Sequence of Speciation

Sequence

of

Speciation

Open Ended

As a scientist, what do you believe is a simple way to determine if two animals are the same species?

Multiple Choice

Are marine and land iguanas the same species?

• YES

• NO

• Speciation: the formation of new and distinct

species in the course of evolution

• Seven steps to become a new species:

1. Reproductive Potential

5. Divergence

2. Variation

6. Isolation

3. Competition

7. New species

4. Natural Selection

Step 1:Reproductive Potential

1. Reproductive Potential

• Reproductive potential: the relative capacity of a

species to reproduce itself under optimum
conditions

• All organisms have the potential to reproduce more

than the environment can support.

• Genetics are biological factors that differentiate

individuals' reproductive potential.

• Environmental factors have the same importance

(e.g., diseases, resources, mates, competitors).

• Reproductive potential charts show how fast

populations can increase.

Start with
2 rats (1
male and
1 female)

Rat
population
after 12
months

Step 2: Variation

2. Variation

• Individuals do not evolve, populations do.
• Population: a group of individuals occupying a

given area and belonging to the same species.

• All individuals in a population have a certain

number of traits in common.

Open Ended

These Austrailian Shepard puppies are from the same litter. Identify 3 variations.

• But all individuals in a population have a certain

number of traits that are different.

• There are three kinds of variable traits:

1. Morphological (morph = form) – External traits
2. Physiological – How their body functions

internally

3. Behavioral – How they

act/react

• There are several possible sources of variation:

1. Genetic mutations (the only one

that creates new species)

2. Abnormal changes in chromosome

structure or number (trisomy, monosomy)

3. Crossing over and genetic recombination during

meiosis.

4. Independent assortment of chromosomes during

meiosis.

5. Fertilization between genetically different gametes.

• Tangerine x Grapefruit =

Tangelo

• Horse x Donkey = Mule

(#2-5 are merely a reshuffling of existing genes.)

• It is unlikely that another person (other than

identical twins) with your exact genetic makeup
has ever or will ever exist. There are more than
10600 combinations of genes possible in human
gametes. There are not
even 1010 humans alive
today.

Step 3: Competition

3. Competition

• In every population there is competition between

individuals for different resources (food, shelter).

• Those who compete and win survive and

reproduce.

• Those who compete and lose die - and do not

reproduce. Their genes are not
passed on to the next generation.

• Types of competition:

1.

Intraspecific competition: Individuals of the
same population must compete for resources.

2.

Interspecific competition: Two species
compete for the same (limited) resource, they
will impact each other’s population.

• Competitive exclusion principle: Two similar

species competing for the same limited resource
cannot coexist together.

Step 4: Natural Selection

4. Natural Selection

• The environment limits the

growth of populations by:
– Increasing the rate of death
– Decreasing the rate of reproduction

• Organisms that have a greater number of

favorable traits tend to leave more offspring.

• Darwin called the different degrees of

successful reproduction among organisms in a
population natural selection.

• A population of organisms adapt to their

environment as their
proportion of genes for
favorable traits increases.

• The resulting change in the genetic make-up

of a population is evolution.

• Natural selection is a passive process.

Organisms do not simply decide
to acquire certain traits.

• The environment “selects” the traits that will

increase in a population.

• Selection conditions change as the demands

of the environment change.

1. Disruptive selection: Individuals

with both extreme forms are
favored

2. Stabilizing selection: Individuals

with the average form are favored

Types of Natural Selection

3. Directional selection: Individuals

with an extreme form are favored

Step 5: Divergence

5. Divergence

• Allele: One of a pair

of genes on
chromosomes

• An allele is dominant

(capital) or recessive
(lower-case).

• Genotype: A pair of

alleles in an
individual.

• One allele comes

from each parent.

• Homozygous:

Both alleles of a

chromosome pair

are same

• Heterozygous:

Alleles of a

chromosome pair

are different

• Allele frequency: the percentage of “A” and

“a” alleles in the total population

• Genotype frequency: the percentage of “AA”,

“Aa”, and “aa” individuals in the total

population

Practice problem #1

In this population of

nine pea plants, how

many times do you

see “WW”?

Answer #1

Six. Therefore, the

genotype frequency

of WW is 6 out of 9,

or 67%.

Practice problem #2

In this population of

nine pea plants, how

many times do you

see “Ww”?

Answer #2

One. Therefore, the

genotype frequency

of Ww is 1 out of 9,

or 11%.

