Bio 1B: Lec 5: How do new species arise?

What are the three species concepts?

Different way to categorize species

1. Biological Species Concept (BSC): populations that can interbreed and are reproductively isolated from other populations (breed)

2. Morphological Species Concept (MSC): groups distinguished by consistent morphological differences (looks)

3. Phylogenetic Species Concept (PSC): smallest diagnosable lineages sharing common ancestry and derived characters (tree)

Strengths and Limitations of the different species concepts

Biological: good for looking at gene flow and reproductive isolation, not good for fossils or asexual organisms

Morphological: good for fossils, asexuals, and field identification, not good for polymorphism, sex, age, convergence, and cryptic species

Phylogenetic: good in integrating DNA and ancestry, not good in determining how much difference is enough to be another species

What is an issue with using a fossil records for chronospecies?

Chronospecies are species that evolves continually over time so that the earliest and latest forms don’t look alike at all

So if you try to use fossils for these and only look morphologically, they look like two different species

Cryptic Species

Populations that look identical, but DNA shows fixed differences and little gene flow where ranges overlap

MSC BAD bc can look like the same species, need PSC to differentiate

Polymorphism

Where different forms occur in the same interbreeding population

Visible difference does not automatically establish separate species (msc thinks different, bsc says same)

Hybridization

Where distinct populations exchange some genes but maintain diagnostic differences over time

Species boundaries can be porous, not absolute

What does speciation require?

In order to speciate, you need divergence between the two to persist despite any homogenizing forces of gene flow

How does gene flow interact with speciation?

Gene flow interferes with speciation, making it so they cannot speciate

Allopatric, Parapatric, Sympatric

-patric = geographic fatherland

Allo- other/different: populations are geographically separated, no gene flow

Para- alongside: adjacent populations experience different environments, limited gene flow

Sym- together/same: divergence within the same geographic area, high gene flow until speciation starts

Allopatry methods

Vicariance: a new barrier splits a once-continuous population

Dispersal: a small subset colonizes a new place

Parapatry methods

Local selection must be strong enough and gene flow weak enough for differences to persist

Sympatry methods

The hardest route because gene flow remains available

Selection has to be super strong

Types of reproductive barriers

Pre-mating barrier: barriers preventing mating (encounter [habitat/geographic, temporal/behavioral])

Prezygotic: prevent formation of a zygote (mating attempt [mechanical], fertilization [gametic])

Postzygotic: reduce hybrid viability or fertility after zygote formation (hybrid development [reduced hybrid viability], hybrid reproduction [reduced fertility/hybrid breakdown])

Dobzhansky-Muller (DM) incompatibilities

Good for location 1, good for location 2, come together, shit in both populations

What could happen when partially isolated populations meet again?

Fusion: barriers are weak and hybrids have similar fitness, gene flow blends populations back together

Stable hybrid zone: hybrids are produced repeatedly, but selection and migration maintain a geographic boundary

Reinforcement: hybrids have reduced fitness, selection favors traits that reduce heterospecific mating