Chapter 26 - Evolutionary Genetics

Question 1

What is evolution?

Answer 1

The genetic change in a group of organisms/population is biological evolution

Genetic variation arises
- Origins in mutation and recombination – both are random and continually produce variation

Change in frequencies of genetic variants
- Changes in allele frequencies arise via mutation, migration, drift, and natural selection (allow evolution to occur)

Question 2

What is anagenesis?

Answer 2

Evolution taking place within a single lineage (a group of organisms connected by ancestry) with the passage of time

Question 3

What is cladogenesis?

Answer 3

The splitting of one lineage into two

When it splits, the two branches no longer have a common gene pool and they evolve independently of each other

Question 4

What is molecular variation?

Answer 4

Measures DNA, RNA, or proteins – the basis of evolution

Very versatile and can be quantifiably applied to all organisms

Applied to huge amounts of variation

All organisms can be compared using molecular data

Amount of data available to us is large and continues to expand

Question 5

What is DNA sequence variation?

Answer 5

Analyzes genetic variation

Many different techniques to visualize this

Question 6

What is population variation?

Answer 6

Measures of genetic variation between individuals

Explanation for high molecular variation through the neutral-mutation hypothesis (neutral theory)

Question 7

What is the neutral-mutation hypothesis (neutral theory)?

Answer 7

Proposes that a lot of the molecular variation that we see is adaptively neutral (individuals with different molecular variance have equal fitness)

Because variants are functionally equivalent, natural selection doesn’t differentiate between them, so evolution is shaped by random processes (drift and mutation)

Doesn’t preclude the importance of natural selection – when selection occurs it will be directionally favored (selects “best” while eliminating others) – natural selection decreasing genetic variation

Question 8

What balancing selection?

Answer 8

Balancing selection is selection that maintains genetic variation

How genetic variation can be maintained

Variance is not functionally equivalent and causes phenotypic changes that affect reproduction

Question 9

What is speciation?

Answer 9

Occurs through cladogenesis

One population separates into two distinct evolutionary groups

Question 10

How is a species defined?

Answer 10

A particular type of organism to which a unique name has been given (genus and species name)

An evolutionary independent group of organisms

Question 11

What is a biological species concept?

Answer 11

Defines a species as a group of organisms whose members are capable of interbreeding with each other but are reproductively isolated from members of other species

Members of the same species have the biological potential to exchange genes

Because those species exchange genes between each other but not between other species, different species evolve independently of each other

Question 12

What is reproductive isolation and its two types?

Answer 12

The existence of biological characteristics that prevent genes from being exchanged between different species

Two types: prezygotic and postzygotic

Question 13

What is prezygotic isolation and its 5 mechanisms?

Answer 13

Isolation before fertilization - prevents gametes from 2 different species from forming a zygote

Ecological isolation – members of 2 different species don’t encounter each other because they inhabit different ecological niches or interact with their environment in different ways

Behavioral isolation – differences in behavior that prohibit breeding (ex. calling songs)

Temporal isolation – reproduction in a species takes place at different times of the year or in different seasons

Mechanical isolation – anatomical differences that prevent successful exchange of gametes

Gametic isolation – mating between individuals of different species can take place, but the gametes don’t fuse to form a zygote

Question 14

What is postzygotic isolation and its 3 mechanisms?

Answer 14

Gametes from 2 different species can fuse and form a zygote but there is not gene flow between the species because the resulting hybrids are inviable or sterile or because reproduction breaks down in subsequent generations
- Often occurs when prezygotic mechanisms have not formed or fail

Hybrid inviability – incompatibility of genomes between 2 species prevent hybrid zygotes from developing

Hybrid sterility – hybrid embryos do form, but hybrid is sterile and gene flow doesn’t occur

Hybrid breakdown – 2 closely related species are capable of mating and produce viable F1 progeny, but further crossing of hybrid fertile F1 progeny results in hybrids that will eventually be produced that are sterile or inviable

Question 15

What are the 2 modes of speciation?

