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Mechanisms of Speciation, Maintaining a Species, and Types of Evolution (Chap. 20)

Mechanisms of Speciation, Maintaining a Species, and Types of Evolution (Chap. 20). Two methods of speciation (formation of a new species) based on geography. 1 . Populations are geographically isolated

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Mechanisms of Speciation, Maintaining a Species, and Types of Evolution (Chap. 20)

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  1. Mechanisms of Speciation, Maintaining a Species, and Types of Evolution (Chap. 20)

  2. Two methods of speciation (formation of a new species) based on geography 1. Populations are geographically isolated • allopatric speciation – a geographic process splits a population so they evolve separately and no longer interbreed • often results in endemic species (species that are found nowhere else) • Two main ways populations become geo. isolated: • I. Individual(s) end up in new hospitable place and survive resulting in new sp. • II. The slow process of geological change that divides a group such as continental drift

  3. Grand Canyon squirrels (Fig. 20.11 in text)

  4. Snapping shrimp in Central America

  5. 2. Populations are overlapping/not geographically isolated • sympatric speciation – occurs in populations occupying the same habitat • Maggot flies – one sp. infects hawthorn trees and the other apple trees Gene flow has been reduced between flies that feed on different food varieties, even though they both live in the same geographic area.

  6. hybridization between two species • polyploidy • disorders in meiosis can result with 2N gametes rather than N • union of these would give 4N zygote; instantly another species! • Plants: very common • Animals: very rare

  7. Geographic models of speciation emphasize the development of reproductive isolation between geographic population components of a species. (A) Allopatric: reproductive isolation develops between populations in physical isolation. (B) Founder Effect: reproductive isolation develops in a small population separated from the main body of the species, in consequence of the biology of small populations. (C) Parapatric: reproductive isolation develops among contiguous components of a population, in consequence of their limited vagility. (D) Sympatric: reproductive isolation develops within the "cruising range" of an existing species, in consequence of special biological properties (e.g., host-parasite specificity).

  8. Reproductive Isolating Mechanisms / Barriers Between Gene Pools • means by which species maintains their integrity and thereby continues as a separate species even though they live in the same habitat • normally do this by preventing hybrids from forming (prezygotic) or ensure that they fail to be successful (postzygotic)

  9. Prezygotic barriers – mechanisms that prevent formation of a zygote Four types (look in text) 1. Ecological barriers – two sp. share same territory but occupy different parts of it • Maggot flies – one sp. infects hawthorn trees and the other apple trees • lions and tigers (also a postzygotic barrier) 2. Behavioral barriers – behavior of two sp. is very different and therefore don’t breed • lions/tigers social tendencies • meadowlarks’ diff. songs • trouts’ diff. mating time(temporal isolation)

  10. 3. Mechanical barriers – genitalia don’t fit or certain insects only pollinate specific plants because they only fit/visit those plants 4. Gametic barriers – even if two different species did mate, fertilization of egg won’t take place because incompatibility between egg and sperm

  11. Postzygotic barriers - mechanisms that act after a zygote is formed to prevent the survival of the hybrid species Three types 1. Hybrid inviability – zygote dies because of genetic differences 2. Hybrid sterility – F1 generation survives but cannot reproduce. • ex. female horse + male donkey = mule • ex. lion + tiger = ligers or tigons 3. Hybrid breakdown – F1 generation survives and reproduces, but F2 cannot produce offspring

  12. Patterns of Evolution • Divergent evolution – process where sp. become increasingly different over time. • adaptive radiation is a form of divergent evolution where a number of diverse species form from a single ancestral one. • ex. Galapagos finches observed by Darwin

  13. Convergent evolution – different species “grow” more alike due to similar environments. Occurs on all levels – biochemical to morphological • ex. Marsupials in Australia have a striking resemblance to placentals on other continents. (Fig. 20.18 in text)

  14. Coevolution – the reciprocal influence of two species on each other’s evolutionary direction • ex. predator-prey relationships • ex. yucca plant is dependent on the yucca moth for pollination and the moth only lays its eggs on the plant where larva eat the seeds (Fig. 20.19)

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