preadaptation - the potential to adapt to a new niche
Organisms do not adapt due to necessity, but because by chance they have the potential to adapt
Specialized species - exhibits little variation and can tolerate minimal change within its niche
Generalized species - exhibits greater diversity and therefore can survive in a variety of niches
Specialized and generalized can also refer to specific traits for an organism
Specialized - Hand or foot suited for specialized function of stable weight support in an upright posture
Generalized - A mammalian limb with five flexible digits, adapted for many possible functions

13. Adaptive Radiation and Extinction
Adaptive Radiation - when a species enters an uninhabited environment or one in which competition does not exist
A species, or group of species, will diverge into as many variations as two factors allow:
Its adaptive potential
The adaptive opportunities of the available niches
In many cases an organisms is too specialized to adapt to changing situations and as a result, they become extinct