Chapter 24 The Origin of Species

What You Need to Know: The difference between microevolution and macroevolution. The biological concept of a species. Prezygotic and postzygotic barriers that maintain reproductive isolation in natural populaitons. How allopatric and sympatric speciation are similar and different. How autopolyploid or an allopolyploid chromosomal change can lead to sympatric speciation. How punctuated equilibrium and gradualism describe two different tempos of speciation.

Do Now # 8 Problem 1: In humans, having the Rh blood antigen is dominant over not having the Rh antigen. In the United States, 84% of the population is Rh positive. How many individuals are homozygous. (Assume it is Hardy-Weinberg)

Homework 7 Problem 2: One in every 1700 Caucasian babies that are born have cystic fibrosis. Cystic fibrosis is an autosomal recessive disorder (c). What is the frequency of the recessive allele? How many of every 1700 babies are heterozygous?

Speciation = origin of species Microevolution: changes within a single gene pool Macroevolution: evolutionary change above the species level cumulative effects of speciation over long periods of time

Biological Species Concept Species = population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring Reproductively compatible Reproductive isolation = barriers that prevent members of 2 species from producing viable, fertile hybrids

Types of Reproductive Barriers Prezygotic Barriers: Impede mating/fertilization Types: Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Postzygotic Barriers: Prevent hybrid zygote from developing into viable adult Types: Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

Types of Reproductive Barriers REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

Types of Reproductive Barriers REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

Other definitions of species: Morphological – by body shape, size, and other structural features Ecological – niche/role in community Phylogenetic – share common ancestry, branch on tree of life

Two main modes of speciation

Two main modes of speciation: Allopatric Speciation “other” “homeland” Geographically isolated populations Caused by geologic events or processes Evolves by natural selection & genetic drift Eg. Squirrels on N/S rims of Grand Canyon Sympatric Speciation “together” “homeland” Overlapping populations within home range Gene flow between subpopulations blocked by: polyploidy sexual selection habitat differentiation Eg. polyploidy in crops (oats, cotton, potatoes, wheat)

Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

Sympatric Speciation by Polyploidy Autopolyploid: extra sets of chromosomes Failure of cell division (2n  4n) Eg. Strawberries are 4n, 6n, 8n, 10n (decaploid)! Allopolyploid: 2 species produce a hybrid Species A (2n=6) + Species B (2n=4)  Hybrid (2n=10) 2n = 6 4n = 12 4n 2n Autopolyploid Speciation

Allopolyploidy

Adaptive Radiation Many new species arise from a single common ancestor Occurs when: A few organisms make way to new, distant areas (allopatric speciation) Environmental change  extinctions  new niches for survivors Eg. Hawaiian archiepelago Founding Parents

Adaptive Radiation: Hawaiian plants descended from ancestral tarweed from North America 5 million years ago KAUAI 5.1 million years OAHU 3.7 HAWAII 0.4 1.3 MAUI MOLOKAI LANAI Argyroxiphium sandwicense Dubautia linearis Dubautia scabra Dubautia waialealae Dubautia laxa N

Hybrid Zones Incomplete reproductive barriers Possible outcomes: reinforcement, fusion, stability

Hybrid zones Where divergent allopatric populations come back and interbreed Biologists look for patterns to study reproductive isolation

Fig. 24-13 EUROPE Fire-bellied toad range Hybrid zone Fire-bellied toad, Bombina bombina Yellow-bellied toad range Yellow-bellied toad, Bombina variegata 0.99 0.9 Fig 24.13 A narrow hybrid zone for B. variegata and B. bombina in Europe Allele frequency (log scale) 0.5 0.1 0.01 40 30 20 10 10 20 Distance from hybrid zone center (km)

Reinforcement: Strengthening Reproductive Barriers The reinforcement of barriers occurs when hybrids are less fit than the parent species that is reproduce less successfully. Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species

Fig. 24-15 Sympatric male pied flycatcher Allopatric male pied flycatcher 28 Pied flycatchers 24 Collared flycatchers 20 16 Number of females Figure 24.15 Reinforcement of barriers to reproduction in closely related species of European flycatchers 12 8 4 (none) Females mating with males from: Own species Other species Own species Other species Sympatric males Allopatric males

Fusion: Weakening Reproductive Barriers If hybrids are as fit as parents, there can be substantial gene flow between species If gene flow is great enough, the parent species can fuse into a single species

Pundamilia pundamilia Fig. 24-16 Pundamilia nyererei Pundamilia pundamilia Figure 24.16 The breakdown of reproductive barriers Pundamilia “turbid water,” hybrid offspring from a location with turbid water

Stability: Continued Formation of Hybrid Individuals Hybrids continue to be produced between the two species in the area of their overlap, but the gene pools of both parent species remain distinct.

Tempo of Evolution Gradualism Common ancestor Slow, constant change Punctuated Equilibium Eldridge & Gould Long period of stasis punctuated by short bursts of significant change