What is a species? • Biological species concept (Mayr) A species is a group of populations whose Chapter 16 individuals interbreed with each other (or at least are capable of interbreeding), but not with members of other such groups. Interbreeding: Includes both mating and production of Origin of species fertile offspring Problems with definition: Can’t always tell whether members of different groups can/do interbreed. Doesn’t help define species that reproduce asexually (i.e. bacteria)
How do new species form? Allopatric speciation • For speciation to occur for a pair of • Geographic isolation Impassible physical barrier populations, two factors are necessary: • Genetic divergence Isolation of populations Natural selection No gene flow between them! Genetic drift Founder effect Genetic divergence of populations • Reproductive isolation Become different enough genetically that they could no Due to accumulated genetic longer interbreed/produce vigorous, fertile offspring if differences reunited
A case of allopatric speciation? Allopatric speciation • Kaibab squirrel and Abert squirrel live on opposite rims of the Grand Canyon. • Known example Are they really different species? Herring gulls (UK) & Molecular data says they’re really subspecies. lesser blackbacked gulls (Continental Europe)
No longer interbreed, even when the two species meet.
1 Sympatric speciation A case of sympatric speciation? • Ecological isolation • Apple-flies and hawthorn-flies (genus Distinct niches Rhagoletis ) show assortative mating. • Genetic divergence Are they really different species? Natural selection (best-fit to niche) Assortative mating Molecular data indicates that they are speciation in process since 1800 (incipient speciation). • Reproductive isolation Due to accumulated genetic differences
Changes in chromosome number lead to Adaptive radiation sympatric speciation • Many species may evolve from an ancestral • Specific to plants species over a short period of time. • Plants can double their • Can occur if a species moves into an area chromosome number and with a variety of unoccupied niches with become new species. differing selective pressures. Plant breeders take advantage Example 1: Darwin’s finches (Galapagos) of this using colchicine. Example 2: Over 300 species of cichlid fish in colchicine causes plants to lake Malawi double their chromosome In both examples, species differences reflect number. adaptations to different food resources.
Adaptive radiation is often seen in islands Adaptive radiation: Darwin’s finches (Galapagos) • Islands, due to natural disasters like volcanoes Beak size and shape were related to food type and hurricanes, are often “cleared” of Other aspects were similar, suggesting the species. birds were related Explanation Species from nearby Ancestral species arrived in mainlands arrive the Galapagos. Unoccupied niches were Speciate to fill empty exploited niches on islands. Sympatric speciation occurred.
2 Adaptive radiation: Maintenance of reproductive isolation Cichlid fishes (Lake Malawi) • Pre-mating isolation • These are three of over 300 Geographic isolation cichlid species! Geographic barriers prevent mating. • Body shape, mouth size and Ecological isolation coloration reflect differences in Ecological barriers prevent mating feeding strategies and habitat Temporal isolation • Explanation: Different species mate at different times of the year. Ancestral species arrived at the Behavioral isolation lake Different species use different courtship signals Unoccupied niches were exploited Physiological barriers Sympatric speciation occurred. Different species don’t fit. Known as a “lock & key” physiology.
Maintenance of reproductive isolation Maintenance of reproductive isolation
• Pre-mating isolation • Pre-mating isolation (cont.) Different habitats prevent cross-breeding Different species breed at different times of year. Example: Bishop pines (in photo) release pollen in the summer, Example: Each species of fig wasps mates and lays while Monterey pines release pollen in the spring eggs within a particular fig species.
Maintenance of reproductive isolation Maintenance of reproductive isolation
• Pre-mating isolation (cont.) • Post-mating isolation Different species may have different reproductive organs. Example: Complex sex organs of insects such as damselflies Sperm may fail to fertilize female’s egg.
Very common in invertebrates.
3 Maintenance of reproductive isolation Evolutionary trees
• Post-mating isolation (cont.) Hybrid offspring may be poor survivors. Particular genetic combination may give rise to intermediate characteristics not well-adapted to the environment.
Hybrid offspring may be infertile. Classic example: Mules, crosses between horses and donkeys, are infertile.
Evolutionary trees Evolutionary tree
• Based on shared characteristics or DNA sequence data. • Trees can be BLOH1 The more shared traits/DNA, the closer related the two based on DNA 63 CANA5 CANA7 species are. SWLMN1 (molecular trees) SWLMN2 SWLMN3 or physical SWLMN4 SWLMN31 characteristics. 83 MFMN5 55 MFMN7 MLMN14 MLMN40 MLMN55 RLWI55 92 WI2WI10 Each node MIQU48 SORMN1 represents an 64 LBMN21 82 LBMN24 LBMN26 evolutionary HLOH2 100 HLOH3 change in traits HLOH4 92 HLOH5 HLOHF3BG HLOH57 HLOHFIT D. albineus D. vitattus
Extinction Extinction • Extinction is the death of all members of a • Great Auk species or subspecies. Breeding colonies once widespread through North • Causes: Atlantic Very limited habitats Population fragmentation by hunting for food/bait Habitat change Scarcity high-price trade Overpredation in skins & eggs
Overhunting Last auk: Iceland, 1844 Breeding pair and egg destroyed
4 Extinction: Tasmanian wolf Extinction • Heath hen • Tasmanian Wolf Eastern species of Marsupial predator prairie chicken.
Extinction caused by Overhunting reduced overhunting by sheep population. farmers Catastrophic fire killed most of females and • Movie link eggs. Last male heath hen looking for a mate in their traditional lek Last male died 1932.
Near Extinctions Near extinctions
• California Condors • Przewalski's Horse or Takhi Population reduced to 22 Population reduced to 31 individuals. individuals. Causes include: 1. Habitat fragmentation Near extinction due to:
2. Lead poisoning from eating 1. habitat loss: grasslands hunting kills. 2. habitat fragmentation 3. poaching limited access to water.
Captive breeding and other Conservation efforts have conservation efforts have increased population to 1,500 increased the population to 326 currently. currently.
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