Patterns of Evolution

There are 3 main patterns of evolution: Convergent Evolution Divergent Evolution Speciation

1. CONVERGENT EVOLUTION Convergent evolution occurs when organism that aren’t closely related evolve similar traits as they both adapt to similar environments. Convergent evolution is also responsible for… Example: The WINGS of the bat, the bird, and the pterodactyl. In each case, the forelimbs of these vertebrates morphed over time into wings, but they did so INDEPENDENTLY.

Example: The different sorts of ANTEATERS found in Australia, Africa, and America. Though not closely related, they all evolved the "tools" necessary to subsist on an ant diet: a long, sticky tongue, few teeth, a rugged stomach, and large salivary glands. In each case, evolutionary adaptations allow them to exploit a food niche of ants and termites, but the developments occurred INDEPENDENTLY.

DIVERGENT EVOLUTION The ADAPTIVE RADIATION of Darwin’s finches is one example of DIVERGENT EVOLUTION. Divergent evolution is the process of two or more RELATED SPECIES becoming more and more DISSIMILAR.

The red fox and the kit fox provide another example of two species that have undergone divergent evolution. The RED FOX lives in mixed farmlands and forests, where its red color helps it blend in with surrounding trees. The KIT FOX lives on the plains and in the deserts, where its sandy color helps conceal it from prey and predators. The ears of the kit fox are larger than those of the red fox. The kit fox's large ears are an adaptation to its desert environment. The enlarged surface area of its ears helps the fox get rid of excess body heat. SIMILARITIES IN STRUCTURE INDICATE that the red fox and the kit fox had a COMMON ANCESTOR. As they ADAPTED TO DIFFERENT ENVIRONMENTS, the APPEARANCE of the two species DIVERGED.

SPECIATION Speciation and extinction were extremely important in determining the history of life on Earth.. (Consider this. Would we be here if the dinosaurs hadn’t become extinct?) Before we can know whether speciation has occurred in a group of organisms, we have to have an idea of what constitutes a species in the first place. “A species is a group of interbreeding individuals or populations that are REPRODUCTIVELY ISOLATED from other such groups.”

One of the central questions in studying evolution is how new species arise. SPECIATION involves ENVIRONMENTAL PROCESSES that lead to SPLITTING A GENE POOL into two or more separate gene pools. The key event in speciation is REPRODUCTIVE ISOLATION between two diverging populations (and, therefore, isolation of their gene pools). Isolating mechanisms are all genetically based and can be either PRE-ZYGOTIC (before a zygote has formed) or POST-ZYGOTIC (after a zygote has formed).

Isolating Mechanisms PRE-ZOGOTIC BARRIERS POST-ZYGOTIC BARRIERS a. prevent fertilization b. include seasonal or habitat barriers to mating, behavioral isolation, or physical isolation. a. come into play despite fertilization b. include gamete mortality, hybrid in viability (premature death) and hybrid sterility

GEOGRAPHICAL ISOLATION Perhaps the simplest mechanism of isolation to achieve Continents collide, mountains are uplifted and suddenly a population finds that it is splintered into a large population as well a number of geographically isolated smaller populations. Most of these small populations eventually go extinct. Distinct flightless birds on different continents exemplify diversification through geographic isolation. [Begon et al., 1990]

COURTSHIP BEHAVIOR Often the behaviors a male must engage in to convince the female of his proper species-hood are quite elaborate and cannot be reproduced by members of a different species. Various behavioral cues allow males and females to recognize their own species. For example, the male bower bird builds a nest, or bower, for female bower birds during mating season. The male is trying to entice the female to mate with him. Males compete for females by building and decorating these elaborate structures. Because different species build different types of bowers, this mating behavior helps maintain reproductive isolation between the different species.

RATE OF EVOLUTIONARY CHANGE The basic evolutionary mechanisms—mutation, migration, genetic drift, and natural selection—can produce major evolutionary change if given enough time. PUNCTUATED EQUILIBRIUM VS GRADUALISM The "PUNCTUATED EQUILIBRIUM" theory was proposed as a criticism of the traditional Darwinian theory of evolution. ELDREDGE and GOULD observed that evolution tends to happen in FITS AND STARTS, sometimes moving very fast, sometimes moving very slowly or not at all. On the other hand, typical variations tend to be small. Therefore, DARWIN saw evolution as a SLOW, continuous process, without sudden jumps. (GRADUALISM)

However, if you study the FOSSILS of SOME organisms found in geological layers, you will see long intervals in which nothing changed ("equilibrium"), "punctuated" by short, revolutionary transitions, in which species became extinct and replaced by wholly new forms. Instead of a slow, continuous progression, the evolution of life on Earth seems more like the life of a soldier or a fire fighter: long periods of boredom interrupted by rare moments of terror.

EXAMPLE OF PUNCTUATED EQULIBRIUM Scientists have scrutinized the fossil records of many organisms looking for evidence of punctuated evolution. One group of coral-like sea organisms in particular, called bryozoan, shows this kind of pattern. The well-preserved fossil record of bryozoans shows that one species first appeared about 140 million years ago and remained unchanged for its first 40 million years. Then there was an explosion of diversification, followed by another period of stability for vast amounts of time.