Ch4: Speciation & Hybridisation

Question 1

Define speciation

Answer 1

evolutionary process in which new species arise through reproductive isolation

Question 2

Define allopatric speciation, what does it prevent?

Answer 2

An ancestral population is divided by a PHYSICAL barrier.

It prevents gene flow between populations.

Question 3

Define founder effect, what kind of speciation does this align with?

Answer 3

When a small number of individuals colonises a new region.

It aligns with allopatric speciation, because the new region is separated (by a physical barrier) from its ancestral population.

Question 4

Define sympatric speciation, what can it be driven by?

Answer 4

When an ancestral population is divided without geographic or physical barriers.

It can be driven by: behaviour differences, different pollinators, different flowering seasons/ mating seasons

Question 5

Define what a fixed allele is:

Answer 5

an allele that is the only variant for that gene (all species are homozygous for that allele)

Question 6

What is chromosomal rearrangement and what does it promote?

Answer 6

a type of chromosome rearrangement, in which two long arms of acrocentric chromosomes fuse to form one metacentric

It promotes speciation:
- Here we see a ancestral species with 3 chromosomes (diploids - 2 copies of each chromosome)
Two independent centric fusions of 1 armed chromosomes occur in 2 sister lineages
Neither centric fusion by itself results in difficulties at meiosis whether the fusion is found in just found in one or both pairs of chromosomes

After fixation of the different fusions in each lineage however, F1 hybrids between the 2 lineages are sterile because the 3 different chromosomes involved in these centric fusions cannot pair normally at meiosis in hybrids.

Question 7

Define prezygotic barriers, give examples

Answer 7

Barriers to reproduction that occur BEFORE the union of nuclei of 2 organisms (no fertilisation)

Ex: geographical isolation, behavioural isolation (courtship, different wing frequencies), mechanical (pollinator), ecological differences (location in same environment~ depth of lake) and mating TIME difference (temporal isolation)

Question 8

Define postzygotic isolation, and give examples. What does it prevent?

Answer 8

Barriers to reproduction that occur AFTER the union of the nuclei of 2 gametes (sperm + egg).

It prevents hybrid zygotes from successfully developing and reproducing themselves.

Ex~ fertilised egg/offspring are inviable

Question 9

Define what a species is

Answer 9

A group of organisms that INTERBREED and produce fertile offspring; hence they share similar characteristics that stem from similarities in DNA

Question 10

Define hybridisation

Answer 10

Interbreeding of individuals from genetically DISTINCT populations/closely related species that produce hybrids (VIABLE offspring) that demonstrate the traits of both parents

Question 11

Are hybrids fertile? What is this an example of?

Answer 11

No they are sterile, and hence, an example of post zygotic reproductive isolation.

Question 12

Define the biological species concept (Hint: it has to do with the definition of what a species is)

Answer 12

If 2 species breed and produce and they produce;
1) fertile offspring = same species
2) Sterile offspring = different species

NOTE: SOME hybrids can be fertile

Question 13

Define adaptive introgression, and pre-adapted traits.

Answer 13

Introgression: When hybrids produced are FERTILE, it can result in the transferring of pre-adapted traits from 1 species to another

Pre-adapted traits: beneficial variation from related species that accelerate adaptation to and survival to new environments.

May assist with adaptation to extreme environments

Question 14

Define hybrid zones:

Answer 14

An area where 2 closely related species cohabit and interbreed

Question 15

How do hybrid zones impact species (hint there are 3 ways), and what affects how they impact them?

Answer 15

RFS: depends on fitness of hybrids

1) Reinforcement: strengthens reproductive barriers, decreasing incidence of hybrids
Ex~ geophysically separated pied and collared flycatchers, males have similar plumage, distinct plumage colours which attract females of OWN species, as well as hybrid females being sterile.
The combination of this decreases the FITNESS of hybrid flycatchers -> reinforces divergence

2) Fusion: weakening of reproductive barriers, until 2 breeding species becomes 1.
Ex~ lake Victoria cichlid females wanted males of a particular colour, but the polluted water reduced their ability to differentiate. So hybrids were not strongly affected and there was a weakening o f the barrier that yielded a single species

3) Stability: hybrids continue to survive and reproduce
Ex~ Barriers a not weak enough for fusion, but not strong enough to prevent hybrids

Question 16

Define Balancing Selection, and name the 2 mechanisms:

Answer 16

An active maintenance of multiple alleles within a population –> allowing for genetic variation.
1) Negative frequency dependent selection
2) Heterozygote advantage

Question 17

Explain Negative Frequency Dependent Selection:

Answer 17

It occurs when a RARE phenotype has higher fitness than common phenotypes, so they do not become extinct (even though they’re rare) because they’re strongly selected for:
- Meaning a phenotype WON’T become dominant nor fixed

Forms a cyclic patterns = fails to reach stability of evolution:
- when they increase in frequency they lose their advantage

Question 18

Explain heterozygote advantage (balancing selection mechanism)

Answer 18

Occurs when heterozygous individuals have a fitness advantage over homozygous individuals:

Ex~ those with sickle cell disease (heterozygotes) have a selected for advantage over homozygous for and without sickle cell disease in their resistance to malaria.

Question 19

Define fitness

Answer 19

the success of an organism at surviving and reproducing

Question 20

Define relative fitness:

Answer 20

describes the success of a genotype in a standardised form, comparing fitness to other genotypes, ranging from: 0-1.0

Ex~ in presence of malaria heterozygotes have an advantage for those without malaria, but in absence of malaria, they no long have an advantage over homozygotes.
- in each case, both parties have a relative fitness fo 1
NOTE: relative fitness of sickle cell homozygotes remains very low in absence and presence of malaria.