Lecture XV: Speciation

Question 1

What is Speciation

Answer 1

The origin of new species

Speciation is a stepping stone between micro- and macro- evolution.

Question 2

What is a species?

Answer 2

A species is:

A population or group of populations whose members have the ability to interbreed and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups.

Question 3

What keeps a species together when populations are spread out?

Answer 3

Active Gene flow between populations of the same species

Reproductive Isolation with individuals of different species.

Question 4

Gene Flow

Answer 4

When individuals from one population of a species move into another and produce offspring they help to maintain the species gene pool as a whole.
How?
When a population is completely isolated from other ones of their species their gene pool tends to change because they are experiencing different pressures from their environment (natural selection) and/or experiencing genetic drift.

Gene flow causes the gene pool to remain more constant amongst all populations of a species.

Question 5

Reproductive Isolation

Answer 5

Not being able to mate with members of other species or not being able to produce viable fertile offspring with other species prevents mixing of species gene pools, keeps them different.
Reproductive isolation keeps different species separate.
Reproductive isolation that arises between populations of the same species may lead to the formation of new species.

Question 6

What are the barriers to Reproductive Isolation?

Answer 6

Prezygotic Barriers
Prior to fertilization

Or

Postzygotic Barriers
After fertilization

Question 7

Types of Prezygotic Barriers

Answer 7

Types of Prezygotic Barriers:
barriers that prevent fertilization between 2 different groups.
Habitat: Separated by where in the ecosystem they live.

Temporal: Mating occurs at different times (day/night, time of year)

Behavioural: Specific mating behaviours (courtship behaviour, different mating calls or signals)

Mechanical: Sexual organs are not compatible or the organisms themselves are too different.

Gametic: Sperm and egg are not compatible.

Question 8

Postzygotic Barriers:

Answer 8

Barrier after fertilization between 2 groups which prevents viable offspring to thrive.
Types of Postzygotic Barriers:

Reduced hybrid viability (hybrids are formed, but they usually do not complete development and if they do they are frail. Due to interactions of combined genes)

Reduced hybrid fertility (hybrids are formed, but they are usually sterile)

Hybrid Breakdown (first generation hybrids fertile, but with each successive generation become more feeble or sterile)

Question 9

a) Reduced Hybrid Viability

Answer 9

Some salamander species that have overlapping habitats can mate and produce hybrid offspring. However, most do not reach maturity and those that do are frail and do not reproduce.

Question 10

b) Reduced Hybrid Fertility

Answer 10

Mules are hybrids of horses and donkeys. They are strong vigorous animals, but they are sterile. Issue happens during meiosis (creating gametes) because the chromosomes inherited from each parent may differ in number and structure.

Question 11

c) Hybrid Breakdown

Answer 11

The first generation hybrids are viable, but with each successive generation they become more feeble or sterile.

Question 12

Limitations of the Biological Species Concept

Answer 12

The biological species concept cannot be applied to all living organisms that have lived or that are living.

Cannot be applied to:

Fossils (can’t tell what groups could mate successfully)
Asexually reproducing organisms

Question 13

Other Definitions of Species

Answer 13

1. Morphological Species Concept

Species defined by physical features
Can be used for all living organisms and fossil remains
Subjective criteria

2. Ecological Species Concept

Species defined by it’s ecological niche. How it interacts with other living organisms and the physical environment.
Can be applied to all living organisms, but not fossil remains.

3. Phylogenetic Species Concept

Species defined by degrees of similarity in DNA and also physical morphology.
Can be applied to all living organisms, DNA not usually available for fossil remains.

Question 14

How do new species arise?

Answer 14

Gene flow must stop between populations of an existing species.

The separated populations must then be exposed to different pressures from their environments (natural selection) or experience genetic drift (more of an influence in small populations (recall founder effect and bottle neck effect).
There are 2 modes of speciation based on how gene flow is interrupted between populations of the existing species.

Question 15

Modes of Speciation

Answer 15

1. Allopatric Speciation
   Reproductive isolation due to a physical environmental barrier
2. Sympatric Speciation
   Reproductive isolation occurs between certain individuals in the same area.

Question 16

Allopatric Speciation

Answer 16

New species arises due to a physical barrier separating populations of an existing species.

Question 17

Sympatric Speciation

Answer 17

New species arise in the same geographic area due to the formation of reproductive barriers.
Reproductive isolation between 2 populations in the same geographic area can arise due to one of the following:

Chromosomal Changes

Habitat Differentiation

Sexual Selection

Question 18

1. Chromosomal Changes

Answer 18

New species is formed when a viable, and fertile polyploid offspring is created. Common in plants. Polyploid offspring are typically isolated from the parent kind reproductively.

Question 19

2. Habitat Differentiation

Answer 19

Sympatric speciation may occur when genetic variation within a population allows a group of individuals to exploit a habitat or resource not used by the rest of the population.

Question 20

3. Sexual Selection

Answer 20

??? When different genders are involved?

Question 21

3 Outcomes of speciation:

Answer 21

Reinforcement: Population above continues to diverge away from the other 3. Fewer and fewer hybrids produced as gene flow lessens. May lead to the isolated population becoming a new species.

Fusion: Gene flow between the isolated population and the other 3 is re-established. Become one shared species again.

Stability: The isolated population and the other 3 populations remain at the same level of divergence. Hybrids are produced where they come in contact with each other.

Question 22

Speed of speciation

Answer 22

Darwin believed that species arose due to gradual changes through natural selection over time.
The fossil record has shown, however, that species remain relatively the same over long periods of time and then new species arise suddenly.
This observation was coined Punctuated Equilibrium by Eldridge and Gould. They suggested that speciation occurs rapidly during moments in time when environmental pressures change dramatically

Question 23

Two types of patterns

Answer 23

Punctuated pattern: sudden change

Gradual pattern: gradual change.