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This week a major ocean sailing race got underway, the Volvo Ocean Race, which has six 70-ft crewed monohulls racing around the world in nine legs. This is the “American” entry, sponsored by Puma. Third Exam scores/grades In counter-current flow systems (widely used in industry), as exemplified by fish respiration, the blood that is richest in oxygen encounters water that is richest in oxygen. That way, it can continue to absorb oxygen throughout the gills. Mammals are not able to do this nearly as efficiently. IB104 - Lecture 32 - Speciation Reading - Chapter 18.9-11 A Median B C D F Taxonomy. Biologists classify organisms into species, group them into genera, then higher levels of the taxonomic system introduced at the start of this class. Broadly speaking, “species” means “a kind of something”, e.g. species of metals or chairs or pasta. For biologists this is also roughly the meaning of the term when applied to asexually reproducing organisms like bacteria, archaea, and many protists. But for sexually reproducing organisms like most eukaryotes and essentially all animals it is a little more complicated. Since the evolution of the extraordinary diversity of organisms of this planet involves the formation of new species, or speciation, we have an entire lecture about it. So formally, speciation is the formation of two or more new species from a single original species. It is the ultimate source of all the biodiversity on the plant. 1 Biological species concept Species numbers The most widely employed species concept for animals is one articulated by a German evolutionary biologist working at Harvard, Ernst Mayr: “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups” This definition leaves open a major problem, which is what about “potentially interbreeding” populations that are currently isolated from each other purely by distance? A classic example is humans, which for thousands of years formed isolated populations on different continents, yet were clearly potentially interbreeding, as is now happening. Over 1 million species have been named by taxonomists, although preciously little is known about the vast majority of them. Estimates of total species numbers range widely, from 3 to 10 million, for example from insecticide fogging experiments in the tropics. The vast majority of these undescribed species are microorganisms, fungi, and invertebrate animals like nematodes and insects. Nevertheless, new vertebrate species are still occasionally described, such as this new bovine from Vietnam in 1994. And sometimes biologists recognize that one “species” is actually two separate genetically distinct, non-interbreeding species, e.g. African elephants split into forest and savannah forms (see handout). Premating isolation Failure to interbreed, or not having the potential to interbreed, means that differences in courtship or mating behavior or physiology prevent most inter-specific mating. This is known as pre-mating isolation, for example: Frog calls – males call and females hear Cricket calls – males call and females hear Dragonfly colors – males are colorful and females see Moth pheromones – females produce pheromone, males smell Marine invertebrate sperm and egg recognition – simultaneous coordinated release into the oceans, followed by specific recognition of eggs by sperm and vice-versa Frog calls 2 Cricket calls Dragonfly colors Moth pheromones Giant clam spawning Coral spawning 3 Postmating isolation How species diverge Hybrids between distant species are inviable, even if one forces mating. Hybrids between closely related species are commonly sterile, and rarely produced, except in unusual situation like zoos. Even fertile hybrids might have ecological disadvantages in the wild. It is impossible to select directly for postmating isolation, because any allele that disposes a hybrid towards sterility or inviability is lost with that individual. Therefore postmating isolation evolves as an incidental consequence of other genetic changes, and especially quickly in spermatogenesis and oogenesis, leading to hybrid sterility. With the passage of lots of time, the genes involved in delicately balanced developmental programs diverge enough that inter-species hybrids don’t even develop, they are inviable. Most premating isolation probably evolves the same way, for example as a byproduct of rapid changes due to sexual selection changing how the sexes recognize each other, or as a result of adaptations to new environments. Mule Liger Geography of speciation Allopatry (“different place”) – populations are geographically isolated from each other, which obviously depends greatly on the biology of the species. Given enough time they will inevitably diverge genetically, perhaps eventually enough to become two species. Sympatry (“same place”) – populations are not geographically isolated in any major way, yet diverge genetically because of divergent selection to exploit different niches, habitats, etc. Parapatry (“adjacent place”) – when divergent populations or species abut each other, persumably after diverging in allopatry. They might fuse, they might remain as is with a hybrid zone between them, or they may diverge enough to coexist. Allopatric speciation This is commonly thought to the the major way that animals speciate. Given enough time, populations isolated by some kind of geographic barrier or simply distance, will inevitably diverge genetically. This results from all the factors we considered in the last lecture on population genetics. For example, founding of a small new population will involve genetic drift, as would population bottlenecks in the separate populations. Random mutations will be different between the two populations. They will likely encounter different pathogens, parasites, and competitors. If sufficiently far apart, they will also experience different habitats and climates, and hence be subject to different selection pressures. Sexual selection will be acting independently in the two populations, perhaps leading to behavioral changes. All of the above are capable of leading to the genetic differentiation that underlies both postmating and premating isolation. 4 Speciation on archipelagos (groups of islands) Allopatric speciation is most convincingly evident in archipelagos, where a single colonizing species has the opportunity to lead to multiple additional species as individuals very occasionally colonize other islands, and eventually, once differentiated, recolonize the original island and co-exist with the first species, etc. The Hawaiian islands and honeycreepers. The main 1 A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat. Later in time, a few 1 individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence. Speciation may also follow colonization of islands 3 and 4. And it may follow invasion of island 1 by genetically different descendents of the ancestral species. 2 3 4 2 1 2 3 4 Honeycreepers (I’iwi) ±50 species Photo by Eric VanderWerf Megalagrion damselflies - 22 species Laupala crickets - 37 species Hawaiian islands are only 5 Myr old, but the island chain is 50 Myr old. Drosophila flies ±800 species FOUNDER SPECIES Sympatric speciation This is generally thought to be a less common route to speciation in animals, although a growing number of biologists think it could be widely important. The difficulty is understanding how two populations can diverge genetically while remaining in the same geographic area and hence are likely to continue to interbreed leading to gene flow homogenizing their gene pools. The answer may be that a fairly radical behavioral shift to a new niche can lead to rapid reductions in hybridization, and selection for adaptation to this new niche can maintain genetic divergence despite some hybridization. 5 The apple maggot fly, Rhagoletis pomonella Rhagoletis pomonella One possible example of sympatric speciation involves a “true” fruit fly in North America, family Tephritidae, which normally lives on hawthorns, but recently colonized a new host, the apple, which was introduced to North America from Europe/Asia about 200 years ago. There is no reason to believe that apple plants in North America were ever geographically isolated from hawthorns, so this colonization and genetic divergence appears to have occurred in sympatry, and relatively rapidly. Because these flies mate on their hosts, it is conceivable that a rapid change in their behavior mediated this event. Today these “host races” or “incipient species” remain morphologically identical, that is, they look exactly alike, but they behave differently. They preferentially seek out hawthorn or apple fruits, mate on them, lay their eggs on them, and even time their emergence from the pupal stage to coincide with when their fruit will be most abundant. Some of the work on this story has been done here, by my colleague Stewart Berlocher. We are sequencing the genome of this fly to examine the genetic differences between these two “species” more precisely. So far there are no absolute genetic differences between them, only frequency differences, e.g. allele A is 80% in hawthorn flies, but 5% in apple flies. Interbreeding and hybridization continue at an estimated 5% per year, which normally should lead to fusion of the gene pools, so divergent selection must be keeping them different, e.g. hybrids are penalized. Mating on a hawthorn tree Siting on an apple Parapatric speciation Adjacent populations might evolve into distinct species while maintaining contact along a common border, although the other two possibilities (fusion, and stasis) are also possible. HYBRID ZONE BULLOCK S ORIOLE BALTIMORE ORIOLE 6