BIOL 214 - Topic 8

Speciation

Mechanisms

• microevolutionary process that alter the pattern and extent of genetic/phenotypic variation within populations (fish game)

  • different processes b/w populations → diverge genetically → different species

  • small changes → small differences of organisms within a population → larger scale of time and space → speciation

Population

any species that lives within the same place at the same time

Morphological Species

• the concept that all individuals of a species share measurable traits that distinguish them from individuals of other species → external traits

• identifying a species based on visible anatomical characteristics (used by paleobiology)

• shortcomings:

  • doesn’t account for sexual dimorphism

  • doesn’t account for variation in a population

  • does not tell us about the evolutionary processes

Biological Species Concept

Reproductive Isolation

• biological characteristic that prevents the gene pools of two species from mixing

• prezygotic & postzygotic mechanisms

• looking at the selection of mates and how they are reproducing for offspring

• 2 populations interbreed and produce viable offspring - same species

  • reproductive isolation means they cannot produce fertile offspring with other populations of the same species → cannot exchange genetic information

  • exception = microbes can share genetic information without needing to produce fertile offspring

Phylogenetic Species Concept

Evolutionary History → smallest group of populations that can be united by shared derived characteristics

• morphological and genetic data

• advantage - applied to any group of organisms (asexually reproducing and extinct species)

• support - morphological and genetic distinctions on the tree reflect the absence of gene flow

• drawbacks - detailed evolutionary trees have only been described for a fraction of species (not very applicable)

Geographical Variation

Subspecies → populations (of a same species) that differ genetically and phenotypically

Neighboring populations → some shared characteristics due to similar environments

• individual exchange, immigration, emigration, comparable patterns of natural selection

Ring Species

organisms live in a ring-shaped geographic distribution → middle/hole is unhabitable

• adjacent populations can exchange genetic material

• harder to get to further populations (more steps)

• indirectly mating → still considered the same species (could be in an intermediate form where they begin to form a different species)

Clinal Variation

• species is distributed over a large, environmentally diverse area → traits exhibit a cline

• Possible Causes = temperature, altitude (environmental stressors)

  • combined with gene flow - adapting to various conditions

Cline

a smooth pattern of variation across a geographical gradient

Prezygotic Mechanisms (premating)

prevents the production of hybrid offspring & limits the frequency of interspecies breeding (only one blocks fertilization from occurring - gametic)

• ecological isolation - species live in different habitats

  • can be in the same geological era

• temporal isolation - species breed at different times

  • can be in the same habitat

• behavioral isolation - species cannot communicate

  • courtship behavior

• mechanical isolation - species cannot physically mate

  • structure of the reproductive organs

• gametic isolation - species have nonmatching receptors on gametes

  • incompatibility between sperm and egg

Postzygotic Mechanisms (postmating)

• reinforcement - individuals have lower fitness than parental species (favoring individuals who choose mates of their own species → their offspring will be able to have viable offspring of their own since they aren’t hybrids)

  • postzygotic mechanisms lead to the evolution of prezygotic mechanisms

reduces the success of hybrid individuals → environmental selective pressures mean the hybrid individuals have lower odds of survival and will be outcompeted by the normal nonhybrid individuals

• hybrid inviability (sickly/prone to death) - hybrid offspring do not complete development

• hybrid sterility - hybrid offspring cannot produce gametes

• hybrid breakdown (chromosome abnormality) - hybrid offspring have reduced survival or fertility (keep species isolated from each other)

Speciation - Geography

• impact on whether gene pools can/will mix

2 modes of speciation

1. allopatric speciation - species are physically separated by a boundary (environments are separated from each other)

2. sympatric speciation - speciation occurs with organisms living in the same environment

Allopatric Speciation

• geographically separated populations

• secondary contact → test if reproductive isolation has occurred (must not mate when meeting again

  • early stages = prezygotic isolation may be weak or incomplete → hybrid zones (fluctuate in how long they exist)

2 outcomes possible

1. species fusion → 2 populations merge into one after secondary contact

2. speciation

Sympatric Speciation

• drivers - changes in the environment, disruptive selection, sexual selection

• mechanisms - polyploidy, habitat differentiation, & behavioral change

  • behavioral changes - differences in selecting mates or feeding behavior

E.g. → founding population responds to selective pressures on a different island (no geographical isolation however) → selective selection (exaggerates morphological and behavioral characteristics being selected for) → forming & maintaining isolation from neighboring populations (differentiation begins)

Speciation - Genetic Mechanisms

Lead to reproductive isolation

1. genetic divergence between allopatric populations

2. polyploidy in sympatric populations

3. chromosome alterations

Genetic Divergence

Allopatric populations

• prezygotic reproductive isolation can have simple genetic basis & such traits trace back to sexual selection (post influences pre)

• sexual dimorphism → exaggerated courtship/structures

Geographical Separations (populations)

• mutation

• genetic drift

• natural selection

Polyploidy - Sympatric Speciation

• Autopolyploidy → from chromosome duplications within a single species → diploid produces tetraploid

  • unreduced gametes from an error in meiosis

  • same # of chromosomes as a somatic cell

• Allopolyploidy → hybridization of different species

  • two or more complete sets from different parent species

  • can be very rapid speciation

• fertilization - self-reproducing OR reproduce with other tetraploid plants