Speciation and Extinction

Transcription

1 Speciation and Extinction

2 Definitions Speciation The origination of new species Extinction The death of all organisms in a species Adaptations Specific morphological changes as a result of environmental pressures Taxonomy Discipline that assigns names to organisms and classifies biological diversity (Linnaeus binomials)

3 What is a species?

4 What is a species? Revisiting a few species concepts Morphological Biological Phylogenetic

5 Morphological Species Concept Species can be distinguished from each other by morphological traits Classical species concept

6 Morphological SC limitations High plasticity

7 Morphological SC limitations Undetected speciation African bush elephant African forest elephant

8 Biological Species Concept Groups of interbreeding natural populations that are reproductively isolated from other such groups

9 Biological SC limitations Asexual reproduction

10 Biological SC limitations Spatially disjunct populations/species: -Reproductively isolated due to spatial segregation, but may be reproductively compatible -Lab tests

11 Biological SC limitations Hybridization: different species often hybridize naturally (very common in plants) Coulter pine Jeffrey pine

12 Biological SC limitations Fossils: impossible to know for sure if similar looking species interbreed

13 Phylogenetic Species Concept the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent (Cracraft 1983) Clusters must be: 1) Monophyletic (ancestor + all descendants)

14 Phylogenetic Species Concept the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent (Cracraft 1983) Clusters must be: 1) Monophyletic (ancestor + all descendants) 2) derived through an evolutionary process of descent from an ancestral lineage 3) diagnosable through examination of derived characters

15 Phylogenetic Phylogenetic tree

16 Phylogenetic SC limitations Can inflate the number of species if species designations are based on a few characters The genetics of a species may not always fully explain the differences in species.

17 Variations below the species level: Subspecies - morphologically and genetically different populations Geographically isolated Trinomial Latin name

18 Contorta Latifolia Murrayana Lodgpole pine (Pinus contorta) Distribution of subspecies

19 Variations below the species level: Ecotype genetic subpopulations adapted to specific physical environmental factors (interfertile with other ecotypes of same species) Differences are genetically based Differences may be morphological, physiological or phenological Ecotypes occur in distinct habitats Genetic differences are adaptive (enhance survival/reproduction)

20 Example: Achillea experiment (Clausen et al. 1948)

21 How does a new species emerge? New species evolve from preexistent species Speciation processes Cladogenetic speciation Reticulate speciation Phyletic speciation

22 Time Time Time From White 2008 Speciation Processes Cladogenetic speciation new species originate from a splitting event in which each branch is a species Reticulate speciation new species results from interbreeding of two existing species Phyletic speciation ancestral species transforms into a single descendant species

23 Time Time Why could it be difficult to distinguish past cladogenetic and phyletic speciation?

24 When can we say speciation happens? When two populations of one species become sufficiently different so they are no longer the same species (this will partly depend on the species concept that you choose) The divergence of an ancestral species into 2 or more daughter species requires genetic change among populations

25 Populations contain individuals with different alleles Brief genetics review Diploid organisms have 2 homologous copies of each chromosome (2n) Allele one of two or more alternative forms of a gene located at a single point (locus) on a chromosome -Homozygote: alleles same -Heterozygote: alleles different

26 Mechanisms of genetic differentiation Mutation Genetic drift Natural selection Gene flow

27 Mutation Random changes in DNA Source of all new alleles If beneficial, allele frequency increases Negative & neutral mutations also occur

28 Genetic Drift Change in allele frequencies that occurs entirely from chance Largest effects in small, isolated populations Jaguars

29 Example

30 Genetic drift 1. Genetic drift acts faster and has more drastic results in smaller populations 2. Can reduce genetic variation in populations 3. Genetic drift can contribute to speciation

31 Natural Selection Genetic traits that enhance the survival and reproduction (fitness) of carriers relative to other individuals in the population will increase in frequency over time

32 Peppered moth, Manchester, UK Pun_Evolution/Chapter3/3.2.htm

33 Gene Flow Movement of alleles within a population or between populations caused by the dispersal of gametes or offspring

34 How does gene flow affect the progression of natural selection?

35 Adaptation and gene flow Niche of species not fixed Expect adaptations at periphery of distribution So, why don t the peripheral populations of all species adapt to local conditions resulting in continual expansion of range?

