1. Mechanisms of Evolutionary Change
Evolution is defined as a change in the frequencies of alleles (Ex: A and a) in a population across generations. Changes in allele frequencies cause changes in genotype frequencies, which cause changes in phenotype frequencies.
Three major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow

2. Modes of Natural Selection:
Stabilizing
Favors the mean
Directional
Favors one phenotypic extreme
Disruptive
Favors two or more phenotypic extremes

3. Modes of selection
(a) No selection (b) Stabilizing selection

4. Modes of selection
(c) Directional selection (d) Disruptive selection

5. Genetic Drift: random allele frequency (gene pool) changes in SMALL populations due to chance alone (not natural selection)
Genetic drift tends to reduce genetic variation through losses of alleles, especially in small populations
Note: The gene pool for a population is defined as all the alleles in a population and their frequencies
Two factors that could cause a drastic reduction in the size of a population, which would result in an increase in genetic drift:
The Bottleneck Effect: When a natural disaster reduces the size of a population and the survivors are determined by chance (Ex: flood or overhunting) b

6. B) The Founder Effect: The founder effect occurs when a few individuals become isolated from a larger population
Allele frequencies in the small founder population can be different from those in the larger parent population due to chance(Ex: colonizing a new island)

7. Gene flow consists of the movement of alleles among populations
Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
Gene flow tends to reduce genetic variation among populations over time

8. Macroevolution and Speciation
12/2/2018
Macroevolution and Speciation
Evolution creates (and destroys) new species, but …
What is a species?
Its not as straightforward a question as most believe.
These are members of different species - eastern (left) and western (right) meadowlark.
G. Podgorski, Biol. 1010

9. And these are all members of a single species.
Macroevolution
12/2/2018
What is a Species?
And these are all members of a single species.
G. Podgorski, Biol. 1010

10. Macroevolution
12/2/2018
What is a Species?
The definition we’ll use is this: A species is a group of individuals capable of interbreeding to produce fertile offspring.
This is the biological species concept. Like all attempts to define a species, it has many problems.
G. Podgorski, Biol. 1010

11. We don’t know. How Many Species Are There?
Macroevolution
12/2/2018
How Many Species Are There?
We don’t know.
About 2 million species have been described.
Estimates of existing species number range from 4 million to 100 million (with million being a more commonly considered upper estimate).
G. Podgorski, Biol. 1010

12. Speciation A population becomes reproductively isolated
The separated gene pools diverge
Genetic exchange stops

13. Macroevolution
12/2/2018
How Do Species Arise?
The key to speciation is reproductive isolation of populations.
Geographic isolation is the primary extrinsic reproductive isolating mechanism.
G. Podgorski, Biol. 1010

14. Reproductive isolation can be classified by whether barriers act before or after fertilization:
Prezygotic barriers block fertilization from occurring by
Impeding different species from attempting to mate
Preventing the successful completion of mating
Hindering fertilization if mating is successful
Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult by
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

15. Figure 22.3
Prezygotic barriers
Postzygotic barriers
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Mechanical
isolation
Gametic
isolation
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
VIABLE,
FERTILE
OFF-
SPRING
MATING
ATTEMPT
FERTILI-
ZATION
(a)
(c)
(e)
(f)
(g)
(h)
(i)
(l)
(d)
(j)
(b)
Figure 22.3 Exploring reproductive barriers
(k) 15

16. Macroevolution
12/2/2018
*Reproductive Isolation May Occur With or Without Geographic Isolation*
Allopatric speciation occurs when geographic isolation creates a reproductive barrier
Sympatric speciation occurs when a reproductive barrier is created by something other than geographic isolation
G. Podgorski, Biol. 1010

17. Allopatric speciation
Geographically separated populations
Most common form of speciation
Genetic drift in small populations

18. Allopatric Speciation
Macroevolution
12/2/2018
Allopatric Speciation
Harris’ antelope squirrel
White-tailed antelope squirrel
Two species of ground squirrel are postulated to have descended from a common ancestral population that was separated by formation of the Grand Canyon.
G. Podgorski, Biol. 1010

19. Squirrel species separated by the Grand Canyon have diverged in fur color

20. Sympatric speciation
Divergence of two populations in the same geographic region
Other reproductive isolating mechanisms at work
Especially common in plants

21. Sympatric speciation in animals
Population occupies a new ecological niche
No gene flow even though species live in the same area

22. Speciation Occurs at Widely Differing Rates
Macroevolution
12/2/2018
Speciation Occurs at Widely Differing Rates
A slow rate of speciation evidenced by a living horseshoe crab (13 extant species) and a 300 million year-old fossil species
A rapid rate of speciation evidenced by Galapagos finches which have diversified into 13 species within the last 100,000 years.
Generalists, like the horseshoe crab, tend to remain as stable species.
Specialists, like the Galapagos finch, tend to be unstable as species.
Speciation also becomes rapid when, as occurred with Galapagos finches, new niches become available.
G. Podgorski, Biol. 1010

23. Punctuated equilibrium
Long periods of stasis (~2 My)
Punctuated by periods of rapid speciation (~100,000 y)
Triggered by changes in the environment
Abrupt appearance of new species in the fossil record

24. Gradualism Continuous evolution over long periods The traditional view
Populations gradually accumulate adaptations
Different selective pressures in different environments

25. Speciation Dynamics - Gradualism or Punctuated Equilibrium?
Macroevolution
12/2/2018
Speciation Dynamics - Gradualism or Punctuated Equilibrium?
Punctuated equilibrium appears to be a more accurate view of speciation dynamics.
G. Podgorski, Biol. 1010

26. Adaptive radiation Speciation fills new ecological niches
New adaptive zones may appear when the environment changes
One species colonizes an island and diversifies into new species