- Islands of land in a sea of water
- Islands of water in a sea of land
- Islands of trees in a sea of grass
- Islands of coolness in a sea of heat
- Islands of warmth in a sea of cold
- Islands of nature in a sea of humanity
- Mountains as barriers
- Rivers and Canyons as barriers
- If a population should become divided into two by a geographic barrier (or if some individuals are transported to a new area outside the parent's range), evolution of each new population continues independently due to the forces of natural selection, genetic drift, migration, nonrandom mating, and mutation. Differences between the two, including differences in reproductive processes, gradually accumulate such that reproductive isolating mechanisms become more and more effective over prolonged periods of time.
- New species arise when genetic differences accumulate to the point when the two groups can no longer successfully mate and reproduce (if and when they come back into contact). Remember: species can be defined as groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups.
- Speciation events lead to the multiplication and diversification of species into higher taxa (e.g., genera, families, orders, classes, phyla, etc.). All species (animals, plants, fungi, and all major groups of microorganisms) can be traced back to a single origin of life on earth. Evolution is a continuing process that explains the history of life on earth, as well as the diversity of life today.
- Stages in the formation of a new species (from Grant, 1963 and 1981, and the University of Alabama). This is basically the same illustration as provided below, with more detail.
- Illustrative example of geographic speciation from the University of Alabama. This poor quality figure shows the separation of two populations by some geographic barrier. Subsequent divergence leads to the formation of different species; they species are reproductively isolated when that barrier is removed.
- Darwin's finches from the University of Alabama.
- Mechanisms of Reproductive Isolation
- Premating (These prevent two individuals from mating)
(some of these differences are the result of single gene mutations)- Ecological or Habitat isolation (The two populations can not survive and reproduce in the same environment)
- Seasonal and Temporal Isolation (Reproduction occurs at different times of the year or even different times of the day)
-
Mechanical isolation (reproductive organs are not compatible in animals or are adapted for different pollinators in plants)
- Behaviorial (or Ethological) Isolation (Mate selection is based on different characteristics)
- Ecological or Habitat isolation (The two populations can not survive and reproduce in the same environment)
- Postmating (If mating does occur, viable offspring may not be produced or they may be infertile because of differences in chromosome structure or number)
- Gametic and Zygotic mortality (sperm may be transferred, but fertilization does not occur. Or the egg may be fertilized but the zygotes are not viable)
- Hybrid Inviability and Reduced Fertility (hybrids may begin to develop, but not survive or if they are viable they are infertile)
What would this infertility have to do with MEIOSIS?
- Gametic and Zygotic mortality (sperm may be transferred, but fertilization does not occur. Or the egg may be fertilized but the zygotes are not viable)
- Premating (These prevent two individuals from mating)
Two populations of the same species living in the same area can become reproductively isolated (and sometimes quite rapidly).
Besides the reproductive isolation mechanisms listed above, gene flow between the two populations can be disrupted by:
- Chromosomal abnormalities such as polyploidy
- autopolyploid (extra chromosome sets from the SAME species)
- allopolyploid (chromosome sets from two or more DIFFERENT species)
These polyploids can self-fertilize or breed among themselves. Remember, in humans and other animals, polyploidy is lethal. In plants, polyploidy is quite common and has given rise to many new species. It is estimated that as many as 50% of extant flowering plant species have evolved via polyploidy.
- autopolyploid (extra chromosome sets from the SAME species)
- and chromosome incompatibility (see page 413, text)
- and choice of host plant or habitat.
- The natural host of the American fruit fly is a hawthorn tree; however, some flies live in apple trees. By eating, courting, mating, and laying their eggs on different host plants, the two groups of flies have become reproductively isolated from one another and are on their way to becoming different species. Genetic differences between these groups have been measured.
- In many other organisms, shifts to new host plants or habitats trigger phenotypic changes that lead to new species.
- The natural host of the American fruit fly is a hawthorn tree; however, some flies live in apple trees. By eating, courting, mating, and laying their eggs on different host plants, the two groups of flies have become reproductively isolated from one another and are on their way to becoming different species. Genetic differences between these groups have been measured.
An Example:
Below is a classic example of speciation by hybridization and polyploidy. The species belong to the genus Spartina(cord grass).
- Extinction: the inability of a species to adapt to a particular environmental challenge.
- The history of the earth is punctuated by several mass extinctions (see Table 20.3, text).
- As we discussed in the biodiversity lectures, the number of organisms on Earth is now being reduced at a rate 1,000-10,000 times higher than any time prior to the evolution of humans (that is, a few decades or centuries rather than millions of years).
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