Darwin & Microevolution

Darwin & Microevolution

Chapter 19-20

Charles Darwin (1809-1882)

Former divinity and medical student Secured an unpaid position as ship's

naturalist on the H.M.S. Beagle Voyage provided Darwin a unique

opportunity to study plants, animals, and their environment

Gathered a great deal of evidence he would later incorporate into a theory of evolution

EQUATOR

GalapagosIslands

Voyage of the Beagle5 yr. mission to chart South America

Darwin’s Theory of Natural Selection Individuals in a population have variable levels

of success in reproducing Left unchecked, populations tend to expand

exponentially, leading to a scarcity of resources In the struggle for existence, some individuals

are more successful (fit) than others, allowing them to survive and reproduce

Those organisms best able to survive and reproduce will leave more offspring

Darwin’s Theory of Natural Selection Over time there will be heritable changes in

phenotype (genotype) of a species These changes may result in a transformation of

the original species into a new species similar to, but distinct from, its parent species

Common Descent, due to these changes similar species have common ancestors. This means that nearly all of life is linked

What is Evolution?

Evolution is a process that results in heritable changes in a population spread over many generations

Evolution can be precisely defined as “any change in the frequency of alleles within a gene pool from one generation to the next”

Populations, not individuals evolve Traits within a population vary among individuals

Variation Within a population most phenotypic traits are

polymorphic, they have two or more forms Those traits that have many forms show

continuous variation Individual inherit different combinations of alleles

leading to different phenotypes All these genes & their alleles within a

population is known as the gene pool. This variation is the raw material for

evolution This variation is also what allows for natural

selection

What Determines Alleles in an Individual? Mutations, many are lethal, but some can be

neutral & some may confer an advantage Crossing over at meiosis I, shuffles alleles Independent assortment, genes that may work

together, but are on different chromosomes will not be inherited together

Fertilization, sexual reproduction Change in chromosome number or structure,

often deleterious, but can be advantageous

Genetic Equilibrium Point when a population is not evolving The opposite of evolution Allelic frequencies are not shifting Calculated using the Hardy-Weinberg equations

p + q = 1p2 + 2pq + q2 = 1

Where p = frequency of Dominant allele (A) and q = frequency of recessive allele (a)

Genetic Equilibrium

Five conditions need to be met: No mutations Random mating Gene does not affect survival or

reproduction Large population No migration

Hardy-Weinberg Equilibrium

AA(p2) Aa(pq)

Aa(pq) aa(q2)

Ap aq

Ap

aq

p2 = frequency of AA (Homozygous dominant) 2pq = frequency of Aa (heterozygous) q2 = Frequency of aa (homozygous recessive)

490 AA butterfliesDark-blue wings

420 Aa butterfliesMedium-blue wings

90 aa butterfliesWhite wings

Hardy Weinberg Equilibrium: Example

Starting population:

Frequencies in Gametes:

A A A a a a

0.49 AA 0.42 Aa 0.09 aa

0.49 + 0.21 0.21 + 0.09

0.7A 0.3a

F1 genotypes:

Gametes:


490 AA butterflies

THE NEXT GENERATION

420 Aa butterflies

90 aa butterflies

THE NEXT GENERATION490 AA butterflies

420 Aa butterflies

90 aa butterflies

NO CHANGE

NO CHANGE

STARTING POPULATION490 AA butterfliesDark-blue wings

420 Aa butterfliesMedium-blue wings

90 aa butterfliesWhite wings


Mechanisms of Evolution

Evolution of a population over time may occur as a result ofNew mutationsNatural selectionNonrandom mating (Sexual selection)Genetic drift because of small

populationGene flow – immigration and emigration

(Opposite of Genetic Equilibrium)

Mutations Mutations that alter protein structure enough

to impact its functionmore likely to be harmful but may be beneficialour genome is product of thousands of

generations of selection Fuel for evolution Mutant allele may enable an organism to fit

its environment better & increase reproductive success

especially likely if environment is changing

Natural Selection Difference in the survival & reproductive success

of different genotypes and/or phenotypes Over time, the alleles that produce the most

successful phenotypes will increase in the population

Less successful alleles will become less common

Change leads to increased fitness Selection is not a “force” it is merely the favoring

of some genetic changes over others

Types of Natural Selection

Directional Selection Stabilizing Selection Disruptive Selection

Num

ber o

f ind

ivid

uals

in th

e po

pula

tion

Range of values for the trait at time 1



Num

ber o

f ind

ivid

uals

in th

e po

pula

tion

Num

ber o

f ind

ivid

uals

in th

e po

pula

tion

Directional Selection Shift in the variation in a

consistent direction within the phenotypic range

Examples: Pesticide resistance in

insects Antibiotic resistance in

bacteria

Num

ber o

f ind

ivid

uals

in th

e po

pula

tion




Stabilizing selection Loss of extreme

forms with stabilization of an intermediate form

Example: Infants greater or less than 7.5 lbs have increased mortality

Num

ber o

f ind

ivid

uals

in th

e po

pula

tion




Num

ber o

f ind

ivid

uals

in th

e po

pula

tion

Num

ber o

f ind

ivid

uals

in th

e po

pula

tion

Disruptive Selection favors individuals at

the extremes with a reduction of intermediate forms

Example: African Finches selection favors small or large beak size

Sexual Selection A type of natural selection Selection that is driven by the competition

for mates

Gene Flow

Movement of alleles into a population Tends to keep the gene pools of

populations similar Counters changes due to mutation, natural

selection, and genetic drift

Genetic Drift

Random change in allele frequencies brought about by chance

Effect is most pronounced in small populations

This can cause similar, but isolated populations to become dissimilar due to the loss or fixation of alleles

Bottleneck Genetic drift is most pronounced when small

populations grow into larger ones, usually after a catastrophe

A bottleneck as only a few alleles survive and are now disproportionally expressed in the population

A founder effect results in rare or even disadvantageous alleles being found in a population at a level higher than normally expected

Macroevolution:Evidence for evolution Biogeography Fossil Record Comparative anatomy Comparative embryology Molecular Biology

Evolution evidence: Biogeography

Geographical distribution of species Indicates that populations that are isolated from

one another geographically evolve separately

Evolution evidence: The Fossil Record

Fossils are created when organisms become buried in sediment, bone and other hard tissue is converted to rock

The fossils contained in sedimentary rock layers reveal a history of life on earth

Evolution evidence: Comparative Anatomy Comparing body forms and structures of

major lineages By studying homology or how similarly

derived body parts have evolved, we can put together an evolutionary tree and find common ancestors even if the body parts no longer serve the same function

Evolution evidence: Comparative anatomy Homologous

structures Similar anatomy,

different functions Indicate divergent

evolution

Evolution evidence: Comparative anatomy Analogous structures

Similar function, different anatomy

Indicated Convergent Evolution

Example: bird’s wing vs. fly’s wing

Evolution evidence: Comparative anatomy Vestigial Organs

Remnants of evolution Organs that were

required in ancestor that are not needed in present-day organism

Examples: appendix, tailbone

Evolution evidence: Comparative Embryology Development of early

embryos of resemble each other due to a common developmental plan

During later embryonic development this plan becomes modified to create the different body types we see

Evolution evidence: Molecular Biology Tracking mutations in

sequences of DNA and/or proteins can trace the evolutionary history of organisms

The more nucleotides/ amino acids in common, the more closely related the species

Darwin & Microevolution

Documents

Transcript of Darwin & Microevolution