The Origin of Species
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The Origin of Species chapter 24 Campbell and Reece Speciation process by which one species splits into 2 or more species Speciation explains both the diversity of life and the unity of living things. Speciation : forms bridge between: MICROEVOLUTION MACROEVOLUTION Evolutionary Evolutionary change below change above the species level species level Example: Examples: change in allele origin of new frequencies in group of population over organisms generations impact of mass extinctions Biological Species Concept Species: a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring members of a species cannot produce viable, fertile offspring with other groups emphasizes the separateness of species due to reproductive barriers What holds the gene pool of a species together? Gene Flow: transfer of alleles between populations of same species exchange of alleles tends to hold populations together genetically Reproductive Isolation existence of biological barriers that keep members of 2 populations from interbreeding over long periods of time Reproductive Isolation hybrids: offspring that result from the mating of individuals from 2 different species or from 2 truebreeding varieties of same species Prezygotic Barriers block fertilization from happening by: 1. impeding members of different species from attempting to mate 2. preventing attempted mating from being completed successfully 3. hindering fertilization if mating was completed successfully Postzygotic Barriers reproductive barrier that prevents hybrid zygotes produced by 2 different species from developing into viable, fertile adults 1. lethal developmental errors 2. infertility in viable offspring Types of Prezygotic Reproductive Barriers Habitat Isolation 2 species that occupy different habitats w/in same area may rarely interact example: 1. Types of Prezygotic Reproductive Barriers 2. Temporal Isolation: species breed during different times of day, different seasons, or different years Types of Prezygotic Reproductive Barriers 3. Behavioral Isolation: Courtship rituals used to attract mates are effective barriers Types of Prezygotic Reproductive Barriers 4. Mechanical Isolation: morphological differences prevent successful completion even if attempted Types of Prezygotic Reproductive Barriers 5. Gametic Isolation: Sperm of 1 species may not be able to fertilize egg of another: ◦ Reproductive tract hostile to sperm ◦ Sperm does not have enzymes to penetrate zona pellicida of another species Types of Postzygotic Reproductive Barriers Reduced Hybrid Viability: hybrids development or survival is impaired 1. Types of Postzygotic Reproductive Barriers 2. Reduced Hybrid Fertility: hybrids may develop and be healthy but they are not fertile Types of Postzygotic Reproductive Barriers Hybrid Breakdown: Some 1st generation hybrids are fertile but those offspring are feeble or sterile 3. Species There is no single, universally applicable species concept that can define, explain, and identify all species. There are multiple ways to think about & define species. Biological Species Limitations unable to use these characterisitics on fossils of extinct species only applies to organisms that reproduce sexually only applies where there is no gene flow Other Definitions of Species These dfns emphasize the unity w/in a species. 1. morphological species concept: characterizes a species by a structural feature applies to species that reproduce sexually or asexually how scientists distinguish most species disadvantage: subjective Morphological Species Concept Problems: ◦ domestic dogs may look very different but are still same species ◦ mouse lemurs look very similar but there are 18 species of them ◦ grey mouse lemur lesser mouse lemur Ecological Species Concept views species in terms of its niche ◦ the sum of how members of the species interact with the nonliving & living parts of their environment ◦ asexual or sexual species ◦ emphasizes role of disruptive NS as organisms adapt to different environmental conditions Phylogenetic Species Concept defines species as smallest group of individuals that share a common ancestor, forming one branch on the “tree of life” determining degree of differences is difficult Species There are >20 other ways to define species Speciation can take place with or w/out geographic separation Speciation can occur in 2 main ways: 1. Allopatric Speciation 2. Sympatric Speciation ALLOPATRIC SPECIATION “other country” Gene flow is interrupted when population is divided into geographically isolated subpopulations Allopatric Speciation Process Once geographic separation has occurred, the separated gene pools will each have their own mutations NS & genetic drift may alter allele frequencies in different ways in each subpopulation group Allopatric Speciation Evidence of Allopatric Speciation There are many studies & examples supporting this type speciation Indirect support: regions that are isolated or highly subdivided have more species than regions w/out those features Drosophila Experiment Sympatric Speciation “same country” occurs in populations in same geographic area less common than allopatric occurs if gene flow is reduced by factors like: ◦ polyploidy ◦ habitat differentiation ◦ sexual selection POLYPLOIDY means extra sets of chromosomes can occur in animals gray tree frog (Hyla versicolor) around Great Lakes Polyploidy much more common in plants estimate: 80% of today’s plants species have ancestors that formed by polyploid speciation 2 forms 1. Autopolyploid Individual has >2 chromosome sets all derived from a single species Plant polyploidy Plant Polyploidy tetraploid plant can produce fertile tetraploid offspring by selfpollinating or mating with other tetraploids 2. Allopolyploid 2 different species interbreed making a hybrid hybrid reproduces asexually over generations sterile hybrid fertile polyploid (called an allopolyploid) Allopolyploids can breed with each other but not with either of their parents so are a new species rare: 5 new plant species since 1850 documented Mimulus peregrinus Allopolyploids include many agricultural crops Triticum aestivum (bread wheat) has 6 sets chromosomes (2 pair from each of 3 parents), an allohexaploid 1st polyploidy event probably occurred ~8,000 yrs ago as spontaneous hybrid Top 2 parents bottom: Triticum aestivum Allopolyploids plant geneticists “create” new polyploids making hybrids with desired characteristics use chemicals that induce meiotic & mitotic errors Habitat Differentiation Sympatric speciation can occur when genetic factors enable a subpopulation to exploit a habitat or resource used by the parent population Rhagoletis pomonella North American apple maggot fly Original habitat was the native hawthorn tree Habitat Differentiation As apples mature faster than hawthorn fruit, NS has favored flies with rapid development have an allele that benefits flies that feed off only 1 or the other not both (post-zygotic barrier to reproduction) The flies feeding on apple trees now show temporal isolation from flies still eating hawthorn fruit (prezygotic restriction to gene flow) Sexual Selection can also drive sympatric speciation: cichlid fish Pundamilia pundamilia Cichlid Fish >600 species found in Lake Victoria originated in past 100 000 yrs hypothesis : subgroups of original population adapted to different food sources genetic divergence female preference for mates may also be a factor: 1 species breeding males have blue back another species has orange back Breeding Cichlids Colors Sexual Selection Study placed the 2 subspecies in same tank used monochromatic orange light so both appeared very similar females bred with either Conclusion: mate choice by females is based on male coloration so it’s the main reproductive barrier (prezygotic behavior) Allopatric & Sympatric Speciation are the 2 main modes of speciation Allopatric geographic isolation NS genetic drift sexual selection Sympatric requires emergence of a reproductive barrier that isolates a subgroup less common polyploidy sexual selection Hybrid Zones
