Speciation

Species

Species

Members of interbreeding populations that are reproductively isolated from other groups and therefore evolve independently

Prezygotic barriers -Prevention of fertilization

Block fertilization by:

• Impeding different species from attempting to mate

• Preventing the successful completion of mating

• Hindering fertilization if mating is successful

Postzygotic barriers - reproductive isolation

Block fertilization by:

• Reduced hybrid viability

• Reduced hybrid fertility

• Hybrid breakdown

Allopatric Speciation

• Evolution of a population into 2 new populations due to physical barrier:

a result of division by a physical barrier. i.e a dam, a mountain, a river, causing GENE FLOW DISRUPTION

(15 pairs of sibling species of snapping shrimp originated from the separation of the Atlantic and Pacific waters)

Sympatric Speciation

• Evolution of populations within the same geographic area into separate species i.e. diploid/tetraploid tree frogs in the U.S

• no physical barrier to gene flow

• often due to chromosomal change (polyploidy)

• can also be a result of new ecological niches

• can also be driven by SEXUAL SELECTION

Gradualism

• Evolution occurs at a slow, steady rate, with small adaptive changes gradually accumulating over time in populations. Species originate through a gradual change of adaptations (not as well supported by fossil evidence)

Punctuated Equilibrium

• Environmental changes lead to rapid changes in the gene pool of a small population that is reproductively isolated from the main population. (supported by fossil evidence)

Speciation

formation of an entirely new species

• focal point of evolutionary theory

Biological species concept

A species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations

• Gene flow between populations holds the phenotype of a population together

Habitat isolation

Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

Temporal isolation

Species that breed at different times of the day, different seasons, or different years cannot mix their gametes

Behavioral isolation

Courtship rituals and other behaviors unique to a species are effective barriers

Mechanical isolation

Morphological differences can prevent successful mating

Gametic Isolation

Sperm of one species may not be able to fertilize eggs of another species

Reduced hybrid viability

Genes of the different parent species may interact and impair the hybrid’s development

Reduced hybrid fertility

Even if hybrids are vigorous, they may be sterile

Hybrid breakdown

Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile

Morphological species concept

Defines species by structural features

Ecological species concept

Views a species in terms of its ecological niche

• It applies to sexual and asexual species and emphasizes the role of disruptive selection

Phylogenetic species concept

Defines a species as the smallest group of individuals on a phylogenetic tree

• It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species

Autopolyploid

an individual with more than two chromosome sets derived from ONE SPECIES

Allopolyploid

Species with multiple sets of chromosomes derived from DIFFERENT SPECIES

Hybrid zone

region in which members of different species mate and produce hybrids

Hybrids

result of mating between species with incomplete reproductive barriers

• often have reduced fitness compared with parent species

Reinforcement

When reproductive barriers strengthen due to hybrids being less fit than parent species

• Reproductive barriers should be stronger for sympatric than allopatric species

Fusion

Occurs when hybrids are as fit as parents

• Gene flow has the potential to be substantial

Stability

Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone

Punctuated equilibria

Periods of apparent stasis punctuated by sudden change