1. Patterns and Mechanisms of Evolution
Natural selection is not the only mechanism of evolution.
EQ: What patterns can be observed in evolution and what factors influence speciation?
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Shaping Evolutionary Theory

2. Patterns of evolution These are models of evolution:
A. Mass Extinctions
B. Adaptive Radiation
C. Convergent Evolution
D. Coevolution
E. Gradualism
F. Punctuated Equilibrium

3. A. Mass Extinctions
Event in which many types of living things became extinct at the same time. 
Huge numbers of species disappeared.
Whole ecosystems were wiped out.
Resulted in burst of evolution of new species in new habitat
Disrupted energy flow throughout the biosphere and caused food webs to collapse
(Possible Causes: Asteroids, volcanic eruptions, continental drifts, changing sea levels)

4. B. Adaptive Radiation (Divergent Evolution)
The evolution of an ancestral species, which was adapted to a particular way of life, into many diverse species, each adapted to a different habitat
Many new species diversify from a common ancestor .
The branching out of a population through variation.
The new species live in different ways than the original species did.

6. C. Convergent Evolution
Opposite of divergent evolution (adaptive radiation)
Unrelated organisms independently evolve similarities when adapting to similar environments, or ecological niches
Analogous structures are a result of this process
Example: penguin limb/whale flipper/fish fin
The wings of insects, birds, pterosaurs, and bats all serve the same function and are similar in structure, but each evolved independently

7. D. Coevolution The mutual evolutionary influence between two species
Bumblebees and the flowers the they pollinate have co-evolved so that both have become dependent on each other for survival.
The mutual evolutionary influence between two species
When two species evolve in response to changes in each other
They are closely connected to one another by ecological interactions (have a symbiotic relationship) including:
Predator/prey
Parasite/host
Plant/pollinator
Each party exerts selective pressures on the other, thereby affecting each others' evolution

8. E. Gradualism
The evolution of new species by gradual accumulation of small genetic changes over long periods of time
Emphasizing slow and steady change in an organism
Occurs at a slow but constant rate
Over a short period of time it is hard to notice
Current living zebras (top), extinct quaggas (bottom)

9. F. Punctuated Equilibrium
Stable periods of no change (genetic equilibrium) interrupted by rapid changes involving many different lines of descent
Opposite of gradualism
It is rare, rapid events of branching speciation
Characterized by long periods of virtual standstill ("equilibrium"), "punctuated" by episodes of very fast development of new forms
Horseshoe crabs have changed little since their first appearance in the fossil record.
They are in a state of equilibrium

10. Gradualism vs Punctuated Equilibrium

11. Mechanisms of Evolution-Evolution occurs at the population level
Population genetics
Hardy and Weinberg showed that evolution will not occur in a population unless allele frequencies are acted upon by forces that cause change.
Hardy-Weinberg principle states that when allele frequencies remain constant, a population is in genetic equilibrium.
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Shaping Evolutionary Theory

12. Mechanisms of Evolution
Population genetics
For a population to be in genetic equilibrium according to the Hardy-Weinberg principle, it must meet five conditions:
No genetic drift
No gene flow
No mutation
Mating must be random
No natural selection
These five conditions are the mechanisms of evolutionary change and VIOLATE the Hardy-Weinberg Principle
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Shaping Evolutionary Theory

13. Mechanisms of Evolution
1. Genetic drift
Any change in the allele frequencies in a population that results from chance is called genetic drift.
In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.
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Shaping Evolutionary Theory

14. Genetic drift
The founder effect results when a group of individuals with a different allele frequency than the original population becomes isolated.
Alleles that were infrequent in the original population may be common in the new population.
Happens when a subset of organisms settles in an area separated from their original population
result in large genetic variations in the separated population
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15. Genetic drift
A bottleneck results when population declines (due to a catastrophe) to a very low level and then rebounds.
The gene pool of the rebound population is similar to the low- level population, which may have reduced diversity.
Bottleneck Effect: Northern Elephant Seal Population
Hunted to near extinction
Population decreased to 20 individuals in 1800’s, those 20 repopulated so today’s population is ~30,000
No genetic variation in 24 genes
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Shaping Evolutionary Theory

16. 2. Gene flow 3. Nonrandom mating 4. Mutation Mechanisms of Evolution
Uncommon in natural populations – organisms migrate/move between populations
Random movement increases genetic variation within a population and decreases the genetic variation between populations.
3. Nonrandom mating
A population in genetic equilibrium must exhibit random mating.
This rarely occurs in populations – mating occurs between individuals in close proximity
Promotes inbreeding
Changes allele frequencies, favoring individuals that are homozygous
4. Mutation
The cumulative effect of mutations in a population might shift allelic frequencies.
Most mutations are harmful, but some may be beneficial and become more common.
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Shaping Evolutionary Theory

17. 5. Natural selection Mechanisms of Evolution
Natural selection acts to select the best adapted for survival and reproduction.
Three types of natural selection:
Stabilizing-eliminate extreme expressions of a trait
Directional-increases the expression of an extreme version of a trait
Disruptive-removes individuals with average traits
Sexual selection -change in the frequency of a trait based on the ability to attract a mate.
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Shaping Evolutionary Theory