Anagenesis vs. Cladogenesis

Changes within a species One or more species arising fromone original species

Defining and Identifying Species

There have been many different attempts to define “species”:

• The biological species concept is based on reproductiveisolation.

• The morphological species concept is based on identifying evolutionarily independent lineages based on appearance of organisms.

• The phylogenetic species concept is based on reconstructingthe evolutionary history of populations.

Biological Species Concept

A species includes all populations that have the potential to interbreed and produce fertile offspring.

Example: A horse and a donkey can mate and produce a mule, but mules are sterile, so horses and donkeys are separate species

Problems with the biological species concept?

Morphological Species Concept Species are defined as groups with measurably different anatomical

features.

Useful for fossil species

Problems with the Morphospecies species concept?

Short-toed treecreeperCerthia brachydactyla

Common treecreeperCerthia familiaris

Cryptic Species

Phylogenetic Species Concept Species are defined as the smallest monophyletic group in a tree diagram

representing populations.

Problems with the Genealogical species concept?

Cryptic Species

A B C D E F G H JI

Why should we care about defining species?

The case of the red wolf

The case of the dusky seaside sparrow

Large ground finch Small ground finch

Ancestral ground finch

Speciation is a splitting event that creates two or moredistinct species from a single ancestral group.

How do species arise?

Allopatric speciation1. One large population is split into two populations that are separated by a

physical barrier (no gene flow)

• A population can colonize a new habitat (dispersal).

• A new physical barrier can split a widespread populationinto two or more isolated groups (a vicariance event).

2. The 2 populations diverge from each other due to drift, selection, mutation

3. The two populations become so different that when they come back into contact with each other, they can no longer reproduce with each other --> they are separate species

Gene flow

Population 1 in environment 1

Population 2 in environment 2

Two different populations of

the same species

Allopatric speciation1. One large population is split into two populations that are separated by a

physical barrier (no gene flow)

• A population can colonize a new habitat (dispersal).

• A new physical barrier can split a widespread populationinto two or more isolated groups (a vicariance event).

2. The 2 populations diverge from each other due to drift, selection, mutation

3. The two populations become so different that when they come back into contact with each other, they can no longer reproduce with each other --> they are separate species

Gene flow

Population 1 in environment 1

Population 2 in environment 2

Two different species

DISPERSAL AND COLONIZATION

1. Start withone continuouspopulation.Then, a colonistfloats to anisland on a raft.

2. Finish withtwo populationsisolated fromone another.

Island

Continent

Dispersal: Hawaiian fruit flies

VICARIANCE

1. Start withone continuouspopulation.Then a chanceevent occursthat changesthe landscape (river changes course).

2. Finish withtwo populationsisolated fromone another.

River

River changes course

What causes populations to diverge from one another? Genetic drift

• Random changes in allele frequencies that typically occur in small populations

Natural selection

• Populations/groups may experience selection for different traits

Sexual Selection

• Different traits may lead to reproductive success in different pops

Gene flow

• No!!! Gene flow (genetic exchange) between populations or groups will keep them similar to one another and prevent them from diverging

Gene flow vs. selection and drift

Hence, in order for 2 populations or groups of the same species to become different from one another the strength of selection and drift must outweigh the level of gene flow

If gene flow > selection & drift --> no divergence

If gene flow < selection & drift --> divergence

Reproductive Isolation

Two populations or groups become different species once they are reproductively isolated from one another

When 2 populations or groups are reproductively isolated, there is no longer any gene flow between them

Therefore, 2 different species have different gene pools

Is there selection for reproductive isolation?

