Anagenesis vs. Cladogenesis One or more species arising from one original species Changes within a species

Defining and Identifying Species There have been many different attempts to define “species”: The biological species concept is based on reproductive isolation. The morphological species concept is based on identifying evolutionarily independent lineages based on appearance of organisms. The phylogenetic species concept is based on reconstructing the 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? Cryptic Species Short-toed treecreeper Certhia brachydactyla Common treecreeper Certhia familiaris

Phylogenetic Species Concept Species are defined as the smallest monophyletic group in a tree diagram representing populations. Problems with the Genealogical species concept? A B C D E F G H I J Cryptic Species

Why should we care about defining species? The case of the red wolf The case of the dusky seaside sparrow 1980- last 6 sparrows were male

Ancestral ground finch How do species arise? Speciation is a splitting event that creates two or more distinct species from a single ancestral group. Large ground finch Small ground finch Ancestral ground finch Figure: 23.1 Caption: When lack of gene flow isolates populations, they tend to diverge. This is because mutation, drift, and selection act on the populations independently. As a result, they acquire distinctive characteristics. For example, the large ground finch and medium ground finch shown here are derived from the same ancestral population. This ancestral population split into two populations that were isolated by lack of gene flow. Because the populations began evolving independently, they acquired the distinctive characteristics observed today.

Allopatric speciation 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 population into two or more isolated groups (a vicariance event). The 2 populations diverge from each other due to drift, selection, mutation 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 Two different populations of the same species Gene flow Population 1 in environment 1 Population 2 in environment 2

Allopatric speciation 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 population into two or more isolated groups (a vicariance event). The 2 populations diverge from each other due to drift, selection, mutation 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 with one continuous population. Then, a colonist floats to an island on a raft. Island Continent 2. Finish with two populations isolated from one another. Figure: 23.8 left Caption: These diagrams illustrate the difference between dispersal and vicariance. When dispersal occurs, colonists establish a population in a novel location. In vicariance, a widespread population becomes fragmented into isolated subgroups.

Dispersal: Hawaiian fruit flies Figure: 23.4 Caption: Water snakes occupy most of the larger islands in Lake Erie, along with mainland habitats on either side of the lake. On island habitats, unbanded individuals like the one in the top photo are quite common. All or most of the water snakes found in mainland habitats are strongly banded, such as the individual in the bottom photo.  Question Banded snakes are most common on the island closest to shore. Why?

VICARIANCE 1. Start with one continuous population. Then a chance event occurs that changes the landscape (river changes course). River 2. Finish with two populations isolated from one another. River changes course Figure: 23.8 right Caption: These diagrams illustrate the difference between dispersal and vicariance. When dispersal occurs, colonists establish a population in a novel location. In vicariance, a widespread population becomes fragmented into isolated subgroups.

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

An Example of reinforcement: 2 species of flycatcher Male pied flycatcher Male collared flycatcher When the 2 species are in allopatry (i.e., apart): the 2 species look very much alike

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

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 that interbreeding will even take place. Postzygotic mechanisms cause hybrids to become sterile or to fail to develop properly. + 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 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. The two groups become separate species

Apple Maggot Flies: The evolution of a new species Rhagoletis pomonella Rhagoletis lays its eggs in fruit to develop --> Original host = hawthorn trees Recently, the species R. pomonella shifted from its native host to introduced, domestic apples (Malus pumila) Now: two genetically distinct groups that do not interbreed

Speciation by Polyploidy is most common in plants

When these gametes combine, most offspring Diploid parent Tetraploid parent (Two copies of each chromosome) (Four copies of each chromosome) Meiosis Mating Haploid gametes Diploid gametes (One copy of each chromosome) (Two copies of each chromosome) Triploid zygote Figure: 23.6 Caption: The mating diagrammed here shows why normal and polyploid individuals are reproductively isolated. The example illustrates just two chromosomes, although most organisms have at least four. Meiosis (Three copies of each chromosome) When these gametes combine, most offspring have 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 they represent evolutionarily independent groups.