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Stages of Natural Selection

Stages of Natural Selection. Variation exists in the phenotypes of a population. There are either brown or yellow shelled snails There is a selective pressure. A predator arrives in the habitat Phenotypes with a higher fitness level pass on genes.

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Stages of Natural Selection

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  1. Stages of Natural Selection • Variation exists in the phenotypes of a population. • There are either brown or yellow shelled snails • There is a selective pressure. • A predator arrives in the habitat • Phenotypes with a higher fitness level pass on genes. • Those that are brown are camouflaged and are more likely to survive • Over several generations the ratio of phenotypes changes in favour of the phenotype with the higher fitness value. • Browns breed and increase the occurrence of brown offspring.

  2. Artificial Selection Brassicas • Humans have controlled the breeding of domesticated animals and plants for centuries. • This practice, called artificial selection or selective breeding, involves breeding (selecting) from individuals with the most desirable phenotypes. • It can result in an astounding range of phenotypic variation over relatively short time periods. • Selection imposed by humans is often more rapid and intense than that occurring in nature. • Strong selection pressures can be applied to produce extremes of diversity and development. Wheat and other grain crops Domestic dogs (pug)

  3. Artificial Selection in Brassica Broccoli(inflorescence) Cauliflower (flower) • Different parts of the wild brassica have been developed by human selection to produce at least six distinctly different vegetables. • All these vegetables form a single species and will interbreed if allowed to flower. • Example: The new “broccoflower” is a cross between broccoli and cauliflower. Cabbage(terminal buds) Brussels sprout (lateral buds) Kale(leaf) Wild FormBrassica oleracea Kohlrabi(stem)

  4. Artificial Selection in Dogs • Dogs were probably first domesticated at least 14 000 years ago from a gray wolf ancestor. • Some 400 breeds have been bred from this single wild species as a result of selective breeding by humans. • Example: The staffordshire bull terrier wasproduced by breeding bulldogs and terriers.From each litter, breeders selected pups withthe characteristics they desired. Bulldog Gray wolf Terrier Staffordshire bull terrier Staffordshire bull terriers combine characteristics of both bulldogs and terriers

  5. Artificial Selection in Dogs • The gray wolf is distributed throughout Europe, North America and Asia. Amongst this species, there is a lot of phenotypic variation. • Selection is based on both physical and behavioral characteristics. In this way, different breeds have been suited to different tasks. • Five ancient dog breeds are recognized, from which all other breeds are thought to have descended by artificial selection. Mastiff-type Originally from Tibet, this breed dates back to the Stone Age Pointer-type Bred for the purpose of hunting small game. Greyhound One of the oldest breeds, originating the Middle East. Sheepdog Originated in Europe and bred for stock protection. Wolf-type Developed in snow-covered habitats in Alaska, northern Europe, and Siberia.

  6. Activity • Answer questions from pages 337 to 339 in Biozone books.

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