When populations have partially diverged, they may come back into contact with each other --> secondary contact

• If the 2 populations have not diverged very much --> they may merge back together

• If the 2 populations have diverged significantly, their hybrid offspring may struggle to survive and reproduce

In the second case, individuals that do not mate with individuals of the other type will have higher fitness

Selection for traits that prevent interbreeding of populations = reinforcement

Male pied flycatcherMale collared flycatcher

When the 2 species are in allopatry (i.e., apart): the 2 species look very much alike

An Example of reinforcement: 2 species of flycatcher

When the 2 species are found in sympatry, the males of the species look very different from each other

Male pied flycatcher

Hybrid offspring have 70% lower survival rates

Females will preferentially mate with males that are most different from the other species --> males that look different have higher fitness

Male collared flycatcher

Conclusion

When there is the potential to interbreed and that interbreeding produces hybrids with lower fitness…

Selection favors traits that prevent the interbreeding, i.e. selecting favors reproductive isolating mechanisms

Reproductive Isolation

Reproductive isolating mechanisms:

• Prezygotic mechanisms make it very unlikely thatinterbreeding will even take place.

• Postzygotic mechanisms cause hybrids to become sterileor to fail to develop properly.

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egg sperm zygote

Prezygotic Isolating Mechanisms

Temporal Isolation- species breed at different times

• Ex: The western spotted skunk and the eastern spotted skunk overlap in distribution. Even though these species are very similar, the western spotted skunk breeds in late summer and the eastern in late winter.

Ecological Isolation

• Ex: two species of garter snake (genus Thamnophis) live in the same area, but one species lives mainly in the water, and the other is mainly terrestrial.

Behavioral Isolation

• Ex: fireflies - if the male flashes its light in the right pattern, he gets the girl. Different species have different light flashing patterns, and only the female of the same species recognizes the pattern as being courtship behavior.

Prezygotic Isolating Mechanisms

Mechanical Isolation

• This is when the male and female sex organs are not compatible, so fertilization cannot occur.

Gametic Isolation

• Ex: many marine species broadcast their eggs and sperm in the water. The gametes of more than one species can encounter each other, but they generally will not fuse.

Postzygotic Isolating Mechanisms

Hybrid Inviability: hybrids (in this case, the offspring of two different species) cannot survive embryonic development, or are weak and without human intervention, do not survive to reproduce.

• Ex: some species of frog (genus Rana) that overlap in habitat use sometime hybridize. Their offspring usually die during development. A few frail offspring sometimes survive development, but die shortly after they “hatch” into tadpoles.

Hybrid Sterility: This is when the offspring of two parent can't reproduce. They are sterile.

• Ex: like the horse and donkey producing a mule, which is sterile.

Hybrid breakdown: this is when the first generation of hybrids can reproduce, but the next generation of hybrids is sterile or the individuals cannot survive.

• Ex: different cotton species can produce fertile hybrid offspring, but the hybrid’s offspring die as seeds or grow into weak defective plants

Sympatric speciation occurs in groups that occupy the same geographic area. Gene flow is occurring

1. Within one population, 2 groups start to diverge from each other

• Natural selection can cause this to occur in spite of gene flow.

• Polyploidy, a type of mutation, can also cause sympatric speciation.

2. The two groups become separate species

Apple Maggot Flies: The evolution of a new species

Rhagoletis pomonella

Recently, the species R. pomonella shifted from its native host to introduced, domestic apples (Malus pumila)

Rhagoletis lays its eggs in fruit to develop -->Original host = hawthorn trees

Now: two genetically distinct groups that do not interbreed

Speciation by Polyploidy is most common in plants

Triploid zygote

Tetraploid parentDiploid parent

Diploid gametesHaploid gametes

Meiosis

Meiosis

Mating

(Two copies of each chromosome)

(Four copies of each chromosome)

(Three copies of each chromosome)

(One copy of each chromosome) (Two copies of each chromosome)

When these gametes combine, most offspringhave incorrect number of chromosomes.

Defining and Identifying Species

Different populations may diverge due to microevolutionary processes (mutation, selection, drift)

Once two groups of organisms are incapable of interbreeding, they are no longer exchanging genes--> 2 separate species (origin of new species = macroevolution)

Lack of gene flow makes a species independent.

Species are distinct types of organisms because theyrepresent evolutionarily independent groups.