BD 3

IMPORTANT TERMS

 Some evolutionary changes do not create new species, but result in changes at the population level. 

A population is a group of organisms of the same species that live in the same area.

A species is a group of organisms that are genetically similar and can mate with one another to produce fertile offspring. 

BACKGROUND

Until a decade or so ago, evolutionary change was generally assumed to happen on a very much longer time scale than ecological change. Our view on biodiversity and ecosystem functioning has often been static, trying to conserve biodiversity as it is, and preferably, as it once was. But the closer we look at adaptive evolution, often with the aid of new biological insights and technological advances, the faster it seems to happen. Evolution and ecology are proving to be so heavily entwined that the distinction is becoming increasingly hard to make. 

This knowledge profoundly affects our thinking on how evolution affects patterns of biodiversity, especially in the face of global change. Adaptations in response to climate change, for example, have been shown to occur within a single generation (Van Doorslaer et al. 2007). Contemporary evolution is probably more important than we assumed and is likely to mediate the response of populations, species, communities and ecosystems to both gradual and sudden environmental change. 

SPECIATION

One of the characteristics that identifies one type of organism as a separate species from another is reproductive isolation: for animals and plants that reproduce sexually, it is impossible for members of two different species to mate and produce fertile offspring (offspring that can successfully reproduce.)

Speciation is the process whereby a single species develops over time into two separate, reproductively isolated species. It is one of the key evolutionary processes and is responsible for the diversity of life that exists on Earth. 

MICRO- AND MACROEVOLUTION

Evolution is the change in species over time, due to the change of how often an inherited trait occurs in a population over many generations.

Evolution encompasses changes of vastly different scales — from something as insignificant as an increase in the frequency of the gene for dark wings in beetles from one generation to the next, to something as immense as the evolution and radiation (spreading out) of the dinosaur lines. These two extremes represent classic examples of micro- and macroevolution.

Microevolution 

Microevolution happens on a small scale - the change in allele frequencies in a population - and across generations. Microevolution does not lead to the creation of a new species.

An example of microevolution is the evolution of pesticide resistance in mosquitoes. Imagine that you have a pesticide that kills most of the mosquitoes in your state one year. As a result, the only remaining mosquitoes are the pesticide resistant mosquitoes. When these mosquitoes reproduce the next year, they produce more mosquitoes with the pesticide resistant trait. This is an example of microevolution because the number of mosquitoes with this trait changed. However, this evolutionary change did not create a new species of mosquito, because the pesticide resistant mosquitoes can still reproduce with other mosquitoes if they were put together. 

Macroevolution 

Macroevolution involves changes at a species level, often across geological time. This large-scale change results from the accumulation of numerous small changes at the microevolutionary scale. 

Despite their differences, evolution at both of these levels relies on the same mechanisms of evolutionary change:

• mutation: changes in the DNA of organisms in the population can introduce new alleles into a population (if the mutation occurs in the sex cells, the organism reproduces and the new allele does not provide a survival disadvantage)

• migration/gene flow: breeding between two populations of the same species that carry unique alleles can add new alleles to the populations

• genetic drift: random increase or decrease of some alleles can occur in a population when certain alleles are not passed on to offspring (organisms with that allele do not reproduce) or when random sampling during meiosis does or does not select that allele)

• natural selection: the process of the organism better adapted to its environment surviving to reproduce more offspring

Macroevolution may happen: 

1. when many microevolution steps lead to the creation of a new species

2. as a result of a major environmental change, such as volcanic eruptions, earthquakes or an asteroid hitting Earth, which changes the environment so much that natural selection leads to large changes in the traits of a species. 

After thousands of years of isolation from each other, some of Darwin’s finch population, which was discussed in the Evolution by Natural Selection lesson, will not or cannot breed with other finch populations when they are brought together. Since they do not breed together, they are classified as separate species. 

The fossil record of the horse starts from about 55 million years ago. 

Around 1870, paleontologist O. C. Marsh described a fossil horse, roughly dog-sized, then known as “Eohippus.” Eohippus represented the far end of a line of fossils that seemed to show a gorgeous and gradual transition from tiny, 4-toed (on the front limbs), low-toothed, forest-dwelling horses to the mighty, 1-toed, high-toothed, plains-dwelling modern horses.

Darwin’s friend Thomas Henry Huxley visited Marsh at Yale University in 1876 and was so impressed by his collection of horse fossils  that he rewrote a speech he was to give in New York on the topic. Marsh developed a visual aid for Huxley’s lecture, upon which the American Museum of Natural History based an exhibit. Both the diagram and the exhibit were designed to show the trends toward modern horse: increasing size, decreasing number of toes, increasing height of the crown of the tooth. There was no branching in this version of events, one horse evolved in to the next horse form until the peak of horse evolution was reached—the living horse. The world had a perfect example of gradual, directional, evolutionary change. Diagrams of the progression appeared in every biology book.

Stephen Jay Gould wrote about this in an essay called "Life's Little Joke". Gould explains that at the time, Cope and Huxley had the trunk of the evolutionary lineage (Eohippus) and the “surviving twig”: modern horses. The mistake the scientists made was in putting known fossils that did not fit the pattern into “side branches” of little to no importance to overall horse evolution. Huxley went even further, and claimed that vertebrates as a whole followed a similar straight-line progression..

The problem is that paleontologists kept finding fossil horses. Herds of them. In fact today, horses are one of the best-documented groups in the fossil record. And it turns out that the once supposedly unimportant side branches were numerous and richly diverse. The simple straight-line example of evolution wasn’t so straight after all. As collections grew, it became clear that the evolution of horses was a bush made up of many branching lineages, not a tree with one  trunk. Some branches showed a general trend toward decreasing size, not increasing, and horses with three toes existed at the same time as horses with one toe.

