Option D questions, help!

[kentpop]

Hey there!

I beg any of you to help me with the three questions regarding the evolution option.

1. How does actually polyploidy lead to speciation? Is it because a e.g. tetraploid plant cannot interbreed with a diploid plant? I understand that the nucleus contains >2 sets of chromosomes, but not why that leads to speciation.

2. Is the process of adaptive radiation just; many similar but distinct species evolve rapidly from a single species? Isnt that the same as divergent evolution, or is divergent evolution a type of adaptive radiation?

Thank you!!

[iblyf]

Hi!

I have tried to give you extensive answers to really explain this properly

To answer your first question,

• Polyploidy is an example of Sympatric Speciation (as opposed to Allopatric), meaning that although a species lives in the same area, members cannot interbread
• Polyploidy refers to an organism that has more than two chromosomes for each chromosome number (e.g. humans are diploid, and we have two chromosomes for each chromosome number)
• If there are any tetraploid individuals in a population (they have four chromosomes at each number), their gametes will be diploid (due to meiosis halving the genetic info)
• However, if there are also diploid individuals in a population, their gametes will be haploid (one chromosome at each number).
• Thus, if a diploid individual mates with, for example, a tetraploid individual (gametes are haploid and diploid respectively), 1 + 2 = 3, meaning that the offspring will be triploid (3 chromosomes at each number)
• Whilst triploid organisms can live, they are sterile, and therefore can't produce fertile offspring, so their genes can't be passed on. This is because meiosis fails.
• Thus, if the population wants to produce fertile offspring, diploids must mate with diploids, and tetraploids must mate with other tetraploids.
• Therefore, this results in selective mating, where only certain organisms can produce fertile offspring together; eventually resulting in a separation of the gene pools which causes speciation.

To answer your second question:

• Adaptive radiation is a kind of divergent evolution (so divergent evolution isn't really a type of adaptive radiation)
• Basically, two new organisms evolve from one common ancestor
• If a certain group has a characteristic that gives it a competitive advantage, it may undergo adaptive radiation
• Adaptive radiation is quite rapid
• For example, Darwin observed that the finches had all adapted different kinds of beaks. This was because it decreased competition between the finches if they ate different foods; thus using their beaks in different ways. Thus, the finches evolved to have different beaks to suit the foods that they were eating. Does that make sense?

If you have any more questions about Option D feel free to ask (HL too), and I also do Option E if you have any questions.

I hope that helps!

Edited October 21, 2013 by iblyf