The Origin of New Species

Transcription

1 The Origin of New Species

2 Introduction If microevolution is small changes in gene frequencies What, then would macroevolution be? And how might that work????

3 The biological species concept emphasizes reproductive isolation In 1942 Ernst Mayr defined species. Can mate and have fertile offspring. Examples??? Same # of chromosomes and the same sequences of genes on them.

4 Species are based on infertility, not physical similarity. For example, these two meadowlarks are different species. But all these folks belong to the same species.. Fig. 24.2

5 2.How can a new species come about? There are two key ingredients to the process of speciation: Reproductive isolation A vacant niche. Then - Evolve (by what mechanisms???).

6 Reproductive isolation can occur for many reasons. But very often it is geography. Fig. 24.5

7 1. Geographic barriers can lead to isolation and the origin of species: Islands are great examples. How about those Grand Canyon squirrels?

8 They also require the means to survive - a niche they can fill. Gause s Law, or the Law of Competitive Exclusion: this town s not big enough for the both of us. The less well adapted population will either?

9 So here s how it works: The isolated populations face different selection pressures and have different random mutations occur. The result is that their gene pools gradually grow different enough and, voila, they are two different species. Even if reunited, their members can no longer mate and produce fertile offspring.

10 Common ancestor??? adaptive radiation??? divergent evolution??? Watch here. Fig

11 Divergent Evolution This is the main pattern we see when we look at the history of life on earth. It is the branching tree pattern of a common ancestor giving rise to different species, orders, kingdoms, etc. Similarities in homologous structures, chemicals like DNA and embryological development all are evidence of evolutionary change with this pattern. Adaptive radiation is divergent evolution that has resulted in the formation of, at the very least, new species. The two terms are almost synonyms.

12 Fig

13 Convergent Evolution, on the other hand is a pattern we see in which groups which, even though they diverged from a common ancestor in the past, were different from each other at one point in time and then later evolved analagous structures which serve similar functions. Two classic examples: The evolution of a streamlined body shape and fins in sharks (fishes), whales (mammals with dog-like ancestors) and reptiles (swimming dinosaurs with 4-legged, land-living ancestors). The evolution of wings in birds, bats and insects, all from ancestors without wings.

14 Now, just to be confusing, let s look at co-evolution. This is not another pattern, really, like divergent and convergent evolution, this is when two organisms are evolving based on the selection pressures provided by the other. 2 examples: Cheetahs and antelopes have both evolved speed related adaptations as a result of each other s speed. Flowers and their pollinators. Great example. 3:40

15 3. The punctuated equilibrium model has stimulated research on the tempo of speciation Traditional evolutionary trees diagram the diversification of species as a gradual divergence over long spans of time. This assumes that big changes occur because of the accumulation of many small ones - the gradualism model. Like sharks.

16 In the fossil record, many species appear as new forms rather suddenly (in geologic terms), persist essentially unchanged, and then disappear from the fossil record. Darwin noted this when he remarked that species appear to undergo modifications during relatively short periods of their total existence and then remained essentially unchanged. He could not explain this fossil evidence in his gradualism theory, but we have learned much since then.

17 In the punctuated equilibrium model, the tempo of speciation is not constant. Species undergo most morphological modifications when they first bud from their parent population. After establishing themselves as separate species, they remain static for the vast majority of their existence. Fig b

18 Periods of mass extinction (2:25) followed by rapid speciation (:30) do exist in the fossil record. Stephan Jay Gould helped pioneer the idea of punctuated equilibrium. Let s watch a bit of video.

19 How did the dinosaurs become extinct?

20 Under this model, changes may occur rapidly and gradually during the few thousands of generations necessary to establish a unique genetic identity. On a time scale that can generally be determined in fossil strata, the species will appear suddenly in rocks of a certain age. Stabilizing selection may then operate to maintain the species relatively the same for tens to hundreds of thousand of additional generations until it finally goes extinct.

21 Most evolutionary novelties are modified versions of older structures The Darwinian concept of descent with modification can account for the major morphological transformations of macroevolution. It may be difficult to believe that a complex organ like the human eye could be the product of gradual evolution, rather than a finished design created specially for humans. However, the key to remember is that that eyes do not need to as complicated as the human eye to be useful to an animal.

22 We have also discovered genes (HOX genes) that act as master switches that turn on and off whole sets of other genes to cause big changes such as in development. Mutations in these genes can cause big changes very quickly.

23 A species of finch has been studied on one of the geographically isolated Galapagos Islands for many years. Since the island is small, the lineage of every bird for several generations is known. This allows a family tree of each bird to be developed. Some family groups have survived and others have died out. The groups that survive probably have A. Interbred with other species B. Been attacked by more predators C. Found new places on the island to live D. Inherited some advantageous variations