David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis Biologia.blu B – Le basi molecolari della vita e dell’evoluzione The Origin of Species

What are species? How do new species arise? What happens when newly formed species come together? Why do rates of speciation vary? The Origin of Species

Species literally means “kinds”. We recognize most species by their appearance. Many species change little over large geographic ranges. The Origin of Species - What are species?

Linnaeus described species based on their appearance—the morphological species concept. Members of species look alike because they share many alleles. He originated the binomial system of nomenclature. The Origin of Species - What are species?

Species can be thought of as branches on the tree of life. Speciation: The process by which one species splIts into two or more daughter species, often gradually. The Origin of Species - What are species?

Speciation may be a gradual process

The biological species concept, proposed by Ernst Mayr: “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” This does not apply to asexually reproducing organisms. The Origin of Species - What are species?

Allopatric speciation occurs when populations are separated by a physical barrier. Also called geographic speciation. Thought to be the dominant mode of speciation. The Origin of Species - How do new species arise?

Allopatric speciation

Barriers can form as continents drift, sea level changes, glaciers advance and retreat, climate changes. The environments in which the isolated populations live are different, and so the populations evolve differently. The Origin of Species - How do new species arise?

Allopatric speciation can also occur if some individuals cross a barrier to form a new, isolated population. The 14 species of finches on the Galàpagos (Darwin’s finches) arose from a single species that colonized the islands from South America. The islands have different environments, and are sufficiently far apart for speciation to occur. The Origin of Species - How do new species arise?

Allopatric speciation among Darwin’s finches (part 1)

The Origin of Species - How do new species arise? Allopatric speciation among Darwin’s finches (part 2)

Sympatric speciation does not require physical isolation. Disruptive selection is required, such as in black-bellied seed crackers. The Origin of Species - How do new species arise?

Sympatric speciation most commonly occurs by polyploidy—duplication of the whole set of chromosomes. Chromosome duplication in a single species is autopolyploidy; combining of chromosomes from two species is allopolyploidy. The Origin of Species - How do new species arise?

Tetraploids are soon reproductively isolated from diploids

The Origin of Species - How do new species arise? The Hawaiian Islands have 800 species of Drosophila, many restricted to one island. Many of these species have resulted from founder events—species are descendents of individuals that dispersed among the islands.

The Origin of Species - How do new species arise? Founder events lead to allopatric speciation

Prezygotic reproductive barriers operate before fertilization occurs. Habitat isolation—e.g., Rhagoletis flies in the Hudson River valley. Temporal isolation—mating periods do not overlap. Mechanical isolation—differences in size and shape of reproductive organs; common in insects. The Origin of Species - What happens when the newly formed species come together?

Gametic isolation - eggs of one species don’t have appropriate chemical signals for sperm of another species; or sperm is not able to attach to and penetrate the egg. Behavioral isolation - individuals reject or fail to recognize potential mating partners. Floral traits of plants can influence the behavior of pollinators, and thus whether plants can hybridize. The Origin of Species - What happens when the newly formed species come together?

Hawkmoths favor flowers of one columbine species (part 1) The Origin of Species - What happens when the newly formed species come together?

Hawkmoths favor flowers of one columbine species (part 2) The Origin of Species - What happens when the newly formed species come together?

If populations are reunited before complete reproductive isolation has developed, interbreeding can occur. If hybrid offspring are fit and interbreed with both populations, gene pools are combined and no speciation occurs. If hybrid offspring are less fit, reinforcement may result in more prezygotic barriers. A hybrid zone can develop, and may persist for a long time while reinforcement develops. Hybrid zones make good natural laboratories for the study of speciation. The Origin of Species - What happens when the newly formed species come together?

Hybrid zones may be long and narrow The Origin of Species - What happens when the newly formed species come together?

The Origin of Species – Why do rates of speciation vary? Rates of speciation among groups vary greatly. Many factors influence the likelihood of speciation. Species-rich groups are more likely to speciate faster than species-poor groups. Speciation rates are likely to be faster in species with poor dispersal abilities, which can be separated by even narrow barriers. Populations with specialized diets are more likely to speciate.

The Origin of Species – Why do rates of speciation vary? Dietary shifts can promote speciation

The Origin of Species – Why do rates of speciation vary? Sexual selection also appears to increase rates of speciation. Example: birds with promiscuous mating systems. A)Birds of paradise—33 species B)Manucodes—5 species

The Origin of Species – Why do rates of speciation vary? Sexual selection in birds can lead to higher speciation rates

The Origin of Species – Why do rates of speciation vary? An evolutionary radiation is the proliferation of many species from a single ancestor. If the resulting species live in a wide array of environments, it is an adaptive radiation. Adaptive radiation is likely to occur in environments with abundant resources. A population may encounter underutilized resources when colonizing a new area that contains few species, such as islands. Adaptive radiations have followed mass extinctions.