Genetic Variation (c)(15)(C) The combination of meiosis and fertilization results in each offspring having one copy of each gene from each parent. Consequently, children tend to resemble their parents and their siblings. However, meiosis results in genetically diverse sperm and eggs which, together with random fertilization, results in genetic diversity of the zygotes and children produced by the same mother and father.

Variation and Evolution Give a definition of Variation Variation is the range of differences that there are between individual organisms.

Variation and Evolution Variation can be within species (Think of all the differences between individual humans) These are different varieties of the same species

Variation and Evolution Or between species: Gorilla Proboscis monkey

Variation and Evolution Variation can be CONTINUOUS, have any value between two extremes. Examples: Height of human Body mass of cats Trumpet length of daffodils Leaf width of shrub Length of bacteria All these features show normal distribution (see graph)

Variation and Evolution Variation can be DISCONTINUOUS, in this case there are only a few possible categories that the characteristic can fall into. EG: Flower color in a single species Human blood group or ear lobe type Bacteria that are or are not resistant to an antibiotic

Variation and Evolution Genetic variation Each tomato is different because it comes from a plant with different alleles, that code for different characteristics Environmental variation These plants are genetically identical, the flower color difference is due to the effects of soil pH.

Variation and Evolution GENETICENVIRONMENTAL Differences that are due to the DNA inside the cells of the organism rather than the effect that its surroundings have on it. Differences that are due to the external environment. For example availability of water, nutrients, light, prevailing winds.

Meiosis creates genetic variation During normal cell growth, mitosis produces daughter cells that are identical to parent cell. Meiosis results in genetic variation by mixing of maternal and paternal chromosomes and crossing over. No daughter cells formed during meiosis are genetically identical to either mother or father. During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

Why do cells divide by Meiosis? To reduce chromosome number Allows for Chromosome Variety Sperm N egg N 2N baby

Meiosis is how haploid gametes are produced Mitosis = ASEXUAL reproduction Meiosis = SEXUAL reproduction Each gamete produced by meiosis is genetically unique.

The Origins of Genetic Variation Offspring of sexual reproduction are genetically different from their parents and from one another.

Independent Assortment of Chromosomes In independent assortment, every chromosome pair orients independently of the others during meiosis.

Random Fertilization The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote.

Crossing Over In crossing over, Homologous chromosomes exchange genetic information. Genetic recombination occurs.

How Accidents During Meiosis Can Alter Chromosome Number In nondisjunction, The members of a chromosome pair fail to separate during anaphase. Gametes with an incorrect number of chromosomes are produced.

The result of nondisjunction

Down Syndrome Is a condition where an individual has an extra chromosome 21. Is also called trisomy 21.

Evolution Connection New Species from Errors in Cell Division Errors in meiosis may have been instrumental in the evolution of many species. Polyploids Are new species. Have more than two sets of homologous chromosomes in each somatic cell.

23 The Gene Pool Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population

24 The Gene Pool Different species do NOT exchange genes by interbreeding Different species that interbreed often produce sterile or less viable offspring e.g. Mule

Adaptations Many of the differences between species are present because they help the individuals to survive. We call these differences ADAPTATIONS. You should understand that there are Adaptations to: ANATOMY or body FORM, PHYSIOLOGY or body FUNCTION BEHAVIOUR

Variation leads to ? Explain the consequences of the four observations made by Darwin in proposing his theory of natural selection. What are the 4 observations that Darwin made? 1.Offspring generally appear similar to parents. 2.No two individuals are identical. (Why not?) 3.organisms have the ability to produce large numbers of offspring 4.Populations in nature tend to remain fairly stable in size. SO what are the consequences? Evolution

Variation and Evolution Because individuals over produce, yet populations remain stable COMPETITION must take place. In COMPETITION there are “winners” and “losers”. Winners are better adapted with more useful characteristics, they are more able to survive and breed, so pass on their beneficial alleles. Over time when this happens continually, accumulated changes can give rise to a new species.

