Flashcards in Evolution and Speciation (Chapter 17) Deck (56)
What is speciation?
The process by which a new species is produced
What is a species
A group of organisms with similar morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring and are reproductively isolated from other species
What is a morphological feature?
A structural feature
What is a physiological feature?
How the body works
What do biochemical features include?
1) the sequence of bases in DNA molecules
2) the sequence of amino acids in proteins
What feature really feature decides whether or not two organisms are the same species?
Whether or not they can interbreed successfully
What must happen for a group of interbreeding organisms (organisms of same species) to produce another group of organisms that cannot interbreed successfully with the first group?
The two groups must undergo reproductive isolation
What are the two mechanisms of reproductive isolation?
Prezygotic and postzygotic
When do prezygotic isolating mechanisms occur?
Before the zygote is formed
What are the 4 prezygotic isolating mechanisms?
1) individuals not recognising one another as potential mates or not responding to mating behaviour
2) animals being physically unable to mate
3) incompatibility of pollen and stigma in plants
4) inability of male gamete to fuse with female gamete
5) geographical isolation
Which isolating mechanism is more wasteful of energy and resources?
What are the 3 postzygotic isolating mechanisms?
1) failure of cell division in the zygote
2) non-viable offspring (offspring that soon die)
3) viable, but sterile offspring
What is almost all the evidence for speciation?
How can you find evidence for speciation?
Can look at populations of organisms at one moment in time (now) and use the patterns to suggest what might have happened or is still happening over long periods of time
What is allopatric speciation?
Speciation which happens when two populations are separated from each other geographically
What does allopatric speciation require?
Geographical isolation e.g. islands (Hawaiian/Galapagos)
How does allopatric speciation by geographical isolation occur?
1) geographical isolation requires a barrier of some sort to arise between two populations of the same species, preventing them from mixing e.g. a stretch of water
2) ∴ the group of organisms on the island interbreed, and because the selection pressure are different from those on the mainland, different alleles are selected for
3) over time, the morphological, physiological and behavioural features of the island population become so different form the mainland population due to mutation and genetic drift that the two populations can no longer interbreed (i.e. reproductive isolating mechanisms may have developed)
4) ∴ a new species has evolved
What are two other examples of physical barriers?
- Large areas of forest being cut down, leaving 'islands' of forest in a 'sea' of agricultural land
- Small-scale barriers for very small or immobile organisms
What is sympatric speciation?
When a new species arises without the original populations being separated by a geographical barrier but who biological factors e.g. chromosomal changes and non-random mating, reduce the gene flow
How does sympatric speciation occur?
1) Through polyploidy (main way)
2) habitat isolation
3) seasonal isolation (breeding seasons are different)
4) temporal isolation
What is a polyploid organism?
An organism with more than two complete sets of chromosomes in its cells - when they spontaneously arise, they are instantly reproductively isolated from their parent population
How can a tetraploid organism form?
1) meiosis goes wrong when gametes are being formed, so that a gamete ends up with two sets of chromosomes instead of one set
2) if two such gametes fuse, then the zygote gets 4 complete sets of chromosomes and is tetraploid
Describe the characteristics of tetraploid organisms
1) they are often sterile bc there are 4 of each kind of chromosomes and ∴ all 4 try to 'pair' up during meiosis 1 and get muddled - ∴ it is very difficult for the cell to divide by meiosis and produce new cells
2) the cell could be able to grow perfectly well and reproduce asexually because mitosis can happen normally, as chromosomes do not need to pair up and behave quite independently
Why does polyploidy happen in plants but rarely in animals?
Because most animals do not reproduce asexually
How could a tetraploid reproduce?
1) sometimes it can produce diploid gametes
2) if one of these fuse with a gamete from a normal, diploid plant, the zygote will be triploid
3) ∴ a new species has arisen in just one generation as the original diploid plant and the tetraploid cannot interbreed successfully (as the triploid is sterile)
Why is a triploid definitely sterile?
Because it cannot produce gametes as it cannot share the 3 sets of chromosomes out evenly between daughter cells (but can grow normally by mitosis)
What is an autopolyploid?
A polyploid containing x number of sets of chromosomes all from the same species e.g. tetraploid
What is an allopolyploid?
A polyploid containing e.g. 2 sets of chromosomes from one species and 2 sets from another closely related species
Why does meiosis happen more easily in an allotetraploid than in an autotetraploid?
1) the chromosomes from each species are not quite identical
2) ∴ the two chromosomes from one species pair up with each other and the two chromosomes from the other species pair up
3) this produces a much less muddled situation than in an autotetraploid, where the chromosomes try and get together in fours ∴ it is much more likely that meiosis will be successful
4) ∴ the allotetraploid is fertile and could produce many gametes