36 Deer Mice, Geographic Variation

37 Geographic modes of speciation Allopatric Sympatric Parapatric

38 Allopatric speciation Geographic isolation cuts off gene flow between populations and generates reproductive barriers Allopatric speciation modes: Vicariance Jump dispersal

39 Allopatric speciation: Vicariance Dispersal barrier forms isolating populations of the species e.g. rising sea level, plate tectonics, mountain building, glaciation, climate change From White 2008

40 Allopatric speciation: Vicariance Gondwanaland breakup and Ranidae family Madagascar India

41 Allopatric speciation: Jump dispersal A few individuals cross a preexisting barrier and start a new isolated population From White 2008

42 Allopatric speciation: Jump dispersal From White 2008

43 Allopatric speciation: Jump dispersal Galapagos tortoises

44 Video

45 Dispersal or Vicariance? Vicariance usually results in multiple species diverging at the same time Dispersal only one (or a few) species diverging at the same time

46 Allopatric speciation What if populations get together again? Fail to interbreed or produce fertile descendants Interbreeding produces fertile hybrids and backcross to parental populations Interbreed but hybrids are less fit so natural selection acts against the hybrids (reinforcement)

47 Sympatric speciation Populations of ancestral species overlap extensively throughout population differentiation From White 2008

48 Sympatric speciation Cichlids in Lake Malawi, eastern Africa

49 Parapatric speciation Overlap occurs only partially & in zone of marginal fitness (i.e. isolating forces operating)

50 Fitness Sympatric and parapatric speciation How to overcome gene flow in a population? 1. Disruptive selection: selection that favors the extreme traits in a population Trait

51 Apple flies Apple Hawthorns

52 Sympatric and parapatric speciation How to overcome gene flow? 2. Chromosomal changes (e.g. polyploidy) Gen 1 Gen 2 2n fuse n 3n fuse n 3n 4n

53 Allopatric vs. Sympatric speciation Allopatric speciation likely most predominant Sympatric/parapatric speciation likely occurs more commonly than suspected but hard to detect

54 Speciation is successful what now? Adaptive Radiation divergence of a taxon into a number of different forms and adaptive zones E.g. silversword plant family

55 Hawaiian Honeycreepers Adaptive radiation, trophic specialization

56 Extinction! The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection; for old forms will be supplanted by new and improved forms. - Darwin

57 Extinction All species eventually go extinct Over 99.9% of all species that have existed are now extinct Counterbalances speciation

58 Animals Driven to Extinction Passenger pigeon Great auk Dodo Dusky seaside sparrow Aepyornis (Madagascar)

59 Old magazine illustration of hunters shooting Passenger Pigeons (Ectopistes migratorius). Note the density of the flight. (From copy in Schorger, 1955.)

60 Historical range of passenger pigeon. The most similar species in existence today is probably the mourning dove (Zenaida macroura)

61 Two kinds of extinction Background extinction Continuously operating Low rate of extinction Mass extinction High rate of extinction (~75%) Short period of time (100,000s-millions years) Affects many lineages

62 Mass Extinction Events K-T Background Extinction Foote 2003

63 Few possible causes of mass extinctions Ice Ages Rapid change in climate Large areas uninhabitable Asteroids Catastrophic damage Debris/climate Volcanoes (massive volcanoes) Change in climate via particulates & SO 2

64 Five or six mass extinctions? Dramatic changes in fossil record Eg Cretaceous-Tertiary event ("K-T extinction") Until 1980s - climate P-Tr O-S Late D Tr-J K-T

65 K-T extinction 1980 Asteroid hypothesis 1. Rock strata enriched with Iridium 2. Selective extinctions 3. Crater

66

67 Rate of extinctions 6 th mass extinction Mag today < Mag prior Rate today = Rate prior

68 Causes of 6 th Mass Extinction Habitat destruction Habitat fragmentation Pollution Overharvesting Invasive species Climate change Human overpopulation