• Theory of evolution explains:
• Adaptation
• Biodiversity
• Short-term and long-term changes in species (microevolution and macroevolution)
• How does evolution work?  Main mechanism is natural selection (Darwin, 1859)
• Populations are variable (source of variation is mutations)
• Overproduction of young is universal in nature, so not all young survive to reproduce
• Those that do are "more fit" than than those that do not, and ...
• They pass on to succeeding generations the genes that conferred fitness, so...
• Over time the population changes.
• Miller, p. 43: "Genes mutate, individuals are selected, populations evolve."
• Ecological niches.  A species' niche includes:
• Range of tolerance for physical and chemical conditions
• Types and amounts of resources it consumes
• How it interacts with other organisms
• Its role in energy flow and nutrient cycling.
• Generalist species have broad niches, specialist species have narrow niches.
• Why isn't adaptation perfect?
• Natural selection can only act on existing traits.
• Environmental change may occur faster than natural selection.
• History matters - pigs can't fly.

• Species concepts -- are species real units in nature?
• Morphological species concept
• Biological (reproductive) species concept
• Level of biodiversity = speciation rate - extinction rate.
• Speciation = the formation of new species.  A model:
• Geographical isolation splits a formerly continuous species range
• Natural selection favors different traits in the geographically separated sub-populations.
• The subpopulations diverge genetically.
• Finally, the subpopulations become reproductively isolated.
• Extinction.  99% of all species that have ever existed are extinct!
• Background extinction: environmental conditions change, some species fail to adapt, go extinct.  Speciation rates in rough balance with extinction.
• Mass extinction: a catastrophe strikes and large numbers of species go extinct over a relatively short period of time.  Some examples:
• 65 MYA, Dinosaur extinction, provided opportunity for adaptive radiation of mammals.
• 250 MYA, over 90% of all species went extinct.
• After mass extinction, speciation restores biodiversity, but this takes 10's of millions of years.
• Human caused extinctions: present extinction rates are 100-1000X background, present period may count as a mass extinction.

Is there a "balance of nature"?  Probably not: systems are constantly changing in response to conditions.  Population dynamics provides a good example of this principle.  Note that we are mostly talking about animal and plant populations in this section, although some of this will apply to human populations as well.

Population change (growth or decline) depends on birth (B), deaths (D), immigration (I) and emigration (E): Population change = (B + I) - (D + E)

Species differ in their potential for population increase.  Species which reproduce early in life, have short generation times, and produce many offspring each time they reproduce have a higher potential for increase (biotic potential, intrinsic rate of increase) than those that are otherwise.  Compare insects and mammals, or annual weeds and trees.

A species' potential for increase is limited by environmental resistance - everything that limits population growth (not enough food, too many predators, etc.)

The carrying capacity of an environment for a particular species is the population size that can be maintained indefinitely in that environment (See Figure 2-38).

Sometime populations overshoot their carrying capacity and crash (See Figure 2.39).

Big question: what is the carrying capacity of the earth for our species?

Adaptation to the environment is a consequence of evolution.  Increasing biological diversity is another, and it's important to remember that evolution accounts both for adaptation and diversity of life.  The main cause of evolution is natural selection which works like this:

Individuals within populations are variable.

Some of these variations are passed on to offspring.

In every generation, more offspring are produced than can survive.

Survival and reproduction are not random:  the individuals that survive and go on to reproduce, or those that reproduce the most are those with the most favorable variations.  They are "naturally selected".

This will result in gradual change in populations over time.

Speciation = the evolution of new species (Figure 2-42).  This generally involves two steps:

Geographic isolation

Reproductive isolation.

Extinction results in loss of biodiversity

Background extinction = the rate at which species go extinct as the result of gradually changing environmental conditions, etc.

Mass extinctions (Figure 2-43) are catastrophic.  We may in the midst of one, and the cause of one, right now.  What will be its consequences?