Topic 1 - Speciation

Question 1

Define parapatric speciation

Answer 1

Speciation that occurs in a continuous population (i.e. not separated by an extrinsic barrier to gene flow) but divergence happens because of reduced gene flow between individuals in one part of the range and others in more distant parts of the population’s range.

Question 2

What is a potential difficulty with sympatric speciation and to a lesser extent, parapatric speciation?

Answer 2

In both modes, there is no physical barrier to prevent mating between individuals and the exchange of alleles between them. Speciation must therefore occur in the face of gene flow which opposes any divergence that occurs. Both modes therefore require something other than a physical barrier to stop gene flow before populations can diverge sufficiently, so that reproductive isolation and speciation occurs.

Question 3

List the evolutionary mechanisms which generate divergence.

Answer 3

Natural selection, sexual selection, random genetic drift & mutation.

Question 4

Explain natural selection as an evolutionary mechanism.

Answer 4

The process by which heritable traits of organisms become more or less common in a population. Natural selection leads to evolutionary change when individuals with certain traits (characteristics) have a greater survival and/or produce more offspring (which inherit the trait) than organisms that lack these traits.

Question 5

Explain sexual selection as an evolutionary mechanism.

Answer 5

A form of natural selection associated specifically with traits that increase an individual’s reproductive success even at the expense of its survival.

Question 6

Explain random genetic drift as an evolutionary mechanism.

Answer 6

Evolution arising from random changes in the genetic composition (frequency of alleles or genotypes) of a population from one generation to the next.

Question 7

Explain mutation as an evolutionary mechanism.

Answer 7

Mutation generates genetic variation on which natural selection, sexual selection and random genetic drift act.

Question 8

List some possible methods of geographical separation which trigger speciation.

Answer 8

Deforestation splitting two areas of habitat, formation of mountain ranges or colonisation.

Question 9

Define ring species

Answer 9

A situation in which two populations that do not interbreed are connected by a geographical ring of populations that can interbreed.

Question 10

Explain ecological speciation

Answer 10

The evolution of barriers to gene flow resulting from ecologically-based divergent selection.

Question 11

Define divergent natural selection

Answer 11

The evolution, by natural selection, of phenotypic differences between populations that exploit different environments.

Question 12

Explain sensory drive in terms of speciation

Answer 12

The sensory drive hypothesis of speciation refers to the adaptation in sensory and signalling systems to different environments that results in premating reproductive isolation between populations.

Question 13

Why is hybridisation between partially diverged populations referred to as being ‘maladaptive’?

Answer 13

Because hybrid offspring have low survival or reduced fertility compared with offspring resulting from matings between individuals from the same population.

Question 14

Explain reproductive character displacement

Answer 14

The phenomenon that occurs where differences among similar species whose distributions overlap geographically so that they co-occur is greater compared to when their distributions do not overlap.

Question 15

What is polyploidy?

Answer 15

When cells or organisms contain more than two paired (homologous) sets of chromosomes.

Question 16

Many eukaryotic organisms are diploid. What does this mean?

Answer 16

They have two sets of homologous chromosomes, one inherited from each parent.

Question 17

What is an autopolyploid?

Answer 17

Individual that exhibits ploidy (having more than 2 chromosome sets) that arises when multiple chromosome sets are derived from a single species. They can also arise by doubling of the genome so that all chromosomes are derived from a single parent.

Question 18

What is an allopolyploid?

Answer 18

Individual that exhibits ploidy (more than two chromosome sets) resulting from interspecific hybridisation.

Question 19

Define hybrid speciation

Answer 19

Speciation that results from hybridisation between two different species which leads to the formation of a new species that is reproductively isolated from both parent species.

Question 20

What is homoploid hybrid speciation?

Answer 20

Occurs when a stable, fertile and reproductively isolated lineage results from hybridization between two distinct species, without a change in ploidy level. Reproductive isolation between a homoploid hybrid species and its parents is generally attained by chromosome rearrangements, ecological divergence or spatial isolation from the parental species.

Question 21

Explain the process of fixation in speciation

Answer 21

Where an allele increases in frequency in a population to such an extent that all individuals carry only that allele (it reaches a frequency of 1) and other alleles are lost.

Question 22

Define epistasis.

Answer 22

A specific kind of genetic interaction between genes at two or more loci, where the phenotype resulting from variation at each locus in combination differs from that which would be expected if the genes were expressed independently. Such interactions may involve one allele negating or enhancing the effects of other alleles or two or more alleles interacting to produce novel phenotypes.

Question 23

Explain introgression

Answer 23

The movement of alleles from the gene pool of one species into the gene pool of another by repeated backcrossing of interspecific hybrids with one of the parental species.

Question 24

What are homeobox genes

Answer 24

A large family of genes that direct the formation of many body structures during early embryonic development of multicellular organisms. Most homeobox genes encode proteins that act as transcription factors. They contain a sequence called a homeobox, which encodes a homeodomain, a part of the protein that binds to DNA and has a role in controlling its transcription and hence gene expression.

Question 25

Explain positive assortative mating

Answer 25

Occurs when there is a positive correlation between male and female phenotypes or genotypes across mated pairs.

Question 26

What are quantitative trait loci?

Answer 26

Stretches of DNA that are correlated with variation in a phenotypic trait. The regions contain genes or are linked to genes that underlie a quantitative trait (a trait controlled by the cumulative action of many genes)

Question 27

Define peripatric speciation.

Answer 27

Speciation by evolution of reproductive isolation in small, geographically isolated (peripatric) populations as a consequence of a combination of genetic drift and natural selection; originally known as founder effect speciation.

Question 28

Explain postzygotic extrinsic reproductive isolation

Answer 28

Hybrids are selected against within nature. Either they have lower survival rates or fail to attract mates.

Question 29

What is a postzygotic intrinsic reproductive barrier?

Answer 29

Hybrid inviability (the foetus dies before birth) or hybrid sterility (offspring are completely sterile, such as the mule)

Question 30

What are the three forms of pre-mating ecological isolation?

Answer 30

Habitat isolation (they are just in different areas and so don’t meet to breed), temporal isolation (breeding at different times) and pollinator isolation (for example bees preferring one colour flower to another and triggering divergence).

Question 31

Explain pre-mating behavioural isolation

Answer 31

They just don’t fancy each other. They may have differences in courtship behaviour such as songs, dances, pick-up lines, fire-fly flashy bums… That kind of shit (yes, obviously I wrote this one and didn’t rip it from the course)

Question 32

Explain post mating prezygotic mechanical isolation.

Answer 32

Penis incompatibility or size differences. Imagine a Great Dane trying to bonk a chihuahua.

Question 33

Explain postmating prezygotic gametic isolation

Answer 33

When mating occurs but the sperm and egg are incompatible.