Scientist report:

Digit reduction is a major trend that characterises horse evolution, but its causes and consequences have rarely been quantitatively tested. Using beam analysis on fossilised centre metapodials, we tested how locomotor bone stresses changed with digit reduction and increasing body size across the horse lineage... from Eohippus to Equus. Test results show that three-toed horses as late as Parahippus would have experienced physiologically untenable bone stresses. The centre metapodial compensated for evolutionary digit reduction and body mass increases by becoming more resistant to bending. These results lend support to two historical hypotheses: that increasing body mass selected for a single, robust metapodial rather than several smaller ones; and that, as horse limbs became elongated, the cost of inertia from the side toes outweighed their utility for stabilisation or load-bearing. 

Reworded: 

Change from three-toed to single toe (hoof) in the horse is a characteristic of the evolution of the horse, but why it happened and what effect it had has rarely been measured. Scientists tested fossil samples of the toes of different forms of horses from the earliest (Eohippus) to modern horse (Equus) against different body sizes. They found that three-toed ancient horses wouldn't have been able to handle the stress on the bones if they were the size of the modern horse. This supported two hypotheses:  

1. as the horse increased in size and mass, the single stronger centre toe would have been able to take more pressure, enabling the horse to move faster, providing an evolutionary advantage over the three smaller toes; 

2. as the leg bones became longer, the side toes would have provided little help and would have slowed the horse down by causing drag.

BACKGROUND: Types of Speciation - Allopatric and Sympatric

Allopatric speciation occurs when populations of a species are separated by a physical barrier - this could be a river for animals that cannot swim, for example. Separation of the populations means that there is very low or no gene flow between them - the proportion of different genotypes in each population is therefore able to change independently of the other (there's no mixing up of genes between the two populations). Over time, these changes may be so drastic that the populations become unable or unwilling to breed with each other, and this physical isolation results in separate species.

Sympatric speciation occurs without a physical barrier to gene flow. This is more common in plant species - plants can mutate in a way which results in them producing offspring with double or even quadruple the number of chromosomes they normally do. The sex cells produced by these individuals cannot fuse with sex cells from a "normal" plant - the plants with unusually high numbers of chromosomes therefore become isolated gentically from the "normal" plants, even though they may be growing right next to each other. This genetic isolation results in the two types of plants developing into species due to lack of gene flow and independent changes in the genotypes of plant populations.

(A way to remember these - Allo- could sound like "Alps"; think about a mountain dividing two populations. Sym- sounds like "similar"; think about species developing in the same place)

Another Way of Classifying Evolution

There are different ways of classifying students in a school, depending on what information is needed: eg year group, number of siblings, type of travel to school, age, etc. There are similarly different ways of classifying evolution. We have already seen classification based on speciation (micro- and macro-), now we look at classification based on effect of evolutionary pressures on phenotypes (physical characteristics).

In this classification there are three classes: convergent  evolution, divergent evolution, and co-evolution. 

a) Convergent evolution is the process of two dissimilar species becoming more phenotypically alike due to similar environmental pressures. 

b) Divergent evolution is the process of two or more related species becoming more and more dissimilar. 

c) Co-evolution is evolutionary changes of interdependent or proximal species. For example, if a plant evolves, so will the bees or birds that pollinate them.

Note: verge = edge, con = together, di = two, apart

a) Convergent Evolution

Convergent evolution is the term used to describe the fact that unrelated species can become more and more similar as they adapt to similar environments.  The wings of the bat, insect, and bird are a good example of convergent evolution.  These species are completely unrelated, but they all evolved wings over time as they became adapted to flying and living in the air.  Cacti and plants also go through convergent evolution.  In adaptation through the process of natural selection in a desert or a rainforest, unrelated plants often evolve with similar traits.

Divergent Evolution

Divergent evolution is the process of a species becoming dissimilar over time.  Although a species may have a common ancestor, environmental differences can select for different characteristics and individuals of a species will diverge.  The red fox and the kit fox, for example, have a common ancestor but have gone through the process of divergent evolution.  The red fox has been selected by its mixed farmland and forest environment where its red color helps it to blend in with the trees.  The kit fox lives in the desert, is sandy in appearance, and has larger ears which help it release excess body heat in the desert.

Charles Darwin believed that evolution was a slow and gradual process.  He did not believe this process to be "perfectly smooth," but rather, "stepwise," with a species evolving and accumulating small variations over long periods of time.  Darwin assumed that if evolution is gradual then there should be a record in fossils of small changes within a species.  

But in many cases, Darwin, and scientists today, are unable to find most of these intermediate forms.  Darwin blamed lack of transitional forms on gaps in the fossil record, a good suggestion, because the chances of each of those critical changing forms having been preserved and then uncovered as fossils are very small.  

In 1972, evolutionary scientists Stephen Jay Gould and Niles Eldredge proposed another explanation for the many gaps in the fossil record. They suggested that the "gaps" were real, there were no fossils to find, that they represented periods of no change in the organisms.  They termed this type of evolution "punctuated equilibrium." This means that species are generally morpholgically (shape, structure) stable, changing little for millions of years.  This stability is "punctuated" by a rapid burst of change that results in a new species.  According to this model, the changes leading to a new species don't usually occur from slow incremental change in the mainstream population of a species,  but occur in those populations living in the edges, or in small geographically isolated populations where their gene pools vary more widely due to the slightly different environmental conditions there.  When the environment changes, these "peripheral" or "geographic isolates" possess variation in morphology which might enable them to have an adaptive advantage, leading to greater reproductive success.  These new successful types spread through the geographic range of the ancestral species, appearing as a new morphology where once the older forms were present.