Speciation - Lecture 4

Question 1

What is the definition of species?

Answer 1

A population of reproducing organisms that is isolated from other populations

Question 2

What is the problem with this definition?

Answer 2

○ Applying this to prokaryotes and eukaryotes that are not strictly sexual
○ Impossible to use with extinct organisms
○ Difficult with bacteria and archaea that have horizontal gene transfer

Question 3

How can species appear?

Answer 3

○ In time along a lineage e.g. humans/chimps and our common ancestor
○ In space due to geographic isolation e.g. animals on an island
○ Various genetic effects
○ These can be identified/detected by comparing populations using population genetics
○ These processes can be observed

Question 4

What is allopatric speciation?

Answer 4

○ When populations of the same species become isolated due to geographical changes
○ Classic mode of speciation

Question 5

What is an example of allopatric speciation?

Answer 5

○ Two different species of antelope squirrels on either side of the Grand Canyon (same environment)
○ Don’t mate so no gene flow

Question 6

What type of selection can happen when two populations are isolated?

Answer 6

○ Directional selection: one extreme phenotype is favoured over others, leading to a shift in a particular direction
○ Disruptive selection: Both extreme phenotypes at both ends of the spectrum are favoured over intermediate traits
○ Stabilising selection: When the intermediate trait is favoured and the extreme phenotypes disappear

Question 7

What is genetic drift?

Answer 7

○ Causes changes in the frequency of alleles in a population over time due to chance events
○ Not selection and is random
○ In a large population: large fluctuations are unlikely
○ In a small population: lose genetic variability as can lose alleles

Question 8

What is the founder effect/bottleneck?

Answer 8

○ When a large population is reduced due to a catastrophic effect
○ Reduces genetic diversity and increases the impact of genetic drift

Question 9

What is sympatric speciation?

Answer 9

Evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region

Question 10

What are some examples of speciation?

Answer 10

Blackcap:
○ Breeds in central Europe
○ In winters: Spain or UK
○ Gentic and morphological differences in Spanish and UK populations
○ Assortative mating = Spanish mating with Spanish birds, UK with UK
○ Likely due to arrival time on breeding grounds (UK birds reach before Spanish birds)
○ An example of “allotemporal” speciation

Question 11

Why are plants different?

Answer 11

○ Can double their chromosome number without causing issues
○ Don’t have specific tissue
○ Allows polyploid speciation

Question 12

What is polyploid speciation?

Answer 12

When an organism gains one or more extra sets of chromosomes, leading to reproductive isolation and the formation of a new species without the need for geographic isolation

Question 13

What is an example of sympatric speciation in plants?

Answer 13

○ Mimulus peregrinus - sterile triploid hybrids in Shortcleuch waters
○ A genome duplication event took place producing an individual which could undergo meiosis
○ Lead to a new population of genetically isolated individuals - a new species

Question 14

What are the genetic factors in speciation?

Answer 14

○ Drifts and bottlenecks combined with long periods of isolation
○ Speciation is not an adaptation

Question 15

Why does it take a long time for species to happen?

Answer 15

○ Selection is often not directly involved
○ 5,000,000 years per speciation
○ Speciation is a branching process and one species can give rise to more than 2 others

Question 16

Why are there so many insect species?

Answer 16

○ Appearance linked to evolution of plants
○ Evolution of larval stage enables insects to exploit new niches
○ Evolution of flowering plants coincides with radiation of the butterflies
○ Changes in ecology means new niches

Question 17

What isolation mechanisms come under prezygotic barriers - premating?

Answer 17

○ Habitat isolation e.g. one species on land and other in water
○ Temporal isolation (differences in breeding schedules) e.g. one species reproduces in Jan - March and other reproduces from May-July
○ Behavioral isolation (differences in behaviour) e.g. female fireflies will only respond to male fireflies that flash a specific pattern

Question 18

What isolation mechanisms come under prezygotic barriers - post-mating?

Answer 18

○ Mechanical isolation (physical barriers between genetical connection) e.g. snails
○ Gametic isolation (eggs and sperm don’t recognise each other)

Question 19

What are examples of postzygotic barriers?

Answer 19

○ Reduced hybrid viability
○ Reduced hybrid fertility
○ Hybrid breakdown

Question 20

What are the possible outcomes of hybrid zones?

Answer 20

○ Reinforcement of isolation: some gene flow, leading to a significant loss of fitness
○ Fusion: lots of gene flow
○ Stability: limited gene flow with negligible fitness