premises of Darwin’s theory The four basic premises of Darwin’s theory The world’s animal and plant communities are dynamic, not static: they change over time with new forms originating and others becoming extinct. 2. The evolutionary process is gradual and continuous. New species arise through slow and steady environmental changes that gradually ‘perfect’ each species to its surroundings. When sudden and dramatic changes occur in the environment, a species’ ability to adapt is usually challenged. Some species adapt and live; others become extinct. 3. All organisms descended from an original and common ancestor. Over time, the process of natural selection has created different species, each specifically adapted to its ecological niche. 4. Natural selection not only causes changes within populations during changing environmental conditions but also acts to maintain the status quo under relatively constant environmental conditions. Two concepts are central to Darwin’s theory of evolution: adaptation (the ability of generations of species to adapt to environmental changes) and natural selection (the transfer of some variations in species from one generation to the next). This slide lists the four basic premises of Darwin’s theory.

Major milestones in human evolution The success of the human species stems predominantly from our ability to walk upright on two feet (bipedalism) and our increased brain size (encephalisation). The interaction between bipedalism and encephalisation allowed humans to exploit new environments and establish well-organised communities. Advances in tool making and hunting, and the use of fire for cooking, protection and warmth helped humans to live longer. The development of language provided the foundation on which all human cultures could be built. As these cultures developed humans became increasingly able to modify their environment. Intellectual resourcefulness prompted the agricultural revolution, the industrial revolution and the technological revolution. Source: Adapted from Lewin, R., Human Evolution: An illustrated introduction. Cambridge, MA: Blackwell Scientific Publications, Inc., 1984.

The genetic determination of sex We inherit 23 individual chromosomes from each of our parents. One pair of these chromosomes - the sex chromosomes - contains the instructions for the development of male or female sex characteristics. (a) The sex of human offspring depends on whether the sperm that fertilises the ovum carries an X or a Y chromosome. An X-bearing sperm produces a girl, a Y-bearing sperm produces a boy. (b) The presence of a Y chromosome in the twenty-third pair of chromosomes in this image indicates that this sample came from a male. A sample from a female would include two X chromosomes. Source: CNRI Science Photo Library/Photo Researchers Inc.

Patterns of inheritance for eye colour (a) (a) If one parent is homozygous for the dominant eye colour (BB) and the other parent is homozygous for the recessive eye colour (bb), then all their children will be heterozygous for eye colour (Bb) and will have brown eyes. Source: Adapted from Klug, W.S. and Cummings, M.R., Concepts of Genetics (2nd edition). Glenview, IL: Scott, Foresman, 1986. © 1986 Scott, Foresman & Co. Reprinted by permission of Addison Wesley Educational Publishers, Inc.

Patterns of inheritance for eye colour (b) (b) If one parent is heterozygous (Bb) and the other parent is homozygous recessive (bb), then their children will have a 50% chance of being heterozygous (brown eyes) and a 50% chance of being homozygous recessive (blue eyes). Source: Adapted from Klug, W.S. and Cummings, M.R., Concepts of Genetics (2nd edition). Glenview, IL: Scott, Foresman, 1986. © 1986 Scott, Foresman & Co. Reprinted by permission of Addison Wesley Educational Publishers, Inc.

Patterns of inheritance for eye colour (c) (c) If one parent is homozygous dominant (BB) and the other parent is heterozygous (Bb), then their children will have a 50% chance of being homozygous for the dominant eye colour (BB) and will have brown eyes, and a 50% chance of being heterozygous (Bb) for the trait and will have brown eyes. Source: Adapted from Klug, W.S. and Cummings, M.R., Concepts of Genetics (2nd edition). Glenview, IL: Scott, Foresman, 1986. © 1986 Scott, Foresman & Co. Reprinted by permission of Addison Wesley Educational Publishers, Inc.

Concordance rates between MZ and DZ twins for various traits This table compares concordance rates between monozygotic (MZ) and dizygotic (DZ) twins for several traits. Where a trait exhibits high concordance for MZ twins but low concordance for DZ twins (e.g. for blood type or eye colour), we can conclude that this trait may be strongly affected by genetics. Where the concordance rates are similar, the effect of heredity is low. Source: Klug, W.S. and Cummings, M.R., Concepts of Genetics (2nd edition). Glenview, IL: Scott, Foresman, 1986.

Concordance rates between MZ and DZ twins for (a) attitudes and (b) personalities This table shows the results of a study of the attitudes and personalities of MZ and DZ twins (Olson et al, 2001). 26 out of the 30 attitude items showed strong genetic effects with MZ twins more likely to share similar attitudes than DZ twins. The same pattern emerged for the measures of personality: 25 out of the 26 personality items showed string genetic effects with MZ twins more likely to share similar personality traits than DZ twins. Source: Adapted from Olson, J.M., Vernon, P.A., Harris, J.A., and Jang, K.L. The heritability of attitudes: A study of twins. Journal of Personality and Social Psychology, 2001, 80(6) 845–860, copyright 2001 by the American Psychological Association, reprinted with permission.

Concordance rates between MZ and DZ twins for (a) attitudes and (b) personalities (continued) Source: Adapted from Olson, J.M., Vernon, P.A., Harris, J.A., and Jang, K.L. The heritability of attitudes: A study of twins. Journal of Personality and Social Psychology, 2001, 80(6) 845–860, copyright 2001 by the American Psychological Association, reprinted with permission.

Reproductive strategies Different numbers of males mating with different numbers of females yields four reproductive strategies: 1. Monogamy - one male mates with one female. 2. Polygyny - one male mates with more than one female. 3. Polyandry - one female mates with more than one male. 4. Polygynandry - several females mate with several males.

Waist-to-hip ratio and physical attraction Research suggests that two potentially critical features of physical attractiveness in women are waist-to-hip ratio (WHR) and weight adjusted for height (the BMI). Some studies have shown that BMI is the more important of the two. Using photographs of real women in which the WHR had been subtly manipulated, Tovee and Cornelissen (2001) found that BMI, not WHR, was the better predictor of attractiveness; this was irrespective of whether the raters were male or female. Furthermore, both sexes preferred the figures with the lowest WHR (i.e. those with a curvaceous figure).

Attractiveness ratings of men against (a) waist-to-chest ratio and (b) body mass index In a study of what makes the ideal male body shape, Maisey et al (1999) found that the best predictor of a man’s attractiveness was his waist-to-chest ratio (as illustrated in graph (a) on this slide). Graph (b) illustrates attractiveness ratings plotted against BMI. Source: Maisey, D.S., Vale, E.L.E., Cornelissen, P.L. and Tovee, M.J., Characteristics of male attractiveness for women. The Lancet, 1999, 353, 1500, reprinted with permission from Elsevier.

Infidelity A sample of 196 homosexual and heterosexual participants was asked whether they would be more upset by sexual infidelity or by emotional infidelity (Harris, 2002). Participants were also asked about their experience of actual infidelity. Results showed that heterosexual men were more likely to be upset by imagined sexual infidelity while women were more likely to be upset by emotional infidelity (graph (a)). When participants recalled actual infidelity, no sex differences were found. Regardless of sexual orientation, men and women focussed more on a partner’s emotional infidelity than on their sexual infidelity (graph (b)). Source: Harris, C.R., Sexual and romantic jealousy in heterosexual and homosexual adults. Psychological Science, 2002, 13(1), 7–12, reprinted by permission of Blackwell Publishers Ltd.