Chapter 24: The Origin of Species1. What is a species?
- A population whose members can interbreed in nature and produce viable, fertile offspring
- aka….reproductive isolation2. 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
Individualsof differentspecies
Matingattempt
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION(b)
(a)(c)
(d)
(e)(f)
(g)
Viablefertile
offspring
Reducehybrid
viability
Reducehybridfertility
Hybridbreakdown
Fertilization
Gameticisolation
GAMETIC ISOLATION REDUCED HYBRID VIABILITY
REDUCED HYBRID FERTILITY HYBRID BREAKDOWN
(h) (i)
(j)
(k)
(l)
(m)
Chapter 24: The Origin of Species1. What is a species?2. What kinds of barriers keep different species isolated so they cannot mate?3. How are new species created?
- Allopatric speciation - when a geographic barrier isolates a population blocks gene flow
- Sympatric speciation- intrinsic factors such as chromosomal changes (plants) or
non-random mating alter gene flow
Figure 24.5 Two main modes of speciation
(a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population.
(b) Sympatric speciation. A smallpopulation becomes a new specieswithout geographic separation.
Chapter 24: The Origin of Species1. What is a species?2. What kinds of barriers keep different species isolated so they cannot mate?3. 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'I5.1
millionyears O'AHU
3.7millionyears
LANAI
MOLOKA'I1.3 million years
MAUI
HAWAI'I0.4
millionyears
Argyroxiphium sandwicense
Dubautia scabra Dubautia linearis
N
Chapter 24: The Origin of Species1. What is a species?2. What kinds of barriers keep different species isolated so they cannot mate?3. How are new species created?
- Allopatric speciation – - when a geographic barrier isolates a population blocks gene flow- 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
- Polyploidy - An individual has more than 2 chromosome sets resulting
from an error in meiosis- IF survivable, then results in a new species
Figure 24.8 Sympatric speciation by autopolyploidy in plants
2n = 64n = 12
2n
4n
Failure of cell divisionin a cell of a growing diploid plant afterchromosome duplicationgives rise to a tetraploidbranch 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.
Sympatric speciation: non-random mating
Figure 24.10
Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orangelight, the two species appear identical in color. The researchers then observed the mating choices of the fish in each tank.
EXPERIMENT
P. nyererei
Normal lightMonochromatic
orange light
P. pundamilia
Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile.
RESULTS
The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate. Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory, the genetic divergence between the species is likely to be small. This suggests that speciation in nature has occurred relatively recently.
CONCLUSION
Chapter 24: The Origin of Species1. What is a species?2. What kinds of barriers keep different species isolated so they cannot mate?3. How are new species created?4. 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)
Chapter 24: The Origin of Species
5. What other mechanisms can influence evolution/speciation?
A) Differences in allometric growth (proportional growth of body structures)
Figure 24.15 B
Chimpanzee fetus Chimpanzee adult
Human fetus Human adult
(b) Comparison of chimpanzee and human skull growth. The fetal skulls of humans and chimpanzees are similar in shape. Allometric growth transforms the rounded skull and vertical face of a newborn chimpanzee into the elongated skull and sloping face characteristic of adult apes. The same allometric pattern of growth occurs in
humans, but with a less accelerated elongation of the jaw relative to the rest of the skull.
DEVELOPMENTAL FACTORS…
Ground-dwelling salamander. A longer timeperoid for foot growth results in longer digits andless webbing.
Tree-dwelling salamander. Foot growth endssooner. This evolutionary timing change accounts for the shorter digits and more extensive webbing, which help the salamander climb vertically on treebranches.
(a)
(b)
Figure 24.16 A, B
5. What other mechanisms can influence evolution/speciation?
Chapter 24: The Origin of Species
A) Differences in allometric growth (proportional growth of body structures)
B) The expression of homeotic genes (which determine the “body plan” of an organism) may change through mutation.
Chicken leg bud Region ofHox geneexpression
Zebrafish fin bud
Figure 24.18
DEVELOPMENTAL FACTORS…
