Ch. 24 The Origin of Species

Transcription

1 Ch. 24 The Origin of Species

2 So what is a species? Biological species concept defined by Ernst Mayr population whose members can interbreed & produce viable, fertile offspring reproductively compatible Distinct species: songs & behaviors are different enough to prevent interbreeding Eastern Meadowlark Western Meadowlark

3 How and why do new species originate? Species are created by a series of evolutionary processes populations become isolated geographically isolated reproductively isolated isolated populations evolve independently Isolation allopatric geographic separation sympatric still live in same area

4 PREZYGOTIC-reproduction barriers Obstacle to mating or to fertilization if mating occurs geographic isolation habitat isolation temporal isolation behavioral isolation mechanical isolation gametic isolation

5 Geographic isolation Ammospermophilus spp Species occur in different areas physical barrier allopatric speciation other country Harris s antelope squirrel inhabits the canyon s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

6 Habitat isolation Species occur in same region, but occupy different habitats so rarely encounter each other reproductively isolated 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats and have different behaviors: lions are live in prides tigers are solitary This results in them not seeing each other as suitable mates.

7 Temporal isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes reproductive isolation sympatric speciation same country Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

8 sympatric speciation? Behavioral isolation Unique behavioral patterns & rituals isolate species identifies members of species attract mates of same species courtship rituals, mating calls reproductive isolation Blue footed boobies mate only after a courtship display unique to their species

9 Recognizing your own species courtship songs of sympatric species of lacewings courtship display of Gray-Crowned Cranes, Kenya firefly courtship displays

10 Well, what can you say?

11 Mechanical isolation Morphological differences can prevent successful mating reproductive isolation Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. sympatric speciation? Plants

12 Mechanical isolation For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer Animals lack of fit between sexual organs: hard to imagine for us but a big issue for insects with different shaped genitals! I can t even imagine! Damsel fly penises

13 sympatric speciation? Gametic isolation Sperm of one species may not be able to fertilize eggs of another species mechanisms biochemical barrier so sperm cannot penetrate egg receptor recognition: lock & key between egg & sperm chemical incompatibility sperm cannot survive in female reproductive tract Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species red & purple are unable to fuse.

14 POSTZYGOTIC- reproduction barriers Prevent hybrid offspring from developing into a viable, fertile adult reduced hybrid viability reduced hybrid fertility hybrid breakdown zebroid

15 Reduced hybrid viability sympatric speciation? Genes of different parent species may interact & impair the hybrid s development Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

16 Reduced hybrid fertility Even if hybrids are vigorous they may be sterile chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes Mules are vigorous, but sterile Horses have 64 chromosomes AP (32 Biology pairs) Mules have 63 chromosomes! Donkeys have 62 chromosomes (31 pairs)

17 Hybrid breakdown sympatric speciation? Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species.

18 Allopatric and Sympatric Speciation Reproductive Isolation is the key! Allopatric speciation geographic separation populations experience different mutations/adaptations over time the two genomes diverge to the point that reproductive isolation has occurred.

19 Sympatric Speciation Organisms still live in same area Can occur by polyploidy speciation (usually in plants) Habitat differentiation a subpopulation exploits a habitat or resource not utilized by the parent population Sexual selection ex: fig

20 Hybrid Zones Populations start diverging genetically. A hybrid zone occurs where interbreeding is possible and gene flow can resume. The outcome depends on the strength of the isolating factor: either reinforcing the isolation, genetically fusing the populations or creating stable areas of hybrid populations.

21 Hybrid Zones Female hybrid Male hybrid Enquiry Moment: Polar bear and grizzly bear hybridization can occur and produce fertile offspring. What does this say about bear evolution and the selection factors involved? Grizzly Polar Bear And, discuss the possible outcome of the Hybrid Zone if climate change is occurring with continued shrinkage of the polar ice pack before the next Glaciation Event occurs?

22 Polyploidy Species A 2n = 6 Species B 2n = 4 Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division Normal gamete n = 3 Normal gamete n = 3 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes New species: viable fertile hybrid (allopolyploid) 2n = 10

23 Polyploidy Polyploidy is much more common in plants than in animals An autopolyploid is an individual with more than two chromosome sets, derived from one species Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

24 Rate of Speciation Current debate: Does speciation happen gradually or rapidly Gradualism Charles Darwin Charles Lyell Punctuated equilibrium Stephen Jay Gould Niles Eldredge Niles Eldredge Curator American Museum of Natural History

25 Gradualism Gradual divergence over long spans of time assume that big changes occur as the accumulation of many small ones

26 Punctuated Equilibrium Rate of speciation is not constant rapid bursts of change long periods of little or no change species undergo rapid change when they 1 st bud from parent population Time

27 Stephen Jay Gould ( ) Harvard paleontologist & evolutionary biologist punctuated equilibrium prolific author popularized evolutionary thought

28 Example: Punctuated Equilibrium- Slide 1 Species A lives in this region as the climatic conditions favor the environmental factors it is adapted to survive within. Original mouse Species Species A Adapted to do best on the wetter, more tropical eastern side of the island under current conditions.

29 Example: Punctuated Equilibrium- Slide 2 Over a few tens of thousands of years, climatic changes expand the wetter, dryer regions suitable for Species A into the Central Valley and Northeast Coast. Over time, Species A increases in numbers and range to expand into these regions. Note: since the suitable environment is expanding, there are no significant selection forces to cause genetic differences to build up. Original mouse Species Species A Adapted to do best on the wetter, more tropical eastern side of the island under current conditions.

30 Example: Punctuated Equilibrium- Slide 3 Climatic changes swing back. The northern coast becomes dryer/colder while the central valley stays warm, but dryer. Over time, most in these populations are not suited to survive the conditions and die out. If the proper mutations occur in the northern coastal area some individuals will vary with larger body size and darker fur, both adaptations to retain body heat and survive colder conditions. These individuals will have higher reproductive success. If they survive, it will create an isolated pocket of these mice. Species A continues on in numbers in its southeastern range. Original mouse Species Species A Adapted to do best on the wetter, more tropical eastern side of the island under current conditions.

31 Example: Punctuated Equilibrium- Slide 4 Note: The Original Species A is back to its original range. The north coastal mice are surviving, but mutations, natural selection forces are creating genetic changes making them larger and darker. The central valley mice are undergoing selection forces that favor drought survival. It may or may not produce visual changes, but you can tell when an overall drought it s the island, as the Central Valley mice will survive, reproduce in higher numbers compared to Species A. Original mouse Species Species A

32 Example: Punctuated Equilibrium- Slide 5 Note: The Original Species A is unchanged. The north coastal mice are undergoing genetic changes making them larger and darker. The central valley mice are undergoing genetic changes making them more drought resistant. They are separated physically in different habitats. All three variants are surviving simulataneously, populations are branching off. One species IS NOT replacing the other. Original mouse Species Species A If enough genetic changes occur that prevent successful reproduction, then they have become reproductively isolated and can be considered as different species!

33 Any Questions??