Chapter 24 The Origin of Species. Macroevolution is the origin of new taxonomic groups, as opposed to microevolution, which is genetic variation between generations. A. What is a species? 1. Biological species concept
The Origin of Species
Macroevolution is the origin of new taxonomic groups, as opposed to microevolution, which is genetic variation between generations.
A. What is a species?
- A species is a population or group of populations whose members have the potential to interbreed with one another and produce viable offspring, but who cannot produce viable offspring with other species.
Figure 24.3 (p. 473) – The biological species concept is based on interfertility rather than physical similarity.
--> Speciation is the process by which a new species originates and involves the creation of a population of organisms that are novel enough to be classified in their own group. There are two processes by which this can occur:
- Anagenesis is the accumulation of heritable traits in a population, that transforms that population into a new species
- Cladogenesis is branching evolution, in which a new species arises as a branch of from the evolutionary tree. The original species still exists. This process is the source of biological diversity.
For a new species to form, there needs to be isolation of some members of a species as a separate population. Forms of isolation, that interfere with breeding include both..
2. Prezygotic and postzygotic barriers (Fig. 24.4, p. 474-5)
Prezygoticbarriers prevent mating or egg fertilization if members of different species try to mate.Examples:
a. Habitat isolation
- Two species that live in the same area, but occupy different habitats rarely encounter each other.
b. Behavioral isolation
- Signals that attract mates are often unique to a species (e.g. different species of fireflies flash different patterns).
c. Temporal isolation
- Two species breed at different times of the day or during different seasons.
d. Mechanical isolation
- Closely related species attempt to mate, but are anatomically incompatable. (Example: flowering plants with pollination barriers; some plants are specific with respect to the insect pollinator, often occurs with butterflies/moths)
e. Gametic isolation
- Gametes must recognize each other. (Example: fertilization of fish eggs, chemical signals between sperm and egg allows sperm to “recognize” the correct egg)
- Postzygoticbarriers prevent a hybrid zygote from developing into a fertile adult.Examples:
a. Reduced hybrid viability
- Abort development of hybrid at some embryonic stage.
b. Reduced hybrid fertility
- Meiosis doesn’t produce fertile gametes in vigorous hybrids.
c. Hybrid breakdown
- First-generation hybrids are fertile, but they cannot produce fertile offspring in the next generation (e.g. different species of cotton).
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Figure 24.4 Reproductive Barriers
REDUCED HYBRID VIABILITY
REDUCED HYBRID FERTILITY
There is a problem with the idea of biological species concept --> How do you get organisms to breed to see whether viable offspring are produced? There are…
3. Alternative concepts of species
a. Ecological species concept
- Species are defined by their use of environmental resources; their ecological niche (e.g. species that are defined by their food source such as butterflies with certain flowers)
b. Morphological species concept
- Takes into consideration factors such as body shape, size, etc.
c. Paleontological species concept
- Species in the fossil record are characterized according to a unique set of structural features.
d. Phylogenetic species concept
- Recognizes species are sets of organisms with unique genetic histories. This idea is based often on molecular analyses such as DNA sequences.
1. Allopatric speciation
- Allopatricspeciation describes speciation that takes place in populations with geographically separate ranges. Gene flow is interrupted and new species evolve.
2. Sympatric speciation
- Sympatricspeciation describes speciation that takes place in geographically overlapping populations. Chromosomal changes and nonrandom mating reduce gene flow.
Remember: Species arise when individuals in a population become isolated one from the other.
Figure 24.6 (p. 477) – Allopatric speciation of squirrels in the Grand Canyon. Animals like birds do not show speciation like those animals that are barred from breeding by the canyon.
Another place where adaptive radiation is apparent is on island chains (e.g. Fig. 24.12). This example is illustrative of what happened on the Hawaiian islands.
Would this example be allopatric or sympatric speciation?
Remember Once geographic isolation has occurred, there still must be changes that reproductively isolate populations of individuals. If the populations evolve so that they are now new species, they cannot interbreed to produce fertile, viable offspring. In other words, they need to be reproductively isolated!
- Sympatricspeciation describes speciation that takes place in geographically overlapping populations. This can occur by chromosomal changes and nonrandom mating. Both can reduce gene flow between organisms and cause populations to evolve to new species.
- Reproductive barriers can arise by polyploidy (greater than 2 sets of chromosomes). This mechanism is most common in plants.
Figure 24.8 (p. 478) – Sympatric speciation by autopolyploidy in plants.
Failure of cell divisionin a cell of a growing diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.
Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.
Gametes produced by flowers on this branch will be diploid.
2n = 6
4n = 12Figure 24.8 Sympatric speciation by autopolyploidy in plants
with 4 chromosomes
with 7 chromosomes
Viable fertile hybrid
2n to n
2n = 4
2n = 10
n = 3
n = 3
2n = 6Figure 24.9 One mechanism for allopolyploid speciation in plants
- Animals diverge mostly due to reproductive isolation. Reproductive isolation is a result of genetic factors that cause offspring to rely upon resources not used by previous generations. (Example: switch to a new food source)
An extremely good example of sympatric speciation in animals occurred in Lake Victoria which has 200 closely related species of Cichlids (fish) which probably all arose from one ancestor with the driving force for speciation being:
Competition for a limited resource (food) within the lake, and adaptation to new food sources. This gave rise to different species that are kept from breeding with each other by distinctive coloration patterns (Fig. 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.
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.
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.
CONCLUSIONFigure 24.10 Does sexual selection in cichlids result in reproductive isolation?
Speciation occurs when divergence leads to reproductive barriers between the new and the ancestral population. And this probably takes vast amounts of time to occur.
But how do evolutional novelties emerge? For example, how did something as complex as the eye first evolve? We need to remember that:
Most evolutionary novelties are modified versions of older structures. And an extremely good example is the eye as shown in Figure 24.14 (p. 483) – A range of eye complexity among mollusks.
Existing structures can be modified for brand new functions. These are called Exaptations: structures that evolve for one purpose but become useful for another function.
Finally, we should
** Remember that evolution is not goal oriented. Differential reproduction is only a reaction of individuals to their environment.
Figure 24.24 (p. 481) – The branched evolution of horses. This figure can give the illusion of goal-oriented evolution of the horse, but it is only an illusion.