Variation and Evolution Outline how variation, adaptation and selection are major components of evolution. Differences in organisms How do they arise Where do these changes happen How can they be passed on

29 Modern Synthesis Theory Combines Darwinian selection and Mendelian inheritance Combines Darwinian selection and Mendelian inheritance Population genetics - study of genetic variation within a population Population genetics - study of genetic variation within a population Emphasis on quantitative characters (height, size …) Emphasis on quantitative characters (height, size …)

Natural Selection Variation in organisms characteristics are caused by the organisms having different alleles of genes. Some differences enable the organism to survive better (compete more successfully) The ones with beneficial alleles survive, breed and pass on their alleles to the next generation Those without beneficial alleles die before they reproduce, so their alleles are less likely to be passed on. The beneficial alleles increase in frequency in the population and may eventually produce a a new species.

31 Modern Synthesis Theory TODAY’S theory on evolution Recognizes that GENES are responsible for the inheritance of characteristics Recognizes that POPULATIONS, not individuals, evolve due to natural selection & genetic drift Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes

32 Microevolution Changes occur in gene pools due to mutation, natural selection, genetic drift, etc. Gene pool changes cause more VARIATION in individuals in the population This process is called MICROEVOLUTION Example: Bacteria becoming unaffected by antibiotics (resistant)

Selective Pressures Competition for food, water Predation Disease Physical and chemical factors Competition for mates Competition for space, nesting sites, territory etc An allele that helps an organism compete better is “selected for”, increases in frequency An allele that fails to help an organism survive is usually “selected against”, decreases in frequency

Natural Selection Darwin proposed that Natural Selection could lead to new species being produced over time. This was his explanation for the large number of different species of finch found in the Galapagos Islands. Each species has adaptations to its body, beak and feet that allow it to feed on specific types of food in specific areas.

Evolution of a new species Speciation occurs when there is a “Reproductive Barrier”, some thing that stops members of an original population passing their alleles freely among all members, this prevents free gene flow. The barrier may be a geographical barrow = Allopatric speciation. The barrier may be behavioural, biochemical, or anatomical = sympatric speciation.

Allopatric Speciation

When there is NO geographical barrier Initially individuals in the original species may have variations that can be divisive of the population. E.g. in birds some might have slightly: different shaped beaks, shorter wings, better nocturnal vision, different color plumage or courting behavior, etc Sympatric Speciation

Speciation in the Genus CANIS

Fossil record – simplest organisms in oldest rocks, old species extinct, new species develop. Reptile/bird link, horse evolution, human evolution. Biochemical – Similar biochemistry of all living things, closely related species have similar biochemistry, the longer ago they diverged the more different the biochemistry. Amino acid sequences in cytochrome c (used in respiration) Regulation of DNA and RNA production. DNA differences are greater between species which are not closely related. Evidence for Evolution in brief

Offspring generally appear similar to their parents. No two individuals are identical Organisms have the ability to produce large numbers of offspring Populations in nature tend to remain fairly stable in size. Darwin’s 4 Observations

In the past, there were species different from those present today.  Old species died out.  New species have arisen.  The new species are often similar to the older ones in the same place (From fossils) Darwin’s Evidence

Darwin felt more modern species had variations that meant they were Better adapted to the environment. Armadillos and Glyptodonts

In Summary: Causes for Genetic Variation and Evolution Genetic Drift - the change in the gene pool of a small population due to chance Natural Selection - success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance) - Cause ADAPTATION of Populations Gene Flow -is genetic exchange due to the migration of fertile individuals or gametes between populations 43

Causes Continue  Crossing Over  Exchange of segments of chromosomes during meiosis  Mutations  a change in an organism’s DNA  Mutations can be transmitted in gametes to offspring  Non-random mating  - Mates are chosen on  the basis of the best traits 44