1. NIS - BIOLOGY
Lecture 67 - Lecture 68 - Lecture 69 - Lecture 70
Shaping Evolutionary Theory
Ozgur Unal

2. Mechanisms of Evolution
Other than natural selection,
how else might species change
over time?
Evolution can be the result of
many different environmental
and genetic circumstances.
In this section you will learn
about the five principle
mechanisms of evolution: natural
selection, gene flow, genetic drift,
mutation and nonrandom mating.

3. Hardy-Weinberg Principle
At the beginning of the 20th century, genes had not been discovered  the allele was understood to be one form of an inherited trait
In 1908, Godfrey Hardy and Wilhelm Weinberg independently showed that evolution will not occur in a population unless allelic frequencies are acted upon forces that cause
change.
According to Hardy-Weinberg principle, when allelic frequencies remain constant, a population is in genetic equilibrium.
Check out Figure 15.4!!

4. Hardy-Weinberg Principle
Example: Consider a population of 100 humans.
40 people  homozygous dominant (EE)
40 people  heterozygous (Ee)
20 people  homozygous recessive (ee)
What are the allele frequencies?
p: the E allele frequency = 0.6
q: the e allele frequency = 0.4
q + p = 1  p2 + 2qp + q2 = 1
p2: equilibrium frequency for homozygous dominant
q2: equilibrium frequency for homozygous recessive
2qp: equilibrium frequency for heterozygous
For our example:
p2: 0.36
q2: 0.16
2qp: 0.48

5. Hardy-Weinberg Principle
Example: In a population of dogs, an allele for short ears (E) is dominant, while the allele for long ears (e) is recessive. For every 100 dogs in the population, 16 have long ears. Use the Hardy-Weinberg equation to calculate the frequencies of the E allele (frequency p) and the e allele (frequency q).

6. Hardy-Weinberg Principle
According to Hardy-Weinberg principle, a population in genetic equilibrium must meet conditions.
There must be no genetic drift, no gene flow, no mutation, mating must be random and there must be no natural selection.
If a population is not in genetic equilibrium, at least one of the five conditions has been violated.
Check out Table 15.3!!
What could cause a population to shrink in size?
What might cause an organism to emigrate from an area?

7. Mechanisms of Evolution
According to Hardy-Weinberg principle, a population is in genetic equilibrium if the following 5 conditions are met:
No genetic drift
No gene flow
No mutation
Random mating
No natural selection
Since genetic equilibrium means no change in allele frequency, it also implies no evolution.
Therefore, we can say that genetic drift, gene flow, mutation, non-random mating and natural selection are the mechanisms of evolution.

8. Mechanisms of Evolution
Genetic Drift:
Any change in the allelic frequencies in a population that is due to chance is called genetic drift.
Remember law of independent assortment  only one of a parent’s two alleles passes to the offspring (randomly)
In a large population, enough alleles ”drift” to ensure that the allelic frequencies of the entire population remain constant.
In small populations, genetic drift becomes more pronounced.
There are two examples of
genetic drift:
Founder effect
Bottleneck

9. Mechanisms of Evolution
Genetic Drift:
The founder effect can occur when a small sample of a population settles in a location separated from the rest of the population.
Alleles that were uncommon in the original population might be common in the new population and the offspring in the new population will carry those alleles.
Example: Amish commmunity in the US
Bottleneck occurs when a population declines to a very low number and then rebounds.
The gene pool of the rebound population often is
genetically similar to that of the population at its
lowest level, that is it has reduced diversity.
Example: Cheetahs in Africa

10. Mechanisms of Evolution
Gene Flow:
A population in genetic equilibrium experiences no gene flow.
It is a closed system  no new genes entering and leaving the population  no immigration or emigration
In reality, few populations are isolated (closed).
Non-random Mating:
Rarely is mating completely random in a population.
Usually, organisms mate with individuals in a close proximity.
Mutation:
Mutation is a random change in genetic material due to mutagens.
The cumulative effect of mutations in a population
might cause a change in allelic frequencies.
Many mutations a lethal, but some provide
advantage to an organism.

