1 2 3 4 5 6 Lecture 3: Chapter 27 -- Speciation Macroevolution Macroevolution and Speciation Microevolution Changes in the gene pool over successive generations; deals with alleles and genes Macroevolution includes The multiplication of The origin of evolutionary Explosive diversification following evolutionary novelty Mass extinctions Speciation The origin of new species What is a species? Unfortunately, there are many definitions Biological Species: species defined by Morphological Species (Morphospecies): characterizes species by Paleontological Species: focuses on morphological species known only from fossil record Phylogenetic Species: species as sets of organisms with unique (monophyletic groups) Our Definition We will mostly use the biological species concept Groups of actually (or potentially) natural populations, which are reproductively isolated from other such groups Reproductive isolation ensures evolutionary ; that is, it keeps the gene pools Limitations Does not work for populations, or Often hard to observe whether members of two groups interbreed Reproductive Isolation The existence of biological barriers that impede or prevent members of two species from producing viable, fertile offspring Usually divided into two groups Prezygotic barriers: before the zygote prevent and/or 1
Postzygotic barriers: after the zygote prevent development of or eliminate fertility of 7 8 9 10 11 Isolation: two species have range overlap but live in different habitats (aquatic vs. terrestrial) Isolation: two species have range and habitat overlap but breed at different time of day, season, or year (diurnal vs nocturnal, spring vs autumn) Isolation: two species have range, habitat, and temporal overlap but courtship rituals/behaviors are totally different (WHAT are you trying to do?!) Examples: different mating displays/dances in birds of paradise https://www.youtube.com/watch?v=w7qznwkqopo Isolation: two species have range, habitat, temporal, and behavioral overlap but morphological differences prevent successful mating (And just what do you think you re going to do with THAT thing?!) 12 13 14 15 16 17 Prezygotic Barriers (Postmating) Isolation: two species have range, habitat, temporal, behavioral, and mechanical overlap but gametes cannot reach each other and/or fuse (no fertilization) Reduced : Fertilization occurs but embryo fails to develop Reduced : Fertilization occurs, embryo develops, but hybrid is sterile A tiglion is a sterile hybrid resulting from cross between a tiger and a lion; a mule is a sterile hybrid resulting from cross between a donkey and a horse Hybrid : Fertilization occurs, embryo develops, hybrid is fertile, but when it mates with another hybrid or the parent species, the next generation is (Rare and only found in plants) 2
18 19 20 21 Speciation The process by which form Fall into two major categories speciation: species form in geographic separation from each other speciation: species form in the same geographic area Allopatric Speciation Allopatric Speciation: Adaptive : The evolution of many diversely adapted species from a single ancestor following (common in isolated habitats like islands) Allopatric Speciation: : Physical splitting of habitat Can isolate segments of populations leading to evolution of separate species from a single ancestor 22 23 24 25 Vicariance: Trumpeters Amazon River didn t exist about 3 million years ago Uplift of Andes Mountains caused Amazon River and its tributaries to form These rivers isolated populations of trumpeter birds More Vicariance: the effect of closing the Isthmus of Panama on Snapping Shrimp Sympatric Speciation Habitat differentiation and/or sexual selection: adaptation to exploit new resources, or to exploit novel traits/behaviors 26 27 28 Figure 24.8: Disruptive Selection on Fruit Preference in Flies Sympatric Speciation Polyploidy: accidents in cell division result in extra sets of chromosomes (more common in plants) Autopolyploid individuals produced when mutation results in double the chromosome number; all the chromosomes from same species Allopolyploid individuals created when parents belonging to different species mate and produce offspring with two different sets of chromosomes Both cases, polyploid individuals are reproductively isolated from the diploid population because breeding between diploids and tetraploids produces sterile offspring 29 30 Patterns of Evolutionary Change 3
: accumulation of changes gradually transforms a species : (branching evolution) splitting of a gene pool into separate pools, no gene flow Only pattern that the number of species 31 Tempo of Speciation Two models Gradualism: species originally quite similar but gradually diverge as they acquire unique adaptations Equilibrium: Species diverge rapidly at first, then reach relative stasis over time 32 33 34 35 36 37 Are these models mutually exclusive? Macroevolutionary changes Cumulative change acquired over many thousands of years Evolutionary novelties are important provides raw material (new alleles/genes) for selection : structures that evolved in one context but are co-opted for another Evolution can only improve structures/traits in the context of current utility Example of exaptation: feathers in birds What are feathers used for in birds? What did the first feathered dinosaurs use their feathers for? What Happens When Isolated Populations Come Into Contact? Unlikely interbreeding will occur Prezygotic isolation mechanisms come into play If some interbreeding occurs, it is rare and gene flow is minimal If no prezygotic isolation exists Populations fuse over time Three other possibilities Reinforcement Hybrids have fitness Selection favors individuals who do not interbreed = Example: experiments with fruit flies Two species that live in same area; these species will not interbreed has occurred Two species that live in different areas; they do interbreed No opportunity for reinforcement to have occurred 4
38 Hybrid Zones Where come into contact, may interbreed and form hybrids that are not dysfunctional and are intermediate in form Hybrid zone = geographical area where occurs and hybrids are common May be wide or narrow, and long- or short-lived New Species through Hybridization Another way in which speciation can occur 39 40 41 42 Key Concepts occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to selection, genetic drift, or mutation Populations can become genetically from each other if they occupy different geographic areas, if they use different habitats within the same area, or if one population is polyploid and cannot breed with the other Key Concepts Populations can be recognized as distinct if they are reproductively isolated from each other, if they have distinct characteristics, or if they form independent branches on a tree When populations that have diverged come back into contact, several outcomes are possible All for today 5