Practice problem #3

In this population of

nine pea plants, how

many times do you

see “ww”?

Answer #3

Two. Therefore, the

genotype frequency

of ww is 2 out of 9,

or 22%.

Practice problem #4

In this population of

18 alleles, how many

times do you see

“W”?

Answer #4

13. Therefore, the

allele frequency of W

is 13 out of 18, or

72%.

Practice problem #4

In this population of

18 alleles, how many

times do you see

“w”?

Answer #5

5. Therefore, the

allele frequency of W

is 5 out of 18, or

28%.

Hardy-Weinberg Principle of

Genetic Equilibrium

• Ideal hypothetical population

• Analogy: The control group of an experiment

Hardy-Weinberg Principle of

Genetic Equilibrium

• Two equations must be true for a population

to be in “Hardy-Weinberg equilibrium”:

To calculate ALLELE frequency:

p + q = 1

p = frequency of dominant allele (A)
q = frequency of recessive allele (a)
To calculate GENOTYPE frequency:

p2 + 2pq +q2 = 1

p2 = Genotype frequency of homozygous dominant (AA)

2pq = Genotype frequency of heterozygous (Aa)

q2 = Genotype frequency of homozygous recessive (aa)

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?

Practice Problem

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?
q2 = Freq of aa = 1/10,000 = .0001

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?
q2 = Freq of aa = 1/10,000 = .0001
q = Square root of .0001 = .01

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?
q2 = Freq of aa = 1/10,000 = .0001
q = Square root of .0001 = .01
If q = .01 and p + q = 1…

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?
q2 = Freq of aa = 1/10,000 = .0001
q = Square root of .0001 = .01
If q = .01 and p + q = 1…then p = .99

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?
q2 = Freq of aa = 1/10,000 = .0001
q = Square root of .0001 = .01
If q = .01 and p + q = 1…then p = .99
Carrier (Aa) = 2pq

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?
q2 = Freq of aa = 1/10,000 = .0001
q = Square root of .0001 = .01
If q = .01 and p + q = 1…then p = .99
Carrier (Aa) = 2pq = 2(.01)(.99) = .0198

If one out of every 10,000 people are born with
a recessive metabolic disorder, phenylketonuria
(PKU), then what percent of the population is a
carrier?

Carrier (Aa) = 2pq = 2(.01)(.99) = .0198
1.98% of the population is a carrier.

Step 6: Isolation

• Isolation - when two parts of a formerly

interbreeding population stop interbreeding

• Two types of isolation:

1. Geographic isolation: physical

separation of members of a
population (e.g., canyon,
river, mountains)

2. Reproductive isolation:

the result of barriers to
successful reproduction
between two groups in
the same population.
This often follows
disruptive selection.

Two types of reproductive isolation:

1. Prezygotic isolation: isolation that occurs

before fertilization, e.g. incompatible behavior
(the wrong mating call), different mating times

2. Postzygotic isolation: isolation that occurs

after fertilization, e.g. the offspring do not
develop fully and die, sterile offspring

Step 7: New Species

The Concept of Species:

What makes one species different from another?

1. Morphological concept of species

– Uses the external structure and appearance

of an organism to classify it as a species

– Positive → Easy to use
– Negative → Does not allow for phenotypic

differences among individuals in a single
population

2. Biological concept of species

– A species is a population of organisms that can

successfully interbreed but cannot interbreed
with other groups

– Positive → A useful definition for living

organisms

– Negative → Useless for extinct organisms;

useless for organisms that do not reproduce
sexually

• As a result of this, we often combine the two

definitions for a species today (morphological
and biological).

• Thus a species is...a population with

morphological similarities that interbreed
among themselves successfully.

1. Gradualism – Speciation requires millions of

years and is caused by evolutionary changes that
occur gradually. This is the Darwinian concept.

2. Punctuated equilibrium – Speciation that occurs

suddenly by rapid shifts in the form of organisms
followed by periods of no change. Most
scientists today agree with this concept.

Rate of speciation: How fast is this process?

Two perspectives:

1. Divergent evolution:

Two or more related
populations or species
become more
different.

Patterns of evolution: How do species
interact throughout evolutionary time?

Three ways:

2. Convergent evolution:

Two organisms that
appear to be very
similar but are not
closely related at all,
e.g., Sharks and
porpoises

3. Coevolution: The

change in two or
more species in close
association with each
other, e.g., Predators
and their prey often
coevolve