Lecture 2- Speciation

October 20

allopatric speciation

divergence following geographic separation

allopatry

geographic ranges that don’t contact or overlap

vicariance

barrier is imposed upon the population- it splits

• vicariance can impose upon multiple lineages, affecting many species in an area

dispersal

a population moves over a barrier to new place

founder populations

• a new population established by a small number of individuals from a larger source population

• often very small

• create a bottleneck that increase the effect of genetic drift

• the “Founder Effect”

ring species/speciation

special case involving clinal variation in a ring, gradual genetic divergence, and reproductive isolation between the ends of the ‘ring’

sympatric speciation

• occurs without geographic isolation,

• involves disruptive selection and assortative mating

• sympatric speciation can evolve by disruptive natural selection

limnetic

the open water zone of a freshwater body, such as a lake, that is too deep for rooted plant growth

ploidy

chromosome number

polyploidy

multiplication or increases in chromosome number

autopolyploidy

polyploidy within a species

allopolyploidy

starts with hybridization between related species

non-disjunction

failure of chromosomes or sister chromatids to separate correctly during cell division

speciation

Speciation is the evolutionary process by which populations evolve to become distinct species.

• rides the line between macro and micro evolution

anagenesis

evolutionary change along a lineage

cladogenesis

Cladogenesis is an evolutionary splitting of a parent species into two distinct species, forming a clade.

polymorphism

polymorphism is the occurrence of two or more clearly different forms or phenotypes in a population of a species

biological species concept

proposed by Ernst Mayr in the 20th century

“biological species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups”

evolutionary species concept

Proposed by GG Simpson

“Lineage evolving separately from others and with its own unitary evolutionary role and tendencies”

cryptic species

one of two or more distinct species that are nearly identical in physical appearance and were once considered to be a single species.

sexual dimorphism

the observable differences in physical and behavioral traits between males and females of the same species

polymorphism

the presence of two or more distinct forms or "morphs" within the same population of a species, which can be physical, genetic, or molecular.

vicariance

A process in which a species' range is divided even though the species has remained in place. This might happen through tectonic action, geologic activity.

secondary contact

Secondary contact is the process in which two allopatrically distributed populations of a species are geographically reunited.

reinforcement

Reinforcement is a process of speciationwhere natural selection increases the reproductive isolation (further divided to pre-zygotic isolation and post-zygotic isolation) between two populations of species.

reciprocal monophyly

Lineages sampled from two groups, A and B, are reciprocally monophyletic if the lineages from group A have a shared common ancestor that is more recent than the most recent common ancestor any of the group A lineages has with a lineage of B , and if the lineages from group B have a shared common ancestor that is more recent than the most recent common ancestor any of them has with a lineage of A.

does speciation require natural selection?

not necessarily!

allopolyploidy

Allopolyploids or amphipolyploids or heteropolyploids are polyploids with chromosomes derived from two or more diverged taxa.

polyploid speciation

the formation of new species through an event that results in a sudden increase in the number of chromosome sets

Dobzhansky-Muller Model

for attaining reproductive incompatability in allopatry

1. ancestral pop separates into two independent lineages

2. a new allele arises at locus 1 in one lineage

3. a different allele arises at locus 2 in the other lineage

4. allele A becomes fixed at locus 1

5. allele B becomes fixed at locus 2

6. Allele A is incompatible with allele B, so hybrids are inviable

Premating/prezygotic isolation

Premating (prezygotic) isolation includes mechanisms that prevent mating or fertilizationfrom occurring in the first place

ex: different mating times, behavioral isolation

secondary contact possible outcomes

• fusion of species

• extinction of one

• formation and stability of a hybrid zone

• reinforcement (further speciation)

• hybrid speciation

hybrid speciation

In rare cases, the hybrids themselves may be more fit than either parent in a new ecological niche, or become reproductively isolated from both parent populations (often through polyploidy in plants). This can lead to the formation of a new, third species.