how do populations evolve? -...

How do populations evolve?....

Are there any trends?....

Gene pool: all of the genes of a population Allele frequency: the percentage of any particular allele in a gene pool A population in which an allele frequency remains the same over many generations is stable and is known as “genetic equilibrium” and therefore is NOT evolving

Gene pool: all of the genes of a population

Allele frequency: the percentage of any

particular allele in a gene pool

Any factor affecting the genes in a gene

pool will result in “microevolution” • Evolution within a population/species

FOUR factors affect evolutionary change…

1. Natural Selection

2. Mutation

3. Gene Flow

4. Genetic Drift

According to the survival of the fittest,

the best adapted phenotype is

“selected,” favouring that particular set

of alleles

Selective Pressures • Disease

• Climate conditions

• Food availability

• Predators

• Choice of mate

A. Stabilizing Selection: (most common) • Limits evolutionary change by

favouring the current population

norm

• Examples:

-human birth weights

-ideal bill length in

hummingbirds

B. Directional Selection: • Produce evolutionary change by favouring

individuals that differ from the population norm

(in one direction)

• Common in artificial breeding

• Results from a sudden change

in environment

• Example:

-Salmon in B.C.

(net fishing)

C. Disruptive Selection: • Works the same as directional selection, but

sometimes favours more than one trait (two

directions)

• Example:

-Finches in Africa

food = soft and hard seeds

D. Sexual Selection: • Selection favours individuals with variation of a

trait that aids in the success of mating

• Leads to “sexual dimorphism”

which is difference in physical

appearance between males and

females

• Example:

-Peacock tails

1. A parrot population has only very large

and very small tail feathers

2. A population of ducks lays eggs of

intermediate mass

3. Most individuals in a population of

hummingbirds have long beaks

4. A population includes only medium-

sized spiders

5. The Peppered Moth

6. Most birds have extremely light weight

and hollow bones

7. Trees in windy areas tend NOT to grow

any bigger each year

8. The brain size of hominids steadily

9. The same species of butterflies tends to

have blue stripes in open areas and

orange stripes in forested areas

10. The average size of salmon due to

over-fishing in British Columbia

1. A parrot population has only very large and very small tail feathers

• disruptive

2. A population of ducks lays eggs of intermediate mass

• stabilizing

3. Most individuals in a population of hummingbirds have long beaks

• directional

4. A population includes only medium-sized spiders

• stabilizing

5. The Peppered Moth • directional

6. Most birds have extremely light weight

and hollow bones • directional

7. Trees in windy areas tend NOT to grow

any bigger each year • stabilizing

8. The brain size of hominids steadily • directional

9. The same species of butterflies tends to

have blue stripes in open areas and

orange stripes in forested areas • disruptive

10. The average size of salmon due to

over-fishing in British Columbia • directional

Occurs when individuals select mates based on their phenotypes

Inbreeding: mating between relatives of a species

Artificial selection: changes to a population caused by deliberate, selective breeding by humans

Leads to… • Decreases () diversity

• the frequency on inheriting recessive abnormalities

• vulnerability to disease and environmental change

• Long term outcomes of artificial selection hard to predict

Mutations: a change in the DNA • A mutation provides new alleles and therefore

produces new variation

• It is necessary for all other mechanisms of evolution

a) Harmful mutations

MOST common

nature selects against them

rare in the gene pool

b) Beneficial mutations

Occur rarely

Nature selects for

Accumulate in gene pool

Mutations: a change in the DNA • A mutation provides new alleles and there produces

new variation

• It I necessary for all other mechanisms of evolution

a) Neutral mutations

Nature selects neither for or against

Gene flow: transfer of alleles from one

population to another via migration of

individuals to and from existing populations

Genetic drift: changes of allele frequencies

due to chance events which change

population size; small populations are

impacted more a) Bottleneck Effect

Some chance even cause extreme in populations size

(disease; natural disasters; human interference etc.)

