Founder Effect Speciation and Chromosomes and Speciation

NOTE: These are lecture notes for Biology 391, Organic Evolution, at The University of Tennesee at Martin.  Anyone outside of UT Martin wishing to use these notes or to contact me for additional information should first read the information obtained by clicking here.

Goals:discuss how speciation may occur in small founding populations and consider the role of large-scale chromosomal change in speciation.

Related Textbook Material: Freeman and Herron (2001) Chapter 12

Lab Manual Questions over this material are in Lab Manual Chapter XIII

The Lecture:

In the last lecture, you learned that one of the ways that allopatric speciation can occur is through a founder or dispersal event -- that is, an event where a small number of individuals disperse to a new location, previously unoccupied by that species. Some evolutionary biologists have argued that such dispersal events are likely to result in rapid evolution in the newly founded population and are, as a result, particularly likely to result in speciation as the new population evolves to become a different species from the original population. Others, however, have argued that most speciation results from vicariance events and that dispersal events are not likely to result in the kind of evolution that will lead to new species evolving. We will consider one argument that suggests that founder events are likely to cause speciation and one argument that suggests that founder events are unlikely to cause speciation. Further tests of these hypotheses are necessary before we can evaluate which is correct.

First, we will consider an argument that founder events are NOT likely to cause speciation. According to this hypothesis:

Now let us consider the argument that founder events ARE likely to result in speciation. According to this hypothesis: Note that both arguments are based on an initial phase of genetic drift that is predicted because populations are likely to be started (founded) by only a small number of individuals so genetic drift should be the main form of evolution occurring initially in these populations. They differ in the predicted effect of genetic drift. If genetic drift occurs for a long enough time so that genetic variation is lost, then these new populations are not likely to evolve to be different species. If, however, genetic drift only occurs for a short period of time, and the population grows large enough so that genetic drift becomes less important relatively quickly, then genetic drift could cause some initial random genetic changes in the population that will subsequently make it more likely to evolve to be very different from the original population so that the two populations will be different species. Both arguments are plausible; both have been modeled theoretically. We need more data on what really happens in populations and how speciation events have most likely occurred to evaluate which is more likely to be generally true.

For the rest of the lecture we will consider a different topic of relevance to speciation. We have considered speciation so far as occurring as a result of relatively small scale genetic changes that build up, through natural selection and genetic drift, so that populations evolve to become different species. There are also, however, larger scale mutations that may cause much more rapid speciation. We will consider these now.

We will define chromosomal mutations as large-scale changes in chromosomal structure or number. These can occur in several ways; we will consider three main kinds of chromosomal mutation:

  1. Fusions occur when two small chromosomes combine to form one larger chromosome. A common way in which this occurs is for two chromosomes with centromeres very near the end of the chromosome to fuse into one large chromosome with the centromere in the middle, as shown in the following picture. Note that when chromosomes become fused like this, the total number of chromosomes is decreased.

    2.  

  2. Inversions occur when a large segment of a chromosome is spliced out, reversed in order, and then spliced back into a chromosome. This is illustrated in the following picture, where the letters represent genes on the chromosome. An inversion generally involves several genes and changes the order of genes on the chromosome.

    3.  

  3. Polyploidy occurs when the total number of chromosome sets increases in number -- typically what occurs is that all chromosomes are duplicated. A species that was originally diploid (two copies of every chromosomes; 2N) would, as a result, become tetraploid, (4N) with four copies of every chromosome.
These chromosomal mutations can result in speciation, as follows. Suppose an individual is produced with one of these chromosomal mutations. It will have all of the genes needed for normal development. In the case of polyploidy, there will be more copies of all of these genes, but they will occur in the same proportions as they did in the original individuals so there will be no imbalance in the doses of the proteins coded by these genes. As a result, all of these chromosomal mutations are likely to produce healthy individuals -- they will not be harmful to the health of the individuals.

When an individual with a chromosomal mutation reproduces with an individual without the chromosomal mutation, however, the offspring, while healthy, may be sterile. Consider how this works with polyploidy. Suppose the polyploid individual is 4N. It reproduces with an original 2N individual. When the 4N individual produces gametes, through meiosis, the gametes have half as many chromosomes as the individual so they are 2N. The gametes of the diploid individual, produced through meiosis, are haploid (1N.) As a result, fusion of the 2N gamete and the 1N gamete results in an offspring that is 3N.

The 3N individual, the hybrid between the 2N and 4N forms, should be healthy because (as noted above) there is no imbalance in the dosages of proteins coded by its genes (it has all the protein products of the genes in the right proportions -- none is out of proportion.) However, when its cells undergo meiosis to produce gametes, there are likely to be major problems. The key event in meiosis that allows production of gametes with half as many chromosomes as the original cells is the pairing of homologous chromosomes. This 3N individual has 3 homologous chromosomes for each type of chromosome. When they try to pair, three things can not form pairs, so different parts of the chromosomes pair with parts of the other chromosomes, and when they separate they are likely to break. The result is that the daughter cells get broken chromosomes and may too many copies of some genes and no copies of other genes. They will not be viable gametes. So these 3N individuals (and, in general, any individual with an odd number of chromosomes) will be sterile because it cannot produce viable gametes because chromosomes can not pair correctly during meiosis.

So this means when an individual that is polyploid reproduces with an individual with the original number of chromosomes, the offspring of this cross are healthy but sterile. By the biological species concept, the polyploid is therefore a different species from the original -- the two can reproduce but their offspring are sterile. By the phylogenetic species concept, being a polyploid means having a trait (in this case polyploidy) that is derived and different from the original form, so it would be a different species from the original by the phylogenetic species concept, too.

There is one problem we have to consider, though. What happens when a polyploid individual is produced in a normal, diploid population? If it reproduces with the diploids, the offspring will be sterile. The first polyploid produced through a mutation would have only diploids to reproduce with. You would think, as a result, that this form would quickly die out. It would, if the only way it can reproduce is sexually. If, however, this species can reproduce asexually, then a polyploid can reproduce more polyploids, like itself, asexually. These polyploids can then reproduce sexually with one another, and their offspring will have the same, even number of chromosomes that they do so they will be fertile as well as healthy. In this case, a new sexually reproducing species can be produced.

We would generally expect, then, that polyploidy could result in new sexually reproducing species primarily in species that can undergo both sexual and asexual reproduction. In fact, we observe that in the flowering plants, a group in which most species can undergo both sexual and asexual reproduction, many closely related species differ in the number of chromosome sets, suggesting that they have been produced as a result of polyploidy. Evidence of polyploidy is much rarer in animals, which often can not reproduce asexually. If polyploidy occurred in a species that can reproduce only sexually, it would quickly die out -- all the polyploid's offspring, with the diploid members of the population, would be sterile.

The other forms of chromosomal mutation (inversion, fusion) can also result in situations in which offspring of the original form and the mutated form are sterile. There are some other complicating factors in these situations, though (not all inversions or fusions have this effect, and they may be involved in speciation for other reasons.) We will not consider these complications in this course. For this course, focus on polyploidy as the main example of how chromosomal mutations can result in speciation.

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