SPECIATION

4 Credits

Evolution

Evolution

The change in the gene pool of a population over time.

Gene Pool

The collection of genes within a population.

Allele Frequency

The frequency of certain alleles within a population or gene pool.

What is the relationship between allele frequencies and evolutionary change?

Evolution can only occur with changes in allele frequencies.

Genetic Drift

Random changes in allele frequency due to chance

How can genetic drift change a gene pool?

It changes the allele frequency, which potentially eliminates a gene and reduces genetic variation.

Founder effect

A small group of individuals from a larger population are separated from that population and form a new gene pool.

Bottleneck effect

When a natural event causes a large portion of a population to be wiped out, leaving a small remaining population with a new gene pool.

Emigration

Leave the original area/habitat (Exit)

Immigrate

Move into a new area/habitat (Into)

Phenotypic Range

The range of phenotypes within a population

Stabilising selection

A form of natural selection where organisms with average phenotypes (not on either extreme) are selected for, compared to organisms with extreme phenotypes.

Disruptive selection

A form of natural selection where organisms of extreme phenotypes are selected for, compared to organisms with the average phenotype.

Directional selection

A form of natural selection where organisms of one extreme phenotype are selected for, compared to organisms with average phenotypes or the phenotype of the opposite extreme.

Ploidy

The number of chromosome sets (pairs) in a cell.

Polyploidy

The state of having 3 or more sets of chromosomes.

How does polyploidy occur?

Due to nondisjunction during cell division, meaning one daughter cell has more chromosomes than usual, and one daughter cell will have less chromosomes than usual.

What does polyploidy result in?

Instant speciation as the offspring becomes reproductively isolated from the original species due to different numbers of chromosome sets.

Nondisjunction

Where chromosomes fail to separate during cell division (meiosis), meaning that instead of moving towards opposite poles of the cell, a pair of chromosomes move to the same pole.

Autopolyploidy

Polyploidy involving the multiplication of identical sets of chromosomes from the same species.

Allopolyploidy

Polyploidy involving the combination of chromosomes from different species to form a (usually sterile) hybrid with chromosomes from each parent species.

Species

A group of organisms within a population that are able to interbreed to produce fertile offspring.

Why is a species hard to define?

Ring species and hybridisation

Ring species

A type of species that can interbreed with neighboring populations, but not with populations that are further away, creating a "ring" of connected populations.

Hybridisation

Where organisms produced by 2 different species can sometimes be fertile.

Speciation

The formation of a new species from an original species due to reproductive isolation.

Allopatric speciation

Speciation which is caused by a geographical barrier or geographic isolation separating members of the same species.

How does geographic isolation lead to allopatric speciation?

Species become isolated and prevented from interbreeding. Gene flow is prevented and different selection pressures favour different traits, meaning allele frequencies for certain genes change over time.

Sympatric speciation

Speciation which occurs when members of the same species occupy different niches in the same geographical location.

How does speciation occur without a geographical barrier?

Polyploidy in plants, and changes in food and habitat preferences within different niches. These factors allow different members of the same species to face different selection pressures, which over time creates barriers to successful interbreeding.

Reproductive isolating mechanism

Barriers to gene flow which prevents successful breeding between species.

Why are geographical barriers not reproductive isolating mechanisms?

They don’t operate through the organisms themselves.

What is the role of reproductive isolating mechanisms in speciation?

When organisms become reproductively isolated, they are no longer able to breed with members of their original species and they become 2 separate populations.

Prezygotic RIM

Acts before fertilisation to prevent successful reproduction.

What are the 5 prezygotic RIM’s?

Ecological, temporal, behavioural, structural and gamete mortality.

Ecological RIM

Species occupy different habitats within the same geographical area.

Temporal RIM

Species have different activity patterns in relation to time, eg. diurnal, nocturnal, or seasonal breeding patterns.

Behavioural RIM

Different behaviours allow different species to recognise potential mates of the same species.

Structural RIM

Organisms need to have similar body structures, appearance and chemical make-up.

Gamete mortality RIM

For fertilisation to occur, sperm and egg need to successfully unite.

Postzygotic RIM

Acts after fertilisation to prevent successful reproduction.

What are the 3 postzygotic RIM’s?

Hybrid inviability, hybrid sterility, and hybrid breakdown.

Hybrid inviability

When the fertilised egg fails to develop properly, causing the death of the offspring, or low viability (survivability).

Hybrid sterility

When the hybrid is viable but sterile (unable to breed).

Hybrid breakdown

When the first generation is fertile, but following generations will be infertile or non-viable.

Divergent evolution

2 or more species which are formed from a common ancestor.

How does divergent evolution occur?

When organisms from the same species become reproductively isolated due to allopatric speciation. When the species becomes separated by a geographical barrier, they are exposed to different environment and selection pressures, causing them to diverge into 2 different species from the same ancestral species.

Homologous structures

Features which are similar in structure and origin, but have different functions.

Convergent evolution

2 different species form common traits, despite no common ancestor.

How does convergent evolution occur?

When different species living in the same environment are subjected to the same selection pressures, which select for similar phenotypes.

Analogous structures

Features which are similar in function, but have different structures and origin. (Bird and butterfly)

Coevolution

When evolutionary changes in one species act as a selection pressure for another, resulting in evolutionary changes of the other species.

Interspecific relationships

Relationships between members of different species.

Mutualism

When both species benefit from an interspecific relationship.

Parasitism and Predation

One species is benefited and one species is harmed by the interspecific relationship.

Competition

Both species are harmed by the interspecific relationship.

How do interspecific relationships cause coevolution to occur?

The species will evolve in response to the selection pressures inflicted by the other species.

Gradualism

Populations slowly diverge from one another by slowly accumulating adaptive characteristics in response to different selection pressures.

Punctuated equilibrium

Most of a species’ existance is spent in stasis (no evolutionary change), followed by short bursts of significant change.

Adaptive radiation

The rapid evolution of a number of species from a single common ancestor.

What is the rapid speciation required for adaptive radiation caused by?

A sudden availability of many niches which were previously occupied by a different species.