Chapter 24: The Origin of Species What is a species? A population whose members can interbreed in nature and produce viable, fertile offspring aka….reproductive isolation What kinds of barriers keep different species isolated so they cannot mate? Figure 24.4 Pre–zygotic barriers – before mating &/or zygote is formed Post–zygotic barriers – after zygote is formed

Figure 24.4 Reproductive Barriers Prezygotic barriers impede mating or hinder fertilization if mating does occur Individuals of different species Mating attempt Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION (b) (a) (c) (d) (e) (f) (g)

Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown Viable fertile offspring Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown Fertilization Gametic isolation GAMETIC ISOLATION REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN (h) (i) (j) (k) (l) (m)

Chapter 24: The Origin of Species What is a species? What kinds of barriers keep different species isolated so they cannot mate? How are new species created? Allopatric speciation when a geographic barrier isolates a population blocks gene flow ex. mountain range emerging, new river dividing a field, island Sympatric speciation intrinsic factors such as chromosomal changes (plants) or non-random mating alter gene flow

Figure 24.5 Two main modes of speciation Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) Sympatric speciation. A small population becomes a new species without geographic separation.

Chapter 24: The Origin of Species What is a species? What kinds of barriers keep different species isolated so they cannot mate? How are new species created? Allopatric speciation – when a geographic barrier isolates a population blocks gene flow ex. mountain range emerging, new river dividing a field, island Adaptive radiation evolution of many diversely adapted species from a common ancestor Seen on islands Sympatric speciation intrinsic factors such as chromosomal changes (plants) or non-random mating alter gene flow

Figure 24.12 Adaptive radiation Dubautia laxa Dubautia waialealae KAUA'I 5.1 million years O'AHU 3.7 LANAI MOLOKA'I 1.3 million years MAUI HAWAI'I 0.4 Argyroxiphium sandwicense Dubautia scabra Dubautia linearis N

Chapter 24: The Origin of Species What is a species? What kinds of barriers keep different species isolated so they cannot mate? How are new species created? Allopatric speciation – when a geographic barrier isolates a population blocks gene flow ex. mountain range emerging, new river dividing a field, island Adaptive radiation evolution of many diversely adapted species from a common ancestor Seen on islands Sympatric speciation intrinsic factors such as chromosomal changes (plants) or non-random mating alter gene flow ex. oats, cotton, tobacco, potatoes, wheat Autopolyploidy An individual has more than 2 chromosome sets derived from a single species from an error in meiosis

Figure 24.8 Sympatric speciation by autopolyploidy in plants Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue. Gametes produced by flowers on this branch will be diploid. Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.

Chapter 24: The Origin of Species What is a species? What kinds of barriers keep different species isolated so they cannot mate? How are new species created? Allopatric speciation – when a geographic barrier isolates a population blocks gene flow ex. mountain range emerging, new river dividing a field, island Adaptive radiation evolution of many diversely adapted species from a common ancestor Seen on islands Sympatric speciation intrinsic factors such as chromosomal changes (plants) or non-random mating alter gene flow Autopolyploidy An individual has more than 2 chromosome sets derived from a single species from an error in meiosis Allopolyploidy 2 different species produce the polyploid hybrid

Figure 24.9 One mechanism for allopolyploid speciation in plants Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes with 7 chromosomes Viable fertile hybrid (allopolyploid) Normal gamete n = 3 Species A 2n = 4 Species B 2n = 6 2n = 10

Chapter 24: The Origin of Species What is a species? What kinds of barriers keep different species isolated so they cannot mate? How are new species created? What is the difference between gradualism & punctuated equlibrium?

Figure 24.13 Two models for the tempo of speciation Gradualism model. Species descended from a common ancestor gradually diverge more and more in their morphology as they acquire unique adaptations. Time (a) Punctuated equilibrium model. A new species changes most as it buds from a parent species and then changes little for the rest of its existence. (b)