11. Mechanisms of Evolution
Natural Selection:
The Hardy-Weinberg principle requires that all individuals in a population be equally adapted to their environment and thus contribute equally to the next generation.
However, natural selection acts to select the individuals that are best adapted for survival and reproduction.
Natural selection acts on an organism’s phenotype and change allelic frequencies.
There are four ways that natural selection alters phenotypes:
Stabilizing selection
Directional selection
Disruptive selection
Sexual selection

12. Mechanisms of Evolution
Natural Selection:
Most common form of natural
selection is stabilizing selection.
It operates to eliminate the extreme expressions of a trait when the average expression leads to a higher fitness.
Example: Human babies with above normal and below normal weights have lower chances of survival than babies born with normal weights  Figure 15.16!!
If an extreme version of a trait makes an organism more fit,
directional selection might occur.
Example: The peppered moth in
industrial England.
Check out Figure 15.17!!

13. Mechanisms of Evolution
Natural Selection:
Disruptive selection is a process that splits the population into two groups.
It tends to remove individuas
with average traits, but retains
individuals with extreme traits.
Example: Northern water snakes
In sexual selection a change in frequency of a trait is based on the ability to attract a mate.
This type of selection often operates in populations where males and females differ significantly in appearance.
Example: Peacock

14. Reproductive Isolation
Remember the 5 mechanisms of evolution.
To what extent each mechanism contributes to the origin of new species is a major topic of debate.
Today, we will consider two types of isolating mechanisms that prevent gene flow among populations.
Prezygotic isolating mechanisms
Postzygotic isolating mechanisms
Prezygotic isolating mechanisms:
These operate before fertilization occurs.
These mechanisms prevent genotypes from entering a population’s gene pool through geographic, ecological, behavioral or other differences.
Example: Eastern and western meadowlarks.
Check out Figure 15.20!!

15. Reproductive Isolation
Postzygotic isolation mechanisms:
These mechanisms operate after fertilization occurs.
When fertilization has occured but a hybrid offspring cannot develop or reproduce, postzygotic isolation has occured.
These mechanisms prevent offspring survival and reproduction.
Example: Lion + Tiger  Liger (sterile)

16. Speciation Speciation is the process whereby some members
of a sexually reproducing population change so much
that they can no longer produce fertile offspring with
members of the original population
There are two types of speciation:
Allopatric Speciation
Sympatric Speciation
Allopatric Speciation:
In allopatric speciation, a physical barrier (such as islands, rivers, lava flows etc) divides one population into two or more populations.
The separate populations eventualy will contain organisms that, if enough time has passed, will no longer be able to breed with one another.
Example: Kaibab and Albert Squirrels

17. Speciation Sympatric Speciation:
In sympatric speciation, a species evolves into a new species without a physical barrier.
The ancestor species and the new species live side by side during the speciation process.
Example: Several insect species  appear
to be diverging based on the type of fruit
they eat
Suppose some members of a tree frog population begin mating a month earlier then the other members of the same population.
What type of isolation will occur?
What type of speciation does reproductive isolation cause?

18. Speciation
Allopatric Specication vs Sympatric Speciation

19. Patterns of Evolution
Speciation is a long process compared to the human life span.
Many details of the speciation process remain unresolved.
However, evidence of speciation is visible in
patterns of evolution.
Adaptive radiation (divergent evolution)
Coevolution
Convergent evolution
Adaptive Radiation: More than 300 species of cichlid fish once lived in Africa’s Lake Victoria.
Data shows that these species diverged from a single ancestor within the last 14,000 years.
Adaptive radiation (divergent evolution) can occur in a relatively short time when one species gives rise to many species in response to the creation of new habitat.
Adaptive radiation often follows large-scale extinctions.

20. Patterns of Evolution
Coevolution: Many species evolve in close relationship with other species.
This relationship might be so close that the evolution of one species affects the evolution of other species  coevolution
Mutualism is one form of coevolution.
Example: Comet orchids and moths
In another form of coevolution, one
species can evolve a parasitic dependency
on another species  coevolutionary arms race
Example: Plants and insect pathogen

21. Patterns of Evolution
Convergent Evolution: Sometimes unrelated species evolve similar traits even though they live in different parts of the world
Convergent evolution occurs in environments that are geographically far apart but that have similar ecology and climate.
Example: Mara and rabbit
Check out Table 15.4!!

22. Patterns of Evolution Rate of Speciation:
Evolution is a dynamic process.
In some cases traits might
change rapidly.
In other cases, traits might
remain unchanged for millions
of years.
Most scientists think that evolution proceeds in small, gradual steps  gradualism
However, the fossil record contains instances of abrupt transitions  Example: certain species of fossil snails
The theory of punctuated equilibrium attempts of explain such abrupt transitions in the fossil record.
Check out Figure 15.25!!