Chapter 4 (Pp ) Heredity and Evolution

Transcription

1 Chapter 4 (Pp ) Heredity and Evolution

2 Modern Evolutionary Theory The Modern Synthesis Prior to the early-1930s there was a break between the geneticists and the natural historians (read ecologists and such). At a pivotal conference these groups began to hammer out a synthesis and this is not called the modern synthesis. The modern synthesis defies evolution as a two-stage process: 1. Stage 1: The production and redistribution of variation (inherited differences between organisms) 2. Stage 2: Natural selection acting on this variation, whereby inherited differences, or variation, among individuals differentially affect their ability to successfully reproduce. The importance of the modern synthesis in the field of anthropology was tremendous. In fact, unlike the text I prefer the term biological anthropology in great part as a rejection of the race typology linked to physical anthropology The discussion concerning the error of thinking in terms of racial categories is discussed extensively in Chapter 12. A paper on the history of this shift can be obtained here. New Synthesis? According to Smith and Ruppell (2011), a new synthesis is emerging. New discoveries are changing the landscape of evolutionary explanations. These include: 1) Horizontal Gene Transfer (HGT) (genes move between species), 2) epigenetics, 3) developmental evolutionary biology (evo-devo) (shared groups of genes), 4) mutualisms and coevolution (such as humans and dogs), 5) phenotypic plasticity and phenotypic integration and 6) niche construction.

3 Factors that Produce & Redistribute Variation 1 A Current definition of evolution Today we define evolution as a change in the frequency of alleles from one generation to the next. Some terms: In a population, the percentage of all the alleles at a locus accounted for by one specific allele is the allele frequency. A population is a community of individuals within a species where mates are usually found. Short-term effects versus long-term effects (These issues are covered more in Chapter 5) The short term changes in allele frequency occurring within a species are defined as microevolution Changes produced only after many generations, such as the appearance of a new species is called macroevolution. Breeding population Dog + wolf = Wolfdog Dog + coyote = Coydog Dog + Jackal = Rare Dog + Fox = Dox Wolf + Coyote = Coywolf Wolf + Fox =? Wolf + Jackal = DNA only Jackal + Fox = Unknown Jackal + Coyote = NO

4 Factors that Produce & Redistribute Variation 2 There are two sources for variation When it goes right: Genetic recombination was discovered by Mendel. Actually, segregation of genes (actually chromosomes, but he did not know this) Independent assortment (random shuffling of the chromosomes) When it goes wrong: Mutation or a change in the DNA sequence that produces an altered gene. Technically, this term refers to changes in DNA bases as well as changes in chromosome number and/or structure. Can be a single change, a cross-over between chromosomes or even additional chromosomes or deletions of them Point mutation is the substitution of one DNA base for another MUST occur in the sex cells to influence evolution. Ultimate source of all variation in organisms Mutation typically effects variability, generally, by increasing it. It is rare to see evolution by mutation alone, except in microorganisms. Mutation rates are very low. In large populations maybe 1 in 10,000 a mutation might be observed. Studies of mutations are at both the DNA level and the chromosomal level.

5 Factors that Produce & Redistribute Variation 3 Gene flow is the exchange of genes between populations Evolution can also occur as a result of genes being transferred from one population to another. So we look at the changes in gene pools. A gene pool is the total complement of genes shared by the reproductive members of a population. Gene flow is studied at the population level, then. Gene flow occurs when there is migration. Large scale migrations are not required. The slow mix of genes by neighboring populations can have a dramatic effect on evolution. The rate of human migration in the last 500 years is unprecedented. More recently: The offspring of U.S. soldiers and Vietnamese women represent gene flow, even though the fathers returned to their native population. When gene flow occurs, the two populations mix genetically and tend to become more similar. The result of gene flow is that there are very few human breeding isolates remain. Genes may occasionally also flow between species (wolves and dogs for instance, as per our earlier example). Gene flow

6 Factors that Produce & Redistribute Variation 4 Genetic drift is evolutionary changes in allele frequencies produced by random factors Based on the randomness of the percentages in a small population. In fact, it is due to the population being small. Probably more important to human evolution many thousands of years ago. Today very big populations Genetic drift is studied at the population level There are different types of genetic drift 1. A type of genetic drift in which allele frequencies are altered in small populations that are taken from, or are remnants of, larger populations is called founder effect. A few members of population break off from the larger group. Due to randomness, the genetic composition is generally The two types not the same. 2. A second type of genetic drift is called bottleneck A bottleneck is the reduction of the size of a population within a single generation The genetic variation within this smaller population is lessened by randomness alone. Often the result of a disaster or other catastrophe Recombination (crossing-over) of DNA within the nucleus Recombination does not change allele frequencies or cause evolution. Recombination changes the composition of parts of chromosomes Recombination affects how some genes act, and slight changes of gene function can become material for natural selection to act upon.

7 Natural Selection Acts on Variation 1 Natural selection is usually the most important mechanism of evolution. We now know that its effect on individuals depends on their phenotypes which in turn are determined mostly by their genotypes. Differential survival combined with differential reproduction Remember it is measured with using fitness. Interactive game The environment ultimately selects individuals with the best suited genotypes to survive to adulthood and to reproduce. Those who have more surviving offspring pass on more of their genes to the next generation. Natural selection provides directional change in allele frequency relative to specific environmental factors. Natural selection is studied at the level of the organism.

8 Natural Selection Acts on Variation 2 Classic example of the effects of natural selection is that of sickle cell anemia (SCA) HbS (hemoglobin S) does not carry oxygen as well as HbA (normal hemoglobin). The HbS mutation occurs at a low frequency in all human populations. Sickle-cell trait results when an individual inherits only one copy of the Hbs allele. Sickle-cell anemia trait results from inheriting two copies of the Hbs allele. The HbS allele increased in frequency in West African populations due to natural selection Up to 100,000 deaths/year are caused by SCA. In the US 40-50,000 persons suffer, most are African American. Worldwide SCA is also seen in Mediterranean populations and in India. So what is the environmental factor that selects for this deadly variant? Malaria Healthy persons (homozygous normal: HbA/HbA) are better hosts, sickly from malaria. SCA occurs in those homozygous for HbS Results in balancing selection as the heterozygote is most fit.

9 Life Cycle of Malaria

10 Review of Genetics & Evolutionary Factors The evolutionary process occurs at four different levels. 1. Mutations of the DNA in the gametes (the molecular level). 2. The chromosomes assort during meiosis and are passed on to the offspring (the cellular level). 3. The individual is subject to natural selection (the level of the individual). 4. Populations are composed of individuals, and evolution is said to occur when allele frequencies change (the level of the population). Genetic drift and gene flow may act to change allele frequencies in populations.