8 April 2005

Lecture 35

Reading, Chapter 11

VII. Biological evolution

A. Variation and Selection

1. Mechanisms the produce genetic variation in populations.

a. Mutations

A mutation is a change in the nucleotide sequence of the DNA in a cell. There are many different kinds of mutations. Mutations can occur before, during, and after mitosis and meiosis. If a mutation occurs in cells that will make gametes by meiosis or during meiosis itself, it can be passed on to offspring and contribute to genetic variability of the population. Mutations are the sole source of genetic variability that can occur in asexual reproduction. Mutations are usually harmful or neutral to offspring but can occasionally be beneficial.

Mutations can result from the insertion, deletion, or substitution of one or a few nucleotides in a gene sequence. Small changes of this sort usually result from errors in DNA replication prior to cell division or from errors in the DNA repair that occurs in response to DNA damage. These small changes are generally known as "point mutations". If the small change occurs in a region of the gene that codes for an important part of its protein, the effect can be large, such as the mutation that causes sickle cell disease.

Mutations also result from gene rearrangements and other large changes in the DNA sequence of a chromosome. A translocation is movement of a segment of DNA from one place to another in a chromosome or between chromosomes. An inversion is a mutation in which a segment of DNA has flipped within a chromosome. A deletion is the loss of a segment of DNA. These large changes are relatively common, at least over long periods of time, and are abundant in genomes that have been sequenced.

b. Crossing over

Crossing over refers to the relatively frequent exchange of chromosome segments between paired homologous chromosomes that occurs during Prophase I. Often, this exchange produces little or no change in the order or number of genes on the chromsomes. It does mean that some of the genes originating on the maternal homologues get mixed in with genes on the paternal homologues, and vice versa. In other words, some of Mom's alleles get into Dad's homologues and vice versa.

Sometimes crossing over is unequal. One chromosome gets a longer piece of its homologue than the other chromosome gets in return. This can result in gene duplication in the chromosome that got more DNA. Gene duplication can give rise to new genes because the extra gene can sustain mutations while the duplicate gene continues to carry out its normal function. Analyses of the genomes of many organisms suggest that genes are often duplicated over evolutionary time. The groups of duplicated genes are referred to as "gene families", owing to the resemblance of their sequences and their origin by descent from a common ancestor gene.

c. Independent assortment

Mutations occur during DNA replication prior to meiosis. Crossing over during metaphase I mixes alleles from different homologues into new combinations.

When meiosis is complete, the resulting eggs or sperm have a mixture of maternal and paternal chromosomes. This is because during anaphase I, the spindle accurately separates a complete set of 23 human chromosomes into each daughter cell but does not distinguish between the 23 from Mom and the 23 from Dad. Mom's and Dad's homologues are randomly intermixed during anaphase such that each egg or sperm cell has a nearly unique combination of Mom's and Dad's alleles. The number of combinations of 23 maternal and paternal homologues that can result from independent assortment is 223, about 8 million. This leaves out variations caused by mutations or crossing over.


d. Fertilization

Fertilization randomly brings together two gametes produced in two different individuals. This means that for a particular man and woman, the number of unique combinations of genes that could occur in their offspring is 8 million times 8 million ( 64 trillion), not counting variation caused by crossing over and mutation. Random fertilization is a further mechanism that produces genetic variation in the process of sexual reproduction.

The genetic variation that results from mutations, meiosis, and fertilization cause the phenomenon with which we are all familiar: even in very large populations, such as the human population, every individual is genetically unique.

e. Other

There are additional mechanisms that generate genetic variation. One is polyploidy, which occurs commonly in plants and leads to new species within one generation. Polyploidy events lead to organisms with more than two sets of chromosomes. More than half of wild plants are polyploid and sao are many domestic plants. On page 240 and 241, your text describes the polyploidy events that lead to modern wheat cultivars.

Another mechanism that produces genetic variation is the transfer of genes between species. This is common between different species of bacteria and may occur in eukaryotes as a result of virus infections in which the virus integrates some of its genes into cells that give rise to eggs or sperm.

2. Selection

The genetic variety produced by sexual reproduction offers many possibilities for how a population of organisms might change over time. The possibility that actually occurs is determined by selection. Those variants that are best suited to prevailing conditions produce more offspring than the others and their combinations of genes therefore tend to prevail in the population, at least until the selection regime changes and another combination of genes is preferred.

Selection is best seen as a filter through which a subset of the genetic variants in a population pass. Some genotypes make it, some don't.


B. Micro- and macro-evolution

Biological evolution is probably the biggest of all biological theories. It has been said that nothing in biology makes sense except in the light of evolution (Theodosius Dobzhansky, 1970). By the same token, modern cell biology and genetics have done much to make sense of biological evolution. Perhaps the first example of this was the discovery of genes by Gregor Mendel, which occurred subsequent to Charles Darwin's hypothesis of natural selection and provided an explanation for the inheritance of traits that were advantageous.

The magnitude of biological evolution has led to two perspectives on how it works: micro-evolution and macro-evolution. They are qualitatively the same but exhibit different scales of change and time.

1. Micro-evolution

Micro-evolution refers to small changes that occur quickly in a population of organisms. The diversity of dogs that has resulted from artifical selection for different physical traits is an example. Microevolution is easy to understand because we practice it in the selective breeding of animals and plants and because it happens in nature on a time scale that we can observe. Micro-evolution proceeds by the action of selection on the genetic diversity of a population, as we have discussed. 

2. Macro-evolution

Macro-evolution refers generally to dramatic changes in the diversity of life on Earth over longer periods of time than humans can perceive. The Cambrian Explosion is an example of macroevolution (we will discuss this later).

Macro-evolution can be considered to consist of two parts: extinction and speciation. Extinction is easy to understand. It is the disappearance of a population when selection overwhelms it. Causes of extinction include, new diseases, new predators (e.g. man), climate changes (e.g. Ice Ages), habitat loss, geological processes (e.g. continental drift), and catastrophic events like asteroid impacts with the Earth.

Speciation is more difficult to understand and is discussed further later. Speciation is essentially lots of microevolution in populations that have become reproductively isolated, i.e. they can no longer share genetic diversity by interbreeding..

Micro and macro evolution are artificial concepts that are defined by a human perspective of time. Because of this, they don't really fit the growing body of evidence and hypotheses about evolution. For example, the Endosymbiont Hypothesis proposes that eukaryotes evolved from prokaryotic ancestors in a matter of moments. Was this micro or macro-evolution?

C. Speciation

Speciation is the process by which new species arise from existing species. Two patterns for the process of speciation have been proposed: phyletic speciation and divergent speciation.