The Origin of Species

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: MICROEVOLUTI ON Evolutionary change below

species level Example: change in allele frequencies in population over generations MACROEVOLUTIO N Evolutionary

change above the species level Examples: origin of new group of organisms 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 block

gene flow between the species & limit formation of hybrids Reproductive Isolation hybrids:

offspring that result from the mating of individuals from 2 different species or from 2 truebreeding varieties of same species

Reproductive Isolation single barrier may not prevent all gene flow combination of several barriers can effectively isolate a species gene pool 2 categories:

1. prezygotic barriers 2. postzygotic barriers 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 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 eastern & western spotted skunks: 1 breeds in late summer other in late winter

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 characteristics 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 mouse lemur lesser

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 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 todays

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 self-pollinating 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

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) 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

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 its the main reproductive barrier Allopatric & Sympatric Speciation are the 2 main modes of speciation Allopatric

Sympatric geographic requires isolation NS genetic drift sexual

selection emergence of a reproductive barrier that isolates a subgroup less common polyploidy sexual

selection Hybrid Zones A region in which members of different species meet & mate, producing at least some offspring of mixed ancestry.

Hybrids are more likely to die as embryos & show a variety of morphological abnormalities Hybrids of yellow-bellied & fire-bellied toads Bombino bambino Hybrids

Bombino variegata are more likely to die as embryos & show a variety of morphological abnormalities Hybrid Zones Hybrid Zones over

Time barriers between parent species may increase (strengthening of reproductive barriers) eventually hybrids no longer formed Hybrid Zones over

Time Or barrier weakens 2 species fuse Hybrid Zone over Time 3rd possibility: status quo is stabilized and hybrids continue to be produced

Speciation Time Course Patterns in the Fossil Record Punctuated Equilibrium: geologic periods of apparent stasis then sudden dramatic changes demonstrated by some species

other species only show gradual changes coined Gradualism Punctuated Equilibrium Punctuated Equilibrium & Gradualism Speciation Rates

Speciation Rates Hybrid sunflower that diverged reproductively from parents to form new species over short time

period F1 generation 5% fertility F5 generation >90% fertile Genetics of Speciation fundamental question for

biologists: How many genes must change to form a new species? Single Gene Examples found in a few cases:

Euhadra snail : single gene resulted in mechanical barrier to reproduction Small # of Genes 2 monkey flower species have different pollinators = strong

barrier to cross-pollination Mimulus lewisii Mimulus cardinalis Large #s of Genes Hybrid

of 2 species of fruit fly: Drosophila pseudoobscura results from gene interactions among at least 4 loci Large #s of Genes Western

sunflower: hybrid species thrives in dry sand but 2 parents live in nearby moister environments

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