Evolutionary Types and Models www.onacd.ca Convergent evolution Convergent evolution occurs when organisms that are NOT closely related live in the SAME environment. They independently evolve similar traits or structures that are adapted to that shared environment. Structures that are the result of convergent evolution are termed ANALAGOUS structures. What similarities are shared by the following animals found in similar environments all over the world that all like to eat ants and termites? Anteater, South America Sloth bear, South east Asia Aardvark, Africa Numbat, Australia Echidna, England Armadillo, USA All of the animals on the previous slide have developed powerful fore claws and long, sticky tongues (most housed in long snouts) that allow them to open the homes of social insects (ants and termites) and eat them. These include the four species of anteater, more than a dozen armadillos, eight species of pangolin (plus fossil species), the African aardvark, one echidna (an egglaying monotreme), the Australian marsupial known as the numbat, the aberrant Aardwolf, and possibly also the Sloth Bear of South Asia, all not related. This is an example of CONVERGENT evolution, common adaptations to similar environments. What common traits do the following animals seem to share? The North American kangaroo rat (top), Australian hopping mice (middle) , and North African and Asian jerboa (bottom) have developed convergent adaptations for hot desert environments; these include a small rounded body shape with very large hind legs and long thin tails, a characteristic bipedal hop, and nocturnal, burrowing and seed-eating behaviors. These rodent groups fill similar niches in their respective ecosystems. Convergent Evolution in Plants Prickles, thorns and spines are all modified plant tissues that have evolved to prevent or limit herbivory, these structures have evolved independently a number of times. The aerial rootlets found in ivy (Hedera, see left) are similar to those of the climbing hydrangea (Hydrangea petiolaris, see right) and some other vines. These rootlets are not derived from a common ancestor but have the same function of clinging to whatever support is available Divergent Evolution Divergent evolution occurs when… related species evolve different traits to the point where they actually become a new species Divergent evolution can result in the formation of…. HOMOLOGOUS structures, structures that are similar in appearance but serve different functions The most well know example of divergent evolution is with Darwin’s finches Remember: when Darwin was on his voyage on the HMS Beagle he spent much time in the Galapagos islands. He noticed that on each island there were finches but the finches on each island, or group of islands, differed in the size and shape of their beaks. Why might the finches below, found on different islands, have developed differently shaped beaks? The birds were all about the same size (10–20 cm). The most important differences between species of finch were in the size and shape of their beaks, and the beaks were highly adapted to different food sources. The birds were all brownish or black. Their behavior differed, and they had different song melodies Darwin theorized that at one time there must have been a common ancestor to the finch species. That common ancestor slowly was dispersed as the Galapagos islands were formed and broke away from one another. Genetic variations amongst the finches beaks were then selected for by the environment over a period of many years, resulting in the formation of new species. Speciation Speciation is the evolutionary process by which new biological species are formed. Speciation can occur in a variety of different methods, termed allopatric, peripatric, parapatric and sympatric speciation During allopatric speciation, a population splits into two geographically isolated allopatric populations (for example, by habitat fragmentation due to geographical change such as mountain building or social change such as emigration). The isolated populations then undergo changes in their genetic makeup as they (a) begin to adapt to different environments or (b) they undergo mutations. When the populations come back into contact, their genetic material has evolved so much that they are no longer capable of exchanging genes In peripatric speciation, new species are formed in isolated, small peripheral populations which are then prevented from exchanging genes with the main population. Genetic drift is often proposed to play a significant role in peripatric speciation. In parapatric speciation, the zones of two diverging populations are separate but do overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time. However, heterozygous (one dominant, one recessive allele) traits are selected against by the environment to the point where the two species can no longer produce offspring together anymore. In sympatric speciation, species diverge while inhabiting the same place. Often cited examples of sympatric speciation are found in insects which become dependent on different host plants in the same area. Rate of Evolution There are two theories that are used to explain how fast evolution is believed to occur. #1. Gradualism • States that evolution of species is slow, continuous and gradual, subtle changes occur over million of years • This theory would explain the adaptations observed in species that are very well adapted to their environments such as sharks, cockroaches and crocodiles. #2. Punctuated Equilibrium • States that during times of climactic or geographic changes in environment organisms evolve more rapidly • Some changes occur in rare, rapid events after a period of little or no change • This is the more accepted theory for most plant and sexually reproducing land animal species. # of C H A N G E S Millions of Years Decide which of the line in the graph above portrays the rate of gradualism and which portrays the rate of punctuated equilibrium? Do the theories of Gradualism and Punctuated Equilibrium seem to oppose one another? Punctuated equilibrium is often mistakenly thought to oppose the concept of gradualism, when it is actually a form of gradualism. This is because even though evolutionary change appears instantaneous in fossils found in neighboring layers of the sediment of the earth, change is still occurring incrementally, with no great change from one generation to the next. Even though the graph lines on the previous slide look to change quickly, we are still talking about changes occurring slowly over millions of years. The changes are simply occurring in intervals rather than continuously. The relationship between punctualism and gradualism can be better appreciated by considering an example. Suppose the average length of a limb in a particular species grows 50 centimeters (20 inches) over 70,000 years—a large amount in a geologically short period of time. If the average generation of that species is seven years, then our given time span corresponds to 10,000 generations. It is therefore reasonable to conclude that if the limb size in our hypothetical population evolved in the most conservative manner, it need only increase at a rate of 0.005 cm per generation (= 50 cm/10,000), despite its abrupt appearance in the geological record.