In past centuries, species were defined by apparent similarities to other known species– i.e., by morphological comparisons

Similar organisms were grouped together, and the most similar kinds of organisms that could somehow be differentiated were classified as different species of the same genus

Lions and tigers live in different places, they look and act differently, and if they mate, their offspring are sterile

But for many organisms, it is not so clear whether populations can be grouped together as one species or divided into two different species, each needing its own scientific name

Historical approach to identifying species

What do biologists mean when they refer to a “species” ?

At least 22 different definitions have been proposed to explain what a “species” is - in different kinds of organisms, different criteria are useful for defining what constitutes a species

We will discuss 2 major concepts of what constitutes a species:

(1) the Biological Species concept

(2) the Phylogenetic Species concept

Species concepts

Gene flow = movement of alleles between 2 populations

Migration movement of individuals between 2 populationsor Dispersal

Reproductive isolation = two groups of organisms that do not produce offspring, either because:

- they do not mate

- they mate, but produce hybrid offspring that are infertile

Big question: what can cause one population to split into 2 new populations that do not reproduce with each other?

Definitions for discussing speciation

Defined a species as a group of actually or potentially inter-breeding individuals The boundaries between species are defined by intrinsic barriers to gene flow that have a genetic basis - “intrinsic barrier” means something innate (built into the organism) that limits gene flow, not something external like a river or mountain range - although a river may form the boundary between two species, it cannot explain why those species aren’t able to reproduce

- reproductive isolation must be due to genetic differences between the two groups

Biological Species Concept (Mayr, 1942)

These barriers to gene flow may be expressed thru effects on: (1) key features of the mating system (mate choice, sperm-egg recognition, timing of gamete release) (2) ecological characteristics like habitat preference, that affect the probability of mating - pre-zygotic isolation: prevents hybridization before the zygote forms (sperm never meets egg) (3) developmental pathways, producing infertility in hybrid offspring - post-zygotic isolating mechanisms act after the zygote

forms, resulting in dead or infertile hybrid offspring

Slight problem with the Biological Species Concept: some hybridization among related species occurs in most groups!

How much is “too much”..? In many cases, organisms might potentially interbreed if they co- occurred, but since they live in different places they never meet

- hard to assess reproductive isolation for non-overlapping populations

However, BSC is still the “gold standard” for describing two populations as separate species: it recognizes that something has to stop gene flow for groups to become different over time

Biological Species Concept (Mayr, 1942)

Evolution is a change in allele frequencies w/in a population

In one sense, a “species” = a boundary to the spread of alleles

- a species could be defined as a unique set of alleles, not all shared with any other species

Different species are evolutionary independent, follow distinct trajectories (we also call these lineages)

Gene flow and speciation

Step 1 – gene flow is reduced or interrupted

Step 2 – populations diverge (especially important: mating preference traits change)

Step 3 – reproductive isolation arises

Step 4 – speciation ultimately results

What can initially interrupt gene flow between 2 populations?

How do species form?

Studies of biogeography revealed that the edges of a species’ range are often defined by a geographical boundary

- species distributions can be determined by geological features

This led to the prevailing opinion for most of the 20th century that speciation requires a long period of geographic isolation

This is termed allopatric speciation – the requirement of physical isolation as the first step in the speciation process

In the allopatric model, speciation results from differences in mating preference that arise from 2 possible sources:

1) genetic drift2) disruptive natural selection

Mechanisms of Isolation

Gene flow can be interrupted by physical barriers preventing migration between two populations

(A) vicariance – chance interruption of migration by changes in geography

- a new river, glacier, or mountain range forms

- an isthmus or land bridge arises, cutting off two previously connected bodies of water (B) colonization of a new, unoccupied habitat by founders

- islands

- postglacial lakes (Canada), volcanic crater lakes (Africa)

Allopatric speciation

Initially, there is migration between 2 nearby populations of a forest-dwelling animal with two alleles controlling color

Allopatric speciation #1: Genetic drift

because of gene flow, the allele frequencies will be the same in the two populations

Over time, a barrier to migration arises between populations:

Allopatric speciation via Genetic drift

mountain range

The allele frequencies in each population will start to change due to genetic drift (Step 1: gene flow has been interrupted)

