Speciation and Extinction

Definitions

• Speciation – The origination of new species

• Extinction – The death of all organisms in a species

• Adaptations – Specific morphological changes as a result of environmental pressures

• Taxonomy – Discipline that assigns names to organisms and classifies biological diversity (Linnaeus binomials)

What is a species?

What is a species? Revisiting a few species concepts

Morphological

Biological

Phylogenetic

Morphological Species Concept

Species can be distinguished from each other by morphological traits

“Classical species concept”

Morphological SC limitations

High plasticity

Undetected speciation

African bush elephant African forest elephant

Morphological SC limitations

Biological Species Concept

Groups of interbreeding natural populations that are reproductively isolated from other such groups

Asexual reproduction

Biological SC limitations

Spatially disjunct populations/species:

-Reproductively isolated due to spatial segregation, but may be reproductively compatible

-Lab tests

Biological SC limitations

Hybridization: different species often hybridize naturally (very common in plants)

Biological SC limitations

Jeffrey pine Coulter pine

Biological SC limitations

Fossils: impossible to know for sure if similar looking species interbreed

Phylogenetic Species Concept

“the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent” (Cracraft 1983)

Clusters must be:

1) Monophyletic (ancestor + all descendants)

Phylogenetic Species Concept

“the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent” (Cracraft 1983)

Clusters must be:

1) Monophyletic (ancestor + all descendants)

2) derived through an evolutionary process of descent from an ancestral lineage

3) diagnosable through examination of derived characters

Phylogenetic

Phylogenetic tree

Phylogenetic SC limitations

Can inflate the number of species if species designations are based on a few characters

The genetics of a species may not always fully explain the differences in species.

Variations below the species level:

Subspecies - morphologically and genetically different populations

– Geographically isolated

– Trinomial Latin name

Lodgpole pine (Pinus contorta)

Distribution of subspecies

Contorta

Latifolia

Murrayana

Variations below the species level:

Ecotype – genetic subpopulations adapted to specific physical environmental factors (interfertile with other ecotypes of same species)

– Differences are genetically based

– Differences may be morphological, physiological or phenological

– Ecotypes occur in distinct habitats

– Genetic differences are adaptive (enhance survival/reproduction)

Example: Achillea experiment (Clausen et al. 1948)

How does a new species “emerge”?

• New species evolve from preexistent species

• Speciation processes

–Cladogenetic speciation

–Reticulate speciation

–Phyletic speciation

Tim

e

Tim

e

Tim

e

Speciation Processes Cladogenetic speciation new species originate from a

splitting event in which each branch is a species

Reticulate speciation new species results from

interbreeding of two existing species

Phyletic speciation ancestral species transforms

into a single descendant species

From White 2008

Tim

e

Tim

e

Why could it be difficult to distinguish past

cladogenetic and phyletic speciation?

When can we say speciation happens?

• When two populations of one species become sufficiently different so they are no longer the same species (this will partly depend on the species concept that you choose)

• The divergence of an ancestral

species into 2 or more daughter

species requires genetic change

among populations

Brief genetics review

• Diploid organisms have 2 homologous copies of each chromosome (2n)

• Allele – one of two or more

alternative forms of a gene located at a single point (locus) on a chromosome -Homozygote: alleles same -Heterozygote: alleles different

• Populations contain individuals with different alleles

Mechanisms of genetic differentiation

• Mutation

• Genetic drift

• Natural selection

• Gene flow

Mutation

– Random changes in DNA

– Source of all new alleles

– If beneficial, allele frequency increases

– Negative & neutral mutations also occur

Genetic Drift

– Change in allele frequencies that occurs entirely from chance

– Largest effects in small,

isolated populations

Jaguars

Example

Genetic drift

1. Genetic drift acts faster and has more drastic results in

smaller populations

2. Can reduce genetic variation in populations

3. Genetic drift can contribute to speciation

Natural Selection

Genetic traits that enhance the survival and reproduction (fitness) of carriers relative to other individuals in the population will increase in frequency over time

Peppered moth, Manchester, UK

www.ibri.org/Books/ Pun_Evolution/Chapter3/3.2.htm

Gene Flow

Movement of alleles within a population or between populations caused by the dispersal of gametes or offspring

How does gene flow affect the progression of natural selection?

