3the origin of species The beginning of new forms of life. Speciation, key process.Explains, macroevolution, the origin of new taxonomic groups.Two patterns:1) anagenesis- linear evolution in which the entire population changes to be different from and to replace the ancestral population. (Lamarckian)2) cladogenesis- branching evolution that creates a greater diversity of sister organisms. Each branch is called a clade.
5populations & species Populations are groups of individuals that are the same specieslive in the same geographical area at the same time.A species is the largest unit of populationreproductively compatibleGene flow: possible to produce viable fertile offspringRegardless of geographical barriers
6A species can be divided into subspecies, if they become reproductively isolated. Subspecies are different due to pre and/or postzygotic barriers:Prezygotic reproductive barriers:impede mating between species or hinder the fertilization of ova.b) Postzygotic reproductive barriers:prevent hybrid zygote from developing into a viable, fertile adult.If subspecies occur together, but remain reproductively isolated, these subspecies may eventually become two distinct species.
7The biological species concept is based on infertility rather than physical similarity. (horse + donkey = mule)
8species species is Latin for “kind” or “appearance” biological species concept:Population or group of populationsmembers have the potential to interbreed with one another to produce viable, fertile offspringcannot produce viable, fertile offspring with members of other species.
10Figure 24.5 A summary of reproductive barriers between closely related species
11prezygotic barriers: Factors that lead to Reproductive Isolation Ecographic Isolation: (geographic) isolationex. Asian and African elephantsB) Habitat Isolation: two species live in different habitats within the same area.ex. Garter snakes- one aquatic, one terrestrialC) Seasonal/Temporal Isolation: two species that breed during different times of the day, seasons, or years cannot mix their gametes.ex. Skunks: S. gracilis mates in late summer;S. putorius mates in late winter.
12Behavioral Isolation: signals to attract mates, elaborate behaviors, courtship rituals differ between species.ex. Eastern & Western Meadowlark songs differ
13Figure 24.3 Courtship ritual as a behavioral barrier between species
14Behavioral Isolation: signals to attract mates, elaborate behaviors, courtship rituals differ between species.ex. Eastern & Western Meadowlark songs differMechanical Isolation: anatomical incompatibility.ex. Insect copulatory organs don’t fit togetherfloral anatomy specialized to one pollinatorF) Gamete Isolation: incompatibility between sperm/egg.ex. Sperm of one species may not be able to survive in the environment of the female reproductive tract of another species.gamete recognition based on complementary molecules found on sperm/egg surfaces.
15postzygotic barriers: Examples that expend (waste) energy and lead to reproductive isolation G) Reduced Hybrid Viability: genetically incompatible hybrid zygotes abort development at some embryonic stage.ex. frogs in genus RanaH) Reduced Hybrid Fertility: results in completely or largely sterile hybrids. Chromosomal differences (structure or number) results in malformed gametes during meiosis.ex. Mule- (sterile) but robust hybrid of a horse and donkeyI) Hybrid Breakdown: first generation hybrids are viable but second generation offspring are feeble or sterile.ex. cotton
16How do new species arise? 1) By geographic isolation:This is the way the flora and fauna of the Galapagos Islands evolved.The barrier prevents gene flow.When two different species arise this way, it is called allopatric speciation.Greek: allos, other & patria, homeland
17Figure 24.8 Has speciation occurred during geographic isolation?
18The factors that lead to divergence: A) size of population (small) Allopatric speciation of squirrelsin the Grand CanyonThe factors that lead to divergence:A) size of population (small)The founder effect- genetic drift attributed to colonization by a limited number of individuals from a parent population.B) ability of organism to move about (isolation)C) harshness/ differences of new environment.
19A famous example of divergent evolution/speciation: Adaptive radiation Adapative radiation is evolution of many diversely adapted species from a common ancestor.Example: Darwin’s FinchesThe 14 species of Finch evolved from one species of ancestral finch.They have adapted to exploit different food sources with differently shaped beaks and feeding behaviors.They exhibit character displacement - evolutionary change driven by competition among species for a limited resource (eg. Food)Gause’s Law- competitive exclusion principle.
222) If two different species arise from a population without geographic barriers, it is called sympatric speciation.Examples of sympatric speciation: balanced polymorphism, polyploidy, hybridization.Polyploidy (having more than the diploid number of chromosomes) and chromosomal changeThis condition is common in plants and less common in animals.It can make offspring reproductively isolated from their parental species. (post-zygotic barrier is created in one generation)Polyploid population can self-pollinate, mate with other polyploids, or reproduce by asexual propagation.
23Figure 24.13 Sympatric speciation by autopolyploidy in plants
24Figure 24.15 One mechanism for allopolyploid speciation in plants
25Causes of Polyploidy:accidents during meiosis (autopolyploidy) results in the wrong number of sets of chromosomes in the gametesthe contribution of two different species to a polyploid hybrid (allopolyploidy) non-homologous chromosomes can’t align during meiosis.The chemical colchicine induces polyploidy.
26summaryIn allopatric speciation, a new species forms while geographically isolated from its ancestor.Sympatric speciation requires the emergence of some type of reproductive barrier that isolates the gene pool of a subset of a population without geographic separation from the parent population.
27PATTERNS OF EVOLUTIONDivergent Evolution- two or more species originate from a common ancestor. homologous traits.Convergent Evolution- two unrelated species that share similar traits. Arise not from a common ancestor but because each species has independently adapted to similar ecological conditions or lifestyles. analogous traits. Ex. Shark, porpoises, penguins bodiesEx. Eyes of squids and vertebrates.Parallel Evolution- two related species making similar evolutionary changes after their divergence. Ex. Marsupial and Placental mammals. analogous traits.Coevolution- tit-for-tat evolution of one species in response to new adaptation that appear in another species.ex. Pollinators-Flowering Plants
28Figure 25.10 Convergent evolution and analogous structures
30Punctuated Equilibrium (proposed by Stephen J. Gould) A catastrophic event or major genetic change occurs, rapid evolution and speciation occurs.The new population works back toward a long period of no evolution (few or no transitional forms.)The Cambrian Explosion represents a period in time(560 MYA) where we see diversification of animal phyla.
31Patterns of macroevolution Phyletic gradualism- evolution occurs by the gradual accumulation of small changes. The intermediate stages of evolution not represented by fossils merely testifies to the incompleteness of the fossil record.Punctuated Equilibrium- evolutionary history consists of geologically long periods of stasis with little or no evolution, interrupted or “punctuated” by geologically short periods of rapid evolution.