Evolutionary genetics

Biological species concept

A species whose population/group of populations have the potential to interbreed in nature and produce viable, fertile offspring

Criteria: Potential reproductive compatibility

Problems with Biological Species Concept

The key to BSC is genetic isolation but does not apply to all species—asexual reproducing species, for example—or when individuals of the same species under different breeds cannot mate

Prezygotic isolation

Barriers to fertilization between two species, such as habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation

Habitat isolation

An animal species in the desert cannot breed with a species they are not able to reach in other habitats

Temporal isolation

Diurnal creatures are not awake to breed with nocturnal creatures

Behavioral isolation

Behaviors involved in mating are unique between two individuals of the same species, this prevents two compatible species to be uninterested in mating with each other (ex. male fireflies signal to female counterparts in a specific pattern that means they will respond only to that pattern—preventing them from mating with other closely related firefly species)

Mechanical isolation

a type of physical reproductive barrier that prevents mating between two organisms—ex. physical barriers between genital connection

Gametic isolation

Different species are reproductively isolated because the sperm of one species cannot find, attach, or fuse with the egg of the other species

Postzygotic isolation

Postzygotic barriers between two species that reduces viability/fertility of interspecific hybrids caused by genetic incompatibilities between diverged parental genomes

Reduced hybrid viability

Less likely to mature into a healthy, fit adult

Reduced hybrid fertility

A common feature of reproductive barriers separating recently diverged species

Hybrid breakdown

a type of reproductive failure that appears after the F2 generation of crosses between different species/subspecies

Does reproductive isolation cause speciation?

No reproductive isolation & speciation are a by-product of genetic changes that occur during changes of populations for other reasons

What processes drive speciation?

• Genetic drift and/or mutation

• Natural selection

• Polyploidy and hybridization

What is evolution?

All species have descended (with modification) over a long period of time from one or a few common ancestors through the process of natural selection; the unifying theme to explain the diversity of life on Earth

Descent with modification

• Evolution only occurs when there is a change in gene frequency within a population over time

• These genetic differences are heritable and can be passed on to the next generation—which is what really matters in evolution: long term change

Industrial revolution and Kettlewell’s Moth Study

Birds eat moths that are more conspicuous on trees; descent with modification = genetic changes in a population over time

Modes of Speciation - allopatric speciation by vicariance

Barrier removed or new species disperse over it, re-establishing sympatry

Peripatric speciation

Range expansion re-establishes sympatry

Parapatric speciation

Range expansion leads to sympatry

Sympatric speciation

Genetic differences result in reproductive isolation

Allopatric speciation

• The evolution of genetic reproductive barriers between populations that are geographically separated by a physical barrier

• Most prevalent mode of speciation in animals

• Populations that begin process of speciation in allopatry can come back into contact, and either interbreed freely at hybrid zones, or not at all

Clades

Monophyletic groups—freshwater fish speciation ex—3 to 4 million years ago when sea level was higher, creating a barrier to gene flow

Homology

A character state shared by taxa without modification from a common ancestor

Homoplasy

A character state that has independently evolved two or more times, or similarity that is NOT from common ancestry

Autapomorphy

Derived character state present in a single lineage

Plesiomophy

Ancestral character state (ex. gelatinous eggs)

Synapomorphy

A homologous character state that supports the monophyly of a group through common ancestry (ex. amniotic eggs)

Genetic changes in closely related species

• Small changes in sequences such as base changes and small deletions

• Duplications of genes and DNA

• Rearrangements on all scales

• Acquisition of foreign DNA and genes

• Loss of DNA and genes

Homologous sequences

Descended from same ancestral sequence through DNA replication across generations

Usually determined if there is a “significant” similarity we assume that the sequences are from a common ancestor

Principle of Parsimony (Occam’s razor)

The simplest explanation is preferred

Maximum parsimony

All mutations are equally likely

Long-branch attraction

Species with many autapomorphies will be chosen as sister groups, when in reality they are not

Maximum parsimony weights all character state changes (mutations) as equal, but reality they are ___________

Biased

Molecular clocks — concept

DNA sequences mutate at a constant rate, and so it is possible to correlate number of mutational changes to a node in a phylogenetic tree, estimating the date of speciation; can be calibrated with fossils by providing a minimal estimate of time since divergence

Molecular clocks — issues

Distantly related taxa and different genes rarely have similar mutation rates, creating problems for molecular clocks. Fossils used in calibrations have inexact ages, making clock estimates inexact

Molecular clocks — recent trends

Scientists are combining fossils calibrations with well-dated geological events to created rough estimates of speciation events