The populations will slowly diverge as different alleles become fixed at many loci throughout the genome (Step 2: populations differentiate)

Allopatric speciation via Genetic drift

mountain range

Different alleles may eventually fix in the 2 populations

Allopatric speciation via Genetic drift

Eventually, differences accumulate by chance at loci controlling mating preference

- purple individuals prefer to mate with other purples

results in assortative mating – individuals sort themselves into separate mating pools, only reproduce with similar individuals

(Step 3: the populations become reproductively isolated)

... but due to assortative mating, the two types do not interbreed

Allopatric speciation via Genetic drift

If the mountain range disappears, the two populations can mix...

(Step 4: speciation has occurred)

Mayr’s model of allopatric speciation says nothing is needed for two populations to evolve into separate species except geographical isolation + time

- this remains the dominant view of how most speciation has taken place over Earth’s history

- note: there is no requirement for natural selection to play a role in species formation (Darwin who?)

Because drift is a random process, it may take a long time for enough fixed differences to build up to prevent two populations from interbreeding

- according to this model, species should only co-exist today if they were previously isolated for a long time

Consider a species found in a desert and a neighboring forest

Allopatric speciation #2: Differential selection

Hot, dry desert

Cool, rainy forest

Mayr also recognized that if populations were isolated in different environments, then selection could quickly fix the changes that ultimately result in speciation

Selection will favor different alleles in the desert and forest

Allopatric speciation via Differential selection

Hot, dry desert

Cool, rainy forest

Natural selection will favor forest-adapted individuals

Natural selection will favor desert-adapted individuals

After selection, the two populations will be genetically different

Allopatric speciation via Differential selection

Hot, dry desert

Cool, rainy forest

forest-adapted individualshave survived here

desert-adapted individualshave survived here

But, migration will keep mixing alleles between the populations

Allopatric speciation via Differential selection

Hot, dry desert

Cool, rainy forest

But, migration will keep mixing alleles between the populations

Allopatric speciation via Differential selection

Hot, dry desert

Cool, rainy forest

Natural selection won’t produce desert-adapted and forest-adapted populations when gene flow is high

Now: a barrier to gene flow arises between the 2 habitats

Allopatric speciation via Differential selection

Hot, dry desert

Cool, rainy forest

mountains

Now: a barrier to gene flow arises between the 2 habitats

Allopatric speciation via Differential selection

Hot, dry desert

Cool, rainy forest

Natural selection will favor forest-adapted individuals

Natural selection will favor desert-adapted individuals

mountains

Each population evolves into a distinct, well-adapted species

Allopatric speciation via Differential selection

Hot, dry desert

Cool, rainy forest

Forest-adapted population Desert-adapted population

mountains

Species #1 Species #2

Caribbean and Pacific oceans were linked until the Isthmus of Panama formed ~3 million years ago Prevented any more gene flow between marine organisms on each side of the new land barrier

Did this result in the evolution of new sister species pairs separated by the Isthmus?

Speciation and the Isthmus of Panama

Google maps

Speciation and the Isthmus of Panama

Knowlton et al. studied pairs of snapping shrimps that were morphologically similar, where one member of the pair was found on the Caribbean side and the other on the Pacific side

Knowlton & Weight 1998

Alphaeus cylindricus

The sister species still closely resembled each other – were they different species?

Sequenced part of a gene and also compared allozyme allele frequencies

Finally, did mating crosses to assess reproductive compatibility

P = pacific species

C = Caribbean species

Knowlton & Weigt 1998

= Pacific member of pair= Caribbean member

(1) based on DNA sequences, the members of 15 pairs were always each other’s closest relatives

(2) members of most pairs snapped at each other instead of mating

pre-zygotic isolation: mating rituals changed so much, sister species don’t look / smell / act right get treated like an enemy, not a potential mate

Speciation and the Isthmus of Panama

(3) when a male of one species was held with a female of its sister species [from the other side of Panama] for a month,

no offspring were produced except for one single pair

This is post-zygotic reproductive isolation –

even if you put sperm + egg together, dad’s alleles and mom’s alleles are too different to work together to build a zygote (= baby)

Thus, 3 million years is enough time for complete reproductive isolation to occur