Adaptation and gene flow

• Niche of species not fixed

• Expect adaptations at periphery of distribution

• So, why don’t the peripheral populations of all species adapt to local conditions resulting in continual expansion of range?

Deer Mice, Geographic Variation

Geographic modes of speciation

Allopatric

Sympatric

Parapatric

Allopatric speciation

Geographic isolation cuts off gene flow between populations and generates reproductive barriers

Allopatric speciation modes:

– Vicariance

– Jump dispersal

Allopatric speciation: Vicariance

Dispersal barrier forms isolating populations of the species

e.g. rising sea level, plate tectonics, mountain building, glaciation, climate change

From White 2008

Allopatric speciation: Vicariance

Gondwanaland breakup and Ranidae family

Madagascar

India

A few individuals cross a preexisting barrier and start a new isolated population

Allopatric speciation: Jump dispersal

1

2

3

From White 2008

From White 2008

1

2

3

Allopatric speciation: Jump dispersal

Galapagos tortoises

Allopatric speciation: Jump dispersal

Video

Dispersal or Vicariance?

• Vicariance usually results in multiple species diverging at the same time

• Dispersal only one (or a few) species diverging at the same time

What if populations get together again?

Fail to interbreed or produce fertile descendants

Interbreeding produces fertile hybrids and backcross to parental populations

Interbreed but hybrids are less fit so natural selection acts against the hybrids (reinforcement)

Allopatric speciation

Sympatric speciation

Populations of ancestral species overlap extensively throughout population differentiation

From White 2008

Cichlids in Lake Malawi, eastern Africa

Sympatric speciation

Parapatric speciation

Overlap occurs only partially & in zone of marginal fitness (i.e. isolating forces operating)

Trait

Fitn

ess

Sympatric and parapatric speciation

How to overcome gene flow in a population?

1. Disruptive selection: selection that favors the extreme traits in a population

Apple flies Apple Hawthorns

Sympatric and parapatric speciation

How to overcome gene flow?

2. Chromosomal changes (e.g. polyploidy)

2n n fuse

3n

Gen 1 Gen 2

3n n fuse

4n

Allopatric vs. Sympatric speciation

• Allopatric speciation likely most predominant

• Sympatric/parapatric speciation likely occurs more commonly than suspected but hard to detect

Speciation is successful…what now?

Adaptive Radiation – divergence of a taxon into a number of different forms and adaptive zones

E.g. silversword plant family

Hawaiian Honeycreepers

Adaptive radiation,

trophic specialization

Extinction! “The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably

follows on the principle of natural selection; for old forms will be supplanted by new and improved

forms.” - Darwin

Extinction

• All species eventually go extinct

• Over 99.9% of all species that have existed are now extinct

• Counterbalances speciation

Animals Driven to Extinction

Passenger

pigeon Great auk Dodo Dusky seaside

sparrow Aepyornis

(Madagascar)

Old magazine illustration of hunters shooting Passenger Pigeons (Ectopistes migratorius). Note the density of the flight. (From

copy in Schorger, 1955.)

Historical range of passenger pigeon. The most similar species in existence today is probably the mourning dove (Zenaida macroura)

Two kinds of extinction

• Background extinction

– Continuously operating

– Low rate of extinction

• Mass extinction

– High rate of extinction (~75%)

– Short period of time (100,000s-millions years)

– Affects many lineages

Foote 2003

Background

Extinction

Mass

Extinction

Events

K-T

Few possible causes of mass extinctions

• Ice Ages

– Rapid change in climate

– Large areas uninhabitable

• Asteroids

– Catastrophic damage

– Debris/climate

• Volcanoes (massive volcanoes)

– Change in climate via particulates & SO2

Five or six mass extinctions?

Dramatic changes in fossil record

Eg – Cretaceous-Tertiary event ("K-T extinction")

Until 1980s - climate

O-S Late D

P-Tr

Tr-J K-T

K-T extinction

1980 – Asteroid hypothesis

1. Rock strata enriched with Iridium

2. Selective extinctions

3. Crater

6th mass extinction

• Magtoday < Magprior

• Ratetoday = Rateprior

Rate

of extinctions

Causes of 6th Mass Extinction

• Habitat destruction

• Habitat fragmentation

• Pollution

• Overharvesting

• Invasive species

• Climate change

• Human overpopulation