The biological species concept defines a species as a group of freely interbreeding individuals that produce fertile offspring but are reproductively isolated from members of other species. Therefore, a new species arises when 2 populations separated for an extended period can no longer interbreed when they are in contact again.
Ecological - grow in different areas or habitats
Temporal - different flowering times; these may be seasonal or daily
Morphological - different sizes or colors of flowers
Physiological - pollen can't survive on the stigma · Genetic- different chromosome numbers
Hybrid inviability - hybrids don't germinate
Hybrid sterility - hybrids germinate and survive but can't reproduce
Hybrid breakdown - hybrids reproduce but subsequent generations are incapable of reproducing
Often these isolating mechanisms are not strong barriers and species are able to hybridize.
The rate at which species diverge is directly related to their dispersal ability.
Organisms that have a means of travelling long distances won't diverge as quickly as more stationary species because the potential for gene flow between populations is great.
Species that are able to disperse large distances require a greater area for divergence to happen than species that don't disperse very far.
1. Allopatric speciation - population divergence is the result of geographic separation. The separation may be due to flood, formation of a mountain, splitting of a lake into several smaller lakes, etc. OR when a group of individuals leaves the population and establishes a new population in a different area.
2. Sympatric speciation - populations occupy a common geographic area, but become separated on the basis of genetic, morphological, or other factors. Because species share a common area, they are likely to hybridize and form polyploids.
A polyploid is an organism with an increased number of chromosomes. There are two ways polyploids are formed.
Allopolyploidy - an increase in the chromosome complement as a result of interspecific hybridization. This mechanism allows for rapid speciation and has led to the evolution of ca. 80% of flowering plants, including common bread wheat and rice.
Autopolyploidy - the entire chromosome complement of a species is doubled because the reduction process of meiosis does not occur. Although not as important for flowering plant speciation as allopolyploidy, this mechanism has resulted in the evolution of potatoes, peanuts, coffee, and grapes among others.
The evolution of lineages is referred to as macroevolution. Evolution at this level occurs by the same mechanisms as microevolution, but the rate of divergence may differ.
1. Rates of evolutionary change
Gradualism = evolutionary divergence is a slow and continuous process that happens over a long period of time.
Punctuated equilibrium = evolution happens all at once in spurts and is followed by long periods of no divergence.
The evolution of major groups of organisms requires an enormous amount of time- more than any person's life. So, experimental manipulation generally is not feasible to study macroevolution.
2. How do we study macroevolution?
Fossils are the primary source of evidence of the evolution of major lineages of organisms.
Adaptive radiation = an ancestral species evolves into many new species that quickly adapt to different niches.
Extinction of species is important for the evolution of new species because once occupied niches are opened up. The fossil record shows mass extinctions were followed by explosions of new life forms.
Biogeography = the study of past and present species distributions.
Continental drift = the separation and movement of land masses that has occurred throughout the history of the earth. The distribution of species throughout time has been dependent upon how close the land masses were to one another. Lineages continue to diverge today as the continents move about, creating and eliminating land bridges.
3. Convergent evolution = different lineages come to have similar characteristics as a result of adapting to similar environments. Often our understanding of lineage evolution is obscured by the close similarity of groups of organisms. For example, cacti are found only in the New World while Euphorbs are found primarily in the Old World. These two groups of flowering plants look very similar. They both have spines produced from modified leaves and succulent stems that carry out photosynthesis. These similarities in morphology are a direct result of the similar arid habitats occupied by these plants.
Return to PB 102 Class Notes Homepage