BIOL 108 - Topic I - Speciation

Biological Species Concept

Populations that can interbreed or have the potential to interbreed with members of the same group in nature, and produce viablle/fertile offspring

Characteristics of the BSC

1. BSC focuses on breeding over generations

2. BSC relies on actual or potential breeding

3. Gene flow between population of the same species contributes to the unity of the species

Reproductive barrier

Existnece of biological factors that impede members of two species from producing viable, fertile offspring. Two main categories;

• Prezygotic barrier

• Post zygotic barrier

Zygote and Hybriids

Zygote - Fertilized egg from the union of reproductive cells

Hybrids - Offspring from the mating of two different species

Prezygotic barrier

Reproductive barrier that impedes mating between species or hinders fertilization if interspecific mating is attempted, 5 categories:

1. Habitat isolation

2. Temporal isolation

3. Behaviour isolaton

4. Mechanical isolation

5. Gametic isolation

Habitat Isolation

Two species that live in different habitats within the same location, but encounter each other rarely or never.

Example: Crucifix Toad and Desert Tree Frog, share the same geographic range yet never encounter one another (Toad lives underground, frog lives in trees)

Temporal Isolation

Two species that breed at different times, leading to rare or no encounters

Example 1: Migratory species arrive for mating at different time

Example 2: Nocturnal species vs diurnal species

Behavioural Isolation

Species use unique courtship rituals and other behaviours to attract mates.
Example 1: Mating dance in blue-footed boobies

Example 2: Honking in Canada geese

Example 3: Colour and tentacle display in large pacific striped octopus

Mechanical Isolation

Anatomical differences prevent successful mating

Example 1: Reproductive openings in some snails (locations/size)

Example 2: Morphology of genitalia in animals (ducks for example)

Gametic Isolation

Sperm of one species cannot fertilize the eggs of another species

Example: Pollen of a species in the Brassicaceae family cannot fertilize another even if pollen reaches the eggs in the ovule

Example 2: Sperms released by the red sea urchins cannot fertilize the eggs of the purple sea urchins

Postzygotic Barrier

A reproductive barrier that prevents hybrid zygotes produced by two different species from developing into viable, fertile adults.

Three main categories;

1. Reduced hybrid viability

2. Reduced hybrid fertility

3. Hybrid breakdown

Reduced Hybrid Viability

Genes from parents of different species may interact to impair the hybrid’s development or survival

Example: Hybrid of native/invasive tiger salamanders show signs of impeded development. Most hybrid animals do not complete embryonic development. The observable hybdrids are typically frail and would most likely be eliminated by natural selection.

Reduced Hybrid Fertility

Hybrids may be sterile and could not reproduce further, even if the hybrids can survive.
Example: hybridisation between a male donkey and a female horse produce the infertile mule, since the mule only has 63 chromosomes (31.5 pairs), issues arise during meiosis.

Hybrid Breakdown

Reproductive failure that appears after the F2 generation of crosses between different species.
Possible Cause: Accumulation of deleterious genetic traits, for example the hybridization across strains of rice

Limitations of Biological Species Concept

1. Cannot be applied to asexually reproducing organisms

2. Cannot be examined in most fossil species

3. Strong emphasis on the absence of gene flow between species, BSC implies different species would not mate and produce surviving offsprings, but hybrids exist in nature.

Morphological Species Concept

Defines species in terms of measurable anatomic features

1. Observable and measureable features of a given species are distinct from other species

2. Applicable to both extant and extinct organisms

3. Applicable to both sexually and asexually reproducing organisms

Ecological Species Concept

Defines species by ecological niches, which encompass how organisms of the same species interact with biological and abiotic aspects of their environment.

1. Individuals of the same species share similar preferences/tolerances to habitat conditions

2. Species are grouped/separated by partitioning of ecological niches

3. Applicable to both sexually and asexually reproducing organisms

Phylogenetic Species Concept

Defines a species as the smallest group of individuals on a phylogeny

1. Definition is based on evolutionary history and shared ancestry

2. Species can be one of the tips/terminal taxa on a phylogeny

3. Applicable to both sexually and asexually reproducing organisms

Challenging due to update frequency and size of groups (example, one tip = an entire monophyletic group itself)

Allopatric Speciation

Formation of new species in populations that are geographically isolated from one another, and gene flow is interrupted. Number of species higher in regions with many geographic barriers

Dispersal

Movement of individuals away from the rest of the population.

Example: flightless cormorant on the Galapagos islands diverged from a cormorant species capable of flight.

This movement expands geographic range of a population

Vicariance

Range of a species is split by a change in the environment, creating two subpopulations.
Example: population of ancestral shrimp got sepereated by isthmus of panama, which leads to formation of new species.

Subpopulations may be subjected to different selective agents and pressures

Sympatric Speciation

Formation of new species in populations that live in the same geographic area. Geograph barrier is absent, individuals stop breeding with others in populations. New species is established once reproductive isolation is established.

Habitat Differentiation

Type of Sympatric Speciation, subpopulations of the same species adapt to different environments, leading to reproductive isolation. Reproductive barriers can be either prezygotic or postzygotic.

Example: Cichlid fish in Africa, various sub-habitats in the lake leading to different adaptations.

Sexual Selection

Type of Sympatric Speciation, individuals with certain inherited characteristics are more likely than others to obtain mates, can be seen as a form of natural selection and drives sympatric speciation.
Examples:

• Calls in Canadian geese, tap dancing in blue-footed boobies

Hybrid Zone

Geographic region in which members of different species meet and mate, producing at least some offspring of mixed ancestry. Hybrids are often less fit, can lead to speciation.

• Modern wheat is an example

Hybrid zone can be a partial overlap in the range of two species

Possible Outcomes of Hybrid Zones

1. Reinforcement

2. Fusion

3. Stability

Reinforcement - Outcome of Hybrid Zone

Hybrid offspring are less fit than the parents, natural selection therefore removes hybrids, strengthening reproductive barriers.

Fusion - Outcome of Hybrid Zone

Hybrid offspring are as fit as the parents, gene flow therefore maintained between hybrids and parents, reproductive barrier is weakened and eventually removed, species then fuse.

Stability - Outcome of Hybrid Zone

Hybrid offspring continously formed by parents, gene flow maintained between parent populations in a stable way.

• This stability may lead to fusion.

Speciation and Natural Selection

1. Speciation can occur with or without natural selection

2. Accumulation of genotypic difference leads to speciation

3. Natural selection acts on traits of the new species

4. New species deemed unfit would be removed