Result in loss of diversity

Surviving genotypes are dictated

by chance

Example: Northern elephant seals

(overhunting in 1890s)

Genetic drift: changes of allele frequencies

due to chance events which change

population size; small populations

are impacted more b) Founder Effect

Several individuals establish a new colony

The new population is small with limited diversity

The founder genotypes are dictated by chance

Example: a few organisms migrate to a NEW location

The Hardy-Weinberg principle can be used

to identify factors causing allele frequencies

to change, leading to evolutionary change…

1. Natural Selection (favours some allele)

2. Small population size

3. Mutation (introduces new alleles)

4. Migration (adds/removes alleles)

Mathematically, a gene pool can be

described by the frequency of each of the

alleles within the population

This relationship between allele frequencies

and the chance that they remain constant can

be represented by an equation:

A2 + 2Aa + a2 = 1

A=dominant a = recessive

A population of sunflower plants has only 2

alleles for the “height” gene.

The allele frequency T is 20%.

Determine the genotype frequencies (TT),

(Tt), (tt) in this population.

In a sample of population of 500 peppered

moths, determine the allele frequencies for

the sampled counts listed below…

Genotype BB Bb bb

# of moths (total = 500) 20 160 320

Genotype frequency

# of alleles in gene pool (total

= 1000)

Allele frequencies

1) A large population consists of

400 individuals, of which 289 are

homozygous Dominant, 102 are

heterozygous, and 9 are

Homozygous Recessive.

Determine the allele frequencies

of M and m?

2)The gene pool of a large

population of fruit flies contains

only two eye-colour alleles; the

dominant red eyes (W) and the

recessive white alleles (w). Only

1% of the population has white

eyes. Determine the allele and

genotype frequencies of this

population.

In a certain population of 1000 fruit flies,

640 have red eyes while the remainder

have sepia eyes. The sepia eye trait is

recessive to red eyes. How many

individuals would you expect to be

homozygous for red eye color?

R2 + 2Rr + r2 = 1

1) r2 for this population is 360/1000 = 0.36

2) r = √0.36 = 0.6

3) If r = 0.6, then R = 0.4

4) The homozygous dominant frequency =

R2 = (0.4)(0.4) = 0.16.

Therefore, you can expect 16% of 1000, or

160 individuals, to be homozygous

dominant.

Speciation: the formation of a new

species • but how do brand new species evolve in the first

place? (“The Blind Watchmaker)

• Recall the definitions of a “species”

• Requires the evolution of distinct, complex

features

CHANCE + CHOICE =

(random change) (natural selection)

Development of

new complex

features

1. Reproductive isolating mechanism:

any biological factor preventing 2

species from exchanging genes • Prezygotic mechanisms - prevents hybrid

offspring formation

o Habitat isolation

o Temporal isolation

o Behavioural isolation

o Mechanical isolation

o Gamete isolation

1. Reproductive isolating mechanism:

any biological factor preventing 2

species from exchanging genes

• Postzygotic mechanisms - prevents hybrid

offspring surviving and reproducing

o Hybrid inviability

o Hybrid sterility

o Hybrid breakdown

Whenever reproductive isolation

develops, separate species have formed

and speciation has occurred

2. Allopatric speciation: populations

evolve into separate species as a result

of geographic isolation (occurs

gradually)

3. Sympatric speciation: populations

evolve into separate species while in

the same geographic area (gradual or

sudden) Example: Hawthorn flies +apples

Predictable outcomes of natural

selection lead to recognizable patterns

A. Divergent evolution: • Populations that were once similar evolve into

many different species

• Occurs as a result of adapting to different

environmental conditions



Adaptive evolution: • A type of divergent evolution in which a single

species evolves into many new species; each

filling an empty ecological niche

• Occurs because new resources become

available or competition



B. Convergent evolution: • Unrelated species evolve similar traits

• Occurs because they occupy similar niches in

different geological locations



C. Coevolution: • One species evolves in response to the

evolution of another species

• Occurs because they are dependent on one

another for survival

Example: Flowers and pollinators, parasites and host

how do populations evolve? -...

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