Answer 15

Allopatric speciation

Sympatric speciation

Question 16

What is allopatric speciation?

Answer 16

Occurs when some geographic barrier splits a population into two groups and blocks the exchange of genes between individuals that were in population

Interruption of gene flow between now isolated groups leads to the accumulation and evolution of genetic differences that further results in reproductive isolation that leads to speciation

If barrier disappears and individuals can disperse through it, populations will come back in contact (secondary contact)

Question 17

What can occur is allopatric speciation when 2 populations are in secondary contact?

Answer 17

If limited genetic differentiation has occurred, reproductive isolation mechanisms may not be fully evolved and gene flow will occur again between those populations, eventually eliminating genetic differences that had accumulated before, and populations will go back to being a single species

If strong genetic differentiation occurred, strong reproductive isolating mechanisms will be in place and populations will remain 2 separate species

Some genetic differentiation occurred that leads to only postzygotic barriers – allows for reproduction, but no gene flow because of mechanisms
- Because selection favors mating with like individuals, eventually prezygotic barriers will arise between populations even though they are in secondary contact – called reinforcement – further isolates species

Question 18

What is sympatric speciation?

Answer 18

Occurs in the absence of an external barrier to gene flow

Typically occurs when reproductive isolating mechanisms evolve within a single population

Can also occur through polyploidy
- Allopolyploidy – 2 diploid species hybridize to form a tetraploid through nondisjunction (not full separation of chromosomes)
- Because tetraploid contains 2 copies of each chromosome, it’s usually fertile, but it usually causes tetraploid to be reproductively isolated from parents

Question 19

What is a phylogeny?

Answer 19

Representation of the evolutionary relationships among organisms

Question 20

What are the 3 parts of a phylogeny?

Answer 20

Branches – evolutionary connections between organisms
- Sometimes length of branches represents different evolutionary amounts of divergence (time, number of mutations)

Nodes – point at which two branches diverge from each other
- Represents common ancestor that existed before divergence took place

Root – a node that is a common ancestor to all of the common ancestors that are being represented in the phylogeny
- Done by including organisms that are distantly related (out group) to the in group

Question 21

What is a gene tree?

Answer 21

A phylogenetic tree that depicts the evolutionary relationship between genes

Question 22

What is a homologous characteristic?

Answer 22

Characteristic that has evolved from the same character in a common ancestor

Question 23

How can phylogenies be built from alignment of homologous DNA sequences?

Answer 23

Distance based approach – overall similarity between each organisms

Maximum parsimony – Fewest number of evolutionary changes since last common ancestor

Maximum likelihood and/or Bayesian methods – Infer phylogenies based on which phylogeny maximizes the probability of obtaining the set of characteristics exhibited by the organisms

Question 24

How are the rates of molecular evolution determined?

Answer 24

Different genes or parts of genes can evolve at different rates

Usually measured by rate of nucleotide substitution – number of mutations occurring at a nucleotide site per year per organism

Rates of nonsynonymous and synonymous substitution
- Rate at which synonymous mutations occur is higher than nonsynonymous because synonymous mutations are not going to directly affect the fitness of the organism (selectively neutral mutation)

Question 25

What are the substitution rates for parts of genes?

Answer 25

Highest rates of substitution are in sequences that have the least effect on protein function

3rd nucleotide of codon region, flanking regions, introns

Can affect gene expression and rate of evolution

Question 26

What are the 4 ways genome evolution can occur?

Answer 26

Exon shuffling (exons of different genes are exchanged) creates new genes that are mosaics of other genes

Gene duplication can also give rise to new genes or new gene functions (multigene families – sets of genes that are similar in sequence but encode different products)

Whole genome duplication – creating 2 copies of every gene
- Genes can acquire new functions or be lost

Horizontal gene transfers – DNA is transferred between individuals of different species in the same generation