15-3: Darwin Presents His Case

Natural Selection and Artificial Selection variation exists in all species in nature and domesticated plants and animals natural variation: differences among individuals of a species Darwin had noticed farmers and breeders routinely selected individual plants and animals with the most desirable qualities for reproduction –artificial selection artificial selection Selecting which individuals that get to reproduce nature provided the variation and humans selected the most useful variations

Evolution by Natural Selection the struggle for existence Darwin believed that high birth rates and limited resources would force organisms to compete Those organisms with variations enabling them to better compete for resources would survive to reproduce offspring with the same beneficial variations survival of the fittest fitness: the ability of an individual to survive and reproduce in its environment adaptation: an inherited characteristic that increases an organism’s chance of survival an organism’s fitness is the result of adaptation

Evolution by Natural Selection survival of the fittest: those individuals with characteristics well suited to the environment will survive to produce many offspring while those less suited will not also referred to as natural selection over time natural selection results in changes in inherited characteristics of a population, thereby increasing its fitness for its environment descent with modification each living species has descended, with changes, from other species over time common descent: all species, living and extinct, were derived from common ancestors a single “tree of life” for all living things on Earth

Summarizing Natural Selection There are variations within population of species A population produces more offspring than nature can support These variations struggle for resources The variations best suited survive and get to reproduce, thus passing on their genes

Evidence of Evolution the fossil record fossils in different layers of rock are evidence that living things have undergone gradual change over time it is also evidence that the Earth is hundreds of millions, rather than thousands, of years old geographic distribution of living species animals on different continents showed similar modifications, even though they descended from different ancestors Darwin reasoned the similar modifications were the result of being exposed to similar ecological conditions and similar pressures of natural selection

Beaver Beaver Muskrat Beaver and Muskrat Coypu Muskrat Capybara Coypu and Capybara NORTH AMERICA Muskrat Capybara SOUTH AMERICA Coypu

Evidence of Evolution homologous body structures additional evidence for evolution is demonstrated by the structural similarities between arms, wings, legs, and flippers constructed from same basic bones even though they have totally different functions homologous structures: structures that have different functions but develop from same embryonic tissues vestigial organs: traces of organs that have no function and are homologous to organs in other animals vestigial organs survive because they have no affect on an organism’s survival

Turtle Alligator Bird Mammals Typical primitive fish

Evidence of Evolution Molecular Biology similarities in early development or (Comparative Embryology) Studying the stages of development “ontogeny recapitulates phylogeny” the embryonic development of an organism recreates its evolutionary development the same groups of embryonic cells develop in the same order and in similar patterns to produce tissues and organs of all vertebrates Molecular Biology Studying DNA, genes and proteins All life has DNA

Evolution of Populations Chapter 16 Evolution of Populations

16-1 Genes and Variation

Darwin’s Ideas Revisited Darwin’s disadvantage was he did not understand the mechanism for heredity he did not know the source for variation he did not how inheritable traits were passed from one generation to the next it was fifty years later that biologists combined Mendel’s work with Darwin’s work

Gene Pool Gene Pools gene pool: the genetic information of all members of a population contains two or more alleles for each inheritable trait relative frequency: the number of times an allele occurs in a gene pool expressed in percents Measuring the gene pool frequencies tells if evolution if occurring

Single-Gene and Polygenic Traits the number of phenotypes produced for a given trait depends on how many genes control the trait single-gene traits are controlled by one gene and have two distinct phenotypes most traits are polygenic traits controlled by two or more genes

The dominant phenotype may be less frequent since allele frequencies may not match Mendelian ratios 100 80 60 40 20 Frequency of Phenotype (%) Widow’s peak No widow’s peak Phenotype

Frequency of Phenotype Various phenotypes of polygenic traits can be grouped into a bell-shaped curve Frequency of Phenotype Phenotype (height)

Sources of Genetic Variation mutations and genetic shuffling are two main sources of genetic variation mutations a change in the genetic sequence of DNA occurring through replication mistakes, radiation, or mutagens can be a few bases or large segments of chromosomes most mutations have a negative affect on organism’s survival

gene shuffling (sexual Reproduction) most variation comes from genetic shuffling occurs during the production of gametes because of independent assortment and crossing over it may change the various combinations of alleles but does not affect